Search

The North China Craton is the oldest continental block, and has suffered from large-scale lithospheric thinning and destruction, which in turn led to gold deposits in northern China. The decratonic gold deposits in the North China Craton became the most important gold deposits in China, and geophysical methods are key means to detect and discover gold deposits there. In this paper, based on the geological and petrophysical characteristics of the North China Craton, the geological model of the decratonic gold deposits is transformed into a geophysical model. At present, two methods of geophysical exploration of decratonic gold deposits are in use: rapid and efficient exploration on the scale of the ore concentration area, and large depth exploration on the scale of the deposit area. In detail, the airborne electromagnetic, magnetic and gravity methods are used to detect the shallow (1,500 m) anomaly area on the scale of the ore concentration area. Through the ground-controlled source electromagnetic and ground magnetotelluric methods, explorations for targets at significant depth (5,000 m) are carried out in the mining area. Then, taking the Liaodong ore concentration area as an example, geophysical methods are used to discover two prospecting areas around the Jianshanzi Fault in the Qingchengzi ore concentration area, Baiyun-Xiaotongjiapuzi deep prospecting area, and Qingchengzi deep prospecting area. Next, three prospecting areas are delineated around the Jixingou Fault in the Wulong mining area, Wulong deep prospecting area, Weishagou deep prospecting area, and Chang’an deep prospecting area. The anomalies in the ore concentration area and mining area are revealed by means of three-dimensional exploration methods, thereby providing technical support for the exploration of metal minerals such as decratonic gold deposits.

Geophysics offers an important means to investigate the physical processes occurring inside the earth. In particular, since the 1960s, electromagnetic (EM) methods have played important roles in mineral exploration and engineering investigation. Such investigation requires extensive data acquisition and experimental analysis based on geophysical techniques. However, high-performance geophysical equipment, particularly EM exploration equipment, has been dominated by large geophysical companies from the United States, Canada, Germany, and other European countries for decades. This has limited the development of deep exploration technology in China. Recently, we have developed a high-resolution acquisition system with a wireless control unit and a high-power transmitting system for surface EM prospecting (SEP). The new system has been tested in the high-intensity, noisy environment in Jian-sanjiang area, Heilongjiang Province. We then conducted a field survey on the western edge of Qaidam Basin, Qinghai Province. A highly conductive anomaly was found in the upper mantle below the Qinmantage Mountains, which indicates a possible northward fluid channel that runs from below the Qinmantage Mountains to the bottom of the western crust of the Qaidam Basin. Identification of this significant feature was made possible by the new SEP for its better resolution than the previous systems. Also, geophysical analysis confirmed that the thick Cenozoic sediments of the Qaidam Basin are underlain by rigid Precambrian basement rocks and are characterized by a series of folds. The resistivity profile indicates that the Qaidam Basin was formed due to the folding structures in the northern part of the Qinghai-Tibet Plateau which provided an additional evidence for the uplifting of the Qinghai-Tibet Plateau.

The Yinchuan Graben (YG) is situated in the transitional area between the Tibetan Plateau, Ordos Block (OB) and Alxa Block (AB). The dynamic mechanism of the YG is controversial because of its unique tectonic environment. This study used a broadband magnetotelluric profile constrained by high-density point spacing to reliably obtain the deep electrical structure across the Helanshan Orogenic Belt and YG and to discuss the dynamic mechanism of the YG. The main research results are as follows. 1) Our profile and previous seismic profiles show a low-resistivity and low-velocity body in the deep YG, which extends upward and cuts through the bottom boundary of the upper crust, and all results indicate that this body may be the result of partial melting due to upwelling mantle material. 2) Our profile also shows a high-resistivity anomaly in the middle-lower crust of the Helanshan orogenic belt, which can be interpreted as intrusive rocks formed by the emplacement of partially melted materials. 3) The Helanshan east piedmont fault dips eastward and intersects with the Yellow River fault at the depth of 20 km, and the typical structure of the intersection may be the seismogenic structure of the Pingluo Ms8.0 earthquake. 4) The evolution of the YG may have been controlled by various dynamic mechanisms, and the initial formation of the YG may have resulted from the upwelling mantle material beneath the northern OB; the evolution of the YG since 10 Ma may also have been affected by lateral mantle flow from the Tibetan Plateau to the gap between the OB and AB.

