Benoit Cordonnier

The properties of fault slip surfaces, gouge characteristics, and fluid-rock reactions are tightly coupled and control earthquake mechanics. To visualise and quantify the role of this coupling, we have developed a new operando imaging approach that allows the documentation of fast direct-shear deformation experiments in time-resolved 2- and 3-dimensional image data at low single-digit micrometer resolution. A direct-shear inset developed for the X-ray transparent Heitt Mjölnir triaxial deformation apparatus enables experiments at 20 MPa normal stress under fluid-pressurised conditions and allows real-time permeability measurements.We apply this platform to three fault systems: 1) Slip surfaces in basaltic rocks, imaged while sliding at 1 mm.s-1, reveal how asperities, phenocrysts, and surface roughness control stick-slip behavior and damage localization during fast slip. 2) Reactive quartz-gypsum gouges imaged during velocity stepping and healing experiments, enable the direct linking of evolving frictional properties to microphysical developments. 3) A shearing, dehydrating gypsum gouge provides insights into transient rheologies and the resulting strain distributions.These datasets demonstrate that 4D imaging resolves coupled mechanical, chemical, and hydraulic fault evolution in real time. Our approach allows documenting microphysical processes underlying the frictional properties of faults and thereby constitutes a potent tool for studying faults in a variety of tectonic settings.

Direct observations of geological processes are often limited by available observation time, the spatial resolution of imaging techniques or the accessibility of active sites. These limitations also apply to fault healing, whereby faults progressively regain strength throughout the interseismic phase of the earthquake cycle. Here, conventional approaches either capture a static snapshot of the final microstructure in exhumed natural fault rocks or focus on the bulk mechanical behaviour through slide–hold–slide or direct shear experiments. However, these approaches generally fail to resolve the dynamic evolution of the microstructural record, and the associated chemo-mechanical feedback that controls a rock’s hydraulic properties. To overcome these limitations and constrain the spatiotemporal coupling between mechanical, chemical, and hydraulic processes in healing fault gouges, we conducted a series of direct shear experiments on analogue fault gouges composed of a quartz–hemihydrate mixture. We then monitored their microstructural evolution using operando 4D synchrotron-based X-ray CT imaging.Our experiments, performed at constant shear rates of 0.3–1 µm/s, were designed to mimic gouge-rich faults in the uppermost continental crust during the interseismic phase. In the presence of a reactive pore fluid, we simulated chemical fault healing through dissolution-reprecipitation and cementation, which are associated with the hydration reaction of CaSO₄ hemihydrate to gypsum. In our deforming samples, these time-dependent healing processes compete with mechanical weakening processes, such as frictional granular flow.Our novel approach combines an innovative experimental setup [1, 2] with high-resolution 4D imaging and advanced image analysis techniques, including digital volume correlation (DVC). In this contribution we discuss the benefits of integrating micromechanical data with high-resolution 4D imaging by linking active deformation mechanisms to the evolving mechanical and hydraulic response of the simulated fault gouge. Further, we demonstrate a shear-rate-dependent competition between time-dependent healing processes and mechanical weakening. [1] Freitas, D. et al. (2024): Heitt Mjölnir: a heated miniature triaxial apparatus for 4D synchrotron microtomography. Journal of Synchrotron Radiation 31, 150-161. doi.org/10.1107/S1600577523009876 [2] Jayawickrama, E. et al. (2026): Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18619.

Volcanic eruptions produce pyroclasts that range from microns to meters, and produce edifices and deposits that extend for kilometers. Characterization of crystal and vesicle textures on millimeter to centimeter scale samples are commonly used to interpret and quantify magmatic storage, transport conditions, and eruptive processes, despite being divorced from their initial context. Due to practical challenges, our understanding of these micro-scale textures has so far been constrained on the basis of experiments limited to millimeters to a few centimeters in total sample size, upon which numerical simulations and empirical models can be calibrated. In this work, we present results from a natural, microlite-bearing, mildly banded, rhyolitic obsidian which was heated to induce ~60 vol% vesiculation. The sample expanded primarily in one direction, along a confined cylinder to impose shear, from an initial size of 14.5 x 14.5 cm to a final experimental size of 15.0 x 36.5 cm. X-ray computed tomography at a resolution of 33 μm/pixel reveals a rich variety of textures including macro-pores up to 1.5 cm in diameter, regions of high vesicularity juxtaposed against denser regions with smaller pores, evidence of differences in vesiculation history between bands with variable initial volatiles, and densification along the sheared margins. This experiment provides new constraints on texture at the decimeter scale, and places individual sub-volumes on the centimeter scale into their broader context, allowing for analysis of shear history and connectivity on neighboring regions. On the basis of these observations, we validate multi-scale numerical simulations of coupled bubble growth, suspension-scale flow, and fluid percolation, improving our reliability in upscaling to volcanic conditions. Comparison of sample textures with the simulated bubble and fluid pressure, temperature, and strain histories results in a comprehensive picture of intra-sample gas transport and segregation, and reveals the complex vesiculation behavior of initially heterogeneous material.

