3D Resistivity Model Research Articles

Imaging and modelling the subsurface structure of the Rungwe Volcanic Province in SW Tanzania with aeromagnetic data: An improved structural map to support geothermal exploration

The Rungwe Volcanic Province (RVP) in the East African Rift System, SW Tanzania, provides a unique opportunity to investigate geothermal resources in the context of particularly complex continental rifting processes. To support geothermal resource targeting in the RVP, we present a revised neotectonic structural map based on an interpretation of aeromagnetic data constrained by 2D-forward modelling of magnetic anomalies integrated with the distribution of previously reported faults, seismic epicentre locations, 3D magnetotelluric resistivity models and surface geothermal manifestations. Magnetic anomalies in the RVP, including the nationally prominent Mbeya anomaly, are related to the high magnetic susceptibility or remanent magnetism of Precambrian rocks and Cretaceous carbonatite intrusions buried in the rift under a varying thickness of non-magnetic sediments and volcanic rocks. Magnetic lineaments are related to structures controlling the geometry of the Precambrian rocks and concealed dikes and the thickness of the sediments and volcanics. The recent Ngozi and Rungwe trachyte volcanics have relatively low magnetic susceptibility comparable to the low susceptibility of the sediments in the rift basins. The revised neotectonic structural map shows prominent NW, NE and NS-trending magnetic lineaments that correlate with previously reported faults and alignments of seismic epicentres in the study area and with the regional trend of the rift segments. The NE-trending magnetic lineaments are consistent with interpretations of the current stress field in the RVP. The main volcanoes in the RVP, Ngozi, Rungwe and Kiejo (also known as Kyejo and Kieyo), are aligned with the NW-trending linear magnetic feature joining the Lupa and Livingstone rift border faults. This lineament is intersected and frequently displaced by the NE and NS-trending lineaments, suggesting that the NE to NS-striking structures are younger. The Rungwe and Ngozi volcanoes as well as numerous ‘‘monogenetic’’ eruption centres and the Mwakaleli caldera, which originated ca. 2 Ma ago (Ebinger et al., 1989) following a large explosive eruption emplacing widespread ignimbrite deposits, are located within a zone of low-intermediate magnitude magnetic features forming a basin-like structure surrounded by magnetic high anomalies of the Precambrian basement structures. We interpret the intersections between the NW-trending intra-rift faults and the NS and NE-trending faults as favourable locations for wells to target high permeability within the geothermal resource conceptual models previously constructed using 3D MT resistivity imaging integrated with supporting geoscientific data. The intersections provide a focus area for follow-up ground mapping of subtle features that may be associated with very recent fault movement.

Resistivity anomalies in the shallow and deep crust beneath West Tasmania from a broadband magnetotelluric 2D transect

West Tasmania has a complex geological history, including a Cambrian tectonic collision and related orogenesis, which formed ore deposits in a volcanic hosted massive sulfide (VHMS) setting. While the topography and dense vegetation covering the area presents challenges for on-ground investigations, the area is relatively well-studied such that 3D geological models are available. This contribution presents results of a broadband magnetotelluric (MT) 2D transect across the area (∼30 stations over a ∼ 80 km line, deployed in early 2016) that enables a combined interpretation of regional-scale geoelectric and geological structures. After accounting for noise, distortion and geoelectric strike; we produce 2D resistivity models using OCCAM2D inversion. We infer low resistivity anomalies in the shallow ( < 3 km ), mid (3–8 km) and deeper crust. Along this MT transect the shallow and mid crust low resistivity zones correspond to major crustal faults. Deep low resistivity features in the east of the transect suggest fluid pathways associated with metamorphism along the western boundary of the Tyennan block during the Cambrian Tyennan Orogeny, while others potentially represent the metasomatism of lower crustal rocks and fluid pathways converging into the mid crust.