The Santa Lucia River Basin Trough is located in the southern sector of the Eastern Republic of Uruguay and has an area of almost 6,000 km2. In this work, the depth to the basement in the SW sector of the Santa Lucia Basin (SLB) was determined, using geophysical methods of exploration. The interest of exploring this geological basin is to determine its depth, recognizing the possible existence of deep aquifers, and the feasibility of hydrothermalism. For this reason were acquired, processed, inverted, and interpreted 10 magnetotelluric (MT) soundings. A 3D inversion based on the ModEM code was performed, and a 2D inversion (NLCG) of the invariant determinant along a profile that crosses the study area was carried out; their comparison yields similar resistivity and structural models results. The resistivity modeling was complemented with a Bouguer anomaly map created from a gravimetric database of 157 stations. Our geophysical results indicate that the Santa Lucia trough deepens toward the E-NE, reaching sedimentary thicknesses that may exceed 950 m. Outside the studied sector, its depth increases according to the gravity minimum, toward the NE of the area. Therefore, sandy or deep fractured episodes could harbor hot springs, whose temperature, considering the normal geothermal gradient of 30 °C/km, would approximately be in the range of 36–40 °C.

Zhongshan Station (69° 22′24.76 ′′s, 76°22′14.28′′E) is located in the Westoden Peninsula, Larsemann Hills area, Princess Elizabeth Islands, East Antarctica. Here, we report results from a magnetotelluric experiment and delineate the deep electrical conductivity structure beside the Zhongshan Station by using the data acquired during the 36th Chinese Antarctic Scientific Expedition. Magnetotelluric (MT) data have the advantages of wide range and large depth in geophysical surveying. Two-dimensional inversion of MT data was conducted using the nonlinear conjugate gradient algorithm, and the conductivity structure under the Larsemann Hills area was described. The simulation results show that the high resistivity level in the shallow layer represents the Quaternary glacial sediments on the surface of bedrock. Further, the underground electrical structure within the depth of 10 km in the middle section shows obvious horizontal difference and drastic longitudinal variation. The depth of 10–40 km is characterized by thick layer and low resistivity, the electrical structure is relatively uniform, and the low resistivity structure is geographically inclined slightly to the northeast. This section is characterized by low resistivity structure. The upper part of the southwest section is characterized by high resistance, and the middle and deep part (below 10 km) is characterized by relatively uniform and low resistance terranes. The discovery can be interpreted as follows: The low resistivity body in the southwest segment represents the core cratonic crustal structure of the southeast polar shield, while the low resistivity body with complex electrical structure in the middle 10 km depth of the section can be interpreted as the collision zone between the Prydz Orogenic Belt and the East Antarctic craton. Our study confirms the position of the Prydz Orogenic Belt under the Larsemann Hills of East Antarctica and depicts the electrical structure of the collisional amalgamation area, which provides evidence for further study of the splicing of the Gondwana archicontinent.

The galvanic distortion caused by small-scale electrical inhomogeneities near the shallow surface perturbs the overall frequency response curve of magnetotelluric (MT) records and adversely affects the imaging of deep electrical structures. Various efforts have been made to suppress the galvanic distortion, but they are still difficult to use in complex regional structures. To effectively recover the deeper electrical structures, we develop an inversion technique in which the shallow electrical structures are constrained by other types of electromagnetic data and used as part of the prior model to remove the galvanic distortion. When other types of electromagnetic data are unavailable, we can also simulate the galvanic distortion by using a refined forward mesh and putting small-scale electrical resistivity anomalies beneath the surface. We validate our inversion approach using synthetic examples and apply it to field MT data collected from the northern Trans North China Orogen. A three-dimensional electrical resistivity model is obtained based on the prior model designed according to the locations of distorted MT stations. In our resulting model, artifacts caused by galvanic distortion are removed as compared to previous work. We demonstrate that our method is effective in improving the reliability of MT inversions in complex regions, leading to better interpretations of deep electrical structures.

