Magnetotelluric Research Articles

Using Resistivity Structure to Study the Seismogenic Mechanism of the 2021 Luxian Ms6.0 Earthquakes

Over the past few years, there has been a noticeable change in the occurrence of seismic disasters in Sichuan, China. The focus has shifted from Western Sichuan to the previously more stable Southeastern Sichuan. The recent Ms6.0 earthquake in Luxian, Southeastern Sichuan, on 16 September 2021, has once again captured the interest of scholars, who are closely examining the seismogenic environment and potential seismic hazards in the region. We conducted a magnetotelluric (MT) array survey in the Luxian earthquake area to explore the deep seismogenic environment of the 2021 Luxian Ms6.0 earthquake zone and understand the potential effects of industrial extraction on seismic activities. Here are the insights we obtained: Underneath the anticline in the Luxian Ms6.0 earthquake area, there is a structure that mainly exhibits high resistance. On the other hand, beneath the syncline, a structure with medium to low resistance is observed. The epicenter of the mainshock was identified near the intersection of high- and low-resistance media within the Fuji syncline area. Smaller aftershocks that followed the mainshock were mainly concentrated in the low-resistance layers at depths of 3–5 km in the Fuji syncline area. MT survey results have confirmed the existence of a detachment zone in the shallow crust near the epicenter of the Luxian Ms6.0 earthquake. It is believed that this detachment layer played a significant role in the seismogenic process of the Luxian Ms6.0 earthquake. During different stress conditions, this layer became active and caused the compression and faulting of a hidden fault below, resulting in the Luxian Ms6.0 earthquake. After the main earthquake, a series of smaller aftershocks with varying focal mechanisms occurred as the stress fields continued to release. It is important to note that the Luxian Ms6.0 earthquake highlights the ongoing high stress levels in the southern region of the Sichuan Basin. This emphasizes the need for continued monitoring and consideration of potential seismic hazards in the southern Sichuan area.

Non-linear Optimization by Generalized Neighborhood Algorithm (GNA) and its Application for Magnetotellurics (MT) Layered-Earth Modeling

Population-based optimization algorithms are often used to resolve highly non-linear inverse problems by performing iterative and stochastic searches of the model space for solutions. These techniques rely on a combination of global and local searches associated with the exploration and exploitation capabilities, respectively. The iterative process usually converges to models very close to each other, leading to unrealistic solution’s uncertainties extracted from the final population. In this paper, we are more interested in the model space exploration leading to more representative uncertainties of the solutions. We divide the search agents into two groups, each randomly distributed in the whole model space and in the vicinity of a promising solution or best model. The additional tuning parameter in the relatively new Generalized Neighborhood Algorithm (GNA) is quite simple to choose, i.e., the extent of search in the neighborhood of the best model. The method was tested to invert magnetotelluric (MT) synthetic data of various representative cases with good results, i.e., recovery of the synthetic models with response that fits the synthetic data. Application of GNA to field data showed that the subsurface resistivity distribution agrees well with the geology of the study area.

Subsurface Faults and Magma Controls on the Jinchuan Ni-Cu Sulfide Deposit: Constraints from Magnetotelluric Data

The Jinchuan Ni-Cu sulfide deposit in the Longshoushan terrane is among the world’s largest magmatic sulfide deposits. This study uses magnetotelluric (MT) survey data imaging combined with previous geophysical data to investigate Segment III of the deposit. The image of MT data reveals a significant low-resistivity anomaly ~200 m beneath Segment III, aligning with known ore bodies, and an additional anomaly to the north, indicating further exploration potential. These findings highlight the deep-seated intrusions and their connection to surface mineralization, emphasizing the critical role of ultramafic rock encapsulation and fault-controlled morphology in ore body formation. The newly identified northern anomaly, resembling the Segment III ore bodies, suggests a promising target for future exploration. Integrating MT surveys with other geophysical methods enhances the understanding of subsurface structures and mineralization, providing a robust framework for discovering concealed mineral deposits and improving exploration efficiency.