IDENTIFICATION OF HYDROTHERMAL DISTRIBUTIONS USING THE WENNER-SCHLUMBERGER CONFIGURATION GEOELECTRIC METHOD IN LEMEU VILLAGE, LEBONG REGENCY, BENGKULU

The hot spring pool, Lemeu Village, Lebong Regency, is one of the geothermal fields. This research aims to identify the study area's geothermal potential and lithology. This is illustrated by the correlation method of resistivity and electrical conductivity values of rocks based on geoelectric measurements. This research uses the Wenner-Schlumberger configuration geoelectric method, as many as four tracks spread around the hot spring hill with a length of 480 meters, which are used to see variations in resistivity values that can indicate the presence of hydrothermal potential. The result is a 2D Resistivity model processed with Res2DinvX32 software. Based on the interpretation results, the subsurface rock lithology in each track is relatively similar in color, texture, grain size, and composition because it is around the hill. The Hulusimpang formation consists of clay, silty sand, tuff, andesite, basalt, and granite. Hydrothermal is found on lines 2 and 4 with resistivity values of 7.3 and 10.1 Ωm at a depth of 26 meters. Geothermal potential in the research area is divided into several temperature categories; some have low, medium, and high temperatures. In the research area, tracks 1 and 3 have low temperatures.

3D modeling of deep borehole electromagnetic measurements with energized casing source for fracture mapping at the Utah Frontier Observatory for Research in Geothermal Energy

AbstractWe present a 3D numerical modelling analysis evaluating the deployment of a borehole electromagnetic measurement tool to detect and image a stimulated zone at the Utah Frontier Observatory for Research in Geothermal Energy geothermal site. As the depth to the geothermal reservoir is several kilometres and the size of the stimulated zone is limited to several 100 m, surface‐based controlled‐source electromagnetic measurements lack the sensitivity for detecting changes in electrical resistivity caused by the stimulation. To overcome the limitation, the study evaluates the feasibility of using a three‐component borehole magnetic receiver system at the Frontier Observatory for Research in Geothermal Energy site. To provide sufficient currents inside and around the enhanced geothermal reservoir, we use an injection well as an energized casing source. To efficiently simulate energizing the injection well in a realistic 3D resistivity model, we introduce a novel modelling workflow that leverages the strengths of both 3D cylindrical‐mesh‐based electromagnetic modelling code and 3D tetrahedral‐mesh‐based electromagnetic modelling code. The former is particularly well‐suited for modelling hollow cylindrical objects like casings, whereas the latter excels at representing more complex 3D geological structures. In this workflow, our initial step involves computing current densities along a vertical steel‐cased well using a 3D cylindrical electromagnetic modelling code. Subsequently, we distribute a series of equivalent current sources along the well's trajectory within a complex 3D resistivity model. We then discretize this model using a tetrahedral mesh and simulate the borehole electromagnetic responses excited by the casing source using a 3D finite‐element electromagnetic code. This multi‐step approach enables us to simulate 3D casing source electromagnetic responses within a complex 3D resistivity model, without the need for explicit discretization of the well using an excessive number of fine cells. We discuss the applicability and limitations of this proposed workflow within an electromagnetic modelling scenario where an energized well is deviated, such as at the Frontier Observatory for Research in Geothermal Energy site. Using the workflow, we demonstrate that the combined use of the energized casing source and the borehole electromagnetic receiver system offer measurable magnetic field amplitudes and sensitivity to the deep localized stimulated zone. The measurements can also distinguish between parallel‐fracture anisotropic reservoirs and isotropic cases, providing valuable insights into the fracture system of the stimulated zone. Besides the magnetic field measurements, vertical electric field measurements in the open well sections are also highly sensitive to the stimulated zone and can be used as additional data for detecting and imaging the target. We can also acquire additional multiple‐source data by grounding the surface electrode at various locations and repeating borehole electromagnetic measurements. This approach can increase the number of monitoring data by several factors, providing a more comprehensive dataset for analysing the deep‐localized stimulated zone. The numerical analysis indicates that it is feasible to use the combination of the energized casing and downhole electromagnetic measurements in monitoring localized stimulated zone at large depths.

Exploring the InSAR signature associated with river-sourced recharge in California’s San Joaquin Valley

