Audiomagnetotelluric Research Articles

Integration of geophysical methods for groundwater exploration: A case study of El Sheikh Marzouq area, Farafra Oasis, Egypt

Newly, the Egyptian government initiated huge national projects for the reclamation and sustainable development of 1.5 million acres. Farafra Oasis is recognized as the focus of the first phase of this project where groundwater is the wellspring of the water for irrigation and drinking. Hence, exploration and assessment of groundwater aquifers are very essential in such area. A reconnaissance geophysical survey was conducted in the centre of Farafra oasis using Vertical Electric Sounding (VES), Transient electromagnetic (TEM), Audio-Magnetotelluric (AMT), Control-source Magnetotelluric (CSAMT), and Very Low Frequency (VLF) techniques for the investigation of shallow and deep groundwater reservoirs. In addition, geologic and stratigraphic information from the nearest well was utilized to confirm the geophysical outcomes. Integrated geophysical measurements succeeded to delineate the three zones A, B and C of the Nubian aquifer system in the investigated area. Also, several fault elements were delineated which control the influx and aggregating of the groundwater in both units; shallow and deeper one. Deeper aquifer (zone C) shows generally higher resistivity values, which consider the major aquifer of Nubian sandstone in the Western Desert of Egypt. Geochemical examinations of the groundwater sample show that the water is remarkable for all agriculture purposes (TDS = 95).

A Comparison of impedance invariants and GB decomposition method for magnetotelluric data analysis; a case study from shallow sedimentary basins of SW Iran

Dimensionality and distortion analysis of the measured magnetotelluric (MT) data are challenging procedures to conclude the main properties of regional geo-electric structure (e.g., its strike direction) and quantify the influence of superficial distorting bodies. In this paper, we applied several analysis methods to the same data set from southwest Zagros to determine the dimensionality and degree of the inherent distortion in the data and to recover the regional responses. We conclude that the regional 2-D responses can be better retrieved in a two-step procedure: we first classify the dimensionality and distortion by impedance invariants and then the Groom-Baily (GB) method is applied to remove distortion effects and estimate the regional strike and principal impedances.We investigate an Audio-magnetotelluric (AMT) data set acquired at 19 stations along a profile in the southwest of simply folded belt (SFB) of the Zagros Mountain. The Bahr and WAL invariants confirm regional 1D and 2D structures with local galvanic distortion, at most periods. The Groom-Bailey (GB) decomposition of MT impedance data reveals approximately period-independent distortion parameters. The strike analysis on all data whose principal phase differences are greater than 5˚ reveals a regional trend of about 30˚. 2-D inductive effects were also retrieved by removing distortion effects from the measured data

Deep learning audio magnetotellurics inversion using residual-based deep convolution neural network

In this study, we developed a novel 18-layers residual full convolutional neural network (18RFCN) for audio magnetotellurics (AMT) data inversion. Different from traditional inversion methods, the 18RFCN-based inversion algorithm is based on big-data training rather than prior-information assumptions, and the prediction is almost no time-consuming after network training. The problem of gradient disappearance was solved by adding shortcut connections when training deep neural networks, and we proposed two novel methods to quickly generate millions of samples for network training to improve the network generalization. We first tested on four synthetic data sets with Gaussian noises, which are entirely different from the data of sample sets, and the inversion results showed that the deep learning algorithm could accurately recover the underground conductivity structure almost no time-consuming compared to the Gauss Newton (GN) algorithm. We further tested the inversion algorithm on a field AMT data set acquired in Qinghai province, China. The inversion results of the deep learning algorithm agree well with the GN algorithm and the drilling data.

Tectonic Setting of the Kenya Rift in the Nakuru Area, Based on Geophysical Prospecting

In this paper, we present results of tectonic and geophysical investigations in the Kenya Rift valley, in the Nakuru area. We compiled a detailed geological map of the area based on published earlier works, well data and satellite imagery. The map was then integrated with original fieldwork and cross sections were constructed. In key areas, we then performed geophysical survey using Electrical Resistivity Tomography (ERT), Hybrid Source Audio MagnetoTelluric (HSAMT), and single station passive seismic measurements (HVSR). In the study area, a volcano-sedimentary succession of the Neogene-Quaternary age characterized by basalts, trachytes, pyroclastic rocks, and tephra with intercalated lacustrine and fluvial deposits crops out. Faulting linked with rift development is evident and occurs throughout the area crosscutting all rock units. We show a rotation of the extension in this portion of the Kenya rift with the NE–SW extension direction of a Neogene-Middle Pleistocene age, followed by the E–W extension direction of an Upper Pleistocene-Present age. Geophysical investigations allowed to outline main lithostratigraphic units and tectonic features at depth and were also useful to infer main cataclasites and fractured rock bodies, the primary paths for water flow in rocks. These investigations are integrated in a larger EU H2020 Programme aimed to produce a geological and hydrogeological model of the area to develop a sustainable water management system.

