Geophysical Journal International

SummaryInversion of geomagnetic anomaly data poses an ill-posed problem, and extremal models such as equivalent source layers or point-source distributions can explain observations to the same degree as volumetric magnetisation distributions. However, the spectral characteristics of magnetic anomalies provide fundamental constraints for magnetic source-depth estimation. Specifically, the maximum detectable depth of crustal magnetic sources is dictated by the longest wavelengths present in the field, which correspond to the low-wavenumber bands of the spectrum. This relationship is often analysed through the log power spectrum versus wave number plot, using the slopes of the linear segment for depth estimation. Methods aiming at reconstructing the depth to the bottom of magnetisation from spectral field characteristics are commonly referred to as spectral methods. However, these methods are based on assumptions about the statistical properties of the source distribution and are prone to misinterpretations. Here, we apply sparsity-constrained 3D inversion of magnetic data using an elastic net regularisation to recover the susceptibility distribution and the bottom of magnetisation. We claim that the elastic net (ℓ2ℓ1 norm) regularisation, when properly tuned to balance the solution’s smoothness with sparsity, stabilises the inversion, avoiding extremal magnetisation distributions and generating a geologically plausible source depth distribution that is consistent with the expected source distribution. The ℓ1 norm brings sparsity and high resolution, while the ℓ2 norm brings inversion stability and structural continuity to the final model. From the recovered 3D elastic net sparse inversion model, we extract the depths of all the deepest non-zero susceptibility values and suggest this to be an alternative estimate to the base of magnetisation. Moreover, we suggest that the resulting 3D model has a value in itself and may aid geological interpretation.

SummaryGaining insight into the centennial evolution of the geomagnetic field over the past 2000 years requires the acquisition of reliable palaeomagnetic data from the study of well-dated archaeological materials or rocks. However, despite previous efforts, palaeointensity data from regions south of 30°S are still underrepresented, potentially limiting the accuracy of global geomagnetic field models and their applications. In addition, a comprehensive understanding of the geomagnetic field evolution in South America is particularly relevant, as the recent geomagnetic secular variation has been mainly characterised by the significant growth of the South Atlantic Anomaly over the past three centuries. The evolution of this low-intensity region, currently centered over central South America, is well understood in detail only during the last few centuries, thanks to the availability of direct measurements. For both the geomagnetic and palaeomagnetic communities, understanding its evolution prior to this period remains a challenge. This study presents new palaeointensity estimates from San Juan Province, central western Argentina, based on the analysis of 23 pottery samples dated between the 3rd and 17th centuries CE using radiocarbon and archaeological constraints. We employed the Thellier-Thellier method, incorporating partial thermoremanent magnetisation (pTRM) checks, TRM anisotropy corrections, and cooling rate adjustments, and obtained 11 mean palaeointensity values of good technical quality for central South America. The results are consistent with the limited number of previously reported high-quality palaeointensity data within an area 900 km in radius centered on San Juan, all showing intensity values ranging from approximately 40 to 55 μT. The new data, combined with these previously published high-quality intensities, do not show anomalously low values in intensity in the region between 200 and 1750 CE, suggesting no significant impact of the South Atlantic Anomaly in the region before the past three centuries. Furthermore, the findings suggest the presence of rapid multidecadal variations between 800 and 1100 CE, a behaviour also observed in other regions worldwide, which may point to a global or dipolar origin for these variations. By enhancing the dataset for this latitude range, this work provides new constraints on the geomagnetic field’s past behaviour south of 30°S over South America and contributes to improving future global geomagnetic reconstructions.

SummaryAbsence of traces tends to reduce the quality and reliability of Ground Penetrating Radar (GPR) data due to equipment, sensor coverage, and acquisition limitations. This is a significant limitation to Full Waveform Inversion (FWI) and Reverse Time Migration (RTM) advanced imaging techniques, which rely on dense and continuous data. To address this challenge, we propose an effective interpolation method using the Projection onto Convex Sets (POCS) algorithm, originally developed for seismic data reconstruction. The algorithm is formulated in a compressed sensing framework, taking advantage of Fourier sparsity and iterative thresholding in the time domain to iteratively update spectral coefficients during reconstruction. We compare its performance on synthetic and real GPR data with various percentages of missing data. Results indicate that the POCS algorithm, in addition to reconstructing missing traces at high precision, significantly improves subsequent RTM imaging structural resolution. We also compare POCS with conventional Kriging and a deep learning-based interpolation model (DL-Net) to benchmark its performance. The proposed method achieves superior reconstruction quality and stability, particularly under high sparsity conditions. This study highlights the practical potential of POCS in enhancing GPR image fidelity and interpretation under real-world acquisition limitations.