Earthquakes threaten people’s lives and property, especially when cities are affected. Thus, detecting the crustal structure and seismogenic background beneath cities is important if we are to lessen losses. Because the city of Zhumadian in Henan Province, China, is a well-developed area with a large population, more attention should be paid to earthquake detection. We therefore collected data from 15 magnetotelluric (MT) stations along a 55-km survey line across Zhumadian. Then, subsequent to acquisition, processing, and data inversion, we developed a preferred electrical resistivity model. We accurately relocated original earthquakes during the period between 1981 and 2019 within the study area using the double-difference earthquake location algorithm. The relocated earthquakes were projected onto our preferred MT model. The new resistivity model presented here reveals that saturated rocks, including rocks with porosities of 4.31–19.69% and moderate salinities of 10 g/L pore fluid and/or graphite, form conductive features in the upper crust. We also show that aqueous fluids derived from metamorphic dehydration within the lower crust and mantle, not melts, constitute mid-crustal conductors and facilitate detachment structures. The data show that the relocated earthquakes are located adjacent to the boundaries between conductive and resistive areas as well as at the bottom of fault fracture zones. These outcomes imply that the combination of rock failure and fault slipping triggers earthquakes.

The Great Sumatran Fault (GSF) activity is a severe threat to Banda Aceh development as the capital city of Aceh Province, Indonesia. The earthquakes originating along this fault trace, despite generating low strength, considerably threaten infrastructure and human lives. Therefore, a detailed study of the GSF activity and presence becomes critical. In this paper, we applied the Global Gravity Model plus (GGMPlus) to map the subsurface structure and modeling of two GSF segments with a resolution of 200 m/px, namely the Aceh and Seulimeum segments toward the north of the Sumatran Island. The Bouguer anomaly data are inconsistent with the geology of the study areas, dominated by igneous rocks on the Aceh segment and volcanic rocks on the Seulimeum segment. Further, the contrast between the Seulimeum segment in the northeast and the Aceh segment in the southwest can be demonstrated by high-pass filtering. The GGMPlus data validation results with field measurements using the Scintrex CG-5 Autograv, the root mean square error obtained via data comparison are 12.32% in the Krueng Raya fault zone, and 26.1% at the Seulawah Agam Volcano area, respectively. We also performed 2D gravity data modeling along with the Aceh and Seulimeum segments in the NW–SE direction. This model was then compared with the geological cross section, seismicity, and magnetotelluric data. The results of Singular Value Decomposition and Occam inversion show three vertical blocks of high densities with an interspersion of lower densities, which can be confirmed as the Aceh and Seulimeum segments. Based on data processing, it can be concluded that the GGMPlus satellite can improve the maps and images of the northernmost GSF structure.

The magnetotelluric (MT) technique is an electromagnetic geophysical method, which is widely used as a complementary to seismic surveys for exploration of hydrocarbon reservoirs. In the inversion process, the method of matrix inverse calculation has a considerable effect on the speed of the inversion and the quality of obtained models. Lanczos Bidiagonalization (LB) method has been reported to be a fast and efficient approach for solving the inversion problems. In this study, we employ LB method for inverting large-scale 2D MT data. In LB algorithm, the full set of equations is replaced by a dimensionally reduced system of equations. As a result, the speed of the solution procedure is increased, while the original problem is solved with a high accuracy. In addition, we employ active constraint balancing approach for determining the optimum regularization parameter. The advantage of the method is that for highly resolvable parameters, a small value of the Lagrangian multiplier is assigned, and vice versa. The results of the synthetic data inversion show that both methods require equal computer memory but LB method is faster and more reliable than conjugate gradient method. The proposed approach is also applied to inverse real MT data collected from the Kashan area. The Kashan area is the most interesting area for oil and gas exploration of the Central Iran Basin. The inversion results obtained by LB are in a good agreement with the geological structure of the study area and the drilling data.