Three-dimensional electromagnetic method in shallow surface exploration in complex piedmont belt

In the complex piedmont area, the quality of seismic data is generally poor, and it is difficult to build the shallow velocity model, which leads to the inaccurate determination of the stratigraphic and structural positions. The combination of seismic and non-seismic exploration is an effective way for oil and gas exploration in complex piedmont belt. In the northwest of Sichuan Province, the joint exploration of three-dimensional audio-magnetotelluric (AMT) and transient electromagnetic (TEM) is carried out. The transient electromagnetic data is used as static correction for audio-magnetotelluric data, and the AMT data is used to detect the shallow surface structure within 1.5km buried depth. The three-dimensional geoelectric model of the study area is obtained by using AMT three-dimensional inversion, and the geological structure interpretation is carried out by combining well logging with the statistical data of rock physical properties, which provides a basis for seismic building the shallow surface model.

Inversion of 1D audio magnetotelluric data based on residual convolution neural network

The 18-layer residual convolution neural network (ResNet18) were used for 1D audio magnetotelluric(AMT) data inversion. In order to avoid the dependence of the traditional iterative algorithm on the initial model and calculate sensitivity matrix, we have trained ResNet18 via providing model parameters instantaneously. The residual network was used to solve the problem of deep network gradient disappearing. Deep network and lots of sample data could improve the generalization of the network. The experimental results showed that it could obtain reliable inversion results for synthetic AMT data.

Geophysical model of Cuenca de la Independencia aquifer

Cuenca de la Independencia (CI) is a semi-arid basin in Central Mexico, subdivided into four administrative hydrogeological regions, and comprised of two hydrogeological units: a granular aquifer of lacustrine origin overlying a fractured acidic igneous aquifer. Although many geophysical soundings were carried out within the basin, no geophysical data exceeding 500 m of depth has ever been reported; the actual conceptual models were generated only from shallower geophysical studies, geological and hydrogeochemical mapping. We performed a geophysical survey in CI to determine the geometry of the hydrogeological units through Audio-Magnetotellurics (AMT), Time Domain Electromagnetics (TDEM), and Vertical Electrical Soundings (VES). Since the acquired data set from the different applied geophysical methods are multidimensional and band-limited, some well-known empirical relations between the applied methods were used to improve the subsurface imaging, turning data into broadband-integrated and space-time scaled into the MT frequency domain. We present a methodology on how to handle this integrated information through a 2-D inversion approach.The results reveal that, even though there are two hydrogeological units, they could not be vertically distinguished, thus is not possible to refute the previous idea of direct vertical communication between the two aquifers. Moreover, a clear lateral transition in resistivity models was not found, so the administrative division of CI would only obey political-administrative purposes.

Imaging strategies to interpret 3-D noisy audio-magnetotelluric data acquired in Gyeongju, South Korea: data processing and inversion

SUMMARY The largest earthquake recorded in South Korea occurred in Gyeongju on 2016 September 12. Since then, understanding the geological structures near the Gyeongju earthquake's epicentre has been demanded. As a preliminary step to understand the geological structures around the epicentre, a 3-D audio-magnetotelluric (AMT) survey was conducted. These 3-D AMT data were imaged through data processing and inversion. However, because the AMT data acquired in Gyeongju were very noisy, conventional imaging procedures did not yield reliable results. To obtain a reliable model inverted from such noisy data, we propose various imaging strategies: an additional data processing technique using the Nyquist diagram after conventional data processing and several inversion strategies related to the selection of data, the weighting of the data, the constraints on the model parameter vector and the Lagrange multiplier used for the regularization. By applying the additional data processing step and several inversion strategies, we were able to successfully invert the noisy field data. The inversion results verify that the data-screening procedure applying the Nyquist diagram remarkably improves the results compared with those obtained using only conventional data processing. The imaging strategies proposed in this case study can be used to image noisy MT/AMT field data for other regions.