Vehicle noise characteristics in magnetotelluric data and vehicle noise removal using waveform fitting

Magnetic field fluctuations due to vehicle noise were observed in magnetotelluric (MT) time-series data measured near roads. The observed vehicle noise had magnitudes ranging from tens to thousands of μA/m, whereas the observed weak natural MT signal magnitudes were approximately tens of μA/m. A small signal-to-noise ratio made it difficult to apply robust processing for removing vehicle noise. In addition, vehicle noise severely distorts the MT response in the MT deadband from 0.01 Hz to 0.3 Hz, where the MT signal is very weak, and methods to remove it are required for deep structure imaging. In this study, magnetic field fluctuations due to moving vehicles were simulated with a magnetic dipole and attempted to be removed using a waveform fitting method. A total of 378 vehicle noises were extracted from the near-road MT data and synthesized with the remote MT data without vehicle noises to investigate the effect of vehicle noise on the MT response. Removal of vehicle noise from synthesized remote MT data resulted in substantial restoration of the apparent resistivity and phase curves around the MT deadband and below 0.001 Hz. In the MT field data, the vehicle noise was simulated and removed with two moving dipoles; the magnitude of the remaining vehicle noise was reduced by approximately half compared to a single dipole, and very stable apparent resistivity and phase curves were obtained. Although electromagnetic noise distortion remains after vehicle noise removal, the waveform fitting method significantly improves the apparent resistivity and phase curve response in the 0.01–0.3 Hz frequency band.

Upper mantle flow and crustal deformation patterns beneath the Dangerous Grounds and Borneo where multiple plates converge in South China Sea revealed by 3-D anisotropic magnetotelluric imaging

SUMMARY A large-scale magnetotelluric (MT) study was carried out in offshore Borneo to understand the lithospheric structure in this geologically complex region where three tectonic plates converge and past crustal studies generated long-standing debates. Marine MT data were acquired at 1416 stations with 3 km spacing along 13 regional lines (covering 4677 line-km) with periodicities of 0.1 to 10 000 s. These were inverted in 3-D incorporating electrical anisotropy with cross-gradients constraints between vertical and horizontal resistivities and the results were validated with resistivity well-logs from several exploration wells. The models reveal widespread presence of electrically resistive upper crustal and uppermost mantle layers, each underlain by a laterally varying conductive and anisotropic layer. The geometries of the anisotropic layers suggest large-scale ductile flow, thrusting and folding forming belts of alternating deep roots and thin lithosphere, consistent with multiple underthrusting/subduction. Our results are in agreement with the seismologically detected lithospheric variations in northern Borneo suggesting onshore–offshore continuity and a common lithospheric–asthenospheric origin for the deformation patterns. Over thinned lithosphere, we found consistent low resistivity and high anisotropy anomalies in the mantle above the spatial locations of fast shear-wave bodies imaged by recent seismological workers which we interpret to indicate post-subduction lithospheric–asthenospheric ductile flow in response to multidirectional regional compression. We demonstrate that these zones are spatially correlated with the distribution of mainly Cretaceous ophiolitic rocks and melanges exposed onshore and suggest that serpentinization of mantle peridotite can explain some of the anisotropic conductivity anomalies. The taper zones of these deep anisotropic conductors are also associated with Neogene sub-basins, high thermal gradients, and intrasedimentary magmatic bodies indicating a link between lithospheric thinning and magmatism. We propose that such anomalies could be important pathfinders for geothermal and natural hydrogen systems in the ongoing global drive for carbon-free energy resources.