Artificial neural networks method (ANNs) is a common estimation tool used for geophysical applications. Considering borehole data, when the need arises to supplement a missing well log interval or whole logging—ANNs provide a reliable solution. Supervised training of the network on a reliable set of borehole data values with further application of this network on unknown wells allows creation of synthetic values of missing geophysical parameters, e.g., resistivity. The main assumptions for boreholes are: representation of similar geological conditions and the use of similar techniques of well data collection. In the analyzed case, a set of Multilayer Perceptrons were trained on five separate chronostratigraphic intervals of borehole, considered as training data. The task was to predict missing deep laterolog (LLD) logging in a borehole representing the same sequence of layers within the Lublin Basin area. Correlation between well logs data exceeded 0.8. Subsequently, magnetotelluric parametric soundings were modeled and inverted on both boreholes. Analysis showed that congenial Occam 1D models had better fitting of TM mode of MT data in each case. Ipso facto, synthetic LLD log could be considered as a basis for geophysical and geological interpretation. ANNs provided solution for supplementing datasets based on this analytical approach.

Magnetotelluric (MT) inversion algorithms are an essential tool in exploration geophysics because they provide us with resistivity models of the subsurface. Inverse problems are ill-posed and non-unique. Resistivity models produced by geologically unconstrained MT inversions are typically smooth, which means that the geological interpretations obtained based on the resistivity models will be ambiguous and uncertain. To solve the above problems, geophysicists have made many efforts over the years to introduce any prior information (such as geological structure information and physical property data) into models, which could help to better constrain the inverted model with the inversion algorithms. In the last few years, there has been renewed interest in machine learning techniques. Various machine learning methods have been applied to inversions. Using the fuzzy c-means (FCM) clustering method, geophysicists proposed new inversion algorithms that are capable of building petrophysical information into the inversion. Although the FCM has been used and advanced in previous work, geophysicists concluded that a priori information of the correct number and value of cluster centers is very important for a successful FCM inversion. In fact, it is difficult to obtain the appropriate clustering information in some geophysical survey areas. In this study, we present an effective way to build the petrophysical information for the MT inversion based on FCM clustering algorithm. When the actual petrophysical information is insufficient, considering the characteristics of MT data, we perform a one-dimensional blocky inversion that can clearly identify the interface with distinct electrical property contrast; then, we obtain the number and value of cluster centers from 1D blocky inversion for the further MT inversion with FCM clustering. The algorithm uses guided FCM clustering to improve the model within the iterative minimization during MT inversion. In our MT inversion method, we integrate the geophysical inversion and geological differentiation into a unified scheme, which interact and enhance each other. The resistivity models obtained from the inversion respect the geophysical and petrophysical data and are easy to geologically interpret. We tested the algorithm using two synthetic examples and a field data example.

This article presents the results of an integrated interpretation of measurements made using Audio-Magnetotellurics and Seismic Reflection geophysical methods. The obtained results were used to build an integrated geophysical model of shallow subsurface cover consisting of Cenozoic deposits, which then formed the basis for a detailed lithological and tectonic interpretation of deeper Mesozoic sediments. Such shallow covers, consisting mainly of glacial Pleistocene deposits, are typical for central and northern Poland. This investigation concentrated on delineating the accurate geometry of Obrzycko Cenozoic graben structure filled with loose deposits, as it was of great importance to the acquisition, processing and interpretation of seismic data that was to reveal the tectonic structure of the Cretaceous and Jurassic sediments which underly the study area. Previously, some problems with estimation of seismic static corrections over similar grabens filled with more recent, low-velocity deposits were encountered. Therefore, a novel approach to estimating the exact thickness of such shallow cover consisting of low-velocity deposits was applied in the presented investigation. The study shows that some alternative geophysical data sets (such as magnetotellurics) can be used to significantly improve the imaging of geological structure in areas where seismic data are very distorted or too noisy to be used alone

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.