3D inversion of audio-magnetotelluric data for mineral exploration: A case study of Layikeleke buried porphyry copper deposit, Xinjiang, China

Layikeleke copper deposit is a large-scale porphyry copper polymetallic buried deposit, which has been discovered recently in the eastern Junggar area of Xinjiang. Mineralization occurred in the Late Silurian and Early Devonian with the copper body mainly confined to tonalite. To find new ore bodies, it is necessary to map the 3D spatial distribution of the tonalite. For this reason, we deployed audio-magnetotelluric (AMT) sounding in the research area, collected 26 profiles and 1198 measuring points, carried out 3D inversion calculations of off-diagonal elements in the impedance tensor data using the nonlinear conjugate gradient method, and obtained a 3D electrical structure model. The electrical structure reveals two groups of NW-SE trending middle and middle low resistivity anomaly zones, which intersect the high resistivity bedrock area in the northwest of the study area. Based on the electrical data, it can be inferred that the medium low resistance tonalite in the northern part of the study area is the host rock of the porphyry copper polymetallic deposit, which has been verified by borehole data. The results of 3D AMT inversion depict the spatial distribution and depth variation characteristics of the tonalite electrical structure model, providing a basis for further prospecting and exploration. Therefore, this method of identifying lithology by 3D inversion can provide an important basis for mineral exploration and mining of buried rock masses, and is an effective prospecting method.

Processing and Interpreting the Data of Deep Frequency Sounding in a Complex with Audiomagnetotelluric Measurements (Murman-2018 Experiment)

This paper aims to study the deep electrical conductivity of the Murmansk block (Fennoscandian Shield) using the controlled source audio-frequency magnetotelluric (CSAMT) and audiomagnetotelluric (AMT) methods. One of the most important tasks in deep sounding studies implying the joint application of the AMT and CSAMT methods is to take into account the influence of surface conductors on the measurement results (static shift). To solve this problem, we develop a methodology for processing and interpreting the measurement data of the CSAMT and AMT system using the results of the deep sounding experiment conducted in the frequency range from 1 to 500 Hz performed in 2018 (Murman-2018) as an example. Measurements with the controlled source in the form of two mutually orthogonal horizontal electric dipoles are taken at distances of 12 to 105 km. The generator-measuring complex consisted of the VMTU-10 measuring station (manufactured by OOO VEGA, St. Petersburg) and the Energy-4 generator (developed at the Kola Science Center, Russian Academy of Sciences, Apatity). Using the synchronous time series of the field components at the observation points and the current in the supply dipole, the power spectra of the autocorrelation and cross-correlation functions of the recorded values are calculated based on the fast Fourier transform (FFT). The obtained spectral characteristics are used (i) to determine the amplitudes of the source field’s components and the phase shifts between them, and (ii) to evaluate the components of the AMT field’s impedance tensor. The values of the apparent resistivity are calculated using the amplitudes of the horizontal components of the electromagnetic field and the impedance ratios. The obtained measurement results are corrected for the static shift. For this, the correction coefficients are calculated from the ratio between the apparent resistivity values for the horizontal component of the magnetic field and the apparent resistivity values inferred from the impedance and electrical field values. The data for the cases of axial and equatorial arrays, together with the data of the AMT and remote sounding, made it possible to establish a high degree of lateral homogeneity of the deep electrical section and to expand the frequency range of the obtained curves of the apparent resistivity and impedance phase. A one-dimensional interpretation is made to obtain estimates of the distribution of the deep resistivity.

Detecting fracture zones in granite with seismic and audio-magnetotelluric methods

Abstract Geological disposal is a feasible and safe method for dealing with the high-level radioactive waste problem at present. The Beishan area is the key area preselected for high-level radioactive waste geological disposal in Gansu Province, China. The Jijicao rock block is currently the most extensively studied area, where there are several fracture zones in borehole BS15. In 2011, a remarkable conductive anomaly near the BS15 was detected using a 4-km-long audio-magnetotelluric (AMT) profile with 50 m station space crossing the borehole. To study the anomaly, a short seismic survey was shot along the part of the AMT profile. This paper presents an example for detection of the fracture zones in granite using AMT and seismic methods at Beishan. The AMT data inversion model agrees with the borehole well-logging data and weak reflections are related to the fractured zones. The seismic detection may help in interpreting the cause of the conductive anomaly, and the conductive anomaly could determine whether there are fracture zones within the weak reflection areas. The simultaneous surveys can complement each other for detecting fracture zones. Besides this, such joint detection may be used to estimate whether there exist aquiferous fracture zones at depth.

