EMRP – Earth Magnetism & Rock Physics

EMRP1.14 – Multiscale rock damage in geology, geophysics and geo-engineering systems

Failure and fault slip in crystalline rocks is associated with dilation. When pore fluids are present and drainage is insufficient, dilation leads to pore pressure drops, which in turn lead to strengthening of the material. We conducted laboratory rock fracture experiments with direct in-situ fluid pressure measurements which demonstrate that dynamic rupture propagation and fault slip can be stabilised (i.e., become quasi-static) by such a dilatancy strengthening effect. We also observe that, for the same effective pressures but lower pore fluid pressures, the stabilisation process may be arrested when the pore fluid pressure approaches zero and vaporises, resulting in dynamic shear failure.

We use acoustic emission locations and our fluid pressure sensors to further detail dilatancy-induced stable failure by tracking the progression of the rupture front (i.e., creation of the fault) and the active slip patches of the newly formed fault. In doing so, we are able to link local pore pressure records to the position of the rupture front where dilation is strongest. We see minimal slip in the wake of the rupture front. Once the fault is completed, we observe that the entire fault slips for up to a few minutes, driven by pore pressure recharge of the fault zone. Hence, we directly observe decoupling of rupture and “after”-slip that would otherwise – in a dynamic failure – occur simultaneously.

All our observations are quantitatively explained by a spring-slider model combining slip-weakening behaviour, slip-induced dilation, and pore fluid diffusion. Using our data in an inverse problem, we estimate the key parameters controlling rupture stabilization: fault dilation rate and fault zone storage. These estimates are used to make predictions for the pore pressure drop associated with faulting, and where in the crust we may expect dilatancy stabilisation or vaporisation during earthquakes. For intact rock and well consolidated faults, we expect strong dilatancy strengthening between 4 and 6 km depth regardless of ambient pore pressure, and at greater depths when the ambient pore pressure approaches lithostatic pressure. In the uppermost part of the crust (<4 km), we predict vaporisation of pore fluids that limits dilatancy strengthening.

How to cite: Aben, F. and Brantut, N.: How dilatancy-induced pore pressure changes control rupture and slip during failure experiments in crystalline rock., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5711, https://doi.org/10.5194/egusphere-egu22-5711, 2022.

EGU22-5822 | Presentations | EMRP1.14

The effect of loading rate on the mechanical behaviour and deformation mechanisms in reservoir sandstones

Mark Jefferd, Suzanne Hangx, and Chris Spiers

Superhot Rock (SHR) geothermal projects (e.g., Japan Beyond-Brittle Project, Iceland Deep Drilling Project, and Newberry Volcano) seek to extract heat from geothermal reservoirs where water reaches a supercritical state (≥ 400 °C). Exploiting such a resource could multiply the electrical power delivered by geothermal wells by almost an order of magnitude. However, SHR reservoirs are hosted in semi-brittle to ductile rocks where fluid flow, porosity, permeability, and rock mechanics are still poorly understood. We conduct experiments in a newly designed, internally heated, gas-confining triaxial apparatus (located at EPFL, CH) where we deform reservoir-type samples under realistic SHR temperature, pressure, and strain rate conditions. Deep well core samples (40 x 20 mm) of andesitic basalts (porosities of 8–10%) from Newberry Volcano (US), were subjected to increasing confinement pressure (25–100 MPa) and temperature (20–500 °C) while continuously recording gas permeability via harmonic permeability. Additionally, triaxial deformation experiments were done at strain rates of 10-6 s-1, confinement up to 100 MPa, temperature up to 500 °C, and up to 8% strain while recording permeability. Results were compared with granite samples from Lanhelin (Fr.). Samples were ductile (e.g., no localization of strain) at relatively low pressure–low temperature conditions (100 MPa, 200 °C). Moreover, permeability in samples subjected to hydrostatic conditions rapidly decreased several orders of magnitude from an initial value of 5.10-20 m2 to less than 10-22 m2  at 50 MPa and 200 °C, effectively impermeable. Thus, permeability decreases rapidly in the ductile regime with strain to reach below measurable values at around 3% strain, and it remains so during subsequent semi-brittle flow up to 8% strain. We interpret this rapid decay of permeability as a result of the conjoined effect of ductile pore collapse and plastic deformation of the poorly crystalline matrix present in the sample. These insights further underline the need for advanced, sustainable reservoir engineering techniques in order to extract heat from high enthalpy geothermal reservoirs.

How to cite: Violay, M. and Meyer, G.: Permeability evoluation at the brittle to ductile transition in newberry volcano basalt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7387, https://doi.org/10.5194/egusphere-egu22-7387, 2022.

EGU22-4697 | Presentations | EMRP1.14

Laboratory assessment of rock fracturing state using infrared thermography

Federico Franzosi, Stefano Casiraghi, Roberto Colombo, Chiara Crippa, and Federico Agliardi

 

The fracturing state of rocks is a fundamental control on their hydro-mechanical properties at all scales and provides a descriptor of the evolution of brittle deformation around faults, underground excavations, and slopes. Its quantitative assessment is thus key to several geological, engineering and geohazard applications.

Descriptors of rock fracturing are diverse depending on considered scale, fracture topology (traces, surfaces) and sampling dimension (linear, areal, volumetric). A complete representation of fracture distribution and abundance in a 3D space can be obtained in the laboratory by non-destructive imaging techniques (e.g. X-ray CT), in terms of volumetric fracture intensity (P32) and porosity (P33). Nevertheless, geophysical imaging is usually unable to resolve small objects in fractured media at field scale. Window and scanline sampling strategies are easily applied in the field to measure fracture intensity descriptors (e.g. P10, P21) or empirical rock mass quality indices (e.g. GSI), but are affected by scale and fracture orientation biases. Some authors suggested that rock mass fracturing states can be characterized by measuring their heating and cooling response through infrared thermography (IRT), but a physically-based, generalized approach to prediction is lacking.

In this perspective, we carried out an experimental study on the thermal response of rock samples with known fracturing state. We studied cylindrical samples of gneiss (7) and schist (8), pre-fractured in uniaxial compression that produced complex fracture patterns constrained by rock composition and fabrics.

Using MicroCT (voxel: 0.625 mm) we reconstructed the 3D fracture network and computed the P32 and P33 of each sample. Then, we set up cooling experiments in both laboratory and outdoor conditions. In laboratory experiments, samples were oven-heated at 80°C and let cool in a controlled environment. Sample surface temperature during cooling was imaged in time lapse using a FLIRTM T1020 IRT camera. In outdoor experiments, samples underwent natural solar forcing in a daily heating-cooling cycle.

The acquired multi-temporal thermal images were processed to extract: a) spatial temperature patterns corresponding to the response of individual features and fracture networks at different cooling steps; b) time-dependent cooling curves, described in terms of Cooling Rate Indices and a Curve Factor. These descriptors show statistically significant correlations with fracture abundance measures, stronger with P33 than with P32 and more robust for gneiss samples, characterized by more distributed fractures than schist. More fractured rocks cool at faster rates and the corresponding cooling curve shapes can be normalized to remove the effects of lithology and boundary conditions to obtain a predictive tool. Experimental results have been reproduced by 3D finite-element modeling of the cooling process in numerical samples including explicit fracture objects. Model results closely reproduce experimental data when fracture surfaces are included as convection surfaces, suggesting that overall sample cooling rates depend on the size of individual blocks forming the sample. Results of outdoor experiments show that differences in thermal response can be significantly detected even in natural conditions. Our results provide a starting point to develop an upscaled, quantitative methodology for the contactless in situ assessment of fracturing state of rock masses using thermal data.

How to cite: Franzosi, F., Casiraghi, S., Colombo, R., Crippa, C., and Agliardi, F.: Laboratory assessment of rock fracturing state using infrared thermography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4697, https://doi.org/10.5194/egusphere-egu22-4697, 2022.

EGU22-1434 | Presentations | EMRP1.14

Multiscale analysis of physical rock properties at Stromboli Volcano: what controls the frictional properties?    

Thomas Alcock, Sergio Vinciguerra, and Phillip Benson

Stromboli volcano, located in the north-easternmost island of the Aeolian archipelago (Southern Italy) and well known for its persistent volcanic activity, has experienced at least four sector collapses over the past 13 thousand years. The most recent activity resulted in the formation of the Sciara del Fuoco (SDF) horseshoe-shaped depression and a tectonic strain field believed to have promoted flank collapses and formed a NE / SW trending weakness zone across the SDF and the western sector of the island. The tectonic strain field interplayed with dyking and fracturing appears to control the episodes of instability and the onset of slip surfaces. This study presents new data identifying areas of damage that could promote fracturing via remote sensing and rock friction measurements taken on rocks around the SDF and the coupled “weak” zone. We have carried out a multiscale approach by integrating satellite observations with block and sample scale physical and mechanical properties and frictional tests carried out in triaxial configuration on cm scale slabs. Over 5000 individual fractures have been at first processed through the MatLab toolbox FracPaQ to determine fracture density, slip and dilatancy tendency around the collapse scarp with results showing that dilation and slip 0.6< is more common the northern side of the SDF as well as around areas of eruptive activity.

Key units have been sampled on the field (Paleostromboli, Vancori and Neostromboli) with reference to SDF and the weak zone. Physical and mechanical properties defined using elastic wave velocities, electrical resistivity, uniaxial compressive strength and elastic moduli have been assessed and inverted for comparison to field scale geophysical investigations. Finally, direct-shear tests in triaxial configuration were carried out to explore the frictional properties using rectangular basalt slabs at 5 – 15 MPa confining pressure in dry and saturated conditions. Preliminary results show a variation in the friction coefficient (µ) between 0.55 and 0.7 with a general µ decrease with increasing confining pressure and saturation. The most porous Neostromboli units show the lowest friction.  This suggests that the textural and pre-existing crack damage variability due to the complex and different magmatic history and cooling rates do control the evolution of the frictional properties and evolving fracturing processes. Further work will structurally quantify the slip evolution throughout post-mortem microstructural observation in order to interpret the relations to the field scale weakness zone and the SDF.

How to cite: Alcock, T., Vinciguerra, S., and Benson, P.: Multiscale analysis of physical rock properties at Stromboli Volcano: what controls the frictional properties?    , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1434, https://doi.org/10.5194/egusphere-egu22-1434, 2022.

EGU22-7622 | Presentations | EMRP1.14

Impact of structural geology on the failure mechanisms of a rock fall site in a metamorphic rock mass (Hüttschlag, Austria)

Reinhard Gerstner, Erik Kuschel, Gerald Valentin, Klaus Voit, Wolfgang Straka, and Christian Zangerl

The case study presented herein is located in the alpine environment of Austria (Hüttschlag), in the geologic unit of the Rauris Nappe, belonging to the Glockner Nappe System. The study site is composed of intensively foliated and fractured calc-mica schists and greenschists. Together with several generations of pre-existing discontinuity-sets, they form a rock mass, which has hosted multiple rock fall events since 2019. The rock fall events show a cumulative volume of 41 000 m3, with individual blocks of up to 200 m3 reaching the valley bottom.

In order to gain insights into the interplay between structural geology and the rock fall failure mechanism, we present a combined approach of methods. They act on multiple observation scales: At the micro-scale, intact rock samples are studied by petrographic microscopy of orientated thin sections. This provides insights into the mineralogy of the intact rocks and their inherent brittle and ductile microstructures (e.g. micro-cracks, folding).

In the field, advanced remote sensing techniques were applied, to perform medium- to large-scale investigations. For this purpose, a ground-based radar interferometer (GB-InSAR) was installed for several months. By this, the actual deformation of the unstable rock face and of the rock fall deposit at the slope´s foot was measured at mm resolution. Additionally, several campaigns of terrestrial laser scanning (TLS) enable us to derive high-resolution recordings of the inaccessible rock face, backed by 3D point cloud processing (LIS Pro 3D) tools. For additional displacement measurements and graphic representation of the results, unmanned aerial system photogrammetry (UAS-P) delivers a 3D model of the rock face.

Geological field investigations complete this combined approach, comprising the recording of lithological, hydrogeological and structural geological features. They embed the rock fall site in its geological setting and allow the creation of a 3D discontinuity network, validating the measurements derived from the advanced remote sensing techniques listed above.

The preliminary results promise interesting insights into the interplay between distinctive structural features and the failure mechanisms of the rock fall site in Hüttschlag, working on variable scales: From micro-structures to well-defined discontinuities, that may be reactivated in course of the rock fall process. This broad database serves as the basis for numerical modelling, intensifying the investigation of failure mechanisms. Furthermore, the high-resolution recordings of the instable rock face derived from UAS-P and TLS allow us to assess the potential failure volume of future rock fall events, contributing to the rock fall site´s hazard assessment subsequently.

How to cite: Gerstner, R., Kuschel, E., Valentin, G., Voit, K., Straka, W., and Zangerl, C.: Impact of structural geology on the failure mechanisms of a rock fall site in a metamorphic rock mass (Hüttschlag, Austria), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7622, https://doi.org/10.5194/egusphere-egu22-7622, 2022.

EGU22-7429 | Presentations | EMRP1.14

Hydromechanical Coupling and Damage at a Retreating Glacier Margin

Marc Hugentobler, Simon Loew, and Jordan Aaron

In deglaciating environments, rock slopes are affected by stress perturbations driven by mechanical unloading due to ice downwasting and concurrent changes in thermal and hydraulic boundary conditions. Since in-situ data is rare, the different processes and their relative contribution to slope damage remain poorly understood. Here we present detailed analyses of subsurface pore pressures and micrometer scale strain time-histories recorded in three boreholes drilled in a rock slope aside the retreating Great Aletsch Glacier (Switzerland). Additionally, we use monitored englacial water levels, climatic data, and annually acquired ice surface measurements for our process analysis.

At the timescale of days, diurnal meltwater cycles and rainfall infiltration into the glacier during summertime cause strong pressure fluctuations in the subglacial drainage channel that diffuse into the adjacent rock aquifer. We show that the pressure diffusion from the subglacial meltwater channel, through the fractured bedrock below the glacier ice, to the ice-free bedrock slope occurs under predominantly confined conditions. In the adjacent ice-free bedrock, rainfall infiltration can cause strong variations in the phreatic groundwater table of the slope. On the seasonal timescale, glacial hydraulic boundary conditions vary with high, relatively constant englacial water levels during wintertime and lower mean englacial water levels during summertime. Above ice elevations, snowmelt infiltration during springtime causes yearly maximum phreatic groundwater tables and a general recession over the rest of the year, that is interrupted by summertime rainfall infiltrations. The seasonality in hydraulic head levels of both the glacier and the rock slope controls the interaction of the two systems. On timescales of decades, phreatic groundwater levels in the rock slope are often assumed to be linked to the ice elevation of temperate glaciers. According to our data, this head boundary effect of the glacier is mainly effective during wintertime when it controls the minimum groundwater level in the slope.

Our results show that the variations in mechanical boundary conditions (or loads) caused by a temperate valley glacier on the adjacent rock slope are more complex than had been previously described. Our observed rapid bedrock strain signals coincide with some of the extreme englacial water level states, and are likely caused by rapid changes in the mechanical load of the glacier with an empty or water filled englacial drainage system. Similarly, but at seasonal timescales, the spring and fall transition time of the englacial hydrological system coincides with characteristic strain reactions in our bedrock slope. Our in-situ data show that these effects also promote progressive rock mass damage, probably similar to hydromechanical effects. Additionally, we show how a single extreme rainstorm event triggers hydromechanical damage exceeding the levels of two years exposition to all the other drivers for progressive rock mass damage in this environment.

The magnitude and impact of coupled cyclic processes in a paraglacial rock slope vary with location on the slope and the process considered. The strongest damage is observed directly at the actively reteating glacer margin and moves through the slope at relatively high speed.

How to cite: Hugentobler, M., Loew, S., and Aaron, J.: Hydromechanical Coupling and Damage at a Retreating Glacier Margin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7429, https://doi.org/10.5194/egusphere-egu22-7429, 2022.

EGU22-9278 | Presentations | EMRP1.14

Multiscale characterization of chaotic rock body for mining backfill remediation

Chiara Caselle, Sabrina Maria Rita Bonetto, Pietro Mosca, Arianna Paschetto, Davide Vianello, Andrea Garello, and Fabio Paletto

The sustainability of geomineral resourses’ exploitation may be assured only in presence of adequate plans for the re-use and reclamation of old or abandoned sites. Among the most commonly used techniques, mining backfill is largely employed for the stabilization of underground sites. This technique recreates the original stress state of the underground, assuring the definitive stabilization of the hypogea volumes, and reduces the risks due to the interference between underground tunnels and ground surface (e.g. possible collapses and surface subsidences). Despite these obvious advantages, careful evaluations are needed to assure the environmental sustainability, with particular attention to the interaction between the hydro-geological and permeability features of the rock body and the chemical properties of the backfill material.

The present research proposes an analysis of the advantages and the risks connected with this technique, examining a case study of mining backfill in an underground gypsum quarry at the end of the active exploitation. The considered quarry is located in Monferrato (NW Italy) and is exploited within chaotic Messinian deposits made of gypsum blocks (from centimeter-size to kilometer-size) included in a marly matrix. The study includes a campaign of field and laboratory tests (i.e. geological and geo-structural mapping and modeling, geophysical surveys, mechanical and permeability tests) that aim at characterize the permeability and mechanical behaviour of the rock mass.

How to cite: Caselle, C., Bonetto, S. M. R., Mosca, P., Paschetto, A., Vianello, D., Garello, A., and Paletto, F.: Multiscale characterization of chaotic rock body for mining backfill remediation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9278, https://doi.org/10.5194/egusphere-egu22-9278, 2022.

EMRP1.15 – Petrophysics and rock physics across the scales: integrating models, laboratory experiments and field geophysical studies

EGU22-228 | Presentations | EMRP1.15

Sensitivity study of C/O logging measurements by the Monte Carlo method

Jozsef Gabor Szucs and Laszlo Balazs

Carbon oxygen ratio (C/O) logging is an important method for the accurate determination of hydrocarbon saturation in the reservoir region. One of the advantages of the measurement, that it is independent of Cl content. Furthermore, the insensitivity of high energy neutrons to the casing, makes it possible to use it in cased boreholes too. In addition to the application in the oil industry, monitoring of CO2 reservoirs is also possible. In our study, the modeling of the time-dependent coupled neutron-gamma field produced by the tool was carried out, using MCNP, a general-purpose Monte Carlo radiation transport code. The energy spectrum of gammas reaching the scintillation detector crystal were simulated in different detector positions, and different tool environments: reservoir rock, reservoir porosity, hydrocarbon saturation, and well status (cased or open). The effect of the parameters above are illustrated by vertical cross sections of the particle fluxes around the tool, and shown by the changes of the interpretation charts. By the introduction of a “goodness” factor, derived from the interpretation chart, the potential, and the limitations of C/O logging are investigated.

How to cite: Szucs, J. G. and Balazs, L.: Sensitivity study of C/O logging measurements by the Monte Carlo method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-228, https://doi.org/10.5194/egusphere-egu22-228, 2022.

EGU22-660 | Presentations | EMRP1.15

The role of pore space topology on ultrasonic wave propagation in volcanic rocks.

Maria Del Pilar Di Martino, Luca De Siena, and Nicola Tisato

At the field scale, petro-elastic models linking seismic velocities with porosity have been widely used to estimate properties of reservoirs and subsurface domains in general. At the laboratory scale, frame elastic properties and porosity are not enough to predict the full ultrasonic wave propagation, and other factors like texture, pore space topology and fluid interactions play a significant role. In dry volcanic rocks characterized by larges vesicles, the heterogeneities triggering the perturbations of the ultrasonic wavefield mainly correspond to the pore space topology. However, the sensitivity of S-waveforms to the pore space has not been examined in volcanic rocks.

To assess the role of the pore space on ultrasonic wave propagation, we performed computational simulations on 2D synthetic samples analogous to volcanic rocks, resembling the acquisition of S-waveforms in laboratory experiments. The computational framework applied is the spectral-element method. The porosity and aspect ratio on the study samples was kept constant along the simulations to focus on the effect that the pore space parameters have on the wave arrival, amplitude, and shape of the waveforms.

This study shows that the pore space topology controls the waveform of ultrasonic waves in dry volcanic rocks, and parameters like amount, size, and even the location of the pores impact the elastic wave propagation independently of the porosity value. This finding has important implications for forward modelling seismic signals of heterogeneous volcanic media at the field scale.

How to cite: Di Martino, M. D. P., De Siena, L., and Tisato, N.: The role of pore space topology on ultrasonic wave propagation in volcanic rocks., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-660, https://doi.org/10.5194/egusphere-egu22-660, 2022.