High-resolution X-ray microtomography (XMT) imaging of rock deformation experiments at micrometer scale provide valuable insights into the coupled evolution of pores, cracks, and fluid pathways (Noiriel & Renard, 2022). A critical step in XMT data processing is the removal of ring artefacts, which is attributed to malfunctioning detector components within the acquisition system (Vo et al., 2018). These artefacts appear as stripes in the raw acquired sinogram domain and concentric circles in reconstructed images. Ring artefacts can adversely affect downstream analyses such as pore and fracture segmentation, and digital volume correlation (DVC) (Mahdaviara et al., 2025). Advances in GPU computing and ML-based image processing have led the synchrotron community to explore deep learning architectures including ResUNET (Fu et al., 2023), and attention-based variants (Zhang et al., 2022) to suppress ring artefacts. Most ML-based denoisers rely on single-domain, pixel-based loss functions, such as L1, L2, or structural similarity index measure (SSIM), applied either in the sinogram or reconstructed image domain.This study investigates a dual-domain loss function that combines loss terms in the sinogram domain with those in the corresponding Fast Fourier Transform magnitude (FFT amplitude) domain, aiming to improve generalization of trained U-Net variants. Existing artefact-free XMT images of basalt were used to simulate stripe artefacts in its raw sinogram domain. Stripe artefact generation was controlled using three parameters: pixel thickness, amplitude, and number of stripes per sinogram. A total of 5,000 paired noisy and clean sinograms were generated and split into training, validation, and test datasets. Three UNET-based architectures were evaluated: a baseline U-Net (baseUNET), a residual U-Net (ResUNET), and a residual U-Net with attention gates (AG-ResUNET). Models were trained for 100 epochs using the Adam optimiser and their performances were assessed using peak signal-to-noise ratio (PSNR), structural similarity index (SSIM), and qualitative inspection of sinograms, reconstructed images, and FFT amplitude spectra. The study compares single- and dual-domain loss functions in terms of ring artefact suppression and generalization beyond the training data distribution, discusses limitations related to the dynamic range of the training data and its implications for denoising experimental XMT datasets.ReferencesFu, T., Wang, Y., Zhang, K., Zhang, J., Wang, S., Huang, W., Wang, Y., Yao, C., Zhou, C., & Qin, Y. (2023). Deep-learning-based ring artifact correction for tomographic reconstruction. Journal of Synchrotron Radiation, 30(3).Mahdaviara, M., Mousavi, M., Rafiei, Y., Raoof, A., & Sharifi, M. (2025). Improving numerical fluid flow simulation by ring artifact removal in micro-CT images of porous media using attention autoencoder–decoders. Transport in Porous Media, 152, 57.Noiriel, C., & Renard, F. (2022). Four-dimensional X-ray micro-tomography imaging of dynamic processes in geosciences. Comptes Rendus Géoscience, 354(G2), 255–280. https://doi.org/10.5802/crgeos.137Vo, N. T., Atwood, R. C., & Drakopoulos, M. (2018). Superior techniques for eliminating ring artifacts in X-ray micro-tomography. Optics Express, 26(22), 28396. https://doi.org/10.1364/oe.26.028396Zhang, J., Niu, Y., Shangguan, Z., Gong, W., & Cheng, Y. (2022). A novel denoising method for CT images based on U-net and multi-attention. Computers in Biology and Medicine, 152, 106387. https://doi.org/10.1016/j.compbiomed.2022.106387