Fully recognize various problems in the observed magnetotelluric (MT) data is the precondition of inverse solutions. In the paper, according to the geomorphological conditions of the observational MT stations in the Guangxi area, we constructed several different kinds of models to conduct a three-dimensional forward simulation of the MT field using the vector finite element method (FEM). First, the variation rule and differences of apparent resistivity ρxy and ρyx in the xy and yx modes were studied and analyzed, and then the geoelectric information reflected by the change of apparent resistivity ρxx and ρyy were discussed. Final, the responses of typical geological structures that cause a static shift problem were presented. The synthetic examples showed that ρxy and ρyx were relevant to the layout of the survey line, for instance, ρxy had different values along the west-east profile compared with that of the south-north profile, Moreover, ρxx and ρyy could subtly show the abnormal body-host rock interface, which could be used to restrict the anomalous domain in the inversion process. In addition to the scale and depth of the top surface of the anomalous body, the widespread rivers and hills, can simulate static shift. Hence, to reduce the influence of static shift on MT data, a reasonable distance between a station and rivers or hills should be considered in accordance with the scale of rivers or hills.

Based on the fact that it is difficult to implement optimum inversion using 2D and 3D forward modeling with magnetic-source transient electromagnetics (TEM), this paper explores a novel approach to the implementation of 2D magnetic-source TEM inversion. In particular, we converted magnetic-source TEM data into magnetotelluric (MT) data and then used a 2D MT inversion method to implement a 2D magnetic-source TEM inversion interpretation. First, we studied the similarity between magnetic-source TEM waves and MT waves and between magnetic-source TEM all-time apparent resistivity and MT Cagniard apparent resistivity. Then, we selected an optimal time–frequency transformation coefficient to implement rapid time–frequency transformation of all-time TEM apparent resistivity to MT Cagniard apparent resistivity. Afterward, we conducted 1D pseudo-MT inversions of magnetic-source 1D TEM theoretical models. The 1D inversion results demonstrated that the difference between the inversion parameters and model parameters was small, while the MT 1D inversion method could be used to conduct magnetic 1D TEM inversion within a certain margin of error. We further conducted 2D pseudo-MT inversions of 3D magnetic-source TEM theoretical models, and the 2D inversion results indicated that selecting a joint 2D pseudo-MT transverse-electric (TE) and transverse-magnetic (TM) inversion method based on measuring the line above a 3D anomalous body can help to accurately implement a 2D inversion interpretation of the 3D TEM response.

Continuously monitored magnetotelluric (MT) time series data were used to identify the short-term earthquake co-seismic and pre-seismic electromagnetic phenomenon. The co-seismic behavior of the MT time series data recorded at 15 Hz sampling frequency is analyzed for the earthquake that occurred on November 24, 2007, of Mw = 4.6. The wavelet analysis of the MT time series data shows significant enhancement at 3–6 Hz frequency band in the scalogram during the earthquake in comparison with pre- and post-time. The significant enhancement in the scalogram is related to the onset of the main shock of the earthquake. In this paper, we have also shown the precursory signatures of several earthquake magnitudes (Mw) ranging from 3.9 to 4.9 and the focal depth extending from 5 to 10 km mainly dominated by normal and strike-slip faulting. The spectral polarization ratio technique was implemented on these events to identify the precursory signatures. A few days before the earthquake, a significant anomaly was identified for most of the earthquakes using this technique. This prominent anomaly is correlated with Dst index, which provides information about the ionosphere and magnetosphere responses in the presence of the solar wind and interplanetary magnetic field. We inferred the unusual behavior prior to the earthquake is related to the precursory signature, but not related to the solar-terrestrial effect. The complex tectonic settings in the study region suggest that both electrokinetic and seismic dynamo mechanisms are the probable mechanisms playing an important role in generation of co- and pre-seismic electromagnetic signals.