EGU22-668 | Presentations | EMRP1.15

Homogenization of Poroelastic Media without a Representative Elementary Volume for Seismic Applications

Edith Sotelo Gamboa, Nicolas D. Barbosa, Santiago G. Solazzi, Marco Favino, J. German Rubino, and Klaus Holliger

The substitution of a heterogeneous poroelastic medium by its homogenized viscoelastic representation is an effective technique to study its seismic response. This homogenization procedure reproduces the dispersive behaviour of the original fast P- and S-waves. This dispersive nature results from energy dissipation that occurs when a wave induces pressure gradients between the heterogenous parts of a poroelastic medium that are equilibrated by fluid exchange. The underlying homogenization approach is to apply oscillatory tests on a representative elementary volume (REV) of the poroelastic medium to find the equivalent moduli. The REV is a sample that is typical of the entire medium under consideration and that ensures results independent of boundary conditions. This is, the REV should be larger than the heterogeneities but much smaller than the medium size. Additionally, in poroelastodynamics, the size of the heterogeneities in the REV is dictated by the scale at which the wave-induced fluid exchange takes place. We focus on the mesoscale. At this scale, fluid exchange occurs between heterogeneities that are larger than the grain size but smaller than the wavelength. However, there are poroelastic media of interest that present heterogeneities of comparable size to that of the domain. Here, the REV concept does no longer apply since the poroelastic sample under examination is affected by the boundaries of the domain. For such scenarios, we propose a novel homogenization method that incorporates the boundary effects produced by the surrounding medium. In this method, we take a sample that consist of the affected poroelastic heterogeneity together with part of the embedding medium. Then, we perform the classical oscillatory tests over this ensemble. Finally, to obtain the homogenized moduli of the poroelastic medium, we perform the averaging of strain and stress only over this domain of interest. As examples, we present a poroelastic system of a single sand layer saturated with gas at the top and water at the bottom that is embedded in impermeable background. We also study a water-saturated poroelastic set consisting of a permeable fracture surrounded by a less permeable damage zone that is also embedded in impermeable background. We idealize these cases as 2D media, assuming that the poroelastic system is infinite along the layering plane but bounded perpendicular to it by impermeable half-spaces. The samples subjected to oscillatory tests consist of a piece of the semi-infinite poroelastic domain together with the corresponding bounding half-spaces. To test the viability and accuracy of the method, we compare reflectivities at the top interface of the half-space and homogenized medium against those obtained at the top interface of the half-space and the original poroelastic system. Results show that errors are of the order of 1 %. The proposed method can be readily extended to 3D and more complex models.

How to cite: Sotelo Gamboa, E., Barbosa, N. D., Solazzi, S. G., Favino, M., Rubino, J. G., and Holliger, K.: Homogenization of Poroelastic Media without a Representative Elementary Volume for Seismic Applications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-668, https://doi.org/10.5194/egusphere-egu22-668, 2022.

EGU22-945 | Presentations | EMRP1.15

Controls on Sonic Velocity in Dolostones

Moaz Salih, Ammar El-Husseiny, John J.G. Reijmer, Hassan Eltom, Abdallah Abdelkarim, and Mike A. Kaminski

Dolostones represent one of the major hydrocarbon reservoirs in the world. Understanding the elastic behavior of these units is crucial for hydrocarbon exploration and/or development. In this study, 100 samples from five formations within the Arabian Platform, were used to examine the main controlling factors on the sonic velocity of dolostones. A combination of field and laboratory analyses were conducted on the collected samples including; thin-section petrography, SEM, XRD, digital image analysis, porosity and permeability measurements, velocity measurements, and rock physics modeling. The studied samples have a wide range of porosity (1- 45%, averaging 18.5 %), and permeability (0.01 2000 mD, averaging 196 mD). Compressional VP and shear wave VS velocity ranges from 3.0 - 6.7 km/s, and 1.6 3.7 km/s, respectively. In general, porosity-velocity trajectory is showing a negative relationship with a coefficient of determination R2 of 0.82. However, some samples are deviated from this trendline due to their inherited and diagenetic parameters. These parameters include texture, mineralogy, pore type, and crystal size. Fabric-preserving dolostones have, relatively, higher velocities than non-fabric preserving dolostones. Although 95% of the studied samples are dominated by dolomite, samples with higher content of calcite and quartz, have lower velocities. Moldic and vuggy pore-dominated samples have, relatively, higher velocities than samples dominated by intercrystalline pores and microporosity. For non-fabric preserving dolostones, samples with larger crystals show higher velocities than samples with smaller crystals. Using equivalent pore aspect ratio (EPAR), a clear distinction between permeable (>10 mD) and tight (< 10 mD) samples can observed, where most of the permeable samples have high EPAR values, while the tight samples have low EPAR values. The result of this study might significantly help in the interpretation and understanding of the sonic logs and seismic data from dolostone strata.

How to cite: Salih, M., El-Husseiny, A., Reijmer, J. J. G., Eltom, H., Abdelkarim, A., and Kaminski, M. A.: Controls on Sonic Velocity in Dolostones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-945, https://doi.org/10.5194/egusphere-egu22-945, 2022.

EGU22-3523 | Presentations | EMRP1.15

How to discover ancient stress-levels preserved within fractures using the stress-memory effect of specific stiffness

Christian Kluge, Lena Muhl, Daniel Schramm, and Guido Blöcher

Stress changes have a large impact on the hydraulic and mechanical properties of fractures and can be caused by varying the fluid pressure in a subsurface reservoir or by tectonic movements. Innovative tools to assess the stress conditions are still of major importance for most subsurface applications. We derived an experimental procedure to reveal stress signals preserved in fractures in the laboratory.

In a set of complex experiments various fractured low-permeability rocks, two sandstones and two crystalline rocks, were cyclically loaded in a MTS tri-axial compression cell. The preconditioned cylindrical samples were split into two halves to generate an artificial tensile fracture and a rigid shear displacement was applied before installing the sample into the apparatus. Two different loading scenarios were applied: “continuous cyclic loading” (CCL) and “progressive cyclic loading” (PCL). During continuous cyclic loading samples were loaded from 2 to 60 MPa in several repeated cycles. In the progressive cyclic loading experiments the hydrostatic confining pressure was increased using a step-wise function (15, 30, 45 and 60 MPa) and was unloaded after every sub-cycle, while the pore pressure was kept constant at a low level. The mechanical fracture closure was monitored continuously during the experiments using axial and circumferential extensometers and the specific fracture stiffness could be calculated at a very high resolution. The fracture permeability was measured continuously using four high-pressure fluid pumps. A 3D surface scanner was used to analyze the fracture surface geometry before and after the experiments to reveal possible changes to the surface topography as well as to quantify changes in aperture and contact-area ratio.

The specific fracture stiffness was shown to be irreversible when a fracture was hydrostatically loaded once. Further, a “stress-memory” effect of fracture stiffness could be shown during progressive loading. It is characterized by a change from non-linear to linear stiffness evolution when a previous stress-level is exceeded. This phenomena can be used to identify previous stress states preserved within fractures. Additionally, this data is important for elasto-plastic contact theories of rough fractures. The impact of progressive loading on fracture permeability evolution showed varying results based on the heterogeneity and mineral composition of each rock type and the resulting fracture geometry.

How to cite: Kluge, C., Muhl, L., Schramm, D., and Blöcher, G.: How to discover ancient stress-levels preserved within fractures using the stress-memory effect of specific stiffness, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3523, https://doi.org/10.5194/egusphere-egu22-3523, 2022.

EGU22-5185 | Presentations | EMRP1.15

NMR investigation of boundary condition effects on spontaneous imbibitionin Longmaxi shale

Yong Liu, Yanbin Yao, Dameng Liu, and Chi Zhang

Spontaneous imbibition is an important process that wetting fluid displaces the non-wetting fluid in the rock by capillary force and is responsible for low flowback efficiency (<30%) of fracturing fluid and severe water blocking effects in shale gas reservoirs. It is crucial to understand the boundary condition effects on imbibition dynamics in shale and the shale-fluid interactions as they provide insights into fracturing fluid loss which can influence gas production. In this study, we designed imbibition experiments and used nuclear magnetic resonance (NMR) to investigate spontaneous imbibition behaviors and water-shale interactions in shale samples with varied boundary conditions including all-side-open (ASO), two-side-open (TSO), one-side-open (OSO), half-side-open (HSO) and two-side-closed (TEC) to. These five boundary effects in imbibition were analyzed by dividing the imbibition stages and comparing the imbibition dynamics. Key imbibition parameters including water saturation, gas recovery factor, residual gas saturation, imbibition capacity, diffusion ability, imbibition rate, and imbibition potential under the respective boundary conditions were selected to compare the imbibition features of five boundary effects. Our results elucidate the existence of three types of water imbibition patterns including the radial counter-current imbibition as shown in TEC boundary condition, the axial co-current imbibition as shown in OSO and TSO conditions, and the compound imbibition which exhibits both radial counter-current imbibition and axial co-current patterns as in ASO and HSO. T2 relaxation times in OSO and TSO shifted to larger relaxation times as imbibition occurred, demonstrating the induced microfractures were generated in water imbibition due to shale-water interactions. Furthermore, imbibition parallel to the bedding plane and imbibition vertical to the beddings have different water migration patterns due to bedding structures of shale. Our experiments contribute to the understanding of the mechanisms of how different boundary conditions affect imbibition dynamics and shale-water interactions in shale gas reservoirs, which is valuable to the interpretation of fracturing liquid retention processes.

How to cite: Liu, Y., Yao, Y., Liu, D., and Zhang, C.: NMR investigation of boundary condition effects on spontaneous imbibitionin Longmaxi shale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5185, https://doi.org/10.5194/egusphere-egu22-5185, 2022.

EGU22-5194 | Presentations | EMRP1.15

The poroelastic response of cracked Westerly granite to cyclical changes in load

Bobby Elsigood, Nicolas Brantut, Philip Meredith, Tom Mitchell, and David Healy

The poroelastic behaviour of cracked rocks is expected to depend on the geometry and properties of the crack network. Any preferred orientation of microcracks produces anisotropy in physical rock properties, including poroelastic parameters. Under conventional triaxial loading there is an alignment of cracks parallel to the vertical direction of compression, leading to vertical transverse isotropy in the cracked rock.

Here, we repeatedly measured transversely isotropic poroelastic parameters during increasing amplitude cyclic loading in a sample of Westerly granite saturated with water. Independent step changes in confining pressure and differential stress were repeated at selected levels of differential stress to measure the change and reversibility in the transversely isotropic parameters throughout the loading and unloading cycles.

We used miniature differential pressure transducers which were located directly around the sample surface, allowing for direct measurement of the pore pressure in the sample. The direct measurements of pore pressure allow us to estimate undrained properties, including Skempton’s coefficients. Axial and radial strain gauges allow for the calculation of elastic moduli from the step changes in axial and radial stress. We determine the undrained moduli from the initial short-term response, and the drained moduli following pore pressure equilibration for each step change in stress.

Results show that the radial Skempton’s coefficient increases with increased differential stress, and the axial coefficient decreases and even becomes negative (where increases in axial stress cause a decrease in pore pressure) at high stress (i.e., about 80% of failure stress). During unloading, the measured Skempton coefficients are observed to be recovered, without hysteresis.

How to cite: Elsigood, B., Brantut, N., Meredith, P., Mitchell, T., and Healy, D.: The poroelastic response of cracked Westerly granite to cyclical changes in load, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5194, https://doi.org/10.5194/egusphere-egu22-5194, 2022.

EGU22-8360 | Presentations | EMRP1.15

Numerical modelling of pressure-dependent permeability in Bentheim sandstone

Mirko Siegert, Marcel Gurris, and Erik H. Saenger

A method for the numerical determination of pressure-dependent permeability in sandstones is developed. The proposed method is restricted to a hydrostatic pressure load that is below the pore collapse pressure.

Our modelling approach is generally based on the idea of digital rock physics. Starting from a µCt-scan, the pore space of a given rock sample is detected and transferred into a numerical model. Subsequently, the stationary Stokes equations are solved, and the permeability is determined from the simulated pressure and velocity fields.

To model the pressure dependence, it is assumed that the deformation of the rock's micro-structure due to pore throat closing has a significant influence on the change in permeability. In our workflow, the respective pore throats between the individual grains of the original CT image are reconstructed using the watershed algorithm and combined in a separate phase of the numerical model. During several simulations, a steadily increasing artificial flow resistance is assigned to the pore throat phase and the respective permeability of the whole sample is determined. Finally, the pressure-dependent permeability curve can be reconstructed via a correlation between flow resistance and pressure load.

The proposed workflow is validated with externally published data of a Bentheim sandstone sample. It is observed that the model is generally able to reproduce the characteristics of a experimentally determined pressure-dependent permeability curve.

How to cite: Siegert, M., Gurris, M., and Saenger, E. H.: Numerical modelling of pressure-dependent permeability in Bentheim sandstone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8360, https://doi.org/10.5194/egusphere-egu22-8360, 2022.

EGU22-8775 | Presentations | EMRP1.15

Numerical investigation of hydro-mechanical responses of a single fracture embedded in a porous matrix

Guido Blöcher, Christian Kluge, Mauro Cacace, Qinglin Deng, and Jean Schmittbuhl

We have conducted a flow-through experiment using a Flechtingen sandstone sample containing a single macroscopic fracture. Based on this experiment, we obtained range of various intrinsic rock parameters, such as permeability and specific stiffness of the combined matrix-fracture system under hydrostatic loading. In order to quantify the processes behind the laboratory observations, we carried out coupled hydro-mechanical simulations of the matrix-fracture system. Navier-Stokes flow was solved in the 3-dimensional open rough fracture domain, and back-coupled to Darcy flow and mechanical deformation of the rock matrix.

To capture the volumetric shape of the fracture, the two fracture surfaces were scanned using a 3D-profilometer (Keyence VR-3200) before and after the experiment. The resulting fracture surfaces were aligned using a grid-search algorithm and subsequently offset to mimic the shear displacement as applied during the laboratory experiment. Based on the obtained 3D representation of the fracture volume embedded in a porous media, the stress path of the laboratory experiment was simulated numerically. By means of the simulation results, values of fracture closure, increase of contact area, fracture permeability and fracture stiffness due to normal load on the fracture surface were obtained.

The results demonstrate that the numerical simulation could capture the elastic and inelastic behaviour as well as the related permeability alteration of the fracture domain. Both, the laboratory experiments as well as the numerical simulation indicate an inelastic deformation of the single fracture even at low normal stress. The inelastic deformation is expressed by an increase of the fracture contact area and therefore fracture stiffness with increasing stress. The increase in the contact area is due to a reduction in mean aperture and is therefore accompanied by a reduction in the fracture permeability. The development of the contact area is irreversible and thus indicates the maximum stress that the sample previously experienced. We call this behaviour "stress-memory effect".

We present the workflow to obtain the numerical results and a comparison with the laboratory experiment to show that the dominant processes were captured by the simulation.

How to cite: Blöcher, G., Kluge, C., Cacace, M., Deng, Q., and Schmittbuhl, J.: Numerical investigation of hydro-mechanical responses of a single fracture embedded in a porous matrix, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8775, https://doi.org/10.5194/egusphere-egu22-8775, 2022.

Magnetotelluric inversions spanning the Pacific-Australian Plate boundary in New Zealand’s South Island indicate there is a localized zone of low electrical resistivity that is spatially co-incident with the mid-crustal part of the Alpine Fault Zone (AFZ), that currently accommodates shear strain by temperature-sensitive creep. We explored the source of this anomaly by measuring the electrical properties of samples collected from surface outcrops approaching the AFZ that have accommodated a gradient of systematic strain and deformation conditions. We investigated the effects of tectonite fabric, fluid saturated pore/fracture networks and grain surface conduction on the bulk electrical response and the anisotropy of resistivity of these samples measured under increasing confining pressures up to 200 MPa. We find that for fault rock protoliths, Haast and Alpine Schist, resistivity and change in anisotropy of resistivity with confining pressure (δ(ρ)/ δ(peff)) increases while porosity decreases approaching the AFZ. This indicates the electrical response is controlled by pore-fluid conductivity and modified during progressive metamorphism. AFZ mylonites exhibit low electrical resistivities at low porosities, and lower δ(ρ)/ δ(peff) than the schists. These reflect changes in both the porosity distribution and electrical charge transport processes in rocks that have experienced progressive grain size reduction and mixing of phases during development of mylonitic fabrics due to creep shear strain within the AFZ.

How to cite: Toy, V., Kluge, E.-K., and Lockner, D.: Electrical properties and anisotropy of schists and fault rocks from New Zealand’s Southern Alps under confining pressure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9616, https://doi.org/10.5194/egusphere-egu22-9616, 2022.

EGU22-9703 | Presentations | EMRP1.15

Inspecting internal magnetic field gradients in volcanic rocks

Nadjib Chibati and Yves Geraud

Nuclear magnetic resonance (NMR) is being used since 1990 in the petroleum industry. NMR is a powerful tool for petrophysical properties estimation (porosity, permeability, pore size distribution, and irreducible saturation). Despite its large success in the conventional carbonate and sandstone reservoirs, some tight sandstones, volcanic and metamorphic rocks, contain a high amount of paramagnetic and clay minerals, which can complicate the interpretation of NMR results. These complications are due to the inhomogeneities of the internal magnetic field generated by the magnetic susceptibility contrast between the pore-fluid and the matrix. The magnitude of the internal gradients depends on the strength of the background magnetic field, magnetic susceptibility contrast, and pore size.

Many studies are focused on the investigation of the effect of clay and paramagnetic minerals on the internal gradient and their implications on the NMR-derived petrophysical properties mainly of the high magnetic susceptibility sandstones. The primary goal of this analysis is to investigate the magnitude of the internal magnetic gradient of volcanic rocks with different alteration grad and its relationship with the rock properties (magnetic susceptibility, iron, and manganese content, pore type, and pore size).

The data were collected using the Minispec q10®, with Larmor frequency of 10 MHz, on the water-saturated samples with magnetic susceptibility between 26.8 10-3 and -0.4 10-3 SI. The average effective internal gradient was calculated from the slope of the mean log relaxation rate (T2gm-1) versus the squared echo time (TE2). The preliminary results show that samples presented a multi-distribution of T2 peaks corresponding to the different pore types observed for these samples (micro, meso, and macropores). The average effective internal magnetic field gradient calculated from the slope of T2gm-1 vs TE2 ranges from 0 to 43.16 T.m-1. The average effective internal gradient increases with the increase of magnetic susceptibility and decreases as the T2gm increase, suggesting that the pore size also impact internal gradient magnitudes. However, No clear relation exists between iron content and average effective internal gradient.

How to cite: Chibati, N. and Geraud, Y.: Inspecting internal magnetic field gradients in volcanic rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9703, https://doi.org/10.5194/egusphere-egu22-9703, 2022.

EGU22-10087 | Presentations | EMRP1.15

Seismic velocity variations generated by controlled hydrological changes : field and laboratory studies based on seismic noise crosscorrelation

Thomas Gaubert-Bastide, Stéphane Garambois, Clarisse Bordes, Christophe Voisin, Daniel Brito, Philippe Roux, and Laurent Oxarango

Continuous seismic noise recordings has demonstrated a remarkable ablitiy to monitor changes of the investigated media at various scales. In this study, we focused on the link between seismic velocity variations (dv/v) derived from seismic noise cross-correlations between pairs of stations and hydrological variations observed both at the field and lab scales. The field-scale experiment was performed at the water supply pumping site of Crépieux-Charmy (Lyon, France). 99 3-C velocimeters were deployed during 20 days around an infiltration basin operated for managed aquifer recharge and designed to generate an hydraulic barrier to prevent a potential contamination from the nearby river. This dense seismic network set-up allowed to dynamically image the seismic velocity variations during two filling/drainage cycles of the basin. Punctual values extracted from computed high resolution tomographies of the velocity variations were compared to local measurements of the water table level using piezometers. A remarkable agreement was found between the 2 observables in particular during the establishment of a 3D dome in the water table. During drainage phases, systematic response delays were observed which are most probably due to variations of water content in the unsaturated zone between the basin and the water table.
To better understand these effects occurring in the critical zone, we tried to reproduce a similar monitoring experiment at the laboratory scale. A tank filled with sand was designed in order to characterize controled hydrological variations (water table depth, water saturation). We used continuous seismic sources deployed on the edges of the tank. The seismic noise was recorded using 10 3-C accelerometers . The combination of these two approaches at different scales provides a better understanding of the links between seismic velocity and hydrological (water table level and water content in the vadose zone) variations.

How to cite: Gaubert-Bastide, T., Garambois, S., Bordes, C., Voisin, C., Brito, D., Roux, P., and Oxarango, L.: Seismic velocity variations generated by controlled hydrological changes : field and laboratory studies based on seismic noise crosscorrelation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10087, https://doi.org/10.5194/egusphere-egu22-10087, 2022.

EGU22-10389 | Presentations | EMRP1.15

Correlation of magnetic pore fabrics with traditional pore fabric characterization and permeability anisotropy in typical sedimentary rocks and hot isostatically pressed samples

Yi Zhou, Michele Pugnetti, Anneleen Foubert, Pierre Lanari, Christoph Neururer, and Andrea Biedermann

Pore-scale experiments are crucial to obtain pore geometry and distribution, i.e., pore fabrics, controlling preferred fluid flow directions in rocks. Pore fabrics are characterized to derive models, which are important for hydrocarbon exploration and geothermal applications. X-ray computed tomography (XRCT) is one typical method to obtain three-dimensional pore fabrics, but limited by its micron-scale resolution in 1-inch cores. Magnetic pore fabrics (MPFs) were proposed as a fast and efficient way to indirectly measure the pore fabrics, and target micropores down to 10 nm. Empirical relationships exist between MPFs and pore space properties, and between MPFs and permeability anisotropy. Previous studies investigated a limited number of rock types or plastic synthetic samples with simplified pores to compare MPFs, pore fabrics and permeability anisotropy. Permeability is commonly estimated from measurements parallel and perpendicular to the macroscopic fabric, and thus the measurement needs a priori information on the fabric orientation. This study integrates complementary measurements to characterize pore fabrics on various scales: pycnometer porosity, MPF, XRCT, and permeability anisotropy measurements. The specimens include various kinds of sandstones and carbonates to cover the main sedimentary lithologies, and hot isostatically pressed (HIP) samples of simple and controlled compositions to bridge the gap between the synthetic samples of previous studies and complex natural rocks. HIP samples were made by mixing calcite and muscovite powders in different proportions and grain sizes, and were cold pressed at 20 MPa and then hot pressed at 160 MPa and 670 °C. Full permeability tensors including confidence angles were determined, and each tensor was calculated from 7 directional permeability measurements for natural rocks. Considering the uniaxial symmetry of the HIP samples, 3 directional measurements are sufficient to calculate a tensor with confidence angles. One additional core from each block was scanned by XRCT with ~5.5 µm pixel size for 3D pore fabric analysis, prior to being impregnated with ferrofluid to measure MPFs. A total shape ellipsoid, representing the average XRCT-derived pore fabric, is compared with other second-order tensors, permeability anisotropy and MPFs. Initial data suggest that the maximum principal directions of permeability anisotropy, total shape ellipsoids and MPFs are coaxial in homogeneous samples with consistent pore space anisotropy. These confirmed quantitative correlations help to apply MPFs as an efficient method to determine pore fabrics and predict preferred flow direction.