SummaryFlooding of abandoned excavation mines implies significant changes in the hydromechanic rock behavior often associated with instantaneous rock instabilities which cause underground and ground failure and collapses, sometimes (but not always) accompanied by induced seismicity. The permanent modification of the hydrogeological setting may, in certain cases, also induce long-term seismic activities persistent over several years. The governing hydromechanic triggering mechanisms are poorly understood in these cases what bares challenges in related seismic hazard and risk assessment. In this study, we provide new insights into this poorly explored field of fluid induced seismicity, by investigating the long-lasting (> 10 years) swarm activity induced by the flooding of an abandoned coal mine at Gardanne in Southern France. The strongest events of the activity have comparatively small magnitudes (Mw < 2) but are felt by the local population due to their shallow source depth (< 1 km). Thanks to full waveform based source analysis we show that the swarm is associated with the permanent activation of preexisting faults situated below the flooded mining voids which act as a very high-capacity anthropogenic reservoir and aquifer. We further show that mine water level changes caused by either natural or anthropogenic driving forces cause seismic triggering which involves direct pore-pressure as well as poroelastic effects. These findings provide constraints for adequate guidelines for safe mine water level management and seismic risk mitigation.

SummaryHigh-resolution seismic reflections are essential for imaging and monitoring applications. In seismic land surveys using sources and receivers at the surface, surface waves often dominate, masking the reflections. In this study, we demonstrate the efficacy of a two-step procedure to suppress surface waves in an active-source reflection seismic dataset. First, we apply seismic interferometry (SI) by cross-correlation, turning receivers into virtual sources to estimate the dominant surface waves. Then, we perform adaptive subtraction to minimise the difference between the surface waves in the original data and the result of SI. We propose a new approach where the initial suppression results are used for further iterations, followed by adaptive subtraction. This technique aims to enhance the efficacy of data-driven surface-wave suppression through an iterative process. We use a 2D seismic reflection dataset from Scheemda, situated in the Groningen province of the Netherlands, to illustrate the technique’s efficiency. A comparison between the data after recursive interferometric surface-wave suppression and the original data across time and frequency-wavenumber domains shows significant suppression of the surface waves, enhancing visualization of the reflections for subsequent subsurface imaging and monitoring studies.

SUMMARYSeismic waves undergo attenuation and dispersion as they propagate through the Earth. These effects are caused by mechanisms such as partial melting in the crust and mantle, and the presence of water in the mantle. Neglecting attenuation effects may result in phase distortion and amplitude anomalies when imaging the Earth’s interior structure. Here, we introduce a novel wave equation for modeling viscoelastic wave propagation in frequency-independent Q media. The proposed viscoelastic wave equation offers several advantages over previous methods: (1) the quality factor Q is explicitly integrated into the wave equation, simplifying the derivation of sensitivity kernels for Q full waveform inversion; (2) the wave equation can be directly solved using the spectral element method, which is computationally more efficient than methods requiring Fourier transforms; and (3) the relaxation time (weighting function) of the wave equation depends only on the selected frequency range, independent on the specific Q values. The accuracy of the proposed wave equation is validated through comparisons with analytical solutions and results from the Generalized Standard Linear Solid (GSLS) method. Furthermore, the method is rigorously tested on two benchmark earth models to assess its capability in handling topographic variations and complex structural configurations in heterogeneous attenuative media. Given its accuracy and reduced computational costs, this new wave equation is expected to be highly beneficial for seismic reverse time depth imaging and viscoelastic full waveform inversion applications.