Three-Dimensional MT Conductive Anisotropic and Magnetic Modeling Using A − ϕ Potentials Employing a Mixed Nodal and Edge-Based Element Method

Magnetotelluric (MT) sounding is a geophysical technique widely utilized in mineral resource surveys, where conductivity and magnetic permeability serve as essential physical parameters for forward modeling and inversion. However, the effects of conductive anisotropy and non-zero magnetic susceptibility are usually ignored. In this study, we present a three-dimensional (3D) MT modeling algorithm using Coulomb-gauged electromagnetic potentials, incorporating a mixed nodal and edge-based finite element method capable of simulating MT responses for conductive anisotropic and magnetic anomalies. Subsequently, the algorithm’s accuracy was validated in two steps: first, it was compared with analytical solutions for a 1D magnetic model; then, a comparison was made with previously published numerical results for a 3D generalized conductive anisotropic model. The results of two tests show that the maximum relative error is below 0.5% for both models. Furthermore, representative models were computed to comprehensively analyze the responses of MT. The findings illustrate the relationship between anisotropic parameters and electric fields and emphasize the significance of considering the impact of magnetic susceptibility in magnetite-rich regions.

Magnetotelluric evidence for the formation of the layered Sask Craton by flat slab subduction

Long-period magnetotelluric (MT) data were collected at 56 locations over the Sask Craton in 2021 and 2022. The data were combined with existing broadband data and inverted to produce a 3-D resistivity model of the Sask Craton and Trans-Hudson Orogen (THO). The model reveals a number of northeast striking electrically conductive crustal structures that extend into the mantle lithosphere. In the mantle lithosphere, these conductors coalesce into a single large low resistivity anomaly in the depth range 70–85 km termed the Northern Sask Craton (NSC) conductor. The resistivity of the NSC conductor is attributed to sulfides deposited along an interface between a flat slab that was accreted to the base of the pre-THO Sask Craton lithosphere during closure of the Manikewan Ocean. Kimberlites have erupted along the margin of the NSC conductor. The boundary of the conductor likely represents deep-seated faults and mantle terrane boundaries formed during flat slab subduction that allowed the ascent of kimberlite melts. The resistivity of the northeast-trending conductors can be interpreted as due to graphite and sulfides precipitated by past fluid or melt flow during ocean closure and orogensis. A number of these conductors are located beneath known mineral districts and trends and may represent source pathways for regional base and precious metal deposits. Other conductors may represent possible, previously unknown, regions hosting mineralization. Many of these conductors are associated with major regional faults and shear zones, which may be deep-seated features that helped to guide both kimberlites and mineralizing fluids. Of the prominent northeast-trending conductors west of the Sask Craton, one corresponds to the previously reported North American Central Plains (NACP) conductor. The new model shows that this conductor abruptly terminates at 54 °N and is not observed farther south in the model. This shows that the NACP is not as spatially continuous as previously suggested, suggesting that the tectonic processes that formed the THO were not as uniform along-strike as shown in existing tectonic models. The connection of one of the northeast-trending anomalies to the NSC conductor suggests that a previously unrecognized phase of east-dipping subduction may have occurred beneath the Sask Craton as the THO was formed.

De-noising magnetotelluric data based on machine learning

The magnetotelluric (MT) sounding is a common geophysical exploration technique, but it is highly polluted by various types of cultural noise. In the realm of MT data processing, traditional techniques often rely on the quality of the measured MT data. Conventional MT time domain denoising methods tend to eliminate valuable signals, potentially leading to unreliable resistivity estimates. To address this concern, we propose employing machine learning to effectively suppress strong noise interference in MT data, thereby preventing the loss of valuable signals. We augment this approach with mathematical morphological filtering (MMF) to capture low-frequency signals, preserving their integrity. We constructed a signal sample library based on a substantial volume of signal samples. Through consistent training, we establish a support vector machine (SVM) classification model that distinguishes high-quality signal fragments from noisy signals. Subsequently, we use adaptive K-singular value decomposition (K-SVD) dictionary learning to extract noise profiles and suppress noisy signals. To validate the feasibility of our method, we apply machine learning to measured data from two distinct observation areas. The measured data were analyzed and processed, and the results were compared with the robust results. This method can effectively eliminate large-scale strong interference in time domain sequences and preserve more low-frequency slow change information and high-quality signals in the reconstructed signals. The apparent resistivity phase curve of synthetic data is smoother and more continuous, and the data quality in the low-frequency range is significantly improved. The results can more accurately and reliably reflect underground electrical structure information.