Songyuan is the most earthquake prone area in northeast China. Since 2006, earthquakes have occurred in the area in the form of swarms, with a maximum magnitude of M L5.8. There is much controversy about the cause of the Songyuan earthquakes. We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area. The L-BFGS inversion method was applied, with a full-impedance tensor data set used as the inversion input. Combined with an evaluation of the earthquake locations, the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area (SEA), which was speculated to be the preexisting Fuyu-Zhaodong Fault (FZF). Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area, which breached the high-resistivity lithospheric mantle and stalled at the base of the crust. A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting, which could release water into the lower crust during later magma emplacements. While weakening the strength of the FZF, these ascending fluids also increased the pore pressure in the fault, further reducing the shear strength of the fault. Shear stress action (a fault strike component of the east-west regional compress), together with possible near-surface disturbances, may drive the fault to slip and trigger the earthquakes in Songyuan. It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms. We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults, regional stress, and deep fluids associated with plate subduction, and near surface disturbances might induce the earthquakes in advance. The Songyuan earthquakes are inherently induced earthquakes, fed by deep fluids.

The magnetotelluric (MT) method is a mainstream geophysical exploration method widely used in deep mineral resource exploration and other fields. However, the MT signal is extremely vulnerable to noise since it employs natural electromagnetic fields as a source. In order to improve the signal-to-noise ratio, a novel MT noise suppression method based on impulsive atoms and a niche particle swarm optimization-orthogonal matching pursuit (NPSO-OMP) algorithm is proposed. First, an over-complete dictionary composed of impulsive atoms is well-developed to match different types of cultural noise. Then, the sparse decomposition process of OMP is optimized with NPSO. Finally, the original MT data are de-noised using the well-developed dictionary and the NPSO-OMP algorithm. By processing the synthetic and measured MT data, and comparing the experimental results with those by traditional methods, we found that the proposed method can effectively suppress the high-intensity impulsive noise in the MT signal under the premise of retaining useful information. The continuity of apparent resistivity and the phase curve are significantly improved, and the results are verified by the remote reference method and robust statistic estimation. When the observation data are subjected to persistent, strongly correlated noise pollution, our method can obtain better results than the remote reference method and robust statistic estimation. Thus, when it is difficult to obtain high-quality MT response curves through the remote reference method and robust statistic estimation, our method is a promising alternative.

One of the well-known geothermal potential areas in Iran is the Abgarm in Mahallat region. Due to existence of several hot springs, the Abgarm area is of particular importance in geothermal studies. To investigate the geothermal mechanism at Abgarm, geophysical surveys of magnetometry and magnetotelluric (MT) were utilized near hot springs to image the geometry of all geological requisites for trapping a geothermal reservoir. The ground magnetic data were inverted in 3D to construct a magnetic susceptibility model, where a deep intrusive igneous rock (as a heat source) and a shallow magmatic lava were inferred from the physical model. Euler depth estimation also provided insight to a shallow granodiorite unit embedded in a thick sequence of sediments. Dimensionality and strike analysis of the MT data were performed through the phase tensor method to investigate the general geometry of the sought reservoir for running a 2D TE + TM mode inversion scheme. Through 2D inversion of the MT data along three profiles across the favorable zones, the electrical resistivity models were used to image sedimentary units (i.e., cap rock, top soil, bedrock and reservoir) and magmatic rocks. Considering both geophysical properties led to illustrate a plausible geothermal reservoir in the area.

The Southwestern area of Fujian Province in China contains a major metallogenic belt. There have been three major tectonic movements since the Late Paleozoic, and the thrusts here are the main mineral deposits. These occurred between the Early Carboniferous and Late Permian eras. Due to dramatic undulations in the surface and associated complex underground structures, a single geophysical method cannot provide reliable imaging results. This is largely due to difficulties in data acquisition and processing. Seismic exploration provides one example. Using this method, raw shot gathers have a low signal-to-noise ratio because of the undulations in the topography. Furthermore, strong lateral velocity variation makes the migration process exceedingly difficult. This means that interpretations of the resulting reflection seismic profile give rise to uncertainty. For this reason, two-dimensional reflection seismology and controlled source audio-frequency magnetotellurics (CSAMT) electromagnetic sounding techniques were performed. After initially completing conventional processing on the seismic reflection data, subsequent pre-stack depth migration (PSDM) over the rugged topography yielded a much better image. It was able to identify thrust faults and magma intrusions. Then, using CSAMT, inversion was conducted with the same topography as the seismic reflection. By combining the migrated seismic profile, inverted resistivity profile, and borehole data, a vertical geological model was constructed. This proved the existence of an overlapped thrust, and it also indicated the possible presence of deeper mineralization.