SummaryWe present a high-order tetrahedral spectral element (SE) method for the computation of three-dimensional (3D) magnetotelluric (MT) forward responses, designed to overcome the limitations of conventional SE methods that rely on hexahedral grids. Our approach utilizes tetrahedral grids, enabling the accurate simulations of large-scale, geophysically complex models, including intricate subsurface anomalies and irregular topography. Starting from Maxwell’s equations, we derive the governing SE equations using a magnetic vector potential A and an electric scalar potential Φ, incorporating the Coulomb gauge to suppress spurious solutions. The computational domain is discretized using the weighted residual Galerkin method, with Proriol-Koornwinder-Dubiner (PKD) polynomials serving as the weighting and shape functions within each tetrahedral element. Two coordinate transformations-affine and collapse transformations are applied during the solution process. To better leverage the properties of the basis functions, both the interpolation and integration nodes are chosen from the same Warp & Blend point set, rather than using two separate sets, which simplifies the computation of the coefficient matrix terms. The resulting global sparse linear system is solved efficiently using the PARDISO direct solver. We assess the accuracy and computational performance of our method through validation against well-established MT community models. Our evaluation, based on misfit (relative error), degrees of freedom (DOFs), computational time, and memory usage for various polynomial orders, demonstrates that the proposed SE method on tetrahedral grids offers a robust and efficient solution for high-precision forward modeling in MT applications.

SummaryWe conduct a seismological monitoring study for groundwater fluctuations within the 12-years period of 2012-2023 in Beijing using relative seismic velocity changes (dv/v) from continuous ambient noise data. Our measured dv/v time series agree with groundwater level changes observed from groundwater wells and reveal significant characteristics on hydrological and other environmental changes. The most intriguing feature is a dv/v increase of ∼0.02% in winter, which is interpreted as the imprint of frozen ground perhaps associated with decoupling between air pressure and groundwater. In addition, a rapid reduction of dv/v during the second half of 2021 indicates the development of a groundwater recharging event resulting from heavy rainfall. The long-term trends of dv/v suggest a groundwater rebound from 2018 to 2023 over the study area, which we attribute to increased precipitation, recharging due to the South-to-North Water Transfer Project, and reduced irrigation.

SummaryWe present a novel method for generating kinematic rupture models for near-source broadband ground motion simulations. Our approach constructs realistic rupture-parameter distributions for slip, rupture velocity and rise time using Von Karman (VK) fields. To more realistically model the slip pattern, we propose rescaling the VK field to follow a truncated exponential distribution rather than a Gaussian, following previous findings on inversion results. For rupture propagation, we initiate the rupture from slip-constrained hypocenter locations, which is crucial for accurately capturing directivity effects. Finally, to characterize the local slip-rate evolution at each computational point on the fault, we propose to employ the regularized Yoffe functions to which small-scale variations are added using 1D VK-fields whose properties are constrained from a database of dynamic rupture simulations. The statistical properties of these fields are calibrated using a database of dynamic rupture simulations, ensuring appropriate high frequency radiation from the generated rupture.Our rupture generator produces kinematic source descriptions to simulate ground motions that successfully reproduce the mean and standard deviation from ground motion models (GMM) for Mw 6.0-7.0 earthquakes. Additionally, our generator allows for the integration of low-frequency source inversions and complements the high frequency radiation of a seismic rupture with physics-constrained stochastic variations. Our broadband pseudo-dynamic kinematic rupture generator facilitates and possibly improves the simulation of realistic high-frequency ground motions to advance seismic hazard analysis.

SummarySeismic scattering waves in random media are usually regarded as noise in conventional seismic imaging, inversion and interpretation. However, the spatial and temporal variation of the scattering energy depends on the stochastic properties of the random media. The extraction of heterogeneity information such as the correlation scale and fluctuation strength from seismic scattering waves remains a challenge. These parameters are inverted from real scattering data by fitting the synthetic envelopes to the observed seismic envelopes. The synthetic envelopes are usually computed using the Monte-Carlo radiative transfer (MCRT) method. However, physical verification of the stochastic parameter inversion based on MCRT theory has not been realized although it is believed to be correct. To this end, we conducted a physical modelling experiment using an ultrasonic acquisition system and recorded the transmitted wavefields through an artificial heterogeneous medium. In this paper, the elastic MCRT method was used to simulate the energy transport, and the correlation length and fluctuation strength of the artificial heterogeneous medium were inverted with a revised objective function, which can better balance the energy level of direct waves and scattering waves in the inversion process. The inversion results of the correlation scale and fluctuation strength match well with true values, suggesting that this method is accurate and reliable. A combination of our physical experiments and the MCRT theory gives strong proof that this inversion method is correct. Therefore, it can be used with confidence to estimate the properties of the heterogeneities from the ‘undesired’ scattering waves, both in the oil/gas exploration and earth structure investigation.