Recharge is a critical component for understanding aquifer systems and the sustainable management of groundwater resources, yet this process is challenging to measure at policy-relevant spatiotemporal scales. Building upon previous research, we tested the hypothesis that InSAR can be used to observe river-sourced recharge if the underlying recharge pathways are associated with sufficient clay content. Our analysis leveraged the decomposition of InSAR time series with interpretations of 3D resistivity models derived from airborne electromagnetic (AEM) surveys. We focused our analysis on two study sites where high density AEM data were available and river-sourced recharge is determined to have occurred during wet years: (1) near Fresno, California and (2) near Visalia, California. Sediment type and hydrogeological structure from AEM supported our hypothesis with the InSAR signature attributed to river-sourced recharge occurring only in the study site with semi-confined to confined conditions and relatively high fraction of interbedded clay within recharge pathways. The timing to peak amplitude, the key feature we wanted to isolate in the InSAR data, near Visalia was interpreted as a pressure pulse front associated with river-sourced recharge propagating into the San Joaquin Valley. This study further validated the potential of InSAR, coupled with AEM data, to map and monitor river-sourced recharge in aquifer systems. As InSAR data become more accessible, this approach holds promise for broader applications in groundwater science and management worldwide.

Enhancing DC resistivity data two-dimensional inversion result by using U-net based Deep learning- algorithm: Examples from archaegeophysical surveys

In this study, we suggested using a convolutional neural network (CNN) based algorithm to enhance two-dimensional (2D) Direct Current Resistivity (DCR) data inversion results. We developed U-net based CNN algorithm, named DCR2D_Net_Archaeo. We generated 1080 sets of 2D resistivity models that simulate buried archaeological remains. We calculated synthetic data for those models for different electrode arrays. We added 2% random noise to apparent resistivity data sets and inverted those data sets. We used the 2D inversion results as input and the corresponding real resistivity model as output. By using those 1080 input and output data sets we developed the DCR2D_Net_Archaeo algorithm. First, we tested this algorithm by using synthetic data. We showed that the developed algorithm improved the 2D classical smoothing regularization inversion and the buried body’'s geometry and depth can be found very close to the real model. Afterward, we also tested the developed algorithm with real data collected from two different archaeological sites. We showed that the buried wall cross-section location and depth are better found by the DCR2D_Net_Archaeo algorithm than by using the conventional inversion codes for the inversion of DCR data if we compare it with the excavated wall structure.

Constraints on Growth and Stabilization of the Western Superior Craton From Inversion of Magnetotelluric Data

AbstractA data set consisting of 376 broadband and long‐period MT measurements was used to generate the first ever 3D resistivity model of the Archean western Superior Craton. The modeled resistivity structure is compared to coincident seismic reflection data. The observed geophysical signatures are interpreted within the context of the late stages of crustal growth and cratonization of the region via the progressive accretion of terranes against the initial cratonic core. The northern‐most terranes comprising the cratonic core exhibit a nearly homogenous highly resistive crust. The lower crust of the southern terranes contains largely continuous low resistivity bands which run subparallel to major terrane boundaries and corresponding fault systems. In some cases, low resistivity features are coincident with dense packages of sub‐horizontal to listric reflections within the mid‐ to lower crust. These resistivity structures are inferred to represent preserved geoelectric signatures of late to post‐orogenic magmatic pulses likely related to delamination of locally overthickened crust. Increased mantle heat flow resulted in partial melting of the lower crust and upper mantle and upward migration of CO2‐rich melts and fluids through crustal weak zones corresponding to shear and/or suture zones formed during terrane amalgamation. Thermal softening of the mid‐ to lower crust led to orogenic collapse and reactivation of the crustal shear zones, resulting in formation and interconnection of graphitic films which were preserved within the stable craton. These results have implications for the tectono‐magmatic history of the western Superior Craton, as well toward the understanding of the geodynamic regime of the Archean Earth.

Electrical resistivity tomography (ERT) exploration data in drought prone areas case study: Buriram Province, Thailand

An electrical resistivity tomography (ERT) investigation was conducted across four drought-prone districts in the Buriram Province, Thailand. The primary objective was to evaluate and map the potential of groundwater reservoirs as sources of water for household and agricultural purposes during the dry season. It was accomplished through the implementation of the Schlumberger array configuration. An electrical resistivity survey instrument was used to generate a 2D resistivity model of the electrical resistance profiles, or pseudo section profiles. The survey instrument included more than 50 electrodes, enabling the investigation of the profile to a depth of up to 50 m from the ground surface. The resistivity values obtained from the field data were recorded and converted or interpreted using RES2DINV software. The data were analysed by comparing them with the geological information about the site and referencing the geological borehole data as at 50 m depth from the surface. The results of the ERT survey indicated that groundwater in the arid areas of the Buriram Province can be found at shallow depths around 10–20 m from the surface and it is deposited in sedimentary and clay layers, and it has remained relatively stable over a 2-year period with the water level measured by an electric probe in the summer, winter and rainy seasons in Thailand.

