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Influence of subduction history and mineral deformation on seismic anisotropy in the lower mantle

Geophysical Journal International - Fri, 11/14/2025 - 00:00

AbstractSeismic observations reveal significant anisotropy in the D″ region, providing direct constraints on mantle flow and deformation. However, the global anisotropy pattern and its relationship with subduction history, mineral deformation, and rheology in the lower mantle remain unclear. We analyze published regional shear-wave splitting and null measurements, along with waveform inversions, which reveal rapid lateral variations in anisotropy near the edges of large low shear velocity provinces (LLSVPs). We combine mineral physics results of temperature- and pressure-dependent elastic tensors, slip systems, and phase transition mechanisms to explore potential deformation scenarios. We set up models that begin with dynamic thermochemical convection, tracking the deformation history driven by the subduction, evolving crystal fabrics, and cumulative seismic anisotropy. Models show that post-perovskite (pPv) with a (001)-dominant slip system, combined with viscosity changes and texture inheritance during the bridgmanite-post-perovskite (Br-pPv) phase transition and the reverse transition, best reproduces the distinct anisotropy patterns observed in upwelling regions such as plume roots and LLSVP edges. The nominal model is time-dependent, showing strong seismic anisotropy when slabs impinge on the CMB that diminishes toward the LLSVP, followed by plume development at the LLSVP edge with significant anisotropy. Within LLSVPs, internal convective upwellings and downwellings can explain the intermittent, spatially clustered anisotropy. We further demonstrate the potential for constraining LLSVP composition through the observed weaker anisotropy within these structures compared to the surrounding mantle, with our results favoring a Br-rich composition. Computations indicate that the bulk of the lower mantle remains nearly isotropic despite significant texture accumulation through dislocation glide, and that seismic anisotropy can extend several hundred kilometers above the core–mantle boundary.

Reverse Time Migration for Microseismic Sources Using Combinational Autocorrelation Imaging Condition

Geophysical Journal International - Fri, 11/14/2025 - 00:00

AbstractReverse time migration based on geometric mean or cross-correlation is a powerful passive-source imaging technique that can produce high-resolution source images even under low signal-to-noise ratio conditions. When the velocity model is inaccurate, a hybrid method combining geometric-mean and arithmetic-mean reverse time migration is typically used to reduce sensitivity to model errors. Conventional hybrid methods usually employ a grouping strategy, in which receivers are divided into groups and multiplicative operations are performed between these groups. However, this strategy essentially utilizes only a subset of receiver combinations, which may compromise imaging quality when the number of receivers is insufficient. To overcome this limitation, a novel combinational autocorrelation reverse time migration imaging condition is proposed. Our method forms multiple combinations of receivers and conducts zero-lag autocorrelation on the extrapolated wavefields of these combinations. The cross terms generated by the autocorrelation operation correspond to all possible receiver combinations. Finally, these autocorrelation results are linearly stacked in order to eliminate interference terms while preserving the cross terms. By including more receiver combinations, the proposed method can provide improved imaging performance. Furthermore, due to the adoption of the autocorrelation algorithm, the new method achieves minimal memory usage among methods based on reverse time migration, which makes it especially suitable for three-dimensional problems. Acoustic numerical simulations verify the effectiveness and advantages of the new method in both two-dimensional and three-dimensional scenarios. Additionally, the mathematical relationship between our method and conventional methods is discussed, clarifying the applicable scope of the new method.

Induced polarization as a tool to characterize permafrost. 2. Applications to low and high-porosity environments

Geophysical Journal International - Fri, 11/14/2025 - 00:00

SummaryIn the previous paper of this series, a petrophysical model named the Dynamic Stern Layer (DSL) model was extended to describe induced polarization phenomena associated with permafrost by capturing direct and indirect effects associated with the presence of ice in porous media. In the present paper, time-domain induced polarization data obtained in field conditions are interpreted thanks to this updated DSL model. We selected three different test sites in order to apply the DSL model to very different conditions of low and high ice contents to see how ice content directly and indirectly affects geoelectrical measurements. A first survey is performed along a cross-section of a ridge in the Kangerlussuaq mountains of Greenland (Site I). In this area, the rock corresponds to a Precambrian granite characterized by a rather low (< 5%) porosity and therefore a low ice volumetric content on the North face of the ridge. We do not see any direct ice polarization contribution in the data obtained with a current injection period of 1 s. We also performed a field survey close to Col des Vés (2846 m a.s.l., Tignes, French Alps, Site II). This site corresponds to a complex ground ice body overlying a substratum made of a low-porosity marble, both having high resistivity values. The front of this body is characterized by a small amount of residual ice while the roots are ice-rich. Therefore the porosity at this site is high and the ice content highly variable. This case study showcases the role of ice in the induced polarization data in terms of high chargeability values (close to 1 as predicted by the theory in which ice behaves as a surfacic protonic semi-conductor) at the roots of the complex ground ice body. A third site (Site III) corresponds to a profile crossing the Aiguille du Midi (3842 m a.s.l., Chamonix), also in the French Alps in a low porosity granitic environment. Laboratory experiments are used to interpret the tomograms of the electrical conductivity and normalized chargeability using the DSL model and water content and Cation Exchange Capacity tomograms are reconstructed at these sites. This study demonstrates the ability of induced polarization to be an efficient tool to characterize permafrost in very different field conditions.

3D EM inversion considering induced polarization effect

Geophysical Journal International - Fri, 11/14/2025 - 00:00

SummaryModelling induced polarization (IP) effects in electromagnetic (EM) data is increasingly becoming a standard tool in mineral exploration, but the industry standard is still based on one-dimensional (1D) forward and Jacobian modelling. We have developed a three-dimensional (3D) electromagnetic forward and inversion method within the EEMverter modelling platform, incorporating IP effects. The 3D computations are performed in the frequency domain using the vector finite element method and then transformed into the time domain via Hankel transformation. This approach enables modeling of any IP parameterization, ranging from the simple constant phase angle model to a full Debye decomposition. Furthermore, 3D forward modeling mesh and inversion mesh are built independently: an Octree forward mesh is designed for efficient spatial segmentation for single or multiple soundings, while the inversion parameters are defined on a structured model mesh, which is linked to the forward meshes via interpolation. In conjunction with the development of a full 3D EM-IP inversion, we introduce a novel 3D inversion workflow. This workflow allows for hybrid 1D-3D computations, both sequentially and spatially, enabling 3D modeling exclusively in the most significant and interesting areas of the survey. We tested the hybrid 1D-3D inversion workflow using airborne electromagnetic (AEM) data acquired by Xcalibur with the HeliTEM system in the Staré Ransko area (Czech Republic), known for its gabbro-peridotite rocks hosting nickel-copper±cobalt, platinum group element (Ni-Cu±Co, PGE) mineralization. The results demonstrate that the hybrid inversion effectively addresses the challenges of 3D modeling on large-scale datasets. It enhances interpretation reliability in regions with strong 3D effects and shows a significant spatial correlation between resistivity and chargeability phase anomalies and known mineral deposits. Moreover, both synthetic and field data indicate that the resistivity parameter is more sensitive to 3D effects than the chargeability phase parameter.

Axial Seamount experiment to test real-time eruption forecasts

Phys.org: Earth science - Thu, 11/13/2025 - 19:59

Currently, scientists struggle to forecast volcano eruption events, as no universally reliable, real-time eruption forecasting framework is available. Instead, researchers often rely on retrospective analysis to evaluate eruptions. And although much has been learned from doing this, it can sometimes introduce biases, such as data snooping, hindsight reinterpretation, and post-eruption model adjustment.