In this study, Shengli fault depression, Tangyuan fault basin, and northern Songliao Basin in Yitong-Yilan fault zone of Heilongjiang province are considered the research areas for geothermal anomaly. Based on the temperature of the deep thermal reservoir, the hydrothermal fluid channel, caprock thickness, and the mode of heat transfer, which are the main factors controlling the geothermal reservoir formation, we examined geothermal resource system of the underground HDR in this area. First, we inversed the aeromagnetic data, calculated the Curie isotherm depth, analyzed the geothermal distribution characteristics, and estimated the temperature of the deep heat source. Second, we applied the controlled source audio frequency magnetotelluric (CSAMT) and magnetotelluric (MT) methods to obtain the deep electrical structure of the study area. We determined the thickness of the caprock and the hydrothermal fluid channel. Finally, we obtained the borehole geothermal steady-state temperature measurement data and water sample chemical analysis data from the logging temperature curves of 24 wells to infer the mode of heat transfer. Based on the results, we built a model of the geothermal system of the sedimentary basin in this area. The results show that the depth of Curie isotherm in the study area is 17–39 km. The resistivity of sedimentary caprock in the north of Songliao basin is low, and there exists a deep heat source, which is mainly thermal convection. In contrast, in Shengli and Tangyuan fault basins, heat conduction is dominant. Based on the geothermal system model, we conclude that the area from Daqing to Lindian in Songliao basin has a thermal-convection-dominated sedimentary basin geothermal system. Heat exchange is realized by the upwelling of mantle-derived thermal materials through fracture channels. The thick sedimentary caprock reduces the heat loss. It can be a target for sustainable development and utilization of HDR.

We propose a new 3D inversion scheme to invert the near- and transition-zone data of CSAMT with topography accurately. In this new method, the earth was discretized into unstructured tetrahedra to fit the ragged topography and the vector finite-element method was adopted to obtain precise responses and good sensitivity. To simulate the attitude and shape of the transmitter, we divided a long-grounded transmitter into dipoles and integrated these dipoles to obtain good responses in the near- and transition-field zones. Next, we designed an L2 norm-based objective functional and applied a standard quasi-Newton method as the optimization method to solve the inverse problem and guarantee steady convergence. We tested our 3D inversion method first on synthetic data and then on a field dataset acquired from select sites near Changbai Mountain, China. In both tests, the new inversion algorithm achieved excellent fitting between the predicted and observed data, even in near- and transition-field zones, and the inversion results agreed well with the true model. These findings reveal that the proposed algorithm is effective for 3D inversion of CSAMT data.

Theoretical magnetotelluric (MT) soundings are investigated for a stratified (five-layered) Earth model consisting of two transitional layers with conductivity varying linearly with depth, and three homogeneous layers with constant conductivity. The analytical expressions for the tangential electric and magnetic fields as well as the surface impedance are derived in terms of Airy functions. The effect of the thicknesses of the two transitional layers and the interlayer between them on the MT responses (apparent resistivity and impedance phase) is examined in detail.

Mapping the static gravity field in the Italian area is fundamental to identify the main lithospheric structures, to delineate the main geological elements at regional level and to infer the regional geodynamic setting. The ongoing tectonic processes highlight nevertheless the need to measure and model the time-variable gravity field, namely the dynamic gravity field, which requires increased accuracy and long time series of observations to separate the secular from the short-term variable components. The first, with a minor impact in Italy, are due to variations of ice mass balance (the viscoelastic response of the Earth to past changes in ice mass loading, and the elastic response of the Earth to present-day deglaciation), and the sea-level rise; the second are due to space/time variations of underground mass distributions, such as those related to seismic deformations, volcanic dynamics/eruptions and water transfer. Local-scale gravity studies along seismogenic faults may provide useful hints to study the seismic cycle and to unravel those areas more prone to seismic release by studying if the crustal volume is undergoing dilatancy (gravity decrease) or overpressure (gravity increase) before earthquake occurrence. This process, however, is accompanied by possible fluid migration, which can be revealed by other geophysical measurements, for example, by magnetotelluric and geoelectrical surveys. In this short paper, we briefly summarize the main sources of gravity variation providing on the same time orders of magnitude, spatial and temporal scales of their effects.