2D magnetotelluric imaging method based on visionary self-attention mechanism and data science

2D magnetotelluric (MT) imaging detects underground structures by measuring electromagnetic fields. This study tackles two issues in the field: traditional methods’ limitations due to insufficient forward modeling data, and the challenge of multiple solutions in complex scenarios. We introduce an enhanced 2D MT imaging approach with a novel self-attention mechanism, involving: 1. Generating diverse geophysical models and responses to increase data variety and volume. 2. Creating a Swin–Unet-based 2D MT Imaging network with self-attention for better modeling and relation capture, incorporating a MT sample generator using real data to lessen large-scale supervised training dependence, and refining the loss function for optimal validation. This method also includes eliminating MT background response to boost training efficiency and reduce training time. 3. Applying a transverse electric/transverse magnetic method for comprehensive 2D MT data response. Tests show that our method greatly improves 2D MT imaging’s accuracy and efficiency, with excellent generalization.

Sediment thickness in the Thrace Basin estimated from the three-dimensional inversion of magnetotelluric data

The time-variation of the magnetotelluric (MT) fields was recorded at 37 measurement stations in the Thrace Basin (North-western Türkiye) by a remote-reference MT system that records the MT data in two frequency bands allowing a data acquisition between 320–7.5 Hz and 6–0.00055 Hz, respectively. The measurement stations were located from the Aegean Sea to the Black Sea seashores along a 207 km profile in the southeast-northwest direction. Three-dimensional inversion of MT data outlined the main geological elements of the region, the interface between sedimentary cover and basement rocks. The total thickness of the sedimentary layers in the basin is interpreted using the inflection and deflection points of the three-dimensional resistivity model in the vertical direction.

A potential deep geothermal reservoir in eastern Ladakh as inferred from the upper crustal geoelectric structure of the region

The Ladakh Himalaya is a repository of the evolutionary history of the India-Eurasia convergent plate margin. We present the results of a magnetotelluric (MT) study carried out in Eastern Ladakh along a 40-km-long profile traversing across various tectonic domains of this plate margin. We modeled the MT dataset by two-dimensional (2-D) and three-dimensional (3-D) inversion algorithms. The upper crustal geoelectric model obtained by 2-D inversion of distortion corrected and decomposed data of 18 sites shows an excellent match with the surface geology. In this model, the Indus Suture Zone (ISZ) appears as a steeply dipping contact and the Ladakh batholith is about 10 to 12 km thick. The model yields a prominent electrical conductor of ∼6 km diameter at 4 km depth beneath the Tso Morari Crystallines with an offshoot of a small conductor that rises upward along the ISZ. The geoelectrical model obtained by 3-D inversion is broadly consistent with the 2-D model but also reveals off-profile features and a connectivity of the shallow conductors with a widespread conductive zone at deeper level of the upper crust. In view of the presence of Puga and Chumathang hot springs towards west and Demchok hot springs towards east of our study region, we infer these conductors to be representing a potential major geothermal reservoir system connected to the widespread deeper conductive zone of partial melts, which extends from the southern Tibet to the eastern Ladakh region.