The brittle–ductile transition, typically located near the base of the seismogenic zone, controls strain localization and the evolution of physical properties, with direct implications for fluid transport and fault mechanics. However, quantitative constraints on strain partitioning and porosity evolution across this transition remain limited, particularly in low-porosity carbonate rocks. We conducted triaxial compression experiments on dry Carrara marble at confining pressures of 5 and 100 MPa using two complementary techniques: (1) acoustic emission (AE) monitoring coupled with P-wave velocity measurement, and (2) in-situ X-ray microtomography combined with digital volume correlation (DVC). AE activity remains low at all pressures, with source mechanisms becoming increasingly compaction-dominated at the highest confinement. P-wave velocities fall markedly during deformation by up to 60% at 5-10 MPa, ∼50% at 40 MPa, and ∼40% at 100 MPa, reflecting pervasive microcrack development. DVC reveals that the proportion of dilative volumetric strain decreases with confinement from nearly 90% at 5 MPa to 50% at 70-90 MPa, yet dilation remains a major component and is comparable to compaction even at the highest pressures. X-ray-derived bulk porosity increases from <1% to ∼1.5% at low confinement prior to failure, and to ≥10% at higher confinements, indicating substantial porosity generation during distributed dilatant cataclastic flow. Together, these results provide the first quantitative link between P-wave velocity reduction, volumetric strain partitioning, and transient porosity generation across the brittle to semi-ductile transition, revealing how the interplay of dilation and compaction governs both elastic properties and strain localization in low-porosity crystalline carbonate.

The inherent instability and risk of liquefaction associated with unstable glaciomarine clay are both scientifically intriguing and societally important. Stabilizing sensitive soils is increasingly necessary with the unfolding climate crisis, which leads to wilder and wetter weather. The relevant length scales extend from nanometer-sized pores to kilometer-sized geological features. Laboratory experiments are important for gaining a better understanding of these phenomena. Here, we present a study of the ultra-soft sandstone Saltwash South , which essentially consists of quartz particles glued together by clay, as a proxy for nanoscopically fine-grained sensitive soil to facilitate time-resolved imaging of the onset of activation and liquefaction. By employing in situ time-resolved combined X-ray and neutron computed tomography (CT), we visualize the structural deformation caused by the presence of water and salts. While neutron imaging was sensitive to the presence of normal and heavy water, simultaneous X-ray imaging was used to measure the porous structure, swelling, and initial liquefaction response of the consolidated rock. Uniform expansion was observed in regions exposed to water, reflecting clay swelling and disaggregation. In tightly confined samples, the swelling and disaggregation were suppressed. Finally, we discuss future perspectives of this promising approach to studying liquefaction phenomena in porous media.

During an earthquake a dynamic rupture propagates, causing fractures and permanent damage that modify the mechanical properties of the fault and its surroundings. Reciprocally, the energy dissipated by the mechanical transformation of the fault rock has a direct feedback on the dynamics of the seismic rupture. We reproduce and image these co-seismic damage processes with sub-microsecond time resolution through dynamic loading experiments on confined Westerly granite samples. Using synchrotron X-ray imaging and ultra-high-speed cameras, we observed that dynamic compression with confinement produces shear faults and a localized granulation of the rock, known as gouge, before significant slip occurs. This fragmentation process begins with volumetric damage, followed by shear deformation less than five microseconds later. Our results indicate that gouge formation is initiated by the dynamic rupture itself, which reduces the strength of a fault prior to other weakening mechanisms that may occur during the earthquake slip.

Fracture development controls the conditions of catastrophic failure of rocks and other heterogeneous materials in the brittle regime. To characterize fracture development during the preparation process of failure, we calculate the volume of fractures propagating in isolation and fractures coalescing throughout eight triaxial compression experiments with in situ synchrotron X-ray microtomography acquisition. We track this behavior in experiments on Westerly granite at ambient temperature with varying confining pressure (5-20 MPa), and fluid pressure (0-10 MPa). In all of the experiments, the coalescing fracture porosity rapidly increases preceding macroscopic failure, on average at 97% of the failure stress. Fractures become increasingly parallel to the maximum compression direction with loading, and increasing volumes of coalescing fractures. The orientation of groups of fractures is more oblique to the maximum compression direction than the orientation of individual fractures, consistent with the development of faults from arrays of en echelon opening mode fractures. The volume of coalescing fractures at each stress increment is more strongly correlated with the volume of fractures with lengths greater than the mean grain size, i.e., the critical fractures, than with the volume of fractures with lengths greater than the spacing, L/S>1, between fracture pairs. Using a grain size of 1025 µm and the condition L/S>3.5 to identify the critical fractures produces the strongest correlations with the volume of coalescing fractures. The mean grain size is thus a more robust metric of fracture network behavior than the spacing between fractures.