Coastal Groundwater Bodies Modelling Using Geophysical Surveys: The Reconstruction of the Geometry of Alluvial Plains in the North-Eastern Sicily (Italy)

The integration of various geophysical methodologies is considered a fundamental tool for accurately reconstructing the extent and shape of a groundwater body and for estimating the physical parameters that characterize it. This is often essential for the management of water resources in areas affected by geological and environmental hazards. This work aims to reconstruct the pattern and extent of two groundwater bodies, located in the coastal sectors of the North-Eastern Sicily, through the integrated analysis and interpretation of several geoelectrical, seismic and geological data. These are the Sant’Agata-Capo D’Orlando (SCGWB) and the Barcelona-Milazzo (BMGWB) Groundwater Bodies, located at the two ends of the northern sector of the Peloritani geological complex. These two studied coastal plains represent densely populated and industrialized areas, in which the quantity and quality of the groundwater bodies are under constant threat. At first, the resistivity models of the two groundwater bodies were realized through the inversion of a dataset of Vertical Electrical Soundings (VES), constrained by stratigraphic well logs data and other geophysical data. The 3D resistivity models obtained by spatially interpolating 1D inverse VES models have allowed for an initial recognition of the distribution of groundwater, as well as a rough geological framework of the subsoil. Subsequently, these models were implemented by integrating results from active and passive seismic data to determine the seismic P and S wave velocities of the main lithotypes. Simultaneous acquisition and interpretation of seismic and electrical tomographies along identical profiles allowed to determine the specific values of seismic velocity, electrical resistivity and chargeability of the alluvial sediments, and to use these values to constrain the HVSR inversion. All this allowed us to recognize the areal extension and thickness of the various lithotypes in the two investigated areas and, finally, to define the depth and the morphology of the base of the groundwater bodies and the thickness of the filling deposits.

A conditioned Latin hypercube sampling design methodology for ground-truthing transient EM resistivity models

Translating 3D resistivity models of the subsurface to relevant hydrogeological parameters such as sediment texture requires applying a rock physics transform that is ideally calibrated using co-located 1D sediment texture logs. It is difficult to select optimal logging sites in the model that span the range of observed resistivity values using expert opinion alone. Instead, we demonstrate a methodology for selecting logging sites using conditioned Latin hypercube sampling (cLHS), a maximally stratified sampling design methodology. cLHS employs simulated annealing to select sampling sites that minimize an objective function by reproducing the statistics of the resistivity model. We implement numerous preprocessing steps to isolate desirable regions within the model for logging, and we use principal component analysis to reduce the dimensionality of the model to aid the convergence of the simulated annealing procedure while preserving the majority of the model variance. Additionally, we impose an additional objective function condition to penalize the selection of sites in the resistivity model that exhibit a high degree of lateral variance. This condition is used to increase the likelihood that the sediment texture logs acquired at the selected sites correlate well with the resistivity model. We tested our approach on a resistivity model obtained using a towed transient EM system in an almond orchard in California’s Central Valley, comparing our selected sets of logging sites against the five cone penetrometer testing logs originally acquired at the site, as well as randomly selected logging sites. The sample sets selected with cLHS were consistently able to reproduce the statistics of the resistivity model and did so more effectively than the true sampling locations or the random sampling locations.

Quasi-2D inversion of surface large fixed-loop transient electromagnetic sounding data

Abstract In many cases, 1D inversion is still an important step in transient electromagnetic data processing. Potential issues may arise in the calculation of apparent resistivity using induced electromotive force (EMF) due to overshoot and the presence of multi-valued functions. Obtaining reliable and consistent inversion results using a uniform half-space as the initial model is challenging, especially when aiming for efficient inversion. Focusing on these problems, we use the land-based transient electromagnetic (TEM) sounding data, which was acquired by using a large fixed-loop transmitter, and adopt a quasi-2D inversion scheme to generate improved images of the subsurface resistivity structure. First, we have considered directly using magnetic field data or converting induced EMF into magnetic field, and then calculating the apparent resistivity over the whole zone. Next, a resistivity profile that varies with depth is obtained through fast smoke ring imaging. This profile serves as the initial model for the subsequent optimal inversion. The inversion scheme uses a nonlinear least-squares method, incorporating lateral and vertical constraints, to produce a quasi-2D subsurface image. The potentiality of the proposed methodology has been exemplified through the interpretation of synthetic data derived from a 3D intricate resistivity model, as well as field data obtained from a TEM survey conducted in a coalmine field. In both cases, the inversion process yields quasi-2D subsurface images that exhibit a reasonable level of accuracy. These images appear to be less moulded by 3D effects and demonstrate a satisfactory level of agreement with the known target area.