SummaryThe attenuation operator t* represents the total path attenuation and characterizes the amplitude decay of a propagating seismic wave. Calculating t* is typically required in seismic attenuation tomography. Traditional methods for calculating t* require determining the ray path explicitly. However, ray tracing can be computationally intensive when processing large datasets, and conventional ray tracing techniques may fail even in mildly heterogeneous media. In this study, we propose a modified fast sweeping method (MFSM) to solve the governing equation for t* without explicitly calculating the ray path. The approach consists of two main steps. First, the traveltime field is calculated by numerically solving the eikonal equation using the fast sweeping method. Second, t* is computed by solving its governing equation with the MFSM, based on the discretization of the gradient of t* using an upwinding scheme derived from the traveltime gradient. The MFSM is rigorously validated through comparisons with analytical solutions and by examining t* errors under grid refinement in both simple and complex models. Key performance metrics, including convergence, number of iterations, and computation time, are evaluated. Two versions of the MFSM are developed for both Cartesian and spherical coordinate systems. We demonstrate the practical applicability of the developed MFSM in calculating t* in North Island, and discuss the method’s efficiency in estimating earthquake response spectra.

SummaryVarious methods for determining the magnetic field at the core-mantle boundary (CMB) from the observed geomagnetic core field have been explored over recent decades. These include the harmonic downward continuation of surface data and the stabilised iterative upward continuation. The instability of the inverted poloidal magnetic field at the CMB for a radial conductivity structure is complemented by the non-uniqueness of determining the toroidal magnetic field at the CMB for a laterally inhomogeneous conductivity model. We reformulate this unstable and non-unique inverse problem as an iterative upward continuation approach, in which the magnetic field at the CMB is successively updated. The uniqueness of the inverse solution is ensured by the initial choice of the toroidal magnetic field at the CMB, while the stability is achieved by stopping the iterations once the desired tolerance is reached between the spectral index of the updated solution and that obtained from numerical geodynamo simulations. We consider two significantly different radial electrical conductivity models of the lower mantle, each with conductance near 108 S: conductivity model A, based on external electromagnetic sounding, which includes a significant conductivity increase in a 10 km thick layer above the CMB, and conductivity model B, characterized by a gradual conductivity increase determined from the Voigt-Reuss-Hill average of the bridgmanite-ferropericlase aggregate, with an additional conductivity increase in the 300 km thick D” layer associated with post-perovskite. Models A and B bracket the lower and upper bounds of conductivity structures derived from thermal and compositional constraints below 1600 km depth. We find that the differences between the magnetic field components at the CMB inverted for models A and B are approximately 1-2 per cent of the total field. To explore lateral variations, we construct a synthetic model of the Pacific and African superplumes by simplifying their geometric shapes, estimating the temperature increase within the plumes and allowing mantle mineral activation energies to vary only with temperature. Our results show that, in the regions of the superplumes, the poloidal and toroidal magnetic fields at the CMB change by approximately 12,000 nT and 2,500 nT, respectively. The changes in the horizontal poloidal field at the CMB are comparable in magnitude to those resulting from substituting model A with model B. However, the changes in the radial field inverted for the three-dimensional plume conductivity model are significantly larger than those arising from replacing model A with model B.