INTERPRETASI GEOFISIKA BIDANG GELINCIR PALEO-ROCKSLIDE PARANGTRITIS

Parangtritis has a rock fall hazard area with a medium to high level of risk. In Parangtritis, there was a large scale paleo-rockslide that involved more than 800 million m3 of rock mass with an area of 1500 m x 2700 m. This is a significant factor in changing the landscape. This research uses 9 Audio-MagnetoTelluric (AMT) primary data points. Using the distance between points 750 m within a 2 km x 2 km study area. The location focused on the western side of the paleo-landslide area in Parangtritis Village. Processing AMT data using the help of MT-INV and RockWorks software. The result of one-dimensional inversion consists of 3 layers. Indicates a low resistivity value (0-10 Ωm) interpreted as beach deposition and sand dune, medium resistivity (50 - 160 Ωm) for Wonosari limestone and high resistivity ( 170 Ωm) for Nglanggran andesite breccias. The average thickness of the first layer is 44 m and the second layer reaches 175 m. Based on the resistivity structure of the AMT data, the interpretation of the boundary contact between the Wonosari limestone and the Nglanggran andesite breccia can act as a slip field in a depth of about 250 m This field is a passive fault field controlled by the Girijati fault in the east of the study area.

Synthetic modelling of downhole resistivity data to improve interpretation of basin morphology from magnetotelluric inversion

Prospective Proterozoic units in the southern Mount Isa Province are concealed by a poorly defined extent of younger basin cover, leading to poor exploration success. Collection of a magnetotelluric (MT) survey in the area containing 809 broadband MT (BBMT) and 855 audiomagnetotelluric (AMT) stations in 2014–2015, offers an opportunity to better model the depth to basement to enable effective exploration. MT inversion models are inherently non-unique, requiring independent geophysical and geological constraint to reduce model uncertainty. Where data are not available to constrain inversion, alternative approaches to dealing in inversion variability are required. This study uses synthetic modelling based on well data combined with two kinds of inversion to generate an interpretation and quantify associated uncertainty. Downhole resistivity logs were obtained from three petroleum wells adjacent to the study area, and 1D resistivity models were generated from the downhole data. A suite of 1D and 2D MT inversion algorithms were tested to determine their ability to resolve basin layering and the basement interface. All inversion algorithms reproduced basin layering, but the basement interface was poorly resolved. A combination of Occam2D and 1D rjMcMC inversions were used to produce interpretation of the base of the Eromanga Basin, an intra-Georgina Basin low-resistivity layer and depth to basement, all of which have associated error estimates. This work highlights the importance of understanding inversion variability during interpretation of geological features, particularly in the absence of constraining information. Distribution of uncertainty between the interpretation features is significantly non-uniform, necessitating careful consideration of inversion results. By quantifying uncertainty rather than ignoring it, we produce an interpretation commensurate with data limitations that still provides valuable new information about the geology of the southern Mount Isa Province.

Electromagnetic characterization of epithermal gold deposits: A case study from the Tuoniuhe gold deposit, Northeast China

