Geophysical Journal International

SummaryA growing body of literature has contributed to linking the presence of bacteria with SIP signals. Yet, there are still unresolved questions concerning the contribution of cell density and microbial metabolic activity in porous media (soils and sediments) to SIP signals. Moreover, there is continued debate on whether cells themselves polarize or whether a cell-sediment interaction is a prerequisite for the measured responses. This study investigates the SIP response of Shewanella oneidensis MR-1 in isolation, that is, in the absence of a mineral porous medium using two setups (i) cells in aqueous suspension and (ii) alginate bead-packed reactors. Results from experiments conducted with static cell suspensions shed light on the strong control of cell settling that drives erratic, poorly reproducible and difficult to interpret SIP signals. However, incubating cells in bead packed reactors yielded reproducible trends in σ″, with strong (3 – 10 mrad) signals that followed the expected cell growth behaviour. Relating σ″ to measured cell density and metabolic activity (using ATP) highlighted the strongly linked contribution of both activity and cell density and SIP. Here, we report a lower frequency polarization peak between 0.01 and 0.1 Hz in the bead reactors, which we attribute to the polarization of cell colonies in the densely packed reactors. In summary, our findings shed light on the direct contribution of cells and their activity to polarization, in the absence of cell-sediment interactions and provide a novel approach for studying cell polarization in static incubation in a porous environment.

SummaryWe present DASPack, a high-performance, open-source compression tool specifically designed for distributed acoustic sensing (DAS) data. As DAS becomes a key technology for real-time, high-density, and long-range monitoring in fields such as geophysics, infrastructure surveillance, and environmental sensing, the volume of collected data is rapidly increasing. Large-scale DAS deployments already generate hundreds of terabytes and are expected to increase in the coming years, making long-term storage a major challenge. Despite this urgent need, few compression methods have proven to be both practical and scalable in real-world scenarios. DASPack is a fully operational solution that consistently outperforms existing techniques for DAS data. It enables both controlled lossy and lossless compression by allowing users to choose the maximum absolute difference per datum between the original and compressed data. The compression pipeline combines wavelet transforms, linear predictive coding, and entropy coding to optimise efficiency. Our method achieves up to 3 × file size reductions for strain and strain rate data in lossless mode across diverse datasets. In lossy mode, compression improves to 6 × with near-perfect signal fidelity, and up to 10 × is reached with acceptable signal degradation. It delivers fast throughput (100–200 MB s−1 using a single-thread and up to 750 MB s−1 using 8-threads), enabling real-time deployment even under high data rates. We validated its performance on 15 datasets from a variety of acquisition environments, demonstrating its speed, robustness, and broad applicability. DASPack provides a practical foundation for long-term, sustainable DAS data management in large-scale monitoring networks.

SummaryInitial stress exerts a crucial impact on the elastic properties and thus the wave reflection in the layered media. However, the stress effect on wave reflection characteristics in such media remain insufficiently understood. To address this issue, we develop a composite matrix reflectivity method incorporating initial overburden stress (CMRMS) by means of acoustoelasticity theory, enabling accurate modeling of seismic wave propagation in stressed layered media. The proposed method can better simulate multiple reflections, converted waves and transmission loss of seismic waves in layered media, compared to the classic stress-dependent reflection coefficient equation for a single interface. Moreover, our method can degenerate into the existing methods in the cases of no initial stress and single interface, which verifies its correctness. We further extended the CMRMS to elastic and viscoelastic non-welded interfaces using the linear-slip theory and standard linear solid model, respectively. The extended method is used to investigate the impacts of non-welded interface compliance, overburden stress, fluid viscosity and frequency on seismic reflection characteristics within layered model. It is shown that the stress effect magnitude on interface reflection significantly depends on the interface depth, due to cumulative transmission losses from overlying layers. Moreover, increasing either the compliance or the number of overlying non-welded interface significantly reduces the reflection amplitude at deeper interface. Our results show the potential of the proposed composite matrix reflectivity method to consider the joint effects of initial stress, multiple waves and transmission loss in both forward modelling and inverse applications.

AbstractThe low-velocity layer confined by surrounding rocks deep in the subsurface acts as a seismic waveguide. The compressional (P-) and shear (S-) waves propagate in the waveguide are reflected on the top and bottom interface, constructively interfered with to formulate the deep-guided wave. Deep-guided wave has high-frequency contents and notable dispersive features. The dispersion represents the kinematics information and can be used to image the low-velocity structures. The Earth media not only shows elasticity but also attenuates seismic waves. This article presents a theoretical study of deep-guided wave propagation and dispersion analysis in viscoelastic media. We utilize the Thomson-Haskell propagator matrix method to theoretically calculate the phase velocity dispersion and attenuation curves of the deep-guided wave in viscoelastic media. We apply the staggered-grid finite-difference scheme to numerically simulate the elastic wavefield propagation in the shale layer for validation. We have conducted a sensitivity analysis of the dispersion and attenuation curves of the deep-guided wave with respect to different media parameters. The theoretical calculation of dispersion and attenuation curves of deep-guided wave opens the doors for the simultaneous inversion of S-wave velocity and quality factor of the low-velocity layer in the future. Deep-guided waves hold the potential for high-resolution imaging of hydrocarbon reservoirs, geothermal reservoirs, coal seams, saline aquifers, and fault zones.