The Pannon LitH2Oscope magnetotelluric array in the Pannonian Basin

The Pannonian Basin is one of the best natural laboratories in the world to study the lithospheric response to continental extension and subsequent tectonic inversion. Here we address the topic of lithospheric structure by a combined geochemical and magnetotelluric analysis, which has been carried out in the framework of the Pannon LitH2Oscope project. The main objective was to detect the resistivity distribution over the entire lithosphere by magnetotelluric measurements, considering the lithological resistivity properties and relate the results to the structure and evolution of the Pannonian Basin. The Pannon LitH2Oscope MT array was used to estimate the depth of the Lithosphere-Asthenosphere Boundary (LAB), considering the legacy MT data and compared to previous estimates for the region. Using the MT and geomagnetic response functions, major structural zones of the Pannonian basin, such as the Mid-Hungarian Shear Zone or fault systems like the Makó Trough and the Békés Basin, were also imaged. In addition, we used the apparent resistivity soundings to compare 1D resistivity models computed from geochemistry and obtained from field MT measurements. This comparison provided new constrains for the composition, fluid and melt content variations at the local lithosphere-asthenosphere boundary. The Pannon LitH2Oscope MT dataset and the results presented in this paper provide input for more complex 3D inversions and further investigations of the lithospheric structure in the Carpathian-Pannonian region.

Imaging leachate runoff from a landfill using magnetotellurics: The Garraf karst case

Electrical and active source electromagnetic geophysical methods have been traditionally employed to approach and tackle environmental problems, such as those caused by landfills. However, since these problems are more consequential and cover broader areas, it is necessary to use deeper penetration methods, such as magnetotellurics. In the Garraf Massif (Catalan Coastal Ranges, NE Spain), an urban waste disposal landfill had been in operation from 1974 to 2006, during which >26 million metric tons of garbage had been deposited. This landfill overlies karstic terrain, thus principally impacting groundwater circulation. Previous electrical resistivity tomography profiles had partially imaged the infill but were not able to penetrate below the base of the original landfill. During 2019 and 2020 we performed a magnetotelluric study over the landfill and its surrounding with the goals of characterizing the electrical resistivity of the infill and below it. The 2D and 3D resistivity models confirmed the highly conductive nature of the leachate and allowed us to identify its presence below the landfill base, which we quantified with maximum thicknesses of 90 m. This proved that landfill leachate had filtered through the original impermeable layer, enhanced by the karstic drainage structure.

Application of ERT, IP and VLF-EM Methods to Investigate Landslide-Prone Structures at Archaeological Sites in Lamreh, Aceh Besar, Indonesia

Since the 21st century, Lamreh, Aceh Besar Regency, Indonesia played an important role during the maritime silk route at the gate of the Malacca strait. This article investigates the subsurface structure of landslide-prone areas in cultural heritage based on 2D resistivity, chargeability and current density models of ERT, IP and VLF-EM methods, respectively. Field data measurements were carried out on 2 crossing profiles along the cliff suspected of experiencing landslides. The length of each profile is 220 with 4 m distance between stations. The 2D models reveal that the subsurface geological structure of Lamreh is composed of a mixed layer of clastic sediment and volcanic material at the top, followed by a layer of calcareous sandstone, and volcanic breccia at the bottom. The 3 layers are most easily distinguished in the resistivity model. The topmost layer is permeable but dry, i.e., characterized by a more resistive layer in the models. While the second layer is characterized by an intermediate conductivity and the bottom layer is highly conductive. The conductivity in these 2 layers is influenced by the degree of water content within the rocks. The chargeability models derived from IP data can distinguish between the dry layer on the surface and the saturated layer below. Meanwhile, the current density models obtained from VLF-EM data have proven the presence of fractures and faults along the profiles due to weathering as also seen in the resistivity models. HIGHLIGHTS Investigation of subsurface structure in landslide-prone area of Lamreh, Aceh Besar Regency, Indonesia, a cultural heritage site with historical significance in the maritime silk route. Utilization of 2D resistivity, chargeability and current density models (ERT, IP and VLF-EM) to analyze the subsurface structure. Field data measurements conducted on 2 crossing profiles along the suspected landslide cliff, with a profile length of 220 and 4 m station distance. Subsurface geological structure of Lamreh identified as a mixed layer of clastic sediment and volcanic material at the top, followed by calcareous sandstone and volcanic breccia at the bottom. Resistivity models most effectively distinguish the 3 layers, highlighting permeable but dry top layer, intermediate conductivity in the second layer, and high conductivity in the bottom layer influenced by water content. Chargeability and current density models confirm surface dryness and presence of fractures and faults due to weathering GRAPHICAL ABSTRACT