A high-resolution electromagnetic study has helped to define the mineralization and alteration system of the Cretaceous volcano-sedimentary hosted epithermal gold (Au) deposit in Tuoniuhe, northeast China. Audio-magnetotelluric (AMT) array data were acquired to map the regional resistivity structure of the Mesozoic volcanic field, whereas an AMT profile and a ground magnetic survey line with denser site spacing were deployed across the deposit to image the alteration and mineralization system. The electrical resistivity model from 2D inversion of the AMT profile data reveals a low-resistivity (approximately [Formula: see text]) cover from the surface to a depth of 0.1 km, which is likely caused by clay and sulfide minerals in the subaerial alteration zone. The magnetic survey and a geologic borehole log assisted in outlining a zone of tonalite and andesite with silicification in the depth interval of 0.1–0.3 km, featuring high resistivity ([Formula: see text]) and high magnetization ([Formula: see text]). This zone is a potential gold target bounded by two channels of moderate resistivity (approximately [Formula: see text]) to its northwest and southeast. The two channels possibly coincide with breccia pipes with fractured stockworks and high permeability to allow gold-bearing fluids to move toward the surface. The 2D and 3D resistivity models reveal regions of low resistivity ([Formula: see text]) at the depth range of 0.5–1.0 km beneath the Cretaceous calderas and the deposit, which might be related to magmatic cryptoexplosion breccia. In the 2D resistivity model, this magmatic cryptoexplosion breccia zone connects to the subaerial alteration zone through the two breccia pipes, indicative of a circulation system of gold-bearing fluids. Given the coincidence of Cretaceous volcanism and the age of mineralization, the Cretaceous magma is inferred to have supplied heat that drove the convective hydrothermal activity and also was a source of magmatic fluids that led to the development of the Tuoniuhe epithermal gold deposit.

Application of audiomagnetotellurics to delineate an iron deposit in the Liguo area, eastern China

A detailed study of the geological and electromagnetic structure of the peripheral area of Liguo iron mine, eastern China, is presented to assess the location and amount of iron ore. This study targeted a strong magnetic anomaly in the Lixi area, west of Liguo, which is located in the northern Yuhuai platform-fold-belt between the Sihu-Jing'an depression and the Xuzhou complex anticline. We carried out a series of audiomagnetotellurics (AMT) prospecting lines to delineate the deposit and assess its potential ore reserve. An integrated analysis of the magnetic anomaly, AMT resistivity, and drill core identified the areas with the great potential for iron ore mineralization. Comparation between magnetic anomaly and the resistivities can reduce the variability of singular geophysics method. The results provide important geophysical evidence regarding the potential iron-ore resources of the Liguo area.

Identification of the structural trap of the hydrocarbon reservoir below volcanic rocks in Majalengka region, Indonesia, using the audio-magnetotelluric method

The exploration of hydrocarbons below volcanic rock is a significant challenge in increasing oil and gas reserves in Indonesia. Because mapping using seismic waves does not produce data of good quality in areas of volcanic rock, in this study, the audio-magnetotelluric (AMT) method was used to map the structural trap of a hydrocarbon reservoir. Geophysical surveys were also used to determine the subsurface resistivity values and phase values. The raw data were in the form of time series measurement obtained using equipment from Phoenix Geophysics. The data were processed further into apparent resistivity and phase curves toward frequency. Data processing involved various filtering methods and corrections. The result was a two-dimensional cross-section of the AMT measurement line. The data from the AMT model were interpreted in an integrated manner using geological data. The results showed a relatively good relationship between the AMT data and the geological data. The results indicate a fault zone in the area of measurement and bedding formations that have produced a petroleum system. The lower part of the Cinambo Formation, which consists of shale-dark, acts as a source rock. While the upper part of the Cinambo Formation which consists of massive sandstone with very fine sand to silty above the shale-dark is thought to be a reservoir. The hydrocarbon trap could be of the structural type because there is a fault zone in the area of measurement.

Application of pseudo-random frequency domain electromagnetic method in mining areas with strong interferences

Abstract Due to the strong electromagnetic interferences and human interference, traditional electromagnetic methods cannot obtain high quality resistivity data of mineral deposits in Chinese mines. The wide field electromagnetic method (WFEM), in which the pseudo-random signal is taken as the transmitter source, can extract high quality resistivity data in areas with sever interference by only measuring the electric field component. We use the WFEM to extract the resistivity information of the Dongguashan mine in southeast China. Compared with the audio magnetotelluric (AMT) method, and the controlled source audio-frequency magnetotelluric (CSAMT) method, the WFEM can obtain data with higher quality and simpler operations. The inversion results indicate that the WFEM can accurately identify the location of the main ore-body, which can be used for deep mine exploration in areas with strong interference.