How to cite: Zhou, Y., Pugnetti, M., Foubert, A., Lanari, P., Neururer, C., and Biedermann, A.: Correlation of magnetic pore fabrics with traditional pore fabric characterization and permeability anisotropy in typical sedimentary rocks and hot isostatically pressed samples, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10389, https://doi.org/10.5194/egusphere-egu22-10389, 2022.

Zeta potential is an important petrophysical property that controls electrostatic interactions between mineral, water, and non-aqueous phase fluids. These interactions play an important role in defining the wetting state of reservoir rocks. The zeta potential can be interpreted from the streaming potential measurements, which are shown to be an efficient means for a broad range of applications including monitoring of single- and multi-phase flows in subsurface settings, characterization of fracture networks, efficiency of CO2 sequestration, hydrogen underground storage and enhanced oil recovery. It is widely agreed that the zeta potential in carbonate rocks is controlled by the concentration of potential determining ions (PDI), but the understanding of this is still poor and there are very limited experimental data on quantitative characterization of the dependence of the zeta potential on concentration of negative potential determining ions (PDI) such as SO42-, CO32-, HCO3-, especially when their concentration is high and exceeds that of the positive PDIs.

In this study, the streaming potential method is used to investigate the zeta potential of natural carbonate rock samples in contact with natural aqueous solutions of low-to-high ionic strength and with varying concentration of sulphate (SO42-) and carbon (C4) related (HCO3-, CO32-) ions. In each set of experiments the total ionic strength was kept constant to eliminate the impact of concentration on the zeta potential. The study probed the concentration of negative PDIs that has never been reported before, with their respective lowest concentration consistent with previously reported values, and the highest concentration equal to the maximum achievable by stripping the tested solutions of Cl-.

Our results demonstrate that zeta potentials strongly depend on concentration of the negative PDIs, thus providing explicit empiric relationship between the zeta potential and a broad range of PDI concentration. Our findings improve the current understanding of the complex physicochemical processes that take place at calcite-water interface and provide important experimental data for surface complexation modelling of carbonate-brine systems.

How to cite: Atiwurcha, N., Derksen, J., Vega-Maza, D., and Vinogradov, J.: Zeta Potential of Intact Carbonate Core Samples Saturated with Natural Aqueous Solutions with Varying Concentration of Negative Potential Determining Ions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13060, https://doi.org/10.5194/egusphere-egu22-13060, 2022.

EGU22-13560 | Presentations | EMRP1.15

Do anomalously high Vp/Vs exist in porous reservoir rocks?

Lucas Pimienta

Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur, interpreted as highly pressurized heavily fractured zones. Assuming the rocks isotropic, Vp/Vs can be directly linked to rocks Poisson’s ratio in the elastic regimes relevant to both the field and laboratory measurements. From dedicated measurements across the frequency range it was shown that such insights hold, in agreement with a micromechanical model for isotropic micro-cracked rocks: Anomalously high Vp/Vs exist for any low porosity isotropic rocks of any mineral content, if heavily micro-fractured and at near-lithostatic fluid pressures, i.e. at very low Terzaghi effective pressure.

Extending that understanding, one could question if such anomalous Vp/Vs could also be observed and similarly explained in isotropic porous reservoir rocks. From the typical micromechanical inclusion models for predictions at the sample’s scale, such is unlikely as Poisson’s ratio should largely decrease with an increasing content of spherical pores. Yet, that is not what is measured in the relevant undrained elastic regime in well-cemented porous sandstones. For these, most rocks Poisson’s ratio remain anomalously high and comparable to that retrieved in low porosity rocks. Moreover, while models would then predict a dependence of Poison’s ratio to the liquid’s bulk modulus that is again not consistent with the measurements.

From comparing literature datasets reporting drained and undrained Poisson’s ratio and bulk modulus for sandstones of varying porosity, the aim of this work is to investigate and discuss (i) how the measured properties compare, (ii) if one property or the two deviate from existing models and why.

How to cite: Pimienta, L.: Do anomalously high Vp/Vs exist in porous reservoir rocks?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13560, https://doi.org/10.5194/egusphere-egu22-13560, 2022.

EMRP1.16 – Open session in Rock Physics

EGU22-1147 | Presentations | EMRP1.16

Open access to Dutch Earth scientific research labs and data through EPOS-NL

Ronald Pijnenburg and Richard Wessels and the EPOS-NL team

Top research into the geo-societal challenges of our densely populated planet requires optimized use of available research data, facilities, and funds. Such optimization is the main aim of the European Plate Observing System – Netherlands (EPOS-NL): the Dutch research infrastructure for solid Earth sciences. EPOS-NL provides free of charge, (inter)national access to a unique cluster of large-scale, geophysical labs and data centers at Utrecht University (UU), Delft University of Technology (TU Delft) and the Royal Netherlands Meteorological Institute (KNMI). Lab access can be requested by applying to one of our biannual calls. EPOS-NL labs that can be accessed include A) The Earth Simulation Laboratory at UU and the Petrophysics laboratory at TU Delft, where experimental rock physics and analogue modelling studies can be performed on the fundamental processes that govern the deformation and transport behavior of the Earth’s crust and upper mantle; and B) The Multi-scale Imaging and Tomography (MINT) facilities, distributed over UU and TU Delft. MINT provides unprecedented capabilities in 2D and 3D imaging and microchemical and crystallographic mapping, down to a resolution of several nanometers. EPOS-NL further works with data centers, researchers and industry to improve open access to essential Earth scientific data and models. These include key data relating to the seismogenic Groningen gas field in the Netherlands, notably Distributed Strain Sensing data of the gas reservoir, a Petrel geological reservoir model, developed by the field operator NAM (Nederlandse Aardolie Maatschappij), and a vast amount of seismological data maintained by the ORFEUS Data Centre. Access is provided in the framework of the European infrastructure EPOS, cf. FAIR (Findable, Accessible, Interoperable and Reusable) data principles. In that way, EPOS-NL contributes to shared and cost-effective research into the geo-societal challenges faced by our densely populated planet. See www.EPOS-NL.nl for more information.

How to cite: Pijnenburg, R. and Wessels, R. and the EPOS-NL team: Open access to Dutch Earth scientific research labs and data through EPOS-NL, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1147, https://doi.org/10.5194/egusphere-egu22-1147, 2022.

EGU22-6004 | Presentations | EMRP1.16

Disclosing the redox conditions in Central Portugal magmatism: the Manteigas granodiorite case study

Cláudia Cruz, Joana Dias, Eric Font, Fernando Noronha, and Helena Sant'Ovaia

The presence of magnetite in granitoids is usually rare in the Iberia Peninsula, but can occur depending on the crystal fractionation processes and redox conditions. Here we studied the Manteigas granodiorite in order to characterize and quantify the nature and abundance of ferromagnetic minerals by using petrographic, Isothermal Remanent Magnetization (IRM) curves, and Frequency-dependent Susceptibility (KfD%) experiments. Igneous rocks of the Variscan age are particularly abundant in Central Portugal, including the Manteigas granodiorite that crops out in the Serra da Estrela region (Central Iberian Zone), Portugal. The Manteigas granodiorite is classified as a medium- to coarse-grained slightly porphyritic biotite rock and was dated by U-Th-Pb methods on zircon at 481.1 ± 5.9 Ma. Petrographic studies show that Manteigas is mainly composed of quartz, Ca-plagioclase, K-feldspar, and biotite. As accessory minerals, apatite, chlorite, magnetite ± hematite, and zircon are identified. Muscovite is rare and most of secondary origin. Monazite, sphene-leucoxene, and brookite-anatase are also present but in minor amounts. Microstructures indicate a slight deformation that is reflected in the undulatory extinction in quartz, microfractures in K- feldspar, and curved biotites. Sometimes microfractures in feldspar are filled by later Fe-oxide/hydroxide. Values of magnetic susceptibility (Km) indicate that the Manteigas granodiorite belongs to the magnetite-type rocks, with Km values higher than 1.9 x 10-3 SI (1.9 x 10-3 SI < Km < 188.37 x 10-3 SI). This is consistent with oxidizing conditions in the magma genesis [1]. Furthermore, the oxygen isotope composition (δ18O) measured on whole-rock samples ranges between 8.8 ‰ and 8.9 ‰ [1,2], which suggests a mantle contribution. The analysis of IRM data through the Cumulative Log-Gaussian (CLG) function with the software developed by Kruiver et al. [3] indicates the presence of a single ferromagnetic s.l. component, with values of mean coercivity (Log B1/2) of 1.61 mT and dispersion parameter (DP) of 0.36, typical of magnetite [4]. The value of the IRM at saturation (SIRM) of 18 A/m indicates a significant contribution of magnetite. We also fitted the IRM curve by using a Skewed Generalized Gaussian function with the MaxUnmix software [5] and obtained similar results. Kfd% analyses were conducted in four samples. Values of the Kfd% are inferior to 6%, suggesting a very weak contribution of superparamagnetic particles. Acknowledgments: The first author is financially supported by UIDP/04683/2020 project (FCT-Portugal). This work is also supported by national funding awarded by FCT under UIDB/04683/2020 project. References: [1] Sant’Ovaia, H., Olivier, P., Ferreira, N., Noronha, F., Leblanc, D. 2010. J. Struct. Geol. 32, 1450-1465. [2] Neiva, A., Williams, I.S., Ramos, J.M.F., Gomes, M.E.P. Silva, M.M.V.G., Antunes, I.M.H.R. 2019. Lithos 111, 186-202. [3] Kruiver, P.P., Dekkers, M.J., Heslop, D. 2001. Earth Planet. Sci. Lett. 189, 269–276. [4] Egli, R., 2003. J. Geophys. Res. 108, 2081. [5] Maxbauer, D.P., Feinberg, J. M., Fox, D.L. 2016.  Comput. Geosci. 95, 140–145.

How to cite: Cruz, C., Dias, J., Font, E., Noronha, F., and Sant'Ovaia, H.: Disclosing the redox conditions in Central Portugal magmatism: the Manteigas granodiorite case study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6004, https://doi.org/10.5194/egusphere-egu22-6004, 2022.

Several hypabyssal dykes and masses, generated during the Permo-Carboniferous, outcrop throughout southwest Europe. In Portugal, this magmatic event is related to the transition from a post-collisional to extensional setting that followed the Variscan orogeny and represented by porphyries, lamprophyres, and dolerites which intruded into older metasediments and granites. The magnetic mineralogy, susceptibility, and fabrics of selected dykes from northern Portugal were studied using methodologies such as IRM curve acquisition and treatment, frequency-dependence of the magnetic susceptibility (Kfd), and low-field AMS.

Based on the bulk magnetic susceptibility, the felsic lithotypes are paramagnetic (Km = 0.9-148.2 µSI) but mostly composed of diamagnetic minerals (i.e., quartz and feldspars). AMS in these rocks is mainly carried by iron-bearing silicates, such as biotite and cordierite, as well as ilmenite. However, other iron oxides, namely hematite, goethite, or fine-grained magnetite, also play an important role in the magnetic anisotropy. By contrast, in most of the mafic lithologies (Km = 238.2-15,640.7 µSI), magnetite is an essential component. Other iron-rich minerals such as biotite, amphibole, and pyroxene also influence the anisotropy of these rocks.

Following the IRM data treatment (Kruiver et al., 2001; Maxbauer et al., 2016), all samples reveal at least one magnetite grain population whose mean coercivities (B1/2) and dispersion parameter (DP) range from 18.2 to 70.8 mT and 0.26 to 0.40, respectively. Petrographic observations suggest that most magnetite composing the mafic rocks is multidomain-type. However, Kfd measurements (4.66-18.18%) indicate that superparamagnetic particles are likely to exist in some lithologies, inducing low bulk susceptibilities and anomalous AMS fabrics. On the other hand, inverse fabrics are probably associated with hydrothermal and/or post-magmatic alterations.

Many felsic specimens display a normal fabric, where the magnetic minerals were oriented along the magma flow direction within the dyke and undisturbed by tectonic strains. Such observation is compatible with the average low anisotropy degree (Ppara% = 0.92-4.28%), implying passive emplacement of the melts, and possibly reflects a weak contribution from single-domain-magnetite. Rare cases where the magnetic fabric is intermediate presumably point to more intense deformations.

Magnetite on the mafic rocks is mainly primary. By contrast, since the felsic dykes derived from anatexis of pelitic sources, their generation occurred under reducing conditions, being similar to Ilmenite-type granites. As such, most magnetite in the felsic samples is probably secondary, having resulted from exsolution from Fe-bearing minerals. However, the presence of primary magnetite cannot be ruled out due to possible magma mixing, as suggested by the coexistence of mantled and non-mantled feldspars. The less evolved member involved in the mixing process is likely to carry a more oxidized composition, bearing the potential to crystallize primary magnetite.

This work was supported by the Portuguese Foundation for Science and Technology (FCT), through the project reference UIDB/UIDP/04683/2020 and ICT (Institute of Earth Sciences). The main author is also financially supported by FCT through an individual Ph.D. grant (reference SFRH/BD/138818/2018). References: Kruiver, P.P., Dekkers, M.J., Heslop, D., 2001. Earth Planet. Sci. Lett. 189, 269–276. Maxbauer, D.P., Feinberg, J. M., Fox, D.L., 2016. Computers & Geosciences 95, 140–145.

How to cite: Oliveira, A., Sant'Ovaia, H., and Brites, H.: Unveiling the magnetic mineralogy and magma flow dynamics within subvolcanic dykes from northern Portugal, Central Iberian Zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6331, https://doi.org/10.5194/egusphere-egu22-6331, 2022.

EGU22-209 | Presentations | EMRP1.16

Adjoint-state Method Based Strategy for Non-linear Seismic AVO Inversion

Nisar Ahmed, Wiktor Waldemar Weibull, and Dario Grana

Seismic amplitude versus offset inversion has gained increased attention over the years and is a pragmatic tool applied to retrieve the seismic and petrophysical properties of the geological layers. The prediction of these petro-elastic properties plays an important role in litho-fluids identification and quantitative seismic reservoir characterization. However, imaging of these subsurface variables from the pre-stack seismic data requires minimizing the objective function and is generally solved by using a gradient-descent based optimization method. This method requires computing the gradients of the cost function with reference to the rock’s variables. We have introduced a model-based non-linear AVO inversion strategy that is based on the computation of the adjoint-state gradients. The forward seismic modelling is carried out by convolving the seismic wavelet with the reflectivity series modelled by using the linearized AVO approximation. The optimization method known as L-BFGS is implemented to attain the best optimal model. The novelty of this work is the adjoint-state solution of the linearized AVO equation. This inversion method has been successfully applied on single and multi-traced seismic data simulated with different seismic noise levels. The modelled examples show that the presented non-linear inversion method accurately extract the seismic properties including seismic (P and S) wave velocities and bulk density. Even at some realistic seismic noise levels, the true and extracted model show good agreement which demonstrates the wide application to solve the AVO inverse modelling problems.

How to cite: Ahmed, N., Weibull, W. W., and Grana, D.: Adjoint-state Method Based Strategy for Non-linear Seismic AVO Inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-209, https://doi.org/10.5194/egusphere-egu22-209, 2022.

Determination of strength and deformation parameters of rocks are crucially important for many engineering structures. In recent decades, numerical modelling and analysis have become a critical part of engineering projects due to significant advances in software technologies and computational infrastructure. As with the dilemmas brought by every new development, differences between 2D and 3D modelling approaches need to be considered. In nature, rocks are exposed to stresses from multi-dimensions. In order to create representative and reliable models to assess the behaviour of rock, this natural phenomenon must have been taken into account. On the other hand, in many studies on rock mechanics and modelling, 2D models are used. The reasons behind this choice can be listed as computational load, time, effort, and the results being in the preferred and reliable range. In addition, it is an inevitable fact that the importance of 3D modelling will increase since developing technologies and engineering structures begin to push the limits of the known engineering experience of engineers. In this study, 2D and 3D models were created by using Particle Flow Code (PFC) for a Castlegate sandstone sample to evaluate the response of 2D and 3D models under similar stress conditions. For this purpose, uniaxial compressive strength, triaxial compressive strength, and tensile strength tests were performed on both intact rock and gapped (circular in 2D and spherical in 3D) models to obtain data from different scenarios. Although both models provide similar test results, 3D models offer much more detail, especially in parameters such as crack initiation, propagation, and stress localization. Here, it can be said that the differences in the behaviours arise from the number of the balls in 3D models, which is more than 3 times in 2D, and the number of contacts which is more than 6 times, respectively. However, because of this resolution, the model response to stress conditions is closer to nature. The computational load for the 3D model is much higher than the 2D model because of the resolution. Even though 3D models have some drawbacks compared to 2D models in terms of computational load, time, and effort, it can be said that the data they provide is much more representative and reliable, especially in terms of model behaviour.

How to cite: Zengin, E.: Representative and Reliable Modeling for Rock Materials: 2D vs 3D , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3008, https://doi.org/10.5194/egusphere-egu22-3008, 2022.

EGU22-12478 | Presentations | EMRP1.16

Exploring new experimental possibilities for the investigation of nonlinear mesoscopic elasticity

Manuel Asnar, Audrey Bonnelye, Christoph Sens-Schönfelder, and Georg Dresen

EGU22-3748 | Presentations | EMRP1.16

Stress triggering and the spectrum of fault slip behaviors

Federico Pignalberi, Marco Maria Scuderi, Corentin Noël, Chris Marone, and Cristiano Collettini

Tectonic fault zones are subject to normal stress variations with a wide range of spatiotemporal scales, resulting in stress field alteration. These perturbations can spread over a wide range of frequencies and amplitudes from the high frequency passage of seismic waves generated by earthquakes, to the low frequency of solid earth tides and underground fluid injection cycles. As a result of these normal stress perturbations, critically stressed faults can be reactivated. The resulting slip mode is then controlled by fault friction and elastic properties of the surrounding rock. Existing works show that complex behaviors may arise from the interplay between friction changes with slip and slip rate and stress perturbations.

To shed light on the mechanics of fault dynamic triggering we performed experiments in a Biaxial Apparatus in a Double Direct Shear configuration under critically stable stiffness conditions (K/Kc~1). We used powdered quartz gouge (Min-U-Sil 40) as starting material, and conducted experiments at reference normal stress of σn = 10-13.5 MPa. After shearing the material and reaching a steady state sliding, normal stress oscillations were applied with various amplitudes, varying from A = 0.5-2 MPa, and periods, T = 0.5-50 s. In addition, we used the laboratory derived friction parameters as input for forward modeling using Rate-and-State friction laws in order to assess if these laws can explain our data. Our results show that creeping faults, under critical stiffness conditions, are sensitive to normal stress perturbations showing a variety of slip behaviors depending on amplitude and frequency of the oscillations:

  • Oscillation frequency has a major effect on fault stability. Low and high frequencies cause a Coulomb-like response of the shear stress, that is accompanied by a complex frictional response with slow events and period doubling. At the critical frequency predicted by the Rate-and-State friction, we observe dynamic weakening resulting in regular stick-slip events.
  • Oscillation amplitude also plays a role with the main effect depending on the magnitude of the perturbation.
  • Using a modified Rate-and-State equation (Linker and Dieterich, 1992), we are able to accurately model the laboratory data.

Our results show that normal stress perturbation on a laboratory creeping fault, at critical stiffness condition, can reproduce the entire spectrum of fault slip behavior depending on the oscillation properties.

How to cite: Pignalberi, F., Scuderi, M. M., Noël, C., Marone, C., and Collettini, C.: Stress triggering and the spectrum of fault slip behaviors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3748, https://doi.org/10.5194/egusphere-egu22-3748, 2022.

EGU22-5818 | Presentations | EMRP1.16

Micromechanics of damage localisation and shear failure in a porous rock: sound and vision

Alexis Cartwright-Taylor, Maria-Daphne Mangriotis, Ian G. Main, Ian B. Butler, Florian Fusseis, Martin Ling, Edward Andò, Andrew Curtis, Andrew F. Bell, Crippen Alyssa, Roberto E. Rizzo, Sina Marti, Derek D. Leung, and Oxana V. Magdysyuk

EGU22-3183 | Presentations | EMRP1.16

Using AE based Machine Learning Approaches to Forecast Rupture during Rock Deformation Laboratory Experiments

Sergio Vinciguerra, Thomas King, and Philip Benson

Parametric analysis of laboratory Acoustic Emission (AE) during rock deformation laboratory experiments has revealed periodic trends and precursory behaviour of the rupture source, as crack damage nucleates, grows and coalesces into a fault zone. Due to the heterogeneity of rocks and the different effective pressures, finding a full prediction of rupture mechanisms is still an open goal. We consider the AE rates and the derived source mechanisms to constrain the stress-strain regime, while scattering and seismic velocity structure define the evolving medium state as the most important attributes for the neural network model to learn. 4x10cm samples of Alzo granite were deformed at confining pressures of 5-40 MPa, whilst AE are recorded. Source mechanisms, as well as AE rates with relation to incremental strain, highlight distinct pre-failure phases. Scattering and seismic velocity measurements indicate the evolving mechanical conditions. A 10MPa simulation test on a model trained with data from 5, 20 and 40 MPa highlights good accuracy when predicting sample failure.