SummarySeismic surface wave tomography, particularly when leveraging dense array data, has become a widely used method for investigating shallow subsurface velocity structures. The shallow structures are usually characterized by rapid seismic velocity changes (i.e. seismic interfaces) due to variations in rock properties, sedimentary environments, or tectonic features. However, the commonly used grid-based parameterization of the velocity field in surface wave tomography often struggles to accurately constrain such interface geometries. In addition, traditional surface wave inversion methods typically rely on 1D inversion at individual stations using dispersion curves, followed by interpolation to construct 2D or 3D models. This approach can sometimes introduce spurious features and reduce model reliability. To address these limitations, we propose a geological and level-set parameterization approach for surface wave tomography, allowing for the explicit consideration of interface structures in inversion. This method is then combined with the Ensemble Kalman Inversion to optimize subsurface structures. Synthetic tests demonstrate that integrating 3D interface parameterization in tomography significantly enhances the reliability of the velocity model and the recovery of interface geometries. Applying this approach to the Woxi gold mine region in China yielded inversion results that closely align with existing borehole data. This study highlights the advantages of level-set parameterization for 3D interface imaging in seismic tomography, underscoring its potential in subsurface mineral exploration.

SummaryIn order to better understand the regional tectonics of western part of Africa (WA) and adjacent islands, joint inversion (Jinv) of body wave and surface wave measurements is conducted to construct new sets of crustal models. Teleseismic P-wave receiver function, receiver function horizontal-to-vertical ratio and Rayleigh wave ellipticity are jointly inverted based on a fast simulated-annealing scheme. All three types of observables are derived from single-station recordings and are primarily sensitive to structures beneath the station. The integration of these datasets through Jinv allows for complementary constraints, thereby improving the resolution of crustal velocity structures and the characterization of velocity variations with depth. We present improved and some new crustal structure parameters including bulk crustal VP/VS ratio, crustal thickness (H) estimates, and shear-wave velocity (VS) models beneath 25 broad-band seismic stations across inland, coastal, and island terrains. Using an improved approach involving the correction of misorientation error effect from seismic waveform data, the data quality is well-enhanced leading to improved resolutions of structures across the different terrains. Results from H-k and crustal models showed a general northward thinning from Congo Craton (> ∼48 km) towards the Lower Benue Trough (∼15 km), and from coastal terrain along Gulf of Guinea (< ∼44 km) towards Mauritanian Belt (> ∼16 km). Compared to other terrains, the islands show very thin depth to the Moho, but higher than the global estimates. In the Mauritanian-Senegal Basin, sharp differential in crustal thickness and Jinv results at neigbouring G.SOK and G.MBO are observed, where slower Vs revealed a LVZ anomaly at G.SOK in contrast with faster Vs at G.MBO—which could be due to local subsidence from sediment loading, or uplift from tectonic activities. In the upper-middle crust, the Jinv imaged structures with faster VS characteristic of felsic to intermediate bulk crustal composition beneath inland terrain (West Africa Craton, Congo Craton, Hoggar), attributed to highly depleted and stable nature of the cratonic lithosphere, contributing to faster VS compared to other terrains. Low velocity structures underlying the island stations are attributed to partial melts and high temperature materials, indicative of volcanic and Basaltic composition. Similarly, the low velocity structures deciphered beneath coastal stations G.SOK and AF.EDA could be related to the structures in their adjacent areas of Tenerife and the Cameroon Volcanic Line, respectively. The nbroad range of VP/VS (∼1.58–1.85) ratio along the coastal terrains demonstrates its complexity; from the low VP/VS which may be attributed to indurated or low porosity sedimentary materials, and high VP/VS —typical of cracks, fluids inundated sedimentary or volcanic materials. Island terrain are associated with higher bulk VP/VS indicative of volcanics and Mafic-Basaltic materials, with the low velocity zones (LVZs) suggestive of the presence of magmatic materials. These broad crustal configuration highlights the complexity and provides new insight for developing more accurate regional model for western Africa and its adjacent islands, and global reference models in future studies.

SummaryThe Xiangshan volcanic basin locates in southeast China hosts the world’s third-largest volcanogenic uranium deposit. However, the structure of the volcanic system remains poorly resolved, limiting insights into the uranium mineralization. To address this, we conducted a joint inversion of gravity and magnetic data collected in the basin. Our inversion results reveal a southeast-dipping porphyroclastic lava conduit beneath the peak of Mount Xiangshan, characterized by low density and high magnetic susceptibility. A southwest-dipping volcanic conduit has also been identified beneath the rhyodacite crater in the Shutang area of the western basin. It connects to the porphyroclastic lava conduit in the deep. Both of these volcanic conduits are controlled by an EW-trending, low-density basement fault zone. This spatial relationship indicates that the volcanic eruptions in the western basin share a common subvolcanic plumbing system, which collectively acted as principal pathways for ore‑forming hydrothermal fluids and uranium enrichment. These results underscore the role of volcanic-intrusive architecture in controlling the mineralization processes in the Xiangshan volcanic basin.