Introduction to the magnetotellurics special issue of exploration geophysics

The geophysical technique of magnetotellurics (MT) was first described in 1953 and the associated technique of geomagnetic depth sounding (GDS) in 1959. Both methodologies have had a long history in Australia both for academic research and resource exploration. Since the early 1990s with advances in digital data storage, 2D inversion codes and 3D inversion codes from the 2010s, there has been a rapid increase in the number of MT surveys across Australia. Regional MT programs around Australia and since 2013 the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) have steadily mapped the 3D resistivity of the Australian continent. To complement the regional AusLAMP MT sites numerous additional transects, and broadband MT grids have been deployed providing remarkable new insights into the tectonic evolution of the Australian lithosphere and provide the link to crustal mineral resources. In this special edition, ten papers explore different aspects of MT research in Australia over the last decade.

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.

Contribution of Magnetotelluric inversion and potential field modeling to map the Gour Oumelalen deep geological Structure, Central Hoggar, Algeria

The Gour Oumelalen area is situated in the northern part of the Egere-Aleksod terrane (Central Hoggar, South of Algeria). This area is one of the best preserved Palaeoproterozoic and Archaean zones in Central Hoggar. The aim of this study based on a survey of 33 Magnetotelluric (MT) soundings, aeromagnetics and gravity data, is to investigate the crustal structure and its architecture beneath this area.As the first stage, we have determined dimensionality of the regional electric conductivity structure. For this purpose, we have used the impedance and phase tensor approaches. Our results suggest that the underling regional conductivity structure is complex. In the central part it presents a 3D geometry, and beyond, it is mainly 2D; with a strike direction oriented in the North-South direction. Thus, the MT data were inverted using the Occam's inversion algorithm (v.3); which is a powerful tool to modeling and inverting the 2-D MT data. The obtained geo-eletric models, show clearly a high resistive upper crust with a resistivity up to 1000 Ω m; overlies a conductive lower crust with a resistivity approximately less then 100 Ω m. Although, our resulting cross-sections confirm the major Precambrian faults, especially the Ounan shear zone, which is characterized by a high conductivity anomaly.At the second stage, using the joint modeling of gravity and magnetic data, we have characterized the different geological units. Although, our modeling reveals that the thicknesse of the upper crust is deeping toward the East, from 16 to 25 km. The Moho depth reaches an average value of 40 Km. Moreover, in the centre of the survey area, the Moho depth decreases and becomes equal to 20 Km. This uplifting of the upper mantle may corresponds to the high magnetic anomaly of the Tisseliline pluton. It seems also related to the magmatic intrusion along the Ounan Shear Zone.

Fuzzy c-mean (FCM) integration of geophysical data from an iron-oxide copper gold (IOCG) deposit under thick cover

Geophysical methods depend on a range of physical and chemical mechanisms, and each method has different sensitivities and resolution of Earth parameters, and over different scale lengths. Additionally, such geophysical methods will also have their own statistical characteristics. Thus, it is a significant challenge to combine different methods through a single inversion framework. Rather than attempting to find an optimal and single model for different geophysical responses, an alternative approach is to use FCM clustering to identify clusters of parameters from two or more different geophysical data sets or models that have similar statistical properties. In this paper, we apply the FCM approach to integrate data and model sets for an array of 100 broadband magnetotelluric (MT) and 100 passive seismic receivers spaced 1 km apart on a 10 by 10 grid above the Vulcan IOCG prospect in the Olympic Cu–Au Province, southern Australia. The challenge for exploration of the Vulcan prospect is that it lies beneath 750 m of regolith sedimentary cover, no single geophysical method provides a unique characterization of the deposit geometry, and drilling is very expensive. Fuzzy c-mean cluster analyses are undertaken in 2D for gravity data and shear-wave velocity model data for basement depths beneath cover and in 3D for shear-wave velocity and electrical resistivity model data. Fuzzy c-mean clustering is shown to provide a simple and efficient method of integrating different geophysical measurements to produce a geological framework that can be verified with drilling information.