It remains a challenge to generate a ‘generic’ model that can be applied over all experimental conditions. Nonetheless, estimation of parameter importance has highlighted that some physical parameters are better for predicting strain, whilst others are better at stress. This importance can vary in time, suggesting a strong sensitivity of AE properties to the dynamic conditions of the fault zone. Small input changes can strongly affect output, therefore multiple models need to be trained in order to confirm the stability of the forecast. We aim to improve the understanding of the analysis through the search of repeating trends and the identification of consistent variations in key time-varying trends. Seismic scattering shows an early relevance, interpreted as due to the breakup of low frequency surface waves as microcracks begin to coalesce. However the reduction of importance at the later phases of deformation is less obvious. Further investigations are needed to identify at which deformation stage individual parameters are more important and segment time series accordingly.

How to cite: Vinciguerra, S., King, T., and Benson, P.: Using AE based Machine Learning Approaches to Forecast Rupture during Rock Deformation Laboratory Experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3183, https://doi.org/10.5194/egusphere-egu22-3183, 2022.

EGU22-3813 | Presentations | EMRP1.16

Coda-Based Estimation of Source Parameters of Laboratory Acoustic-Emission Events

Tatiana Kartseva, Nikolai Shapiro, Andrey Patonin, Vladimir Smirnov, and Alexander Ponomarev

We propose a coda-based estimation of source parameters of acoustic events recorded in laboratory experiments on rock deformation. Coda-waves are considered as the reverberation of the acoustic field in the tested sample. After multiple reverberations, the resulting wavefield can be approximated as nearly homogeneously distributed over the sample and with signal amplitudes decaying exponentially in time (linearly in a logarithmic scale). Within the framework of this model, the frequency-dependent coda amplitude at any moment of time is described as combination of a source spectra, of a decay rate combining internal attenuation with reverberation losses, and of a sensor response. One of the main difficulties with the laboratory experiments is that acoustic sensors are very difficult to calibrate and their absolute response function in most of cases remains unknown. With the simple reverberation model, the logarithms of coda amplitudes at different times and sensors and for multiple events are described by a system of linear equations that we solve in a least-square sense to find frequency-dependent decay rates and relative source spectra and sensor responses. In a next step, we compute spectral ratios between different events to eliminate the sensor responses and to estimate main source parameters such as corner frequencies and relative seismic moments. Additionally, we propose a new method for computing relative magnitudes (energy classes) of acoustic emission events from the coda envelopes and argue that it might be more robust comparing with estimations based on first arrivals.

We provide details of our data analyses tehnique and present first results of our new coda-based method applied to 30-600 kHz signals recorded during experiments carried out in the Research Equipment Sharing Center of IPE RAS “Petrophysics, Geomechanics and Paleomagnetism” on a controlled hydraulic press INOVA-1000 of the Geophysical Observatory ”Borok”, IPE RAS with granites of the Voronezh massif and Berea sandstones.

How to cite: Kartseva, T., Shapiro, N., Patonin, A., Smirnov, V., and Ponomarev, A.: Coda-Based Estimation of Source Parameters of Laboratory Acoustic-Emission Events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3813, https://doi.org/10.5194/egusphere-egu22-3813, 2022.

EGU22-5890 | Presentations | EMRP1.16

The effect of pore fluid chemistry on limestone deformation.

Jon-Danilo Kortram, Auke Barnhoorn, and Anne Pluymakers

Active use of the subsurface alters the in-situ pore-fluid composition. For limestone, chemical interaction between the pore fluid and the rock has been shown to alter some of the mechanical parameters, though the exact nature of this mechano-chemical interaction is not yet fully understood. To address this, we performed tri-axial compressive experiments on two highly pure (>97% CaCO3) and well documented limestones: Indiana Limestone and Edwards White, which were saturated with different fluid compositions. We selected our samples to have a porosity within a narrow range: 23.2 ± 0.3% for Edwards White and 12.9 ± 0.5% for Indiana Limestone. Prior to testing, the rock samples were saturated under vacuum with a fluid solution, and left to equilibrate under vacuum at room temperature for 18 hours. The fluids used in our experiments are 1) CaCO3-saturated water, 2) a brine which has a composition representative for the Dutch subsurface, and 3) a solution of industrial corrosion inhibitor. Samples were tested at room temperature and confining pressures of 2.5, 5 and 10 MPa. In addition to the stress and strain data observed from these experiments, thin sections were made from the deformed samples to perform micro-structural analysis on the damage zone.

Our results show no mechano-chemical effects for Edwards White. However, the rock strength of the Indiana limestone samples changes due to the different pore fluids: At a confining pressure of 2.5 MPa the sample saturated with to CaCO3 solution failed at 49 MPa, compared to 47 MPa and 54 MPa for the samples that were saturated with the brine and the inhibitor solution respectively. At a confining pressure of 5 MPa both the sample tested with the CaCO3 solution and the brine solution failed at 57 MPa and the sample exposed to the inhibitor solution failed at 56 MPa. The samples tested at a confining pressure of 10 MPa respectively failed at: 76, 79 and 73 MPa.  These differences of 5 to 10% lead to a shift in the resulting failure envelopes depending on the pore fluid used in the experiments when describing the failure behaviour of these samples using the Mohr-Coulomb failure criterion: The group tested with CaCO3 solution had a cohesion of 11 MPa and the coefficient of friction of 0.67. For the samples tested with brine solution these values are 10 MPa and 0.71 respectively. For the group tested with inhibitor solution these values equal 15 MPa and 0.47 respectively. The experiments presented here serve as a baseline from which we can further determine which ions or compounds interact with the rock, and the nature of this interaction. For our follow-up work, we will continue by performing a detailed microstructural analysis to better understand the overall controls on the mechano-chemical interactions or the lack thereof. In follow-up experiments, we will narrow down the complexity of the fluid solutions so we can identify the effect of specific ionic species.

How to cite: Kortram, J.-D., Barnhoorn, A., and Pluymakers, A.: The effect of pore fluid chemistry on limestone deformation., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5890, https://doi.org/10.5194/egusphere-egu22-5890, 2022.

EGU22-9866 | Presentations | EMRP1.16

Determination of parameters characteristic of dynamic weakening mechanisms during coseismic slip

Chiara Cornelio, Elena Spagnuolo, Stefan Nielsen, Stefano Aretusini, Francois Passelègue, Marie Violay, Massimo Cocco, and Giulio Di Toro

While sliding at seismic slip-rates of ca. 1 m/s, a natural fault undergoes an abrupt decrease of its strength called enhanced dynamic weakening. Asperity-scale (<< mm) processes related to flash heating & weakening and meso-scale (mm-cm) processes involving shear across the bulk slipping zone related to frictional melting or viscous flow of minerals, have been invoked to explain pronounced velocity-dependent weakening. Here we present a compilation of ca. 100 experiments performed with two rotary shear apparatuses, i.e. SHIVA installed in INGV (Rome, Italy) and HVR installed, at the time of experiments, in Kyoto University (Japan). Cohesive rock cylinders of basalt, gabbro, tonalite, granite and calcitic marble were sheared under a range of effective normal stresses (sn=5-40 MPa), target slip-rates (Vt=0.1-6.5 m/s) and fluid pressures (from room humidity conditions RH or Pf=0, to Pf =15 MPa). We fit the measured shear stress evolution with slip with two dynamic weakening mechanisms models, which include, depending on rock type: (1) flash heating and bulk melting (granitoid, gabbro and basalt), (2) flash heating and diffusion creep (calcitic marble), (3) flash heating and dislocation creep (calcitic marble). We provide a set of optimized parameters, specific for each mechanism, that control the dynamic weakening.

Lastly, the modelling procedure allow us to estimate the slip-switch distance d0, i.e. the slip necessary for the complete transition from the asperity-scale to bulk slipping zone dynamic weakening mechanism. Our analysis shows that (1) the d0 decreases with increasing effective normal stress acting on the fault and, (2) for the same type of transition between dynamic weakening mechanisms (e.g., from flash heating to bulk melt lubrication) the d0 is a function of rock composition. The decrease of d0 with normal stress indicates that during earthquakes, bulk mechanisms dominate over asperity scale weakening mechanisms with increasing crustal depths. This study provides constitutive law parameters to be included in physically- and geologically-based dynamic earthquake rupture simulations.

How to cite: Cornelio, C., Spagnuolo, E., Nielsen, S., Aretusini, S., Passelègue, F., Violay, M., Cocco, M., and Di Toro, G.: Determination of parameters characteristic of dynamic weakening mechanisms during coseismic slip, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9866, https://doi.org/10.5194/egusphere-egu22-9866, 2022.

EMRP1.17 – The interplay between brittle and ductile deformation – the semi-brittle regime from Earth layers and laboratory experiments

EGU22-5863 | Presentations | EMRP1.17

Velocity-dependent friction of granitoid gouge under hydrothermal conditions: A contribution to understanding of fault zone seismicity

Weijia Zhan, André Niemeijer, Natalia Nevskaya, Alfons Berger, Chris Spiers, and Marco Herwegh

Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. Their velocity-dependent friction is crucial for understanding earthquake nucleation and the depth distribution of fault-related seismicity in granitoid shear zones (Wehrens et al. 2016; Blanpied et al. 1998). In the framework of rate-and-state friction laws (RSF), the friction parameter (a-b) is measured in sliding experiments to describe the velocity dependence of friction in fault gouges (Scholz, 1998). A velocity-strengthening system is frictionally stable, (a-b) >0, whereas a velocity-weakening system can be frictionally unstable, (a-b) <0. In earthquake mechanics, velocity weakening is prerequisite for stick-slip deformation, i.e. the nucleation of earthquakes. Although (a-b) values of granitoid gouge are sensitive to varying temperature conditions and sliding velocities, only a few studies have examined this velocity-dependence under hydrothermal conditions.

To address this issue, we conducted velocity stepping sliding experiments under hydrothermal conditions by using a ring shear apparatus. The powdered starting gouge was derived from a granitoid mylonite collected at the NAGRA Grimsel Test Site (Central Swiss Alps). The applied velocity steps were 1-3-10-30-100 μm/s. Pore fluid pressure and the effective normal stress were 100 MPa. Temperatures explored ranged from 20-650 °C. Values of (a-b) were obtained from RSF model inversions of the evolution of friction coefficients at mechanical steady state conditions. Our experiments showed pronounced changes in (a-b) values with across the full range of temperatures up to 650 °C and velocities investigated. At temperatures below ~100 °C and above ~400 °C, we observed mostly velocity strengthening with positive (a-b). In contrast, velocity weakening with negative (a-b) was observed between ~100 °C and ~400 °C. Samples deformed at a sliding velocity of 100 μm/s deviated slightly from this trend, as (a-b) values were negative between ~200 °C and ~400 °C.

The presented experimental study demonstrates a significant influence of temperature and sliding velocity on velocity-dependence during deformation of granitoid gouge. We suggest that the observed transitions in velocity dependence reflect an interplay of interactions. In terms of crustal faulting, our data suggest the existence of a seismogenic window that limits the depth distribution of earthquakes on faults in granitoid shear.

 

REFERENCES

Wehrens, P. C., Berger, A., Peters, M., Spillmann, T., Herwegh, M. 2016: Deformation at the frictional-viscous transition: Evidence for cycles of fluid-assisted embrittlement and ductile deformation in the granitoid crust, Tectonophysics, 693, 66-84.

Blanpied M. L., Tullis T. E., Weeks J. D. 1998: Effects of slip, slip rate, and shear heating on the friction of granite.

Scholz, C. H. 1998: Earthquakes and friction laws, Nature, 391, 37-42.

How to cite: Zhan, W., Niemeijer, A., Nevskaya, N., Berger, A., Spiers, C., and Herwegh, M.: Velocity-dependent friction of granitoid gouge under hydrothermal conditions: A contribution to understanding of fault zone seismicity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5863, https://doi.org/10.5194/egusphere-egu22-5863, 2022.

EGU22-3971 | Presentations | EMRP1.17

On the stability of carbonate-bearing faults at the brittle-to-ductile transition

Francesco Figura, Carolina Giorgetti, Gabriel Meyer, and Marie Violay

The majority of the seismic events in the Mediterranean region are hosted in carbonate-bearing rocks at depths representative of the semi-brittle regime. Within this regime, both brittle behavior (i.e. deformation is localized on the fractures and on the faults) and ductile one (i.e. deformation is distributed and accommodated in the rock core) coexist. The influence of this interplay on the nucleation and propagation of seismic events is poorly studied. Up to now, most experimental work has been conducted far from in-situ conditions, mostly at room temperature and low confining pressure.

Here we constrain the frictional behavior of faults in carbonate rocks under conditions relevant for their brittle-to-ductile transition. Velocity-step experiments are performed through the HighSTEPS (Strain, TEmperature, Pressure, Speed) biaxial apparatus installed at EPFL, investigating sliding velocities from 10-6 m/s to 10-2 m/s. Experiments are conducted under different values of confining pressure (Pc 15 MPa and Pc 50 MPa) and normal stress (σn 29 MPa and σn 95 MPa) on the experimental faults, keeping the ratio between them constant (around 2). The local strain field along the fault was measured with strain gauges. The collected data were modeled with rate-and-state friction laws (RSFLs) to define the rate and state parameters relate to the critical condition for fault stability. Moreover, microstructural observations of the post mortem sample were conducted at the SEM, to investigate the deformation mechanisms active during the experiments.

These results shed light on the evolution of rate-and-state frictional parameters with depth, as well as their dependence on the strain partitioning between on-fault slip and bulk-accommodated deformation with increasing depth.

How to cite: Figura, F., Giorgetti, C., Meyer, G., and Violay, M.: On the stability of carbonate-bearing faults at the brittle-to-ductile transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3971, https://doi.org/10.5194/egusphere-egu22-3971, 2022.

EGU22-8365 | Presentations | EMRP1.17

Weakening mechanisms in dry, lower-crustal pseudotachylytes

Kristina G. Dunkel, Luca Menegon, and Bjørn Jamtveit

Earthquakes are often regarded as agents of rheological weakening of the dry and mechanically strong lower crust. The weakening is typically attributed to fluid infiltration and resulting fluid-mediated metamorphism along the seismic fault.

On Moskenesøya in SW Lofoten (Northern Norway), we observe lower-crustal pseudotachylytes (frozen frictional melts that record fossil earthquakes) that are unusually dry. This presents us with an exceptional opportunity to study the processes affecting the rocks during and after an earthquake:

  • We can observe the pristine microstructures of the pseudotachylytes, not overprinted by later metamorphism, to elucidate the earthquake-generating mechanism.
  • We can study the further development of these dry pseudotachylytes after the seismic event.

We have previously described the composition and microstructures of the pristine pseudotachylytes, and concluded that transient stress pulses caused by shallower earthquakes are the most likely explanation for the occurrence of fossil earthquakes in the analysed rocks from Lofoten, with no evidence of other mechanisms such as thermal runaway or dehydration embrittlement.

In this contribution, we focus on the evolution of the pseudotachylytes after their formation. We study their development from the initial, pristine pseudotachylytes, via pseudotachylytes with slightly mylonitized margins, to ultramylonites. We use compositional and microstructural analyses, including electron backscatter diffraction (EBSD), to understand the weakening mechanisms in this dry system.

In the mylonitized margins of the pseudotachylytes, a slight shape-preferred orientation is developed and the quenching microstructures, such as microlites, are lost. The mineralogical composition (dominantly feldspars and pyroxenes) stays the same as in the pristine pseudotachylytes. In the ultramylonite, quartz and amphibole appear as accessory minerals, which means that we cannot completely exclude the presence of minor amounts of hydrous fluids; however, feldspars and pyroxenes persist as the main components of the rock. The foliation of the ultramylonite is not defined by phyllosilicates, but by a compositional banding, which suggest a phase separation and aggregation during shearing. EBSD data indicate that the main constituent phases deformed dominantly by grain size sensitive creep.

Our preliminary results suggest that even in the absence of fluids, pseudotachylyte-bearing seismic faults represent weak zones in the lower crust that are localizing viscous shear during post- and interseismic deformation, presumably due to the intense grain size reduction that facilitates grain-size sensitive mechanisms. 

How to cite: Dunkel, K. G., Menegon, L., and Jamtveit, B.: Weakening mechanisms in dry, lower-crustal pseudotachylytes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8365, https://doi.org/10.5194/egusphere-egu22-8365, 2022.

EGU22-13212 | Presentations | EMRP1.17

Experimental deformation of talc at near-seismic deformation rates

Charis Horn and Philip Skemer

Talc is a hydrous magnesium silicate with an extremely low coefficient of friction.  In recent years, the recognition that talc is present in many fault systems has led to the suggestion that talc strongly influences the strength of faults.  To understand the role of talc in the seismic cycle, we conducted high pressure and temperature torsional deformation experiments on specimens of natural talc at shear strain rates relevant to slow-slip earthquakes (~10-4 s-1).  Scanning transmission electron microscopy revealed decreasing talc grain sizes (from ~3-5 mm to <100 nm), alongside delamination and kinking of individual talc grains.  This microstructural evolution with progressive strain greatly increases the density of planar defects (including grain-boundaries), and is consistent both with observations of natural, talc-rich faults, and prior experimental work.  Nanoindentation tests at room temperature were performed on deformed specimens to assess precisely whether the observed microstructural changes also affect rheology. At these conditions, nanoindentation is assumed to produce deformation predominantly by intercrystalline frictional slip.  However, bulk hardness data determined from nanoindentation show that there is no change in indentation hardness with increasing strain or defect density, both for indents made parallel to and perpendicular to the shear plane.  Although the talc grains become increasingly damaged with strain, the overall strength of deformed talc does not change.  This suggests that accumulated slip on talc-bearing faults does not change their mechanical response or hazard potential.

How to cite: Horn, C. and Skemer, P.: Experimental deformation of talc at near-seismic deformation rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13212, https://doi.org/10.5194/egusphere-egu22-13212, 2022.

EGU22-5029 | Presentations | EMRP1.17

The localized to ductile transition in porous rocks : experimental investigation on Volvic basalt.

Gabriel Meyer, Marie Violay, and Michael Heap

With increasing depth, the rheology of rocks gradually transitions from brittle (localized, fractures) to ductile (homogeneous flow). Recently, it was demonstrated that, in the crust, the transitional zone might extend to shallower depth than previously thought (2km) with a zone where the deformation regime can be both localized and ductile (the LDT). In this regime, both extremely localized (fault slip) and distributed (cataclastic flow and/or plasticity) deformation may occur concurrently. This observation had great importance since the ductile regime is commonly thought to be aseismic and to mark the maximum depth of earthquake nucleation.

However, this observation was made experimentally in non-porous rocks; porous rocks on the other hand display an additional characteristic in that their ductile behaviour may consist in the formation of compactions bands which greatly impact the behaviour of porous reservoirs and systems (e.g., volcanoes). Moreover, ductile rocks are commonly believed to be aseismic, the potential coexistence of both ductile and localized regimes in reservoir rocks might therefore have great implications for induced seismicity mitigation.

Here, we present three conventional triaxial experiments on Volvic basalt (homogeneous, istropic, fine grain). We deformed cylindrical cores equipped with strain gages at 5MPa and room temperature until a sample-scale fracture nucleated and propagated. Subsequently, we increased confining pressure step wise, loading the sample every step until 0.2% irrecoverable strain was accumulated in the sample. In between confinement steps, the differential stress was unloaded. A pair of Linear Variable Differential Transformers (LVDTs) was used along with the strain gauges to accurately monitor the deformation behaviour of the samples.

We show that Volvic basalt transitions from being purely localized to being purely ductile over a rather narrow pressure range from 40 to 80 MPa. The transition initiates when the frictional strength of the fault equates the yield strength of the bulk and terminates when it becomes greater than the maximum strength of the bulk. In this pressure range, deformation is initially accommodated in the bulk (most likely by compaction bands) until strain hardening eventually leads to fault reactivation. Once both fault sliding and bulk flow are active, the partitioning of strain between the two can be described by the same empirical ratio as that already established for non-porous rocks, i.e. (σf - σy)/ (σflow - σy).

We conducted a second experiment at a faster strain rate (10-4 s-1) and show that faster deformation promotes brittle behaviour which pushes the LDT to greater confinement (i.e., greater depth).

Additionally, we conducted a similar experiment in the presence of water. In this case, the LDT occurs at lower confinement, showing that, fluids, by promoting ductile processes such as stress-corrosion, bring the LDT to shallower depth.

Our results are crucial for the understanding of reservoirs where ductile deformation (compaction bands) and induced earthquake mitigation have to be finely tuned.

How to cite: Meyer, G., Violay, M., and Heap, M.: The localized to ductile transition in porous rocks : experimental investigation on Volvic basalt., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5029, https://doi.org/10.5194/egusphere-egu22-5029, 2022.

EGU22-5065 | Presentations | EMRP1.17

The Meran-Mauls segment of the Periadriatic Fault System, Italy: pure thrusting across the brittle-plastic transition

Andrea Zanchi, Stefano Zanchetta, Chiara Montemagni, Volkmar Mair, Corrado Morelli, and Claudia Mascandola

EGU22-8320 | Presentations | EMRP1.17

The effects of grain size on semi-brittle flow in calcite rich rocks

Christopher Harbord and Nicolas Brantut

Grain size is in an important microstructural parameter affecting both brittle and plastic deformation processes. In the low temperature brittle regime, larger grain size materials typically have lower strength, whereas in the plastic flow regime smaller grain size materials tend to be weaker. It is not clear how grain size impacts at intermediate conditions where deformation of rock is accommodated by coupled brittle and plastic deformation processes.