New MT surveys and 3D resistivity imaging beneath the Ngozi-Rungwe volcanoes at the triple rift junction of the East African Rift System in SW Tanzania: Support for integrated interpretations of geothermal conceptual models

The geothermal system related to the Ngozi and Rungwe volcanoes, SW Tanzania, lies at the intersection of the west and east branches of the East African Rift System and has been investigated by many geoscientists for decades. Here we present a 3D electrical resistivity model based on 190 magnetotelluric resistivity soundings that have been integrated with geochemical and geological results to support the development of the geothermal resource conceptual model presented here. The model includes two separate reservoirs, a larger system located beneath the Rungwe volcano and a smaller chloride water reservoir located under the Ngozi caldera, which contains a neutral chloride hot spring with geothermometry >230°C. An extensive conductive clay cap with variable thickness extends along the 30 km long NW-SE trending Ngozi-Rungwe Fault Zone from the Kiejo area SE of the Rungwe volcano to the Ngozi caldera. The absence of geothermal surface manifestations directly over the inferred Rungwe upflow zone is consistent with effective sealing of the proposed underlying geothermal reservoir by the clay cap. The scarcity of thermal manifestations on the up-dip margins of the low-resistivity clay cap can be explained by coincidence of the base of the clay cap with impermeable Precambrian formations and by structural boundaries. This interpretation implies that the area with the highest geothermal resource potential is the Rungwe volcano where proposed drilling sites might intersect the proposed high-temperature reservoir.

PyMERRY: A Python solution for an improved interpretation of electrical resistivity tomography images

Electrical resistivity tomography (ERT) is a widely used geophysical method for studying geologic hazards, civil engineering, and environmental remediation. It provides information about the subsurface’s resistivity distribution by analyzing electrical data collected at the surface or in boreholes. However, interpreting ERT images can be complex due to ambiguities in their resolution. To address this issue, we develop a postprocessing method called Python iMprovement of Electrical Resistivity tomography ReliabilitY (PyMERRY) to improve the reliability of ERT images. The PyMERRY code can be applied to any 2D resistivity model obtained from ERT inversion software. It computes a coverage mask that defines the domain well constrained by the data and the inversion process. It also evaluates the resistivity uncertainties in the ERT models. In addition to the sensitivity approaches, PyMERRY provides low- and high-resistivity values for all covered cells. Synthetic tests indicate that the approach is efficient and highlight the importance of resistivity contrasts, mesh selection, electrode spacing, and profile length in the reliability of ERT images. Compared with previous studies, using PyMERRY in south-central Bhutan allows a more accurate interpretation of ERT images. It confirms a high-resistivity contrast across the topographic frontal thrust and reveals the existence of small-scale variations.

Audio‐Magnetotelluric Survey for Groundwater Investigation in the Al‐Jaww Plain in Eastern Abu Dhabi, Al‐Ain, United Arab Emirates