Subsurface structural trends in Central-Western India: Inferences from magnetotelluric data using the complex MT apparent resistivity tensor

The central-western part of the Indian subcontinent exhibits significant geological diversity and possesses a rich record of different tectonothermal events in the earth's evolutionary history, spanning from the Archean to the present. Precambrian geology in the region is mostly obscured by Cretaceous-Tertiary/Paleocene volcanic eruptions and Proterozoic to Quaternary/recent sedimentary cover. Magnetotelluric (MT) impedance and tipper responses from 146 stations, covering central-western India in a grid fashion with a spacing of ∼55 km, were analyzed to evaluate the subsurface structural trends, dimensionalities, and qualitative characteristics of the electric lithosphere in the region. An advanced Complex Apparent Resistivity Tensor (CART) approach, along with the popular Phase Tensor (PT) approach and induction arrows are utilized to achieve the above goals. The analysis revealed a high degree of 3D induction effects in the data at almost every station and period. The directionality information retrieved from the tensor properties showed that the structural trends in the deep crust and upper mantle of the major geologic domains correlate with the known surface tectonic trend of the respective geologic domain. This study provides vital evidence to support the suspected bifurcation of the Precambrian Aravalli-Delhi Mobile Belt (ADMB) tectonic trend and its extensions into neighboring Kutch, Saurashtra, and Central Indian Tectonic Zone (CITZ) domains. The analysis results indicate a lithospheric scale enhanced conductivity beneath the Malwa plateau, which marks the first major phase of the Reunion mantle plume eruption in India. A critical finding of this study is that the Resistivity Phase Tensor better defines the subsurface resistivity structure and provides much clearer structural information than the conventional Phase Tensor.

An assessment of controlled source EM for monitoring subsurface CO2 injection at the wyoming carbonSAFE geologic carbon storage site

We evaluate if electromagnetic (EM) geophysical methods for monitoring geologic carbon storage (GCS) efforts at the Wyoming CarbonSAFE project adjacent to the Dry Fork Station power plant near Gillette, Wyoming. This first involved acquiring both electric and magnetic fields at eleven different locations ranging in distance from immediately adjacent to 4 km from the plant. Passive EM measurements were made to provide spectral EM noise measurements generated by electricity production at the plant and to determine if useful magnetotelluric (MT) data can be successfully collected in the region. The processed data indicate that useful MT data can be collected as long as the site is located more than 2km away from the power plant as well as active roads and rail lines. Controlled source EM data were collected using three different source configurations, two of which connected to steel casings used to complete the injection wells. Comparing the EM noise measurements to the CSEM data show measurable electric and magnetic field signals at all sites. Next a series of three-dimensional (3D) numerical models were built that simulate resistivity changes caused by the proposed CO2 injection at depths ranging from 2.4 to 3.0km. These models were used to simulate various EM measurement configurations. The modeling shows that casing-source CSEM monitoring can provide sensitivity to the injected CO2 if source electrodes are connected to the bottom of one or both of the injection wells.

Lithospheric resistivity structure of the Xuefeng Orogenic Belt and its implications for intracontinental deformation in South China