To investigate the role of grain size in the semi brittle regime we deformed three calcite-rich rocks, spanning 3 orders of magnitude in grainsize (0.006-2 mm). A gas medium triaxial apparatus was used at a range of confining pressures (200-800 MPa) and temperatures (20-400°C), and samples were loaded at a constant axial strain rate (1×10-5 s-1). Axial measurements of P-wave speed were performed during tests in order to infer the in-situ microstructural state of the sample.

Nearly all tests show strain hardening behaviour after yield, typical of semi-brittle deformation, which is quantified using the hardening modulus (h = ∂σ/∂ε). Grain size has a first order control on rock strength, with yield stress and h following a Hall-Petch type relationship at all P-T conditions. For a given temperature, h is low at low pressure (200 MPa) and accompanied by large decreases in wavespeed, and h increases at high pressure (>400 MPa) whereas velocity decreases by a smaller magnitude. This suggests that, at low temperature, strain hardening is relieved by microcracking. At constant pressure, wavespeed decreases significantly at 20°C with progressive deformation, but remains nearly constant at 400°C indicating a transition from dominatly brittle to fully plastic deformation with increasing temperature, in some cases with little change in the macroscopic strength.

Given that both strength and strain hardening behaviour depend on grain size, our data suggests that grain size dynamically impacts the long term rheology of the crust. Larger grain sizes will broaden the depth distribution of the brittle ductile transition and result in a weaker peak crustal strength.

How to cite: Harbord, C. and Brantut, N.: The effects of grain size on semi-brittle flow in calcite rich rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8320, https://doi.org/10.5194/egusphere-egu22-8320, 2022.

EGU22-162 | Presentations | EMRP1.17

Viscoplastic Rheological Modelling- A Realistic Approach to Natural Ductile Shear Zones

Arnab Roy, Puspendu Saha, and Nibir Mandal

Crustal deformations generally undergo a brittle-ductile transition with depth, producing fault-dominated structures at shallow depths, replaced by ductile shear zones at middle and lower crustal levels. One of the keys to shear zone modelling concerns the choice of rheological approximations that can successfully reproduce the characteristic features of natural ductile shear zones in the models.  With the help of 2D FE (finite element) simulations, this study shows viscoplastic rheology as a suitable rheological approximation to predict the competing growth and orientations of multiple sets of secondary shear bands in a ductile shear zone. The viscoplastic rheology is modelled by combining bulk viscous weakening of the shear zone material and plastic yielding (Drucker-Prager criterion) to replicate strain-softening behaviour, where the instantaneous viscosity decreases nonlinearly with increasing strain. The cohesive strength of the material is also assumed to reduce with progressive plastic strain. This rheological combination allows us to replicate the various shear band networks found in crustal-level ductile shear zones. It also addresses the conditions for fluid flow into ductile shear zones, which leads to metamorphic reactions, mineralisation, etc. We validate our model results with field observations of similar shear band structures from the Eastern Indian Precambrian craton. The present study finally leads us to conclude that a pressure-dependent viscoplastic rheology is an ideal rheological approximation to model ductile shear zones extensively found in this craton.

How to cite: Roy, A., Saha, P., and Mandal, N.: Viscoplastic Rheological Modelling- A Realistic Approach to Natural Ductile Shear Zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-162, https://doi.org/10.5194/egusphere-egu22-162, 2022.

EGU22-5198 | Presentations | EMRP1.17

Rheology of plagioclase transforming under high pressure and temperature conditions: insights from deformation experiments

Marie Baisset, Julien Gasc, Damien Deldicque, Arefeh Moarefvand, Alexandre Schubnel, and Loïc Labrousse

EGU22-7954 | Presentations | EMRP1.17

Experimental study of the impact of hydration extent on the strength of the lower continental crust

Lisa Katharina Mohrbach, Joerg Renner, and Sarah Incel

Previous experimental data and field observations demonstrate that fluids have a significant influence on rock strength. The relation between strength and hydration extent of the lower continental crust is still poorly constrained and thus a matter of an ongoing debate. We tested the impact of hydration extent on the strength of rocks representing the lower continental crust by performing deformation experiments on various plagioclase-epidote mixtures as well as on natural granulite samples in a Grigg's type deformation apparatus. In these samples, the plagioclase component represents rocks of the lower continental crust and epidote reflects hydration extent, because, alongside with quartz, kyanite and jadeite or albite, it forms as a decomposition product of plagioclase at high-pressure/ high-temperature conditions in the presence of even small amounts of fluids. To quantify the relation between strength and epidote content, we conducted the tests on plagioclase-epidote powders with a grain size of 90-135 mm and plagioclase-epidote ratios of 100:0, 99:1, 98:2, 95:5, 90:10, 85:15, and 0:100. The pre-dried powders were first hot-pressed at 550 °C and a confining pressure of 1 GPa for 3 h in the Griggs apparatus. Mixtures were subsequently deformed at 1 GPa and 550 to 650 °C at strain rates of 5·10-6 to 5·10-5 s-1. All stress-strain curves show pronounced maxima followed by strain softening towards a final strength. The deformation data yield an exponential decrease of the ultimate strength with increasing epidote content. Investigations of the microstructures of samples deformed at 550 °C and 5·10-5 s-1 using the SEM and polarized light microscopy reveal cataclastic flow by grain-scale fracturing of both epidote and plagioclase and the rotation and alignment of epidote grains at angles between 60° and 70° to the maximum principal stress  σ1. In addition, plagioclase grains show pronounced undulatory extinction but we found no evidence for deformation twinning. Some samples exhibit networks of conjugate bands of fine-grained plagioclase surrounding larger plagioclase grains oriented at an angle of around 50° towards σ1. These bands are mostly visible in samples without epidote.

How to cite: Mohrbach, L. K., Renner, J., and Incel, S.: Experimental study of the impact of hydration extent on the strength of the lower continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7954, https://doi.org/10.5194/egusphere-egu22-7954, 2022.

Amphibole is an important mineral in rocks of the lower crust and in subduction zones, forming as the product of metamorphic reactions and hydration of mafic rocks. As such, the textural and rheological properties of amphibole are of relevance for assessing the physical properties of these tectonic provinces. Aggregates containing amphibole grains often exhibit a strong texture, i.e., a crystallographic preferred orientation (CPO). Since amphibole possesses inherent anisotropic properties, the CPO will affect the bulk strength and elastic properties. However, amphibole’s rheological behavior is not well understood as its capability to deform purely via plastic deformation remains unresolved, previous studies suggesting numerous deformation mechanisms such as semi-brittle and cataclastic flow, dissolution precipitation, dislocation creep, recrystallization, micro-twinning, and diffusion assisted creep. Here, we use pre-textured natural samples cored at 60° to the foliation and lineation to investigate the deformation mechanism/s activated in a polycrystalline aggregate/rock of well-oriented amphibolite-rich hornblende. Samples from the Mamonia complex (Cyprus) with hornblende as the dominant mineral (> 70 % modal fraction) and strong initial alignment of the [001] axis were deformed using a Griggs-type solid-medium apparatus. Experiments were run at 1 GPa confining pressure, temperatures of 400 to 800 °C, and a strain rate of ~10-5 1/s. Samples show temperature-dependent differential stress that falls below the Goetze criteria (i.e., below the confining pressure, 1 GPa) - ~700, 500, and 200 MPa for samples deformed at 400, 600, and 800 °C, respectively. Microstructural analysis using Electron backscatter diffraction (EBSD) reveals folding and kink bands, accommodated by both plastic mechanisms, via dislocation glide on the hornblende easy slip system, and brittle mechanisms, via micro-fracturing along the crystal cleavage (110). We discuss the implications of the interplay and contribution of different deformation mechanisms for our ability to translate laboratory experiments to flow laws for the lower mantle and subduction zone interfaces.

How to cite: Boneh, Y., Sarah, I., and Renner, J.: Mechanism/s of deformation and strength of experimentally deformed hornblende-rich amphibolite with a strong pre-existing texture, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6966, https://doi.org/10.5194/egusphere-egu22-6966, 2022.

EGU22-10023 | Presentations | EMRP1.17

Seismic attenuation across the brittle-ductile transition

Maria Aurora Natale Castillo, Magdala Tesauro, Mauro Cacace, Francois X. Thibault Passelegue, Lucas Pimienta, and Marie Violay

Rocks mechanical behaviour, and in particular, their transition from a brittle to a ductile deformation has been prevalently investigated through rheological experiments and numerical models. In conjunction with rocks mechanical studies, the analyses of seismic wave propagation can improve our knowledge of physical rocks behaviour and provide an alternative assessment of the brittle ductile transition (BDT).

In this study, we investigate the quantitative relationships between seismic attenuation and viscous rocks' rheology, especially across the BDT domain. For this purpose, we rely on the Burgers and Gassmann mechanical model to derive shear wave attenuation (1/Qs ), for several dry and wet crustal rheology, thermal conditions, and different strain rates values. This allows us to establish geothermal and mechanical conditions at which the BDT occurs and to cross-correlate this transition to computed shear seismic wave attenuation values. We observe that the variation with depth is related much more to the input strain rate than to the rock‘s rheology and thermal conditions, so that a fixed amount of Qs reduction can identify the average BDT depths for each strain rate used. Below the BDT depth, we observe a significant increase of the Qs reduction (up to 10-4 % of the surface value), depending also on rocks temperature and rheology. Since the greatest Qs reduction is estimated for the greatest input strain rate (10-13 s-1) and hot thermal conditions, the proposed method can find more applicability in tectonically active/geothermal areas.

We tested the obtained results by performing triaxial lab experiments, while monitoring ultrasonic P-waves, on a sample of Carrara marble, at ambient temperature and 180 MPa confining pressure. The transition from brittle to semi-brittle conditions is characterized by the increase of crack-density with a progressive rate reduction. At the same time, both the seismic velocity and energy significantly decrease during the first phase of deformation (brittle regime) and tend towards an asymptotic value, when the sample approaches the ductile deformation. We interpret the absence of an increase of energy loss at the BDT, as due to the persistent effect of the microfracturation. The last one usually accompanies the deformation mechanisms that occur at the BDT (e.g., pressure solution, twinning), masking the expected increase of attenuation at the beginning of the ductile conditions. This is a matter that still needs to be investigated.

How to cite: Natale Castillo, M. A., Tesauro, M., Cacace, M., Passelegue, F. X. T., Pimienta, L., and Violay, M.: Seismic attenuation across the brittle-ductile transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10023, https://doi.org/10.5194/egusphere-egu22-10023, 2022.

EGU22-12465 | Presentations | EMRP1.17

Diopside microfabric development in lower-crust oceanic detachment fault zones

Rhander Taufner, Claudia Trepmann, and Gustavo Viegas

Exhumation of oceanic core complexes occurs through large-scale extensional shear zones that expose parts of the deformed gabbroic lower crust. However, it is not well understood how these high-temperature shear zones nucleate and develop. Since diopside is traditionally described as a load bearing phase in deforming systems, its microstructures may record the deformation mechanisms involved in the progressive stages of shear zone development. In this study, we focus on the fabrics of diopside in both the host coarse-grained gabbro and the adjacent high-temperature shear zone from the Atlantis Bank (IODP Exp 360), in order to better constrain the role of diopside during strain localization in deep crustal detachment fault zones.

In the host rock directly in contact to the shear zone, diopside porphyroclasts display microfractures filled with fine-grained diopside (~ 65 µm) and minor amounts (~10%) of plagioclase, amphibole and Fe-Ti oxides with grain size ~ 30 µm that occur as interstitial phases. Diopside grains in the microfractures have little internal deformation and are interpreted as “new” grains. On the other hand, fragments of the host diopside within the fracture are distinguished by their larger diameters of ~200 µm and dominant cleavage planes that is systematically missing in the new grains. These microstructures indicate cataclastic deformation with later precipitation of plagioclase, amphibole and Fe-Ti oxides. Other diopside porphyroclasts in the host rock show undulatory extinction, low-angle grain boundaries and new grains with crystallographic orientations controlled by the host, indicating dislocation creep.

Diopside porphyroclasts within the shear zone show undulatory extinction as well as bent cleavage planes and exsolution lamellae. New grains of diopside (~35 µm) that occur rimming the porphyroclasts - concentrated at sites of strong undulatory extinction - have long axes correlating the orientation of the bent cleavages within the host. These new grains have a crystallographic orientation with poles of (100) planes close to the X-axis and [001] axes close to the Z-axis, and high angle boundaries (>140º) with misorientation axes clustered between [001] and [100]. We propose that these new grains are a result of dislocation glide and growth due to bending of the host diopside during the early stages of shear zone nucleation.

In the strain shadow of the porphyroclasts within the shear zone, new grains of diopside (~20µm) occur together with amphibole, plagioclase and Fe-Ti oxides. They are rounded, strain free, have random orientations and the amount of diopside decreases with distance from the host. These grains are interpreted to have precipitated from the pore fluid during ongoing deformation of the shear zone.

We suggest that diopside in the host rock was deformed by cataclasis associated with dislocation glide during nucleation of the shear zone at probably high stress, as indicated by the similar microfabric of diopside porphyroclasts in the shear zone compared to those in the host rock. Unlike, ongoing deformation localized within the shear zone is due to dissolution and precipitation, as indicated by the polyphase aggregates in the strain shadows and in the matrix. 

How to cite: Taufner, R., Trepmann, C., and Viegas, G.: Diopside microfabric development in lower-crust oceanic detachment fault zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12465, https://doi.org/10.5194/egusphere-egu22-12465, 2022.

We provide here in situ evidence from a network of well-preserved extensional shear zones cutting a rift related lower crustal Reinfjord Ultramafic Complex, Seiland Igneous Province, that formed in the late Ediacaran. Our results can explain seismic events well below the seimic zone of continental rifts and associated CO2 emissions. Processses leading to catastrophic failure of the weakened rocks led to extremely high strain rates and the formation of pseudotachylites can be traced from a netwok og mm-m scale steeply dipping transtensional shearzones associated with gabbronoritc dykes to a 2km long low angle extensional shearzone. Deformation, initiated through a priming of the dyke-host rock interface by magmatic fluids, exploits subgrains and microfractures in olivine, with reactive CO2-bearing fluids leading to volume expanding reactions such as olivine + diopside + CO2 = Dolomite + enstatite, enhancing olivine grain fracturing. Fragmentation of the olivine grains and addition of weaker phases facilitated strain localization and local increases in strain rate by two orders of magnitude. Catastrophic failure of the weakened rocks led to extremely high strain rates and the formation of pseudotachylites in several cyclic events. The frictional heat raised the temperature above the dolomite forming reaction, causing release of CO2 and H2O along the fault, but also in the surrounding mafic-ultramafic rocks, forming veins around the shearzone. Fluid-rock interaction surrounding shear zones is highly variable and depends on bulk rock compositions. Thermodynamic modelling demonstrates that mineral reactions involving hydration and carbonation differ between dunitic rocks and the pyroxenitic dykes which intersect them. Alteration of dunitic rocks results in the formation of dominantly magnesite-anthophyllite-talc and talc-magnesite assemblages causing approximately 12% volume expansion, resultinig in a sharp reaction front contacts with the host rock. When the alteration zones cross the dunite-pyroxenite boundary the associated alteration has a more gradual boundary towards the unaltered rock and the alteration zone widens by approximately 40%. In contrast to the simpler dunite alteration assemblage, the pyroxenenitic dykes are altered to a complex mixture of cummingtonite-anthophyllite, magnetite and chlorite. Additionally, orthopyroxene is completely pseudomorphed by a mixture of cummingtonite and magnetite, whereas olivine xenocrysts are partly preserved and surrounded by a magnesite-anthophyllite assemblage. Other, open cavity-like areas are filled by chlorite, amphibole, and Mg-MgCa carbonates, indicating volume reduction during alteration of the pyroxene.Accordingly, dunite alteration effectuates a significant volume expansion, and are therefore only altered locally during seismic creep events. The pyroxenites are near volume neutral throughout interaction with the same fluids, and are thus more homogeneously altered. The formation of chlorite in hybrid compositions, such as the dykes in the lower crust, may create weak permeable zones that are consequently exploited as pathways for fertile mantle fluids and will hence also be the locus of ore bearing fluids moving to the upper crust.  We conclude that catastrophic failure along shear zones in lower crustal continental rifts is possible without remote stress events in the presence of pre-existing heterogeneities and volatiles. These zones also acted and transport conduits for volatiles from the lower crust to atmosphere.

How to cite: Sørensen, B. E., Ryan, E. J., Larsen, R., and Grant, T.: Infiltration of volatile-rich mafic melt in lower crustal peridotites provokes deep earthquakes, initiates km scale shearzones and volatile transfer from the lower crust to the atmosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10545, https://doi.org/10.5194/egusphere-egu22-10545, 2022.

EMRP1.18 – Rock physics modelling and inversion using multiple physical properties

EGU22-6760 | Presentations | EMRP1.18

Probing the micromechanical features of a fracture interface using a multi-physics approach: A numerical investigation relating asperity deformation with fluid flow

Clay Wood, Chun-Yu Ke, Andy Rathbun, Jacques Riviere, Derek Elsworth, Chris Marone, and Parisa Shokouhi

The focus of this study is to elucidate the relation between elastodynamic and hydraulic properties of fractured rock subjected to local stress perturbations in relation to fracture aperture distribution. The goal of our integrated numerical and experimental investigations is to understand the mechanisms responsible for changes in fault zone permeability and elasticity in response to dynamic stressing in the subsurface (anthropogenic or seismic in origin). High-resolution (micron-scale) optical profilometry measurements combined with pressure sensitive films have been used to characterize fracture properties such as ‘true’ contact area, aperture distribution and morphology, as well as asperity deformation under applied loads in our experiments. These measurements allow a direct correlation between fracture properties and our lab measurements of fracture elastic nonlinearity and permeability. Using micron-resolution profilometry of centimeter-scale samples, we calculate the elastic deformation of fracture asperities to varying applied  stresses (static and dynamic) using Hertzian contact mechanics. Then, permeability is calculated for each applied stress (deformed asperities) using the parallel plate approximation, in which the Reynolds equation is solved using the finite difference method. This study is uniquely constrained, wherein we investigate the effect of measured deformation of real asperities on creating flow pathways through a fracture. Future work will include implementing contact acoustic nonlinearity (CAN) to model the change in transmission of acoustic waves across the fracture interface during stress perturbation.

How to cite: Wood, C., Ke, C.-Y., Rathbun, A., Riviere, J., Elsworth, D., Marone, C., and Shokouhi, P.: Probing the micromechanical features of a fracture interface using a multi-physics approach: A numerical investigation relating asperity deformation with fluid flow, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6760, https://doi.org/10.5194/egusphere-egu22-6760, 2022.

EGU22-418 | Presentations | EMRP1.18

Comparison Between Gravimetry and Radiometry Results: Alto do Sobrido-Ribeiro da Serra Case Study

Ana Carvalho, Ricardo Ribeiro, Rui Moura, and Alexandre Lima

The Alto do Sobrido (AS) and Ribeiro da Serra (RS) Mines are old Sb-Au explorations. These are located in Gondomar, Portugal, on the inverse limb of the well-known structure called Valongo Anticline. In the AS Mine, the mineralization occurs near the contact between the Schist-Greywacke Complex (CXG) (Precambrian and/or Cambrian(?)) and the breccia of the base of the Carboniferous. In the RS Mine, the mineralization occurs only on the CXG. In both mines, the Sb-Au mineralization occurs in quartz veins and some stockworks.

A spatial correlation between the Sb-Au mineralization and the post-orogenic granites occurs in the Dúrico-Beirã Region according to Gumiel & Arribas (1987). Couto et al. (2007) also acquired data that suggests a genetic connection between this mineralization and non-outcropping granites. These granites may have been the source of fluids and a heat source that improved hydrothermal circulation and they have been observed in one of the RS Mine’s galleries.

With this hypothesis in mind, we intend to compare the data from a radiometric survey, which is a method that is radiometrically sensitive to K, Th and U at the near-surface, to the data from a gravimetric survey, which is a method that is sensitive to density anomalies at greater depths, in order to show if these granites could have chemically influenced its embedding rocks.

To make this comparison, we used the residual anomaly map from our gravimetric survey and the four maps obtained in the radiometric survey (total concentrations, K, eTh and eU). Firstly, we normalized all the grid maps to obtain grids with values between -1 and 1. Once this was complete, we multiplied each of the four radiometry maps to the residual anomaly map, obtaining the comparison maps.

On the resulting maps, we can observe high values in 3 different areas. The first corresponds to a lower value of gravimetric anomaly and a lower value of concentrations of all the elements. This area is located where the hypothesized non-outcropping granites are situated. The second area corresponds to high values on both methods. This matches the location of the lithologies from the Middle Ordovician to the Carboniferous, which are rocks of higher densities and higher concentration values of K, eTh and eU. The third area consists of lower gravimetric anomalies and lower concentrations of K and eU, and coincides with the location of the Ordovician quartzites. This area isn’t as visible on the eTh map, which is consistent with what was observed on the field.

We consider this approach to be a practical method to correlate the results of these two methods and an attempt to understand how the granite located at depth could have influenced these lithologies that today outcrop.

References

Gumiel, P., Arribas, A., 1987. Antimony Deposits in the Iberian Peninsula. Economic Geology, Volume 82, pp 1453-1463.

Couto, H., Borges, F. S., Roger, G., 2007. Late Palaeozoic orogenic gold-antimony deposits from the Dúrico-Beirã area (North Portugal) and their relation with hidden granitic apexes. Ninth Biennial SGA Meeting, Dublin. pp 609-612.