SummaryA strong earthquake sequence in Storfjorden, south of Svalbard, was initiated by an Mw 6.1 event on 21 February 2008. Earthquake distribution and fault plane solutions indicate that seismic activity is controlled by unmapped NE-SW striking oblique-normal faults, contrasting with the major N-S oriented faults mapped onshore Svalbard. We present a geophysical model derived from an ocean bottom seismometer profile crossing the seismogenic zone to identify structures in the crust and uppermost mantle that potentially control the earthquake source mechanism. Travel-time forward modeling using raytracing, combined with travel-time tomography and gravity-magnetic modeling, reveal distinct crustal domains across the earthquake region. Crystalline crustal P-wave velocities range from 6.1 km/s to 6.7 km/s at the Moho depth in the eastern section. The western profile section exhibits a higher Vp velocity lower crust (6.6–7.0 km/s) with Vp/Vs ratios of 1.75–1.8 and high density (∼3100 kg/m³). Basement depth reaches 8 km in the west, forming a sedimentary basin, and shallows eastward. The Moho remains relatively flat at 29-32 km depth throughout the profile. The N-S oriented Caledonian suture, identified from deep seismic and potential field data, traverses the Storfjorden earthquake zone. The lithological contacts within the suture zone, inferred from the new OBS data, may facilitate seismic failure oblique to the N-S oriented structure, following the regional stress field.

SummaryWe developed a new amplitude correction method for receiver function imaging to analyze velocity contrasts along dipping interfaces. Because receiver function imaging typically assumes a horizontally layered structure, corrections are needed for amplitude and polarity variations of P-to-S converted phases when analyzing dipping interfaces. However, previous studies have not adequately addressed these effects, and improved receiver function analysis is required to better delineate dipping structures, such as subducting plate surfaces and the oceanic Moho. Therefore, we propose formulae that quantify converted S-wave amplitude variations between horizontal and dipping interfaces. This relationship is expressed as a function of the back azimuth, the ray parameter of an incident P wave, and the dip angle and dip direction of a dipping interface, and in this study, the geometry of the dipping interface (dip angle and dip direction) is assumed. We applied these formulae to receiver function imaging using synthetic and observed data and confirmed that the amplitude of seismic discontinuities was successfully reproduced. This method enables the use of numerous receiver functions regardless of the back azimuths of incident P waves, thereby providing more detailed amplitude estimations for dipping interfaces.

SummaryTo better understand the mechanics of injection-induced seismicity, we developed a two-dimensional numerical code to simulate both seismic and aseismic slip on non-planar faults and fault networks driven by fluid diffusion along permeable faults, in an impervious host rock. Our approach integrates a boundary element method to model fault slip governed by rate-and-state friction with a finite-volume method to simulate fluid diffusion along fault networks. We demonstrate the capabilities of the method with two illustrative examples: (1) fluid injection inducing slow slip on a primary rough, rate-strengthening fault, which subsequently triggers microseismicity on nearby secondary, smaller faults, and (2) fluid injection on a single fault in a network of intersecting faults, leading to fluid diffusion and reactivation of slip throughout the network. This work highlights the importance of distinguishing between mechanical and hydrological processes in the analysis of induced seismicity, providing a powerful tool for improving our understanding of fault behavior in response to fluid injection, in particular when a network of geometrically complex faults is involved.

SUMMARYIn mineral exploration, induced polarization and self-potential are two broadly used active and passive geophysical methods, respectively. In the case of ore bodies, both methods are associated with charge distributions associated with a secondary electrical field (induced polarization) and a source current density (self-potential). Both the chargeability and volumetric source current density distributions bring information regarding the shape of ore bodies. Therefore the joint inversion of these datasets is expected to better tomograms of ore bodies. A joint inversion approach is developed to combine both methods. The objective function to minimize includes two independent components plus a cross-gradient joint function. The use of the cross-gradient is justified from the underlying physics of the two geophysical problems at play. The structure of the cost function is tailored to overcome some problems like convergence and parameter determination in the inverse process. Two synthetic tests and a laboratory experiment are used to benchmark the proposed algorithm. We demonstrate that the joint inversion algorithm performs better than the localizations obtained from independent inversion approaches. To refine the interpretation of the shape of ores, we introduce an ore presence index using the chargeability and source current density resulting from the joint inversion algorithm. The K-Medoids clustering algorithm is used to automatically categorize the calculated ore presence index into different clusters. The cluster with larger values successfully identifies the ore bodies associated with strong chargeability and/or volumetric source current density.

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.