SummaryThe South American continent (SAC), a region of pronounced geodynamic and hydrological activity, exhibits crustal deformation and gravity field anomalies driven by the interplay of tectonic forces and surface/subsurface mass redistribution. While previous studies have mainly focused on gravity changes driven by terrestrial water storage (TWS), mass variations of the solid Earth remain inadequately addressed. In this study, we resolve deep-seated mass transport Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry, hydrological model outputs, GPS-derived vertical crustal motions, and glacial isostatic adjustment (GIA) correction. Our results reveal an internal mass variation of 0.21 ± 0.45 cm yr -1 in equivalent water height (EWH), independent of surface hydrological contributions. Interpreting this signal as predominantly driven by crust-mantle boundary (Moho) displacements, we estimate an average Moho depth uplift rate of 0.37 ± 0.80 cm yr -1 across SAC, based on the crust–mantle density contrast. The Moho interface depth variations exhibit significant spatial heterogeneity. Through uncertainty analysis, four distinct regions (A, B, C, and D) are identified: Region A exhibits Moho uplift and Region B exhibits subsidence, with part contributions from the isostatic adjustment. Key uncertainties in these estimates stem from sedimentation effects and the accuracy of current observations or models. Subsidence in Region C and uplift in Region D are related to the co-seismic and post-seismic effects of the 2010 Chile earthquake. These findings underscore the significance of solid Earth mass flux in active continental regions and unravel the mechanisms governing crust-Moho mass redistribution.

SummaryRecent advancements in high-resolution Digital Elevation Models (DEMs) derived from LIDAR and satellite radar technologies have added a new dimension to the determination of height systems and geoid models. However, their benefits are limited by simplified assumptions inherited from past practices. In mountainous areas, taking into consideration of topography as the Bouguer plate or employing inaccurate terrain corrections can constitute to a problematic approach. Even though the gravity reduction procedures mentioned above have been enhanced in geoid determination studies, the Helmert orthometric heights based on them are still used in some countries such as Türkiye and Taiwan. It is inevitable that this contradiction will negatively affect geoid modeling studies that are intended to be verified or combined with GNSS/leveling data. Another issue arises by ignoring density variations of topographic masses. Through a comparative analysis, this study reviews combined and individual impacts of terrain roughness and density variations on geoid models in the Konya Closed Basin (KCB) and the Auvergne regions, with a focus on their distinctive topographical characteristics. Using 1″ DEMs of the SRTM mission and 30″ UNB_TopoDensT lateral density models, we reveal that terrain corrections in gravity reductions significantly affect geoid heights, with deviations of up to 11.9 cm in KCB and 4.2 cm in Auvergne. Incorporating lateral density models has resulted in geoid height discrepancies of up to 26.8 cm in KCB and 6.7 cm in Auvergne. A validation strategy implemented through GNSS/leveling paths showed that terrain corrections markedly improved geoid model accuracy, particularly in relation to elevation. However, the contribution of the UNB_TopoDensT model to geoid accuracy is questionable in terms of accuracy. Notably, applying density values below 2.4 g·cm⁻³ in high-altitude regions can lead to disruptive effects on geoid determination. This result is underscoring of the need on a realistic modeling of topographical densities in high elevated and rugged terrains. A further conclusion that emerged from these analyses is that gravimetric geoid models should be verified by rigorous orthometric heights, which are observed to fit them better at the 1-2 cm level, instead of the Helmert orthometric heights.