How to cite: Carvalho, A., Ribeiro, R., Moura, R., and Lima, A.: Comparison Between Gravimetry and Radiometry Results: Alto do Sobrido-Ribeiro da Serra Case Study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-418, https://doi.org/10.5194/egusphere-egu22-418, 2022.

EGU22-2416 | Presentations | EMRP1.18

Implications of reservoir heterogeneity for CO2 storage monitoring: A combined experimental and theoretical rock physics study 

Ismael Himar Falcon-Suarez, Michael Dale, and Nazmul Haque Mondol

Carbon Capture Utilization and Storage (CCUS) is an essential technology to meet net-zero carbon emission targets. Due to CO2 injection, original reservoir properties are altered. Early warning of potential CO2 injection-induced reservoir instability depends on our correct interpretation of the geophysical remote sensing data, particularly seismic and electromagnetic datasets. Using joint elastic-electrical datasets is a proven effective approach to simultaneously characterize mineral skeleton properties and pore fluid distribution, and therefore a powerful reservoir monitoring tool for CCUS.

To interpret large-scale elastic-electrical datasets, original rock properties and fundamental CO2-fluid-rock interactions are  preliminarily investigated by combining available well-logging data, lab-controlled experiments using rock samples, and rock physics theories; the latter two are inevitably dependent on one another. Despite we can mimic changing reservoir conditions in the lab and generate datasets that provide essential information to understand specific processes at the micro- and meso-scales (and serve as inputs for large-scale reservoir simulations), every experiment carries limitations inherent to the particular lab capabilities, together with the obvious time- and space-scale related uncertainties (i.e., core-scale experiments only partially describe the events occurring in the field). Then, we need theoretical rock physics to make experimental assumptions, and reciprocally we use the experimental data to validate models.

Physical and petrographic properties of reservoir rocks condition the degree of heterogeneity and anisotropy of the CO2 storage unit that, in turn, influence the total storage capacity and fluid migration. Original clay content, grain size distribution, mineralogy, porosity and permeability are among the most influencing parameters, particularly for low reactive siliciclastic formations (i.e., desired CO2 storage reservoirs). But these properties randomly change to some extent within any reservoir formation.

Here, we investigate how reservoir heterogeneity influences our geophysical interpretation of the potential CO2 storage site Aurora, offshore Norway. Recent studies suggest high clay content and porosity variability within the Johansen Formation sandstone, Aurora’s primary reservoir. Due to lack of Johansen Fm. samples, we selected three sandstone samples from the Central Graben, Offsore UK (Forties Formation), formed in a similar depositional environment, with similar mineralogical composition, and porosity (20 to 28%), clay content (10 to 26%) and permeability (1 to 8 mD) ranges. The elastic (from P- and S-wave velocities) and transport (from permeability and resistivity) properties of the tested samples were used to assess the influence of their intrinsic properties on the pore fluid distribution during CO2 injection and the permanent CO2-induced changes in the Aurora reservoir complex. We apply well-known rock physics theories, including effective stress law, Archie’s relationship, and the Biot-Stoll and White and Dutta-Ode models, for both to impose the most similar reservoir conditions according to our lab limitations and to assess the experimental results. We observe (i) elastic and transport properties variations (up to 15% and 30%, respectively) between samples, mainly related to porosity differences, and (ii) more significant permanent alterations post-CO2 injection in those with higher porosity and clay content. Our results show the importance of accounting for heterogeneity-related changes in sandstone reservoirs during/after subsurface CO2 storage activities for enhanced geophysical interpretation.  

How to cite: Falcon-Suarez, I. H., Dale, M., and Mondol, N. H.: Implications of reservoir heterogeneity for CO2 storage monitoring: A combined experimental and theoretical rock physics study , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2416, https://doi.org/10.5194/egusphere-egu22-2416, 2022.

EGU22-4190 | Presentations | EMRP1.18

Joint inversion of DC resistivity and potential field data under different model weighting functions 

Maurizio Milano, Ramin Varfinezhad, and Maurizio Fedi

In this study we analyze the role of model weighting functions for resistivity and potential field data in both separate and joint inversion. We show that the model weighting function built with depth weighting and compacting factor, formerly formulated for the gravity and magnetics inversion, can be useful also for DC resistivity data modelling. The comparison was made using the depth weighting with different exponents and the roughness matrix under L1- and L2-norm Constrained Optimization. We then analyze the 2-D joint inversion of DC resistivity and potential field data, based on the above model weighting function and the cross-gradients constraint. We provide a number of synthetic cases to discuss the pro and cons of each model-weighting function and to examine the feasibility of the joint inversion algorithm. We then provide results from two real case datasets for mining and archeological exploration. The results show that the value of the β exponent is decisive for potential field problems, but it also leads to a faster convergence for the resistivity data inversion. Similarly, the role of compactness is important for modelling compact source from gravity and magnetic, and to warrant an even faster and compact solution for DC resistivity. On the other hand, the results of the joint inversion reveal that the cross gradient constraints allow a successful joint inversion even when resistivity and magnetic data are often not easily comparable.

How to cite: Milano, M., Varfinezhad, R., and Fedi, M.: Joint inversion of DC resistivity and potential field data under different model weighting functions , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4190, https://doi.org/10.5194/egusphere-egu22-4190, 2022.

EGU22-9251 | Presentations | EMRP1.18

Assessing the pressure (in)dependence of cross-property relations

Phillip Cilli and Mark Chapman

It is known that geological reservoir characterisation can be improved by the joint modelling and inversion of both electrical and elastic data, however the relationship between a rock’s electrical and elastic properties, which is intrinsic to these methods, is relatively uncertain. On top of this, estimating reservoir pressure from geophysical measurements is an essential part of the 3D and 4D monitoring of CO2 injection and hydrocarbon production, and while electrical and elastic properties are affected by pressure, the effect of pressure on electrical-elastic relations is less obvious.

Here we use the Cross-Property Differential Effective Medium approximation to model public-domain electrical-elastic laboratory measurements made on brine-saturated clean and mixed sandstones cores at 6 effective pressures ranging from 8 MPa to 60 MPa. Although the approximation is able to realise a large proportion of the electrical-elastic space bounded by the Hashin-Shtrikman bounds using a range of permissible parameter values, we find the model parameter, equivalent pore aspect ratio, varies very little as a function of pressure when modelling the measured data. Interestingly, we see equivalent pore aspect ratio changes exponentially as a function of pressure with an R2 value of over 0.99 when modelling clean sandstones, a trend which has been observed previously in single-property inclusion modelling. This small variance in the model parameter as a function of pressure corresponds to an observably small change in the samples’ electrical-elastic measurements with pressure.

We conclude the electrical-elastic properties of the examined clean and mixed brine-saturated sandstones are only weakly dependent on pressure and we demonstrate how a single, pressure-independent model parameter is able to model the electrical-elastic measurements of both the clean and mixed sandstones with reasonable accuracy over the full range of experimental pressures.

How to cite: Cilli, P. and Chapman, M.: Assessing the pressure (in)dependence of cross-property relations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9251, https://doi.org/10.5194/egusphere-egu22-9251, 2022.

EGU22-4536 | Presentations | EMRP1.18

Transient rheology of the continental crust

Sagar Masuti, Jun Muto, and Erik Rybacki

Postseismic relaxation after large earthquakes induces transient deformation of the solid Earth, particularly in the deeper part of the crust. The deformation of the upper and lower crust are mainly controlled by the rheological behavior of quartz and feldspar, respectively. The mechanical properties of quartz and feldspar at steady-state creep conditions are well constrained and flow law parameters are known from experimental calibrations. However, the physical mechanism underlying transient creep is poorly understood and the corresponding flow law parameters are unknown so far. Here, we constrain a constitutive framework that captures transient creep and steady state creep consistently using the mechanical data from laboratory experiments. The constitutive framework represents a Burgers assembly with a thermally activated nonlinear stress versus strain-rate relationship for the dashpots. Using the Markov chain Monte Carlo (MCMC) method, we uniquely determine the flow law parameters for both quartz and feldspar. We find an activation energy of 70±20 kJ/mol and a stress exponent of 2.0±0.1 for transient creep of quartz. For feldspar, the best-fit activation energies are 280±30 and 220±20 kJ/mol with stress exponents of 1.0±0.2 and 0.9±0.1 under mid- and high-temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady-state creep for both quartz and feldspar. The flow law parameters determined in this study could be used to quantify the contribution of transient creep in the postseismic deformation following a large continental earthquake. 

How to cite: Masuti, S., Muto, J., and Rybacki, E.: Transient rheology of the continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4536, https://doi.org/10.5194/egusphere-egu22-4536, 2022.

EMRP2.12 – Open Session in Geomagnetism

Rapid secular variation pulses in the Earth’s geomagnetic field have been identified during the last decade. In particular the 2019-2020 event is the latest in a series of rapid secular variation events observed at the Earth’s surface which are thought to be the result of rapid oscillations at the core surface approximately at a depth of 3000 km. In Southern Africa the 2019-2020 pulse has been analysed using data from 4 observatories located at Hermanus, Hartebeesthoek, Keetmanshoop and Tsumeb, and found that the 2019-2020 event occurred with varying strengths in the different components at a particular observatory, while different observatories in the region showed strong individual characteristics. These rapid changes in the secular variation patterns at individual magnetic observatories in this study can also be influenced by the South Atlantic Anomaly (SAA) where the geomagnetic field has been diminishing at a very rapid rate over the past 400 years in comparison to regions at similar latitudes around the globe. Results will be compared to the global CHAOS field model derived from ground and SWARM satellite data.

How to cite: Nel, A. and Kotzé, P.: The 2019-2020 geomagnetic jerk as observed by southern African magnetic observatories. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3976, https://doi.org/10.5194/egusphere-egu22-3976, 2022.

EGU22-9173 | Presentations | EMRP2.12

Geomagnetic Observatory at Lampedusa Island: Characterization of local magnetic activity and comparison with the other Italian observatories

Domenico Di Mauro, Mauro Regi, Stefania Lepidi, Alfredo Del Corpo, Guido Dominici, Paolo Bagiacchi, Giovanni Benedetti, and Lili Cafarella

At present, the geomagnetic observatory at Lampedusa (south of Sicily — Italy, geographic coordinates 35°31′N; 12°32′E, altitude 33 m a.s.l. - provisional IAGA code: LMP) is the southernmost point of observation in European territory, and since 2007 it contributes at filling the spatial observational gap in the whole south Mediterranean and North African areas. A signature of very low electromagnetic noise is expected at LMP, since it is located in the inner part of a wild park with limited access, far away from the urbanized areas of the island. LMP lies in the middle of the Mediterranean Sea, while the other two Italian observatories (Castello Tesino – CTS and Duronia - DUR, in North and central Italy, respectively) are located in the continental territory.

Comparisons among the three observatories, in both time and frequency domains, allow to magnetically characterize the Italian territory. Both 1-minute and 1-second data for the years 2017-2020 are analyzed under a statistical approach and also single event analysis is performed. Superposed Epoch Analysis (SEA) of geomagnetic data from the three observatories returns individual responses to external triggers during geomagnetic storms as well as SSC and SI events, indicating that in correspondence to impulsive inputs a peculiar feature arises at LMP, probably as contribution of electric currents in the surrounding sea salt water. Magnetic responses in the Ultra-Low-Frequency (ULF, 1 mHz–5 Hz) range from spectral, local Signal-to-Noise Ratio (SNR) analyses under different local time are computed, showing that the signal emerges mainly during morning hours, as expected for upstream waves related ULF source waves: in particular, the distinct narrow band characteristic of SNR at LMP indicates that the ULF signals are here mainly uncontaminated by local Field Line Resonance (FLR) as at DUR and CTS, while lower noise levels estimated at LMP suggest a smaller anthropogenic contamination in this frequency range. Moreover, for the first time at such low latitudes in the Mediterranean region, we find evidence of FLR events on Duronia–Lampedusa intermediate field line with the application of the gradient method, a consolidated technique that provides estimates of the ULF standing wave frequencies.. Results from data retrieved by geomagnetic observatories, whose long time series of data are of primary importance, demonstrate a unique contribution in characterizing the magnetospheric response to external events.

How to cite: Di Mauro, D., Regi, M., Lepidi, S., Del Corpo, A., Dominici, G., Bagiacchi, P., Benedetti, G., and Cafarella, L.: Geomagnetic Observatory at Lampedusa Island: Characterization of local magnetic activity and comparison with the other Italian observatories, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9173, https://doi.org/10.5194/egusphere-egu22-9173, 2022.

Geomagnetic data at Kiruna station (KIR) in Sweden has been expected to be affected by the iron ore mine because high conductance underground generally means depressed ΔZor dZ/dt compared to ΔH or dH/dt, where Z and H are downward and horizontal components, respectively, Δ indicates deviation, and we took 1-min average of 1-sec resolution data when defining d/dt values. We examined the 1-sec resolution magnetometer data using both frequency-domain (i.e., standard magnetotellurics) and time-domain analyses, and compared general behaviours with the other high-latitude stations on the same longitude (Hornsund: HRN, Abisko: ABK, Lycksele: LYC, Uppsala: UPS, Nurmijärvi: NUR) for the same period (September 2014-2020).

Surprisingly, we found KIR anomaly only in time-domain derivative dB/dt and not in the frequency domain spectrum. To quantify this anomaly, we examined the standard deviation of each parameter (1-min average of 1-sec resolution values) over 3 hours. With this quantification, the level of anomaly was about the same between old magnetometer until 2019 and new magnetometer from 2020. The anomaly is somewhat present in both dZ/dt and dH/dt but is the clearest in the ratio of dZ/dt to dH/dt.  On the other hand, neither ΔZ nor ΔH showed anomaly. Furthermore, no anomaly is recognized in the inclination I (=atan(Z/H)), i.e., ΔI nor dI/dt. From all of these, we believe that the observed anomaly is caused by underground iron ore deposit and not by the magnetometer filtering setting. The reason why the anomaly is found only in d/dt values is not clear, but we suspect that the iron ore deposit might cause time delay between dZ/dt and dH/dt when step-like variation dominates as the input variation, which is often the case with auroral activity. In such variation, neither the frequency domain analyses, nor simple time domain analyses (ΔB) show any anomaly.

We applied this method to the other meridians (three meridians in North America). We could not find any anomaly similar to what KIR data showed. However, we found another type of anomaly (on dI/dt) in Barrow, Alaska. It can be related to its location, surrounded by the arctic sea in both east and west, but we have not yet found an appropriate interpretation.

[Acknowledgements:  This work resulted from a 2021 summer internship study at the Swedish Institute of Space Physics, Kiruna.   The 1-sec resolution geomagnetic data are obtained from INTERMAGNET and are originally provided by SGU (Sweden: UPS, LYC, KIR, ABK), FMI (Finland: NUR), PAS (Poland: BEL, HLP, HRN), GSC (Canada: BLC, CBB, FCC, IQA, OTT, RES, STJ, YKC), USGS (USA: BRW, CMO, FRD, SHU, SIT), IPGP (France: CLF), ZAMF (Austria, WIC), and ASCR (Czech: BDV).]

How to cite: Longeon, B. and Yamauchi, M.: Iron deposit effect observed in Kiruna geomagnetic fluctuations: Indications for an improved approach of magnetotellurics searching methods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-168, https://doi.org/10.5194/egusphere-egu22-168, 2022.

EGU22-10727 | Presentations | EMRP2.12

Exploration of spectral energy from marine and modeled magnetic anomalies

Andreina Garcia-Reyes and Jérôme Dyment

The spectral method has been widely used in various branches of science since it simplifies the analysis of periodic signals. In the case of geophysics, Fourier transform is used to decompose the signal into different wavelengths, and to associate the gradient of the spectral energy to the depth of the source (after Spector and Grant, 1970). This last association is mostly applied in gravity and magnetism, that is, from sources that produce a contrast of density or magnetic susceptibility with respect to the medium. Large part of the mathematical operators used in geophysics rely on the Fourier analysis.

In this work, we set out an analysis of the spectral energy of the magnetic signal in three and two dimensions. We apply this analysis to three cases: the first case, the classical approach, corresponds to the spectral energy calculated from synthetic magnetic anomalies, produced by bodies of simple geometry. In the second case, we use marine magnetic anomalies on a regional scale, specifically of the Caribbean plate, and finally, a third case, where we apply the method on a marine area covering a few square kilometers. Our objective was primarily to explore and characterize the spectral response from another perspective: that of the spectral cube and to estimate depths of magnetic sources from methods previously used to derive only the depth of the Curie isotherm. A first trial in the application of this method was carried out by Garcia-Reyes and Dyment (2020). However, the application of this in other areas allows evaluating its sensitivity to factors such as scale, resolution and quality of the data, the proximity of the source and geometry of the source. This exercise allows us to validate the approach in estimating depths of sources in the subsurface, and in turn, it is a step forward in understanding the spectral cube and the information it provides.

Our results allow us to offer a mapping of the depth of the magnetic sources (detectable in the spectra), and in turn, a three-dimensional view of their spectral energy. These sources are generally correlated with geological structures, as is the case with the results obtained for the Caribbean plate. Beyond the major developments of the spectral method in geophysics, we suggest that the information inscribed in the spectral signature of magnetic anomalies can still be further explored.

 

References:

GARCIA-REYES, Andreina and DYMENT, Jérôme (2020). Spatial Power Spectral Density Distribution of Magnetic Sources in the Gulf of Mexico and Caribbean Plate. In : AGU Fall Meeting Abstracts. p. GP012-0009.

Spector, A., & Grant, F. S. (1970). Statistical models for interpreting aeromagnetic data. Geophysics, 35(2), 293-302.

How to cite: Garcia-Reyes, A. and Dyment, J.: Exploration of spectral energy from marine and modeled magnetic anomalies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10727, https://doi.org/10.5194/egusphere-egu22-10727, 2022.

EGU22-3153 | Presentations | EMRP2.12

On the influence of rotational processes on the tectonic activation of the Earth

Тамара Литвинова

The capacity of oceanic crust to record geomagnetic polarity reversals makes sea-surface magnetic anomalies an essential tool to study plate tectonics. The anomalies are usually well-defined at magmatic spreading centers, but are distorted and eventually disappear on magma-poor mid-ocean ridges such as the ultraslow Southwest Indian Ridge (SWIR), making their interpretation difficult. We attribute the variability of the SWIR sea-surface magnetic anomalies to the alternance of magmatic spreading and detachment faulting. A three-layer magnetic model is used to simulate the influence of such an alternance on the sea-surface magnetic anomalies. Conversely, observed magnetic profiles at the SWIR are modelled to unravel their off-axis crustal structure and past mode of spreading. The intruding gabbro bodies on the footwall of detachment faults play a major role in explaining the variability of sea-surface magnetic anomalies at slow and ultraslow spreading ridges.

How to cite: Zhou, F. and Dyment, J.: Variability of Sea-surface Magnetic Anomalies at Ultraslow Spreading Centers: Consequence of Detachment Faulting and Contrasted Magmatism?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11119, https://doi.org/10.5194/egusphere-egu22-11119, 2022.

The paper considers the reason for the displacement of the magnetic axis relative to the axis of rotation of the Earth for the case of the quartz nature of the dipole magnetic field. A model based on a ring with a current with an uneven distribution of charges along the circumference of the ring is considered. It is shown that the magnetic axis shifts from the axis of rotation towards a greater concentration of charges and, conversely, with a decrease in the concentration of charges. The issue of reducing the number of gravity-oriented quartz crystals in areas of volcanic activity is discussed. The temporal correlation of the beginning of accelerated drift of the Earth's north magnetic pole with the development of volcanic and tectonic activity in the Yellowstone caldera is shown. Attention is drawn to the fact that the Earth's north magnetic pole is shifting towards the geographical pole relative to the geographical coordinates of the Yellowstone caldera.

How to cite: Zhumabayev, B. and Vassilyev, I.: The relationship of the displacement of the north magnetic pole with volcanic activity in the Yellowstone caldera, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4530, https://doi.org/10.5194/egusphere-egu22-4530, 2022.

EGU22-8344 | Presentations | EMRP2.12

Analysis of the Last Reversal, Last Excursions and important HoloceneAnomalies of the Geomagnetic Field using the Eccentric Dipole and a 360-Dipole Ring Mode

Alicia González-López, Maria Luisa Osete, Saioa A. Campuzano, Pablo Rivera-Pérez, Alberto Molina-Cardín, and F. Javier Pavón-Carrasco

Eccentric dipole can be considered the next approximation of the geomagnetic field after the
generally used geocentric dipole. Considering that during reversals, excursions and important
anomalies the non-dipole contributions are relevant, we study the evolution of the eccentric
dipole during the last reversal (Matuyama-Brunhes transition, ~780 ka), last excursions
(Laschamp, ~41 ka and Mono-Lake, ~34 ka), the Levantine Iron Age Anomaly (LIAA, ~1000 BC)
and the South Atlantic Anomaly (SAA, from 700 AD to present day) according to
paleoreconstructions (IMMAB4, LSMOD.2, SHAWQ-Iron Age and SHAWQ2k, respectively). In
order to get as much as information as possible from the eccentric dipole, we design a simple
model based on 360-point dipoles evenly distributed in a ring close to the Inner Core Boundary
that can be reversed and/or changed their magnitude. We calculate the evolution of the
modeled eccentric dipole according to the 360-dipole ring model reproducing the eccentric
dipole from the paleoreconstructions. If we consider that each point dipole could be associated
to convective columns in the outer core of the Earth, we can relate the evolution of the eccentric
dipole with potential variations in the outer core that cause its displacement. We observe that
the modeled eccentric dipole moves away from regions where dipoles start to reverse (which
are the cases for the reversal, excursions and the SAA) and towards regions where there are
anomalous high-moment dipoles (such as the LIAA). The results show that the eccentric dipole
paths during the events studied correlate well to Core Mantle Boundary low heat flux regions
that is consistent with the development of instabilities in the geomagnetic field.