AbstractThe United Arab Emirates (UAE), located in an arid climate zone with low rainfall, relies on shallow aquifers for freshwater. Understanding the depth and extent of such aquifers is crucial for meeting water supply needs. The UAE's hydrogeology is influenced by neighboring mountains in Oman. The Al‐Jaww Plain in southeast of Al‐Ain city is an essential groundwater source, characterized by a large, flat area of gravel and sand deposits from the Oman Mountains. This study aims to map groundwater aquifers in the Al‐Jaww Plain by integrating the audio‐magnetotelluric (AMT) method, seismic reflection profiling, and borehole data. AMT data were collected along an 11‐km ENE–WSW profile and a 2D resistivity model was generated. The resulting model delineates three distinct geo‐electrical zones from the surface to a depth of 5 km. First, a shallow layer with low resistivity (0–15 Ωm) represents the Quaternary and Pliocene aquifers, in addition to the Upper Cretaceous Simsima and Tertiary groundwater aquifer zone, extending to a depth of 1.5 km. Second, a moderately resistive layer (15–250 Ωm) is recorded beneath the first layer, corresponding to the Upper Cretaceous Aruma foreland basin sequence. Finally, a high‐resistivity region (&gt;250 Ωm) at depths exceeding 3 km is attributed to the allochthonous Hawasina thrust sheet, which is associated with Late Cretaceous obduction of the Semail ophiolite. These findings have practical implications for managing groundwater resources in Al‐Ain.

Transdimensional Bayesian Inversion and Uncertainty Estimation of DC Resistivity Data for Undercover Cavern Characterization

The presence of subsurface karst caverns could pose significant threat to highway construction safety, and the knowledge about the locations and volumes of the caverns is crucial for evaluating potentially hazardous areas. In this work, we present a transdimensional Bayesian approach for characterizing undercover caverns by inverting direct current resistivity data., This approach provides a robust measure of uncertainty associated with the resulting resistivity model. We evaluate the effectiveness of the algorithm on a synthetic 2D resistivity model. The results demonstrate that quantitative uncertainty information on the derived model parameters is vital for reliable data interpretation and facilitates decision making.

Effect of frequency combination on the resolution of 3D land-based CSEM and its optimal design

To maximize the parameter resolution of a 3D resistivity model, we present an experimental design algorithm to achieve a set of frequencies that is considered optimal for targeting the model by using a land-based controlled-source electromagnetic method (CSEM). An objective function (OF) is defined by combining the maximum energy and flatness of the singular values of a sensitivity matrix, which is used to evaluate the quality of one given survey layout. A genetic algorithm (GA) is employed to minimize the OF searching for the optimal frequency combination. The novelty of our algorithm is extending the optimization theory for CSEM survey system from 1D model to the case of 3D model and by using genetic algorithm to deal with the optimization of a huge-scale unknowns. The algorithm can be applied in two fundamental scenarios, i.e., to select (1) the optimal transmission frequencies before field data acquisition and (2) the optimal inverted frequencies after field data acquisition. The algorithm is first tested with one synthetic example, and the results show that more frequencies and a wider band do not ensure an improvement of the inversion result, but instead increase the computational cost. A further tested with real data acquired in a net array shows the advantages of the optimized frequencies that improve the spatial resolution of the resistivity anomalies in the inversion model. Our study provides theoretical and methodological support for the frequency optimization of a land-based CSEM.

Assessment of three mixed arrays dataset for subsurface cavities detection using resistivity tomography as inferred from numerical modelling

The present study deals with the evaluation of a three-mixed array dataset for the detection of subsurface cavities using conceptual air-filled cavity model sets at different depths. Cavity models were simulated using the forward modelling technique to generate synthetic apparent resistivity data for three common individual arrays. These arrays are dipole–dipole (DD), pole–dipole (PD), and Wenner–Schlumberger (WS). The synthetically apparent resistivity data obtained from two different individual arrays were merged to form a high-resolution single model. Consequently, three possible mixed arrays datasets can be obtained: the dipole–dipole-Wenner–Schlumberger (DD+WS), pole–dipole, and Wenner–Schlumberger (PD+WS), and dipole–dipole and pole–dipole (DD+PD). The synthetically apparent resistivity data for both the individual and mixed arrays were inverted using Res2dinv software based on the robust constrain inversion technique to obtain a 2D resistivity model section. The inverted resistivity sections were evaluated in terms of their recovering ability of the model’s parameters (e.g. resistivity, and geometry). The results show that the individual arrays can resolve the location and dimensions of the cavity within reasonable accuracy only at a depth not exceeding 6 m below the surface. On the other hand, a significant resolution enhancement in model resistivity with increasing depth was observed when the mixed arrays were used. The (DD+WS) mixed arrays dataset brings up better model resistivity and shows closer parameters to the true actual model among the other mixed arrays. So it is strongly recommended for cavity detection studies.