AbstractThis study introduces a novel method for performing 3D inversion of magnetotelluric (MT) data. The proposed method, referred to as the radiation boundary scheme, employs a two-step simulation strategy for the computation of both forward and adjoint responses. One of the key advantages of the scheme is its ability to handle arbitrarily shaped inversion domains, thereby optimizing the number of unknown model parameters by discarding model parameters that are not constrained by the data. Consequently, it significantly improves accuracy and computational speed as compared to traditional inversion algorithms. The effectiveness of the developed algorithm is demonstrated through a comprehensive analysis of 3D inversion using synthetic and continental-scale (SAMTEX) MT data. The method’s efficiency facilitates a detailed analysis of large-scale MT data inversion. Through numerical experiments, it is observed that using a coarse mesh for inversion, the resolution is compromised in the shallower part, resulting in inferior imaging and, consequently, affecting the estimation of resistivity value in the deeper subsurface. The detailed numerical experiments indicate that performing a fine-scale inversion on a small portion of the survey data utilizing a coarsely inverted model may run into a local minimum. Hence, caution should be exercised in employing such an approach. Instead, the investigations suggest simultaneously executing a fine-scale inversion on the entire data set. The forward/adjoint problem can be solved with a two-order higher tolerance as compared to the conventional finite-difference-based inversion algorithm. Therefore, the proposed algorithm holds significant value for the MT inversion of large data sets.

SummaryMineral exploration is frequently centred around delineating discrete geological units with, typically, sharp boundaries that could represent economic targets. In the case of complex resistivity (CR) inversions, the choice of regularisation and model parameterisation significantly impacts the inversion’s ability to delineate targets. Initially, however, a prudent researcher may not wish to bias their inversion towards sharp distinct units without prior justification. Here we explore how a suite of regularisation approaches to the CR inverse problem allows to encompass different classes of prior beliefs. We present these as a progression as more information becomes available regarding the likelihood of distinct geological units. The most weakly informed approach with respect to the delineation of geological units we consider is the classic ℓ2-type regularisation, tend to produce smeared-out fuzzy images. However this is typically not what is expected for distinct geological units, and we compare this with schemes that increasingly resolve sharp boundaries. We test a range of ℓ1-type regularisations, which have been frequently touted in the geophysics and optimisation literature as being well-suited for such tasks. We experiment with using a so-called overcomplete parameterisation of the CR field, which aims to separate smooth background and sharp foreground features. These ℓ1 schemes are shown to produce generally sharper images than ℓ2. In the most informed case, where strong assumptions can be made about the local geology, we represent the CR field as a foreground ellipse in a homogenous background. This approach significantly reduces the size of the parameter space, and tends to have a simple geometric interpretation. While the anomaly parameterisation has some unique challenges, we show it clearly resolves distinct units compared to both the ℓ2 and ℓ1 regularisations. Applications first to synthetic data and then to field data from Century Zinc Deposit in northern Australia, demonstrate the progression from weakly informed to strongly informed regularisation and parameterisation and the sharpness of the recovered geological units.

SummaryFollowing reanalysis of data from 8 seismic networks that operated in the region surrounding the Pampean flat slab during the past several decades, we generated 3D images of Vp, Vs, and Vp/Vs from a combination of arrival times of P and S waves from local earthquakes, and Rayleigh wave dispersion curves from both ambient noise and existing shear wave models. Among the robust features in these images is a low velocity, root-like structure that extends beneath the high Andes to a deflection in the flat slab, which suggests the presence of an overthickened Andean crust rather than a hypothesized continental lithospheric root. Most of the larger scale features observed in both the subducted Nazca plate and the overriding continental lithosphere are related to the intense seismic activity in and around the Juan Fernandez Ridge Seismic Zone (JFRSZ). Vp/Vs ratios beneath, within, and above the JFRSZ are generally lower (∼1.65–1.68) than those in the surrounding Nazca and continental lithosphere (∼1.74–1.80). While the higher continental lithosphere ratios are due to reduced Vs and likely a result of hydration, the lower JFRSZ related ratios are due to reduced Vp and can be explained by increased silica and CO2 originating from beneath the slab, perhaps in concert with supercritical fluid located within the fracture and fault networks associated with the JFR. These and related features such as a region of high Vp and Vs observed at the leading edge of the JFRSZ are consistent with a basal displacement model previously proposed for the Laramide flat-slab event, in which the eroded base of the continental lithosphere accumulates as a keel at the front end of the flat slab while compressional horizontal stresses cause it to buckle. An initial concave up bend in the slab facilitates the infiltration of silica and CO2-rich melts from beneath the slab in a manner analogous to petit spot volcanism, while a second, concave down bend, releases CO2 and supercritical fluid into the overlying continental lithosphere.