How to cite: González-López, A., Osete, M. L., A. Campuzano, S., Rivera-Pérez, P., Molina-Cardín, A., and Pavón-Carrasco, F. J.: Analysis of the Last Reversal, Last Excursions and important HoloceneAnomalies of the Geomagnetic Field using the Eccentric Dipole and a 360-Dipole Ring Mode, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8344, https://doi.org/10.5194/egusphere-egu22-8344, 2022.

The International Geomagnetic Reference Field (IGRF) is a multi-institute model of the Earth’s magnetic field, compactly described by sets of up to 195 spherical harmonic (Gauss) coefficients to degree and order 13, which allows the continuous evaluation of the field at any location and time on or above the surface. It is developed from satellite and ground-based magnetometer data and describes the large-scale variation of the magnetic field in space and time under quiet conditions. While much effort has been made on improving the forecast of the secular variation of the field over the five-year intervals between release and renewal, less emphasis has been placed on understanding the spatial errors from a user point of view. We estimate the large-scale time-invariant spatial uncertainty of the IGRF based on the globally averaged misfit of the model to semi-independent ground-based measurements at repeat stations and observatories between 1980 and 2021. As the ground measurements are reduced to quiet-time values, the external field is minimized. We find the 68.3% confidence interval is 87 nT in the North (X) component, 73 nT in the East (Y) component and 114 nT in Vertical (Z) component. Due to the Laplacian distribution of the residuals, the standard deviations are larger at 144, 136 and 293 nT, respectively.

How to cite: Beggan, C.: Evidence-based uncertainty estimates for the International Geomagnetic Reference Field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4172, https://doi.org/10.5194/egusphere-egu22-4172, 2022.

EGU22-5000 | Presentations | EMRP2.12

Disturbance Storm Time (Dst) index estimation using deep learning applied to Swarm satellite data

Gianfranco Cianchini, Alessandro Piscini, Angelo De Santis, and Saioa Arquero Campuzano

Computed from the intensity of the globally symmetrical equatorial electrojet (Ring Current) measured by a series of near-equatorial geomagnetic observatories, the Dst (Disturbance Storm Time) is an hourly index of magnetic activity. To give the estimation of the Dst index through the magnetic data measured by the Swarm three-satellite mission, we selected and trained an Artificial Neural Network (ANN). From November 2014 to December 2019, we collected a big Swarm magnetic dataset, confined in space to three very narrow belts of low-to-mid latitude, to better resemble the geographic distribution of the four geomagnetic observatories used to estimate at ground Dst. We also extended the analysis to mid latitude locations to increase the number of satellite samples. By using a Deep Learning architecture and based on its performance, we selected the best topology and trained the network testing its modelling capabilities. The outcomes show that the ANN is able to give a reliable fast estimation of the Dst index directly from Swarm satellite magnetic data, especially during magnetically disturbed periods.

How to cite: Cianchini, G., Piscini, A., De Santis, A., and Arquero Campuzano, S.: Disturbance Storm Time (Dst) index estimation using deep learning applied to Swarm satellite data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5000, https://doi.org/10.5194/egusphere-egu22-5000, 2022.

EGU22-3867 | Presentations | EMRP2.12

The evaluation of crustal field influence to geomagnetic cutt-off rigidities

Patrik Jakab and Pavol Bobík

The cut-off rigidities of cosmic rays usually do not reflect the influence of the crustal geomagnetic field. Due to the weakness of the crustal field effect to the cosmic rays trajectories are very minor. However, two regions of the world have a crustal field with significantly higher values. The effect of the crustal field in those regions is evaluated. The consequences for the approach of cosmic rays to Earth's surface (top of the Earth atmosphere) in the last decades are analyzed and discussed. Presented are suggestions for possible modification of models for evaluation of Earth's magnetosphere transparency for cosmic rays.

How to cite: Jakab, P. and Bobík, P.: The evaluation of crustal field influence to geomagnetic cutt-off rigidities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3867, https://doi.org/10.5194/egusphere-egu22-3867, 2022.

EMRP2.13 – Modelling and measuring Geomagnetically Induced Currents in grounded infrastructure

EGU22-6243 | Presentations | EMRP2.13 | Highlight

Estimating the Geoelectric Field, Transmission Line Voltages, and GICs During a Geomagnetic Storm in Alberta, Canada

Darcy Cordell, Martyn Unsworth, Benjamin Lee, Cedar Hanneson, David Milling, Hannah Parry, and Ian Mann

Estimating the effect of geomagnetic disturbances on infrastructure is an important problem since they can induce damaging currents in electric power transmission lines. In this study, an array of magnetotelluric (MT) impedance measurements in Alberta and southeastern British Columbia are used to estimate the geoelectric field resulting from a magnetic storm on September 8, 2017. The resulting geoelectric field is compared to the geoelectric field calculated using the more common method involving a piecewise-continuous 1-D conductivity model. The 1-D model assumes horizontal layers, which result in orthogonal induced electric fields while the empirical MT impedance data account for fully 3-D electromagnetic induction. The geoelectric field derived from empirical MT impedance data demonstrates a preferential polarization in southern Alberta, and the geoelectric field magnitude is largest in northeastern Alberta where resistive Canadian Shield outcrops. The induced voltage in the Alberta transmission network is estimated to be ~120 V larger in northeastern Alberta when using the empirical MT impedances compared to the piecewise-continuous 1-D model. Transmission lines oriented northwest-southeast in southern Alberta have voltages which are 10-20% larger when using the MT impedances due to the polarized geoelectric field. As shown with forward modelling tests, the polarization is due to the Southern Alberta British Columbia conductor in the lower crust (20-30 km depth) that is associated with a Proterozoic tectonic suture zone. This forms an important link between ancient tectonic processes and modern-day geoelectric hazards that cannot be modelled with a 1-D analysis. The geoelectric field model and resulting line voltage is compared to differential magnetometer GIC measurements on one transmission line near the Heartland transformer in northeastern Alberta.

How to cite: Cordell, D., Unsworth, M., Lee, B., Hanneson, C., Milling, D., Parry, H., and Mann, I.: Estimating the Geoelectric Field, Transmission Line Voltages, and GICs During a Geomagnetic Storm in Alberta, Canada, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6243, https://doi.org/10.5194/egusphere-egu22-6243, 2022.

EGU22-6442 | Presentations | EMRP2.13

Investigating the levels of Geomagnetically Induced Currents in the Mediterranean region during the most intense geomagnetic storms of solar cycle 24

Adamantia Zoe Boutsi, Georgios Balasis, Ioannis A. Daglis, Kanaris Tsinganos, and Omiros Giannakis

Geomagnetically Induced Currents (GIC) constitute an integral part of space weather research and are a subject of ever-growing attention for countries located in the low and middle latitudes. A series of recent studies highlights the importance of considering GIC risks for the Mediterranean region. Here, we exploit data from the HellENIc GeoMagnetic Array (ENIGMA), which is deployed in Greece, complemented by magnetic observatories in the Mediterranean region (Italy, France, Spain, Algeria and Turkey), to calculate values of the GIC index, i.e., a proxy of the geoelectric field calculated entirely from geomagnetic field variations. We perform our analysis for the most intense magnetic storms (Dst < -150 nT) of solar cycle 24. Our results show that GIC indices do not exceed low activity levels despite the increase in their values, at all magnetic observatories / stations under study during the selected storm events.

How to cite: Boutsi, A. Z., Balasis, G., Daglis, I. A., Tsinganos, K., and Giannakis, O.: Investigating the levels of Geomagnetically Induced Currents in the Mediterranean region during the most intense geomagnetic storms of solar cycle 24, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6442, https://doi.org/10.5194/egusphere-egu22-6442, 2022.

In this research the analysis of shutdowns in long power line SS147AL169A in Almaty power grid for 2016-2021 was done. For the analyzed period, there were 16 emergency shutdowns. 6 of them occurred due to evident external causes. Other 10 cases analyzed for the possibility of a connection with the geomagnetic environment. Initial analysis showed a possible connection between the automatic operation of relay protection and the presence of geomagnetically induced currents. This is due to the geomagnetic situation, which was before the moment the relay was triggered. At the moment, more detailed calculations are being carried out.

This research has been/was/is funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP00000000).

How to cite: Nurgaliyeva, K.: Analysis of correlations between geomagnetic storms and emergency shutdowns in the part of Almaty power grid for 2016-2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3317, https://doi.org/10.5194/egusphere-egu22-3317, 2022.

EGU22-3006 | Presentations | EMRP2.13 | Highlight

Modelling space weather impacts on UK railway signalling systems

Cameron Patterson and Jim Wild

Track circuits are widely used signalling systems that use electrical currents to detect the presence or absence of a train in predefined sections of a railway network, as such, they are susceptible to interference from geomagnetically induced currents.

This work aims to determine the impact space weather has on realistic track circuits across geologically different regions of the UK under various storm conditions by using the Spherical Elementary Current System method of geomagnetic field interpolation, a ground conductivity model of the UK, a 1D-layered model to provide estimations of the geoelectric field and track circuit modelling techniques developed by Boteler (2021).

Early results of a modelled section of the West Coast Main Line in North West England will be presented.

How to cite: Patterson, C. and Wild, J.: Modelling space weather impacts on UK railway signalling systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3006, https://doi.org/10.5194/egusphere-egu22-3006, 2022.

EGU22-3218 | Presentations | EMRP2.13

Assessing the impact of weak and moderate geomagnetic storms on UK power station transformers

James Wild, Zoe Lewis, and Matthew Allcock

It is well documented that space weather can impact electricity infrastructure, and several incidents have been observed in recent decades and directly linked to large geomagnetic storms (e.g. the Hydro Quebec incident in 1989). However, less is understood aboutthe impact of lower-level geomagnetically induced currents (GICs) on the health of transformers in the long term. In this study, the long term impact of geomagnetic activity  on 13 power station transformers in the UK is investigated. Dissolved gas measurements from 2010–2015 were used to look for evidence of a link between degradation of the transformer and heightened levels of the global SYM-H index and dB as measured at Eskdalemuir magnetometer station in southern Scotland. First, case studies of the most significant storms in this time period were examined using dissolved gas analysis (DGA) methods, specifically the Low Energy Degradation Triangle (LEDT). These case studies were then augmented with a statistical survey, including Superposed Epoch Analysis (SEA) of multiple storm events. No evidence of a systematic space weather impact can be found during this time period, likely owing to the relatively quiet nature of the Sun during this epoch and the modernity of the transformers studied.

How to cite: Wild, J., Lewis, Z., and Allcock, M.: Assessing the impact of weak and moderate geomagnetic storms on UK power station transformers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3218, https://doi.org/10.5194/egusphere-egu22-3218, 2022.

EGU22-9495 | Presentations | EMRP2.13

Combining geoelectric field modelling and differential magnetometer data to validate GIC modelling in the UK High voltage power transmission grid

Juliane Huebert, Ciaran Beggan, Gemma Richardson, Natalia Gomez Perez, and Alan Thomson

Geomagnetically induced currents (GICs) have been identified as a hazard to the UK power grid and the security of electricity supply during severe geomagnetic storms. In order to monitor, model and forecast GICs, sophisticated models of the ground electric field and the network topology are required. We present a detailed analysis of differential magnetometer (DMM) and magnetotelluric (MT) data in the UK that allow the verification and validation of our network model for the UK power transmission grid. Combining the observation of line GICs measured with DMM in the past three years and the MT impedance tensor estimated at several locations in the UK shows an excellent fit of prediction and observation of GICs when using realistic modelled ground electric fields. This validates our whole network model allowing us to use it with confidence for real time and forecasting as well as extreme event analysis.

How to cite: Huebert, J., Beggan, C., Richardson, G., Gomez Perez, N., and Thomson, A.: Combining geoelectric field modelling and differential magnetometer data to validate GIC modelling in the UK High voltage power transmission grid, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9495, https://doi.org/10.5194/egusphere-egu22-9495, 2022.

During geomagnetic storms rapid magnetic variations cause large, sharp enhancements of the magnetic and geoelectric field at mid-latitudes. These present a potential hazard to grounded technology such as high voltage transformers, pipelines and railway systems. Spatio-temporal quantification can provide insight into the magnitude and configuration of their potential hazard. We perform a wavelet decomposition on both European ground-based magnetometer measurements and modelled Geomagnetically Induced Currents (GICs) from the high voltage grid of Great Britain (GB).  A wavelet decomposition localizes the signal in the time-frequency domain, and we show that in both magnetometer observations, and modelled GIC response, the Haar wavelet extracts the signal power and waveform at the signal fastest rate-of-change.

We then use Haar wavelet cross-correlation of the GIC in the grounded nodes to build a time-varying network of GIC coherent response around the GB grid during intense geomagnetic storms [1]  including the 2003 Halloween storm. We find a highly intermittent (few 10s of minutes duration) long-range coherent response that can span the entire physical grid at most intense times. The spatial pattern of coherent response seen in the GIC flow network does not simply follow that of the amplitude of the rate of change of B field that is estimated via the Haar wavelet. Coherent response is excited across spatially extended clusters comprised of a subset of nodes that are highly connected to each other, with a tendency for east-west linkages following that of the physical grid, simultaneous with  the overhead presence of the auroral electrojet and the inducing component of the magnetic field. This can quantify the spatial and temporal location of increased hazard in specific regions during large storms by including effects of both the geophysical and engineering configuration of the high voltage grid.

[1] L. Orr, S. C. Chapman, C. Beggan, Wavelet and network analysis of magnetic field variation and geomagnetically induced currents during large storms, Space Weather (2021) doi: 10.1029/2021SW002772

 

How to cite: Chapman, S., Orr, L., and Beggan, C.: Wavelet cross-correlation dynamical network of the coherent GIC response to intense geomagnetic storms in the high voltage grid of Great Britain, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1769, https://doi.org/10.5194/egusphere-egu22-1769, 2022.

EMRP2.15 – Measuring space weather condition with geomagnetic data

EGU22-4570 | Presentations | EMRP2.15 | Highlight

Building a GIC forecasting tool based on geomagnetic and solar wind data: challenges and future avenues

Rachel L. Bailey, Roman Leonhardt, Christian Möstl, Ciaran Beggan, Martin Reiss, Ankush Bhaskar, and Andreas Weiss
Measurements of geomagnetically induced currents (GICs) in the Austrian power transmission grid have been carried out since 2014 at multiple locations. Following an analysis of the scales of GICs across the grid, we now look into forecasting the GICs from incoming solar wind data. Using nearby geomagnetic field measurements stretching back 26 years, we can estimate the local geoelectric field and consequently the GICs over longer time periods. We apply a machine learning method based on recurrent neural networks to this dataset combined with solar wind data as input. In this talk, we present the final method to forecast both the local geoelectric field E and the GICs in substations in the Austrian power grid, with our model results being compared to GIC measurements from recent years. We will discuss the current status of the model, outline limitations, and consider future applications.

How to cite: Bailey, R. L., Leonhardt, R., Möstl, C., Beggan, C., Reiss, M., Bhaskar, A., and Weiss, A.: Building a GIC forecasting tool based on geomagnetic and solar wind data: challenges and future avenues, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4570, https://doi.org/10.5194/egusphere-egu22-4570, 2022.

EGU22-5792 | Presentations | EMRP2.15

Utilisation of geomagnetic data and indices for GIC applications

Larisa Trichtchenko

The development of mitigation capabilities to counteract the detrimental impacts of space weather on critical ground infrastructure, such as power lines, pipelines and cables, depends on the availability of the observations of their causes as well as monitoring of the subsequent results.
Although direct monitoring of critical infrastructure response to GeoMagnetic Disturbances (GMD) has become more advanced in recent years, observations of geomagnetic variations continue to play the most important role in all aspects of development of safe and robust operational procedures and technology, from the forecast of geomagnetically induced currents (GIC) to their climatological studies.
This presentation shows how different types of geomagnetic data are utilised, from 3-hour and 1-hour geomagnetic indices to 1 sec. geomagnetic data, and from real-time to multi-year climatology in order to provide forecasts of GIC, identify the effects of different geomagnetic patterns on infrastructure response or provide “climatology” for network design considerations.  

How to cite: Trichtchenko, L.: Utilisation of geomagnetic data and indices for GIC applications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5792, https://doi.org/10.5194/egusphere-egu22-5792, 2022.

The space environment near Earth is constantly subjected to changes in the solar wind flow generated at the Sun. Examples of this variability are the occurrence of powerful solar disturbances, such as coronal mass ejections (CMEs). The impact of CMEs on the Earth's magnetosphere perturbs the geomagnetic field causing the occurrence of geomagnetic storms. Such extremely variable geomagnetic fields trigger geomagnetic effects measurable not only in the geospace but also in the ionosphere, upper atmosphere, and on the ground. For example, during extreme events, rapidly changing geomagnetic fields generate intense geomagnetically induced currents (GICs). In recent years, GIC impact on the power networks at middle and low latitudes has attracted attention due to the expansion of large-scale power networks into these regions. This work presents the analysis of the geoelectric field determined by the use of the MA.I.GIC. (Magnetosphere - Ionosphere - Ground Induced Current) model, on May 12, 2021 Geomagnetic Storm over the northern hemisphere. In addition, we discriminate between the ionospheric and magnetospheric origin contribution on the geoelectric field in Europe and on the Northern America in order to evaluate their relative contribution to the GIC amplitude.

How to cite: Piersanti, M., D'Angelo, G., and Recchiuti, D.: On the possible magnetospheric and ionospheric sources of the geoelectric field variations during the May 2021 Geomagnetic storm., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1249, https://doi.org/10.5194/egusphere-egu22-1249, 2022.

EGU22-3338 | Presentations | EMRP2.15

Geomagnetically Induced Currents over Kazakhstan during Large Geomagnetic Storms

Saule Mukasheva, Alexey Andreyev, Vitaliy Kapytin, and Olga Sokolova

The paper shows that during very large magnetic storms (VLMS), the energy systems of Kazakhstan are exposed to geomagnetically induced currents for quite a long time (from tens of minutes to several hours). The minute values of the magnetic field vector B or its components Bx, By, Bz during four very large geomagnetic storms with a local geomagnetic activity K-index≥7 were used to calculate the values of geomagnetically induced currents:

- September 26-28, 2011, VLMS, Sc, duration 54 h 00 min, K-index =7;

- June 22-25, 2015, VLMS, Sc, duration 78 h 30 min, K-index =8;

- October 24-28, 2016, VLMS, duration 93 h 00 min, K-index =7;

- May 12-17, 2021, VLMS, Sc, duration 17 h 25 min, K-index =7.

Sc – a sudden commencement of strong storms.

The data of four magnetic observatories of the INTERMAGNET network, whose geomagnetic latitudes are close to the geomagnetic latitudes of the southern and northern borders of Kazakhstan were considered: Alma-Ata Observatory, Kazakhstan (code AAA, 43.25°N, 76.92°E); Novosibirsk Observatory, Russia (code NVS, 54.85N, 83.23E); Irkutsk Observatory, Russia (code IRT, 52.17°N, 104.45°E) and the Beijing Ming Tombs Observatory, Beijing, China (code BMT, 40.3°N, 116.2°E).

Variations of the Bx component of the geomagnetic field during the four considered very large magnetic storms according to the observatories AAA, NVS, IRT, BMT showed variability from 50 nT to 150 nT for several hours.

Also, based on measurements of geomagnetic observatories AAA, NVS, IRT, BMT, the analysis of variations of the horizontal component H of the magnetic field vector and its time derivative (dH/dt) was carried out. Histograms of the distribution dH/dt and histograms of the distribution of the directions H and dH/dt are constructed.

It is shown that the energy systems of Kazakhstan are exposed to geomagnetically induced currents when dH/dt/ varies from 17 nT/min and more. The geomagnetic-induced current is estimated based on the calculation that the electromotive force of self-induction is proportional to the rate of change in the magnetic field strength. According to preliminary calculations, the values of geomagnetic-induced currents are fractions of mA. For more accurate calculations, it is necessary to take into account the topology of the electrical system, the composition of the underlying surface and other factors that determine the degree of susceptibility of individual elements of the power system.

This research has been/was/is funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP09259554).

How to cite: Mukasheva, S., Andreyev, A., Kapytin, V., and Sokolova, O.: Geomagnetically Induced Currents over Kazakhstan during Large Geomagnetic Storms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3338, https://doi.org/10.5194/egusphere-egu22-3338, 2022.

EGU22-8015 | Presentations | EMRP2.15

Towards an improved proxy for geomagnetically induced currents (GICs)

Fernando Jorge Gutiérrez Pinheiro, Marta Neres, M. Alexandra Pais, Joana Alves Ribeiro, Rute Santos, and João Cardoso

The irregular variation of geomagnetic activity caused by the solar wind interaction with the magnetosphere/ionosphere (space weather) occurs in wide temporal and amplitude ranges. Major geomagnetic storms can induce geoelectric fields in the Earth conducting layers (through the lithosphere and down to the mantle), which may, in turn, be responsible for generating geomagnetically induced currents (GICs). The vulnerability of grounded conducting infrastructures, particularly electrical power transmission systems, to GICs, makes it important to understand the relation between the varying geomagnetic field components and the generated GICs, as well as the role of the local conductivity, i.e., geology, on the inducing process. Looking for proxies that better translate this relation is an open matter of debate.