The Xuefeng Orogenic Belt (XFOB), located in the central part of the South China Block, is a typical Mesozoic intracontinental orogen in the central Jiangnan Orogenic Belt. By collecting magnetotelluric (MT) data across the XFOB, we obtained the resistivity structure of the lithosphere, which sheds light on the Mesozoic intracontinental orogenic processes in the XFOB. The resistivity structure reveals a low-resistivity body (<10 Ω∙m), beneath the XFOB, dipping southeast wards from a depth of 10 km to the bottom of the crust. This conductor is interpreted as a relic of the lower detachment zone, which coincides with low-density areas obtained from joint inversion of seismic models. It is believed to result from mineral fluids migrating along the thrust fault and squeezing sulfides into folds. Four low-resistivity bodies were identified at three extensional locations along the Jiangshan-Shaoxing Fault and at the Cili-Baojing Fault. The low-resistivity body (<10 Ω∙m) at the junction of the Shaoyang and the Hengyang Basin is located at the point where the Moho depth thins. The variation trend of the terrestrial heat flow values, with this low-resistivity body as the plate boundary, is consistent with the average variation of the terrestrial heat flow values within the block. We propose that the low-resistivity body under the Qidong-Yongzhou-Guilin fault conforms to the characteristics of the suture zone in the resistivity structure. Its existence indicates that the missing location of the Jiangshan-Shaoxing suture zone of the Yangtze and Cathaysia Block in the middle-southwest section of the South China Block is the Qidong-Yongzhou-Guilin fault. The Yangtze Block and the Hengyang Basin show high resistivity, the depth of which reaches 100 km and 40 km, respectively. Based on the resistivity model and geological data, the XFOB experienced Triassic compression, leading to basement decollement, thrusting, and nappe structures due to low-angle Paleo-Pacific Plate subduction. This compression also led to the uplift of the orogenic belt. Moreover, under the tension caused by the high-angle retreat of the Paleo-Pacific Plate, the Cretaceous extensional tectonics led to detachment along the thrust faults, forming half-graben and basin structures along the margins.

Misidentified subglacial lake beneath the Devon Ice Cap, Canadian Arctic: a new interpretation from seismic and electromagnetic data

Abstract. In 2018 the first subglacial lake in the Canadian Arctic was proposed to exist beneath the Devon Ice Cap, based on the analysis of airborne radar data. Here, we report a new interpretation of the subglacial material beneath the Devon Ice Cap, supported by data acquired from multiple surface-based geophysical methods in 2022. The geophysical data recorded included 9 km of active-source seismic-reflection profiles, seven transient electromagnetic (TEM) soundings, and 17 magnetotellurics (MT) stations. These surface-based geophysical datasets were collected above the inferred locations of the subglacial lakes and show no evidence for the presence of subglacial water. The acoustic impedance of the subglacial material, estimated from the seismic data, is 9.49 ± 1.92 × 106 kg m−2 s−1, comparable to consolidated or frozen sediment. The resistivity models obtained by inversion of both the TEM and MT measurements show the presence of highly resistive rock layers (1000–100 000 Ω m) directly beneath the ice. Re-evaluation of the airborne reflectivity data shows that the radar attenuation rates were likely overestimated, leading to an overestimation of the basal reflectivity in the original radar studies. Here, we derive new radar attenuation rates using the temperature- and chemistry-dependent Arrhenius equation, and when applied to correct the returned bed power, the bed power does not meet the basal reflectivity threshold expected over subglacial water. Thus, the radar interpretation is now consistent with the seismic and electromagnetic observations of dry or frozen, non-conductive basal material.

Low resistivity anomalies in the upper crust of the Midlands of Tasmania from combined magnetotelluric datasets

Heat flow data obtained in connection with geothermal resource exploration suggests anomalous upper crustal structure and processes in parts of central east Tasmania. The regional scale crustal geology of the Midlands of Tasmania is, however, mostly obscured at the surface by the Permo-Triassic sedimentary sequences of the Tasmania Basin together with extensive exposures of Jurassic dolerite. We investigate controls on undercover crustal processes in this region by combining long period and broadband magnetotelluric (MT) datasets in 3D inversions for the geoelectric structure; followed by an interpretation that is informed by aspects of the pre-existing 3D regional geological and geophysical model. The new 3D model allows improved resolution of low resistivity anomalies together with a qualitative appraisal of spatially variable model sensitivity. The most robust features ( < 1 Ω m ) in the 2–3 km depth interval occur where N–S and E–W faults intersect with a high point in the topography of the upper surface of a deep seated granite body. Enhancement of conductivity in this zone by clay, graphite or mineralisation, or a combination thereof, is likely. Other low resistivity features suggest that conductive pathways exist where major or multiple faults are present. These interpretations provide support for continued exploration in the Midlands of Tasmania for a variety of resources related to crustal fluids and fracturing.