Exploration of permafrost with audiomagnetotelluric data for gas hydrates in the Juhugeng Mine of the Qilian Mountains, China

In China, gas hydrates in onshore permafrost areas have so far only been found in the Juhugeng Mine of the Qilian Mountains. However, their subsurface distribution remains unclear. Electrical resistivity logs have revealed that zones containing gas hydrates have higher resistivity than surrounding zones, which makes electromagnetic methods viable for detecting gas-hydrate deposits. We have deployed a natural-source audio-magnetotelluric (AMT) survey at the Juhugeng Mine. AMT data were collected at 176 sites along five profiles, and resistivity models were derived from 2D inversions after detailed data analysis. After the available geologic and geophysical observations were combined, the inversion results from profile 1 suggested that permafrost near the surface with high resistivity and thickness is essential for underlying gas hydrates to be present. The decrease in resistivity and/or thickness of permafrost due to climate change may lead to gas-hydrate dissociation. The other four AMT transects suggested three prospective gas-hydrate sites. Our results indicate that the AMT survey technique is suitable for exploring gas hydrates in permafrost areas and analyzing the impact of permafrost characteristics on gas-hydrate occurrence.

Investigation of auriferous mineralisation over greenstone schist belt of Dhanjori Basin, Eastern India using 2D electrical resistivity tomography and induced polarisation surveys

Dipole–dipole resistivity and induced polarisation (IP) surveys were conducted over a conductor identified from an audiomagnetotelluric (AMT) survey run in a greenstone schist belt of the Dhanjori Basin, Eastern Singhbhum, India. The shallow conductors identified in the AMT were not well resolved due to insufficient high-frequency data. The geology of the area suggests that gold mineralisation is associated with quartz pebble conglomerates (QPC) or sulfides. The resistivity and IP surveys were conducted over an area of 1.2 × 0.4 km with an electrode spacing of 10 m and a line spacing of 50 m. This data set was inverted using Gauss–Newton optimisation. Based on the resistivity and IP results, responses were categorised into five distinct geological/geophysical signatures defining local lithology and associated mineralisation. Low resistivity and low to moderate chargeability at depths up to 20 m from the surface indicate the weathered zone (which is not a strong feature in some of the IP models). High resistivity and high chargeability zones are identified as quartz vein/QPC with disseminated sulfide ± auriferous concentrations. Moderate resistivities and chargeabilities are interpreted as being related to the greenstone schists that make up the major part of the geology in the Dhanjori Basin. Zones with high resistivity and low chargeability are interpreted as Proterozoic gabbroic anorthosite ultramafics (PGAU) and/or younger dolerites. PGAU is intrusive to the local greenstone schist/phyllites; we have observed that its resistivity is considerably high and there is no significant gain in its chargeability due to the hindrance in ionic and electronic movement in the formation. Low resistivity and low to moderate chargeability zones below the weathered layer are attributed to mineralisation associated with massive sulfides. The integrated geophysical study indicates the presence of quartz vein/QPC and sulfide hosting auriferous mineralisation over the greenstone schist belt of the Dhanjori Basin, Eastern Singhbhum.

AMT survey in the Outokumpu ore belt, eastern Finland

ABSTRACTThe Outokumpu belt in eastern Finland is known for numerous polymetallic (Cu–Co–Zn–Ni–Ag–Au) sulphide ore deposits and it is a potential area for new discoveries. Audiomagnetotelluric (AMT) data have recently been collected to image the subsurface conductivity structure of the belt. The data were acquired along five profiles aiming to explore deep conductors, study their geometry and investigate the deep conductivity structure of the belt.The acquired AMT data were mostly adequate for 2D inversion, as the survey area is characterised by thin, laterally extensive conductors indicated by airborne electromagnetic data, deep drillings and regional strike analysis of the impedance tensor data. 2D smooth inversion was carried out jointly for transverse electric and transverse magnetic data. The results show southeast-dipping and sub-horizontal continuous conductors southeast of the Outokumpu old mine, suggesting the presence of prospective Outokumpu assemblage rocks. One ∼ 1 km deep sub-horizontal conductor is verified by a deep drill-hole located ∼ 8 km southeast of the mine. The results demonstrate a similar southeast-dipping structure in the Vuonos mine area, which is located ∼ 7 km northeast of the Outokumpu old mine. An eastwards-dipping conductor detected in the Miihkali serpentinite area indicates deep exploration potential in this area. Besides the smooth 2D inversion, a sharp-boundary 2D inversion was performed and the resulting model was assessed with the existing geological interpretation of the Miihkali area. In the area of the Sotkuma gneiss inlier, conductors are absent from the uppermost ∼ 6 km. This feature may represent an uplifted fault block rather than a thin thrust sheet of Archaean basement rocks.