In this work, we present a comprehensive study of several possible candidates for GIC proxies. We use geomagnetic time series from the Portuguese mid-latitude Coimbra observatory (COI) to calculate geomagnetic indices considering different periods (whole-storm duration, 3-h, 1-h and 1-min), with different focuses on the field components or their derivatives, and discuss their advantages and limitations. We compare the computed indices with both GIC simulations of the Portuguese mainland high voltage power network (150, 220 and 400 kV) (Alves Ribeiro et al., 2021), and observations from a Hall effect sensor based system installed at a power transformer located in the vicinity of Coimbra. 
We then propose a better GIC proxy, an index obtained from geomagnetic field components filtered by convolution with a uniform conductivity Earth model filter (EGIC index), based on previous work by Marshall et al (2010,2011). We search for empirical parameters that may contain information on local conductivity effects and power network geometry.

This study is funded by national funds through FCT (Portuguese Foundation for Science and Technology, I.P.), under the project MAG-GIC (PTDC/CTA-GEO/31744/2017). FCT is also acknowledged for support through projects UIDB/50019/2020-IDL, PTDC/CTA-GEF/1666/2020 (MN) and PTDC/CTA-GEO/031885/2017 (MN). CITEUC is funded by FCT (UIDB/00611/2020 and UIDP/00611/2020). We acknowledge the collaboration with REN (Redes Energéticas Nacionais).

References:
Alves Ribeiro J., F.J. Pinheiro, M.A. Pais, 2021. First Estimations of Geomagnetically Induced Currents in the South of Portugal. Space Weather, 19(1)
Marshall R. A., C. L. Waters, M. D. Sciffer (2010). Spectral analysis of pipe‐to soil potentials with variations of the Earth’s magnetic field in the Australian region. Space Weather 8.5
Marshall, R. A., et al (2011). "A preliminary risk assessment of the Australian region power network to space weather." Space Weather 9.10

How to cite: Gutiérrez Pinheiro, F. J., Neres, M., Pais, M. A., Alves Ribeiro, J., Santos, R., and Cardoso, J.: Towards an improved proxy for geomagnetically induced currents (GICs), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8015, https://doi.org/10.5194/egusphere-egu22-8015, 2022.

EGU22-13154 | Presentations | EMRP2.15

Time derivative of the geomagnetic field has a short reset time

Mirjam Kellinsalmi, Ari Viljanen, Liisa Juusola, and Sebastian Käki

Space weather, like solar eruptions, can be hazardous to Earth’s electric grids via geomagnetically induced currents (GIC). In worst cases they can even cause city-wide power outages. GIC is a complicated phenomenon, closely related to the time derivative of the geomagnetic field. However, behavior the time derivative is chaotic and has proven to be challenging to predict. In this study we look at the geomagnetic field orientations at different magnetometer stations in the Fennoscandian region during active space weather conditions.  We aim to characterize the magnetic field behavior, to better understand the drivers behind strong GIC events. One of our main findings is that the direction of time derivative of the geomagnetic field has a very short “reset time“, only a few minutes. We conclude that this result gives insight on the time scale of the ionospheric current systems, which are the primary driver behind the time derivative’s behavior.

How to cite: Kellinsalmi, M., Viljanen, A., Juusola, L., and Käki, S.: Time derivative of the geomagnetic field has a short reset time, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13154, https://doi.org/10.5194/egusphere-egu22-13154, 2022.

EGU22-9999 | Presentations | EMRP2.15

Long-term Trends and Occurrence Distributions of Geomagnetic Fluctuations as Revealed by 35 Years of CARISMA Observations at 5s Cadence

Stavros Dimitrakoudis, Ian R. Mann, Andy Kale, and David K. Milling

The rate of change of the horizontal component of the geomagnetic field is a useful proxy for determining the severity of geomagnetically induced currents (GIC). While contemporary measurements for geomagnetic disturbances (GMD) are available from a number of arrays, short timescale datasets are not ideal for the characterisation of extreme events since their data sets are rarely indicative of the most extreme geomagnetic conditions. In the absence of long duration data sets, statistical methods have to be employed to assess the overall longer timescale historical power occurrence distributions, so as to extrapolate the behaviour of their high-end tail and which is required for the assessment of extreme events. Conversely, the CANOPUS array, subsequently expanded and operated as the CARISMA magnetometer array (www.carisma.ca), has been in continuous operation in Canada since 1986, first with a 5-second and then more recently with a 1-second cadence. Using that long timebase dataset we are able to evaluate the occurrence distributions of 5-second cadence measurements for over 10,000 operational days for each of several stations. Of particular significance for the expected magnitude of extreme events is an assessment of whether the disturbances follow a power law or log-normal distribution. Such indications can inform risk assessments on the potential for extremely hazardous GICs, for example in the estimation of a 1-in-100-year event. The CANOPUS/CARISMA GMD occurrence distributions, overall, appear to be well-approximated by log-normal rather than power law distributions. However, for extreme events, the local time at which the largest GMD typically occurs rotates away from the midnight sector, such that the largest events in the tail of the distribution most often occur instead at dawn. This has significant implications for assessing the size of expected extreme GMD events, and indeed the local time of the largest vulnerability, with clear applications for assessing extreme space weather impacts on the electric power grid. 

How to cite: Dimitrakoudis, S., Mann, I. R., Kale, A., and Milling, D. K.: Long-term Trends and Occurrence Distributions of Geomagnetic Fluctuations as Revealed by 35 Years of CARISMA Observations at 5s Cadence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9999, https://doi.org/10.5194/egusphere-egu22-9999, 2022.

EGU22-2259 | Presentations | EMRP2.15

Power density dissipated by field-aligned currents in the topside ionosphere

Fabio Giannattasio, Giuseppe Consolini, Igino Coco, Michael Pezzopane, and Alessio Pignalberi

The response of the magnetosphere-ionosphere (MI) system to the forcing by plasma of solar origin gives rise to several phenomena relevant to Space Weather. In particular, part of the energy injected into the ionosphere by means of field-aligned currents (FACs) connecting the magnetosphere with the high-latitude ionosphere is converted into mechanical energy and dissipated via Joule heating. Under reasonable assumptions, in the direction parallel to the geomagnetic field the only relevant contribution to dissipation is from the Ohmic term. Dissipated power density may significantly affect the physical parameters characterizing the upper ionosphere, such as electron temperature and density, and alter its chemical composition. This can result, for example, in the increased atmospheric drag and affect the satellite orbits. For this reason, understanding the dissipation of FACs in the topside ionosphere is important to shed light on the physical processes involved in MI coupling. Power density dissipated by FACs in crossing the topside ionosphere can be estimated by using Swarm data. Here, for the first time, we show statistical maps of power density features dissipated by FACs by using six-year time series of electron density and temperature data acquired by the Langmuir Probes onboard the Swarm A satellite (flying at an altitude of about 460 km) at 1 s cadence, together with the field-aligned current density product provided by the ESA’s Swarm Team at the same cadence. Maps of the same quantity under different levels of geomagnetic activity are also shown and discussed in light of the previous literature.

This work is partially supported by the Italian National Program for Antarctic Research under contract N. PNRA18 00289-SPIRiT and by the Italian MIUR-PRIN grant 2017APKP7T on "Circumterrestrial Environment: Impact of Sun-Earth Interaction".

How to cite: Giannattasio, F., Consolini, G., Coco, I., Pezzopane, M., and Pignalberi, A.: Power density dissipated by field-aligned currents in the topside ionosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2259, https://doi.org/10.5194/egusphere-egu22-2259, 2022.

EGU22-6096 | Presentations | EMRP2.15

Pressure-Gradient current at high latitude from Swarm measurements 

Giulia Lovati, Paola De Michelis, Giuseppe Consolini, and Francesco Berrilli

The pressure-gradient current is among the weaker ionospheric current systems arising from plasma pressure variations. It is also called diamagnetic current because it produces a magnetic field which is oriented oppositely to the ambient magnetic field, causing its reduction. The magnetic reduction can be revealed in measurements made by low-Earth orbiting satellites flying close to ionospheric plasma regions where rapid changes in density occur. This type of current can be revealed at both low and high latitudes and more generally in all those regions where the plasma pressure gradients are greatest. In the recent past, most studies have focused on low latitude, in the equatorial belt, while only a few papers have focused on high latitudes. Here these currents, although weak, may pose additional challenge since they seem to appear preferentially at the same geographic locations.

Using geomagnetic field, plasma density and electron temperature measurements recorded onboard ESA Swarm constellation from April 2014 to March 2018, we reconstruct the flow patterns of the pressure-gradient current at high-latitude ionosphere in both hemispheres, and investigate their dependence on magnetic local time, geomagnetic activity, season and solar forcing drivers. Although being small in amplitude, these currents appear to be a ubiquitous phenomenon at ionospheric high latitudes, characterized by well defined flow patterns, which can cause artifacts in main field models. Our findings can be used to correct magnetic field measurements for diamagnetic current effect, to improve modern magnetic field models, as well as understanding the impact of ionospheric irregularities on ionospheric dynamics at small-scale sizes of a few tens of kilometers. All these points are important in the framework of space weather effect modeling and confirm the key role of Swarm mission in providing information even on phenomena of very weak signature.

How to cite: Lovati, G., De Michelis, P., Consolini, G., and Berrilli, F.: Pressure-Gradient current at high latitude from Swarm measurements , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6096, https://doi.org/10.5194/egusphere-egu22-6096, 2022.

EGU22-6269 | Presentations | EMRP2.15

Dawn-dusk asymmetry of solar flare-driven ionospheric current at high latitudes

Pierre Cavarero, Masatoshi Yamauchi, Magnar G. Johnsen, Shin-Ichi. Ohtani, and Janet Machol

Solar flares are known to enhance the ionospheric electron density and thus influence the D- and E-region electric currents in the sunlit hemisphere.  The resultant geomagnetic disturbances (called "crochet") are found at both low latitudes and high latitudes with a minimum in between.  The subsolar response, with short-lived and symmetric changes around the subsolar region, is understood as a temporal re-distribution of the electron density.  However, no systematic study has been made of the high-latitude responses, covering the auroral oval, the cusp, and the dayside sub-auroral region.  Even global patterns are not well described or understood.  

Using data from GOES satellites and SuperMAG, we made a statistical study of the high-latitude geomagnetic responses to X-class solar flares in the northern polar region.  First, we needed to create a reliable X-flare database that we could use to get precise timings of when the flares start and when they stop. We merged XRS databases from different GOES satellites to create a X-class solar flare database between 1984 and 2017, gathering 331 X-flares over 34 years.  

For all these X-flares, we plotted the geomagnetic disturbance (∆B) on a polar map during the periods when the X-ray flux exceeds 1e-4 W/m2 (>X1 flare).  Plots were made also for merged data, i.e., different events on the same map organized by geographic coordinates and local time to obtain the average disturbance pattern caused by the flares.  Large events (∆B >300 nT) were excluded to minimize the contamination from substorm events.  

In these "merged" plots, we classify the data by season (summer - 4 months, equinox ±2 months, winter 4 months), flare intensity (X1-X2 flares and >X2 flares), and maximum ∆B among all stations > 65° GGlat (< 100 nT and 100-300 nT). 

 

Except for winter, we found a large poleward ∆B which peaks at 13-16 LT, particularly for > X2 flares, but no enhancements in the pre-noon sector.  This asymmetry, surprisingly, remains even after we consider IMF By polarity.  We do not have any plausible explanation for this result, and we will discuss it during the presentation.

 

[Acknowledgement: This work is resulted from a 2021 summer internship study at the Swedish Institute of Space Physics, Kiruna.   The GOES X-ray data is provided by NOAA (USA). The geomagnetic data at high latitudes are obtained from SuperMAG and are originally provided by DTU (Denmark), TGO (Norway), FMI (Finland), SGO (Finland), SGU (Sweden), GSC (Canada), USGS (USA), AARI (Russia), PGI (Russia), IZMIRAN (Russia), BAS (UK), BGS (UK), IPGP (France), PAS (Poland), ZAMF (Austria), and ASCR (Czech)]

How to cite: Cavarero, P., Yamauchi, M., Johnsen, M. G., Ohtani, S.-I., and Machol, J.: Dawn-dusk asymmetry of solar flare-driven ionospheric current at high latitudes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6269, https://doi.org/10.5194/egusphere-egu22-6269, 2022.

EGU22-4733 | Presentations | EMRP2.15

Swarm Fast Track spherical harmonic model of the external magnetic field to degree and order 3

Natalia Gomez Perez and Ciaran Beggan

The development of satellite measurements over the past four decades has allowed us to understand the magnetic field in the Earth environment at higher temporal and spatial resolution than before. This is most evident for satellite ensembles such as ESA’s Swarm constellation which allows simultaneous global coverage with three independent satellites.

Thanks to Swarm’s particular configuration, we can take advantage of the Local Time sampling difference between Swarm A/C and Swarm B in order to estimate the low degree variation of the external magnetic field in latitude and longitude. We separate the external and induced fields measured at satellite altitude, and obtain the spherical harmonic decomposition of each source to degree and order 3 twice per day. However, there is a trade-off between spatial and temporal resolution and clear disadvantages occur when the measured field varies rapidly during a geomagnetic storm, since the method used will result in coefficients of the averaged field over the chosen time interval rather than the peaks.

We compare our results with previous models of the external field during the St Patrick storm 2015, which used up to four different local time simultaneous coverage, as well as during quiet times and lesser storms using our own solutions. We find good agreement in each case.

In this talk we will describe the algorithm and methodology used and show results over the lifetime of the Swarm mission to date (2013-). A new daily product for the Swarm mission (MMA_SHA_2E) is being developed.

How to cite: Gomez Perez, N. and Beggan, C.: Swarm Fast Track spherical harmonic model of the external magnetic field to degree and order 3, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4733, https://doi.org/10.5194/egusphere-egu22-4733, 2022.

EGU22-10681 | Presentations | EMRP2.15

Causality and information transfer in interactions of solar wind, radiation belts and geomagnetic field

Pouya Manshour, Constantinos Papadimitriou, George Balasis, Milan Palus, Simon Wing, Ioannis A. Daglis, Reik Donner, Adamantia Zoe Boutsi, Giuseppe Consolini, Juergen Kurths, and Bruce T. Tsurutani

Understanding physical processes that drive dynamics of the radiation belts - the high-energy charged particle population trapped by the geomagnetic field in the inner magnetosphere, is of great importance for science and society. In fact, this population dynamically interacts with the solar wind and geomagnetic field over various temporal and spatial scales, and can have significant impacts on its surrounding environment, including hazards to satellites and astronaut health. Understanding the relevant acceleration mechanisms of these particles can help not only to uncover the underlying physics, but also to improve our ability to predict and to protect. Despite numerous attempts over several decades, unfolding the dynamics of interactions in such systems is still one of the challenging research areas and has not yet been achieved, due to the complex and nonlinear underlying physics of the radiation belts. However, information theory is not constrained by such limitations and has proven itself to be a powerful non-parametric approach to discover the causal interactions among different nonlinear complex systems, and can be considered complementary to physics-based approaches. In this work, we apply entropy-based causality measures such as conditional mutual information to determine the information transfer between various variables including different solar wind parameters and geomagnetic activity indices obtained from NASA’s OmniWeb service and omnidirectional electron fluxes from the MagEIS units onboard Van Allen Probe B in the outer radiation belt, ranging in energy from a few keV to several MeV. We find significant information flow from low energy electrons into high energy ones as well as from some solar wind/geomagnetic field parameters into electron fluxes of various energies. We are confident that our results provide great prospects for future targeted research on the dynamical mechanisms underlying radiation belts dynamics.

This work has benefitted from discussions within the International Space Science Institute (ISSI) Team # 455 “Complex Systems Perspectives Pertaining to the Research of the Near-Earth Electromagnetic Environment.”  P.M. and M.P. are supported by the Czech Science Foundation, Project No. GA19-16066S and by the Czech Academy of Sciences, Praemium Academiae awarded to M. Paluš.

How to cite: Manshour, P., Papadimitriou, C., Balasis, G., Palus, M., Wing, S., Daglis, I. A., Donner, R., Boutsi, A. Z., Consolini, G., Kurths, J., and Tsurutani, B. T.: Causality and information transfer in interactions of solar wind, radiation belts and geomagnetic field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10681, https://doi.org/10.5194/egusphere-egu22-10681, 2022.

EGU22-10887 | Presentations | EMRP2.15

Swarm as an EMIC Wave Monitor: Applications for Radiation Belt Modelling and Specification

Ivan Pakhotin, Ian Mann, Louis Ozeke, Leon Olifer, and Stavros Dimitrakoudis

The outer belt electron radiation belt is highly dynamic, responding to a superposition of a variety of acceleration and loss processes imposed along the electron drift orbits to produce increases and decreases in flux on timescales from minutes, to hours, days and years. These trapped relativistic so-called ‘satellite killer’ electrons can penetrate spacecraft shielding and cause damage to internal electronics and single-event upsets. Understanding and predicting the radiation belt environment, therefore, is valuable for the understanding and mitigation of these potentially catastrophic impacts. Magnetic measurements from the constellation of Swarm satellites in low-Earth orbit (LEO) can be used to monitor the populations of electromagnetic ion cyclotron (EMIC) waves along their orbits. This is significant for radiation belt applications since these waves are believed to be potentially responsible for some fast losses of radiation from the Van Allen belts through fast scattering into the loss cone. Despite being far from the equatorial plane where most of the radiation belts are trapped, the propagation of EMIC waves along field lines allows an assessment of these wave populations from LEO, Swarm and similar satellites in LEO traversing the radiation belts four times in each approximately 90-minute orbit. Here we demonstrate how Swarm can be used to detect and characterize the EMIC wave populations, and compare the observed EMIC wave populations to simulations of two strong magnetic storms where radiation belt modeling based on radial diffusion demonstrated the likelihood of a missing fast loss process and which might be explained by EMIC wave-particle interactions. The current state-of-the-art for the incorporation of EMIC-related wave losses is based on empirical means, related for example to solar wind compressions. Here we investigate, despite the often spatio-temporally localized character of some EMIC wave populations, whether magnetic field data from the Swarm constellation could be used in an observational data-constrained approach for the inclusion of EMIC wave losses in radiation belt modelling. LEO satellites have the advantage over high-apogee near-equatorial satellites in that the latter only cross L-shells comparatively slowly; similarly, the interpretation of EMIC wave location from ground-based magnetometer networks is complicated by propagation in the ionospheric duct. Through the use of multi-spacecraft techniques, and/or those which utilise electric and magnetic data together, we demonstrate how it is possible to reliably disentangle EMIC waves from nearby field-aligned currents. Such techniques provide hitherto unprecedented observation capability for the specification of EMIC waves from LEO for use in radiation belt modelling. Future work could examine the utility of such data for both improving the accuracy of radiation belt models, and for the nowcasting and even forecasting of belt dynamics.

How to cite: Pakhotin, I., Mann, I., Ozeke, L., Olifer, L., and Dimitrakoudis, S.: Swarm as an EMIC Wave Monitor: Applications for Radiation Belt Modelling and Specification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10887, https://doi.org/10.5194/egusphere-egu22-10887, 2022.

EGU22-6336 | Presentations | EMRP2.15

Swarm-derived indices of geomagnetic activity

Constantinos Papadimitriou, Georgios Balasis, Adamantia Zoe Boutsi, Alexandra Antonopoulou, Georgia Moutsiana, Ioannis A. Daglis, Omiros Giannakis, Giuseppe Consolini, Jesper Gjerloev, and Lorenzo Trenchi

Ground-based indices, such as the Dst, ap and AE, have been used for decades to describe the interplay of the terrestrial magnetosphere with the solar wind and provide quantifiable indications of the state of geomagnetic activity in general. These indices have been traditionally derived from ground-based observations from magnetometer stations all around the Earth. In the last 7 years though, the highly successful satellite mission Swarm has provided the scientific community with an abundance of high quality magnetic measurements at Low Earth Orbit (LEO), which can be used to produce the space-based counterparts of these indices, such the Swarm-Dst, Swarm-ap and Swarm-AE indices. In this work, we present the first results from this endeavour, with comparisons against traditionally used parameters. We postulate on the possible usefulness of these Swarm-based products for a more accurate monitoring of the dynamics of the magnetosphere and thus, for providing a better diagnosis of space weather conditions.

How to cite: Papadimitriou, C., Balasis, G., Boutsi, A. Z., Antonopoulou, A., Moutsiana, G., Daglis, I. A., Giannakis, O., Consolini, G., Gjerloev, J., and Trenchi, L.: Swarm-derived indices of geomagnetic activity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6336, https://doi.org/10.5194/egusphere-egu22-6336, 2022.

EGU22-11344 | Presentations | EMRP2.15

Comparing long short-term memory and convolutional neural networks in SYM-H index forecasting

Federico Siciliano, Giuseppe Consolini, and Fabio Giannattasio

Geomagnetic indices can have a central role in the mitigation of ground effects due to space weather events, for instance when their reliable forecasting will be achieved. To this purpose, machine learning techniques represent a powerful tool. Here, we use two conceptually different neural networks to forecast the SYM-H index: the long short-term memory (LSTM) and the convolutional neural network (CNN). We build two models and train both of them using two different sets of input parameters including interplanetary magnetic field components and magnitude and differing for the presence or not of previous SYM-H values. Both models are trained, validated, and tested on a total of 42 geomagnetic storms among the most intense that occurred between 1998 and 2018. Results show that both models are able to well forecast SYM-H index 1 hour in advance. The main difference between the two stands in the better performance of the one based on LSTM when SYM-H index is included in the input parameters and, contrarily, in the better performance of the one based on CNN for predictions based only on interplanetary magnetic field data.

How to cite: Siciliano, F., Consolini, G., and Giannattasio, F.: Comparing long short-term memory and convolutional neural networks in SYM-H index forecasting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11344, https://doi.org/10.5194/egusphere-egu22-11344, 2022.