The Wulong gold district (WGD) is located at the northeastern margin of the North China Carton (NCC) on the Liaodong Peninsula, China. The results of early mineral exploration, mining, and metallogenic modeling research show that gold deposition was associated with lithospheric thinning of the NCC and the emplacement of voluminous Early Cretaceous intrusions. In this study, multi-source information on the ore formation and geology (including geophysical, geochemical, and remote sensing data) was used to construct two-dimensional (2D) prospectivity maps at multiple levels (surface (0 m), − 500 m, − 1000 m, − 1500 m, − 2000 m, − 2500 m, and − 3000 m) for subsurface mineral exploration in the WGD. The methodology employed consists of four steps. Firstly, three-dimensional (3D) geological and geophysical models were constructed using UBC-GIF and SKUA-GOCAD software. 3D geophysical modeling included 3D gravity and magnetic inversions, and 3D resistivity models were based on 2D magnetotelluric (MT) inversion. Secondly, multi-source, ore-forming, and geoscientific data were integrated using radial basis function link net modeling technology within ArcGIS-SDM. Thirdly, multiple-level 2D prospectivity maps were integrated using the discrete smooth interpolation method within the SKUA-GOCAD software to produce 3D prospecting models. Finally, posterior probabilities were classified to delineate 3D exploration targets using the concentration–volume fractal method (GeoCube software, v. 2.0). The results suggest that: (1) the Early Cretaceous intrusions are key ore-forming and ore-controlling objects within the WGD, and a potential target is the western part of the Sanguliu intrusion at depths from 1000 to 3000 m, in the zone where the Wulong and Yangjia gold deposits cross; (2) there are potential targets within 2500 m of the Wulong gold deposit; however, (3) there are no potential targets from 1000 to 3000 m in the Sigdaogou gold deposit.

To explore the tectonic framework and features of stratigraphic distribution in the Tuolai Sag, Yin-E Basin, a 47-km-long magnetotelluric (MT) sounding measurement was performed around Well MAZD1 in the sag. During field data acquisition, a remote reference technique was used to ensure data quality, with apparent resistivity and phase curves of all measuring points obtained using the Fourier transform, power spectrum selection, robust estimation and other methods. After MT data processing, dimensionality analysis and the degree of two-dimensional deviation indicated that the study area had the dimensionality needed for two-dimensional inversion. The major electrical axis in the sag was determined, using a multipoint–multifrequency point statistical imaging technique, to be in the WNW direction. Within the constraints of the resistivity log data for Well MAZD1, inversion results for TE and TM models were compared, after which the TM model, which corresponded well to geological conditions, was selected for conducting the nonlinear conjugate gradient method inversion and a reliable resistivity model was finally obtained. Based on regional petrophysical properties, resistivity logging, and near-well bathymetric data, the electrical characteristics of different formations within the sag were obtained and a set of low-resistance clastic rock identified in the lower Carboniferous strata. Based on an integrated analysis of the regional surface geology, tectonic setting, and depositional environment, and within the constraints of gravity to fit with electrical structure, a tectonic framework of two subsags sandwiched by an uplift is proposed for the Tuolai Sag. The scale of the northern subsag is large, with development of pre-Carboniferous nappe as well as of Carboniferous–Permian strata within the lower part of the nappe. The southern subsag is small and filled mainly with Carboniferous–Permian strata.

Geomagnetic disturbances cause perturbations in the Earth’s magnetic field which, by the principle of electromagnetic induction, in turn cause electric currents to flow in the Earth. These geomagnetically induced currents (GICs) also enter man-made technological conductors that are grounded; notably, telegraph systems, submarine cables and pipelines, and, perhaps most significantly, electric power grids, where transformer groundings at power grid substations serve as entry points for GICs. The strength of the GICs that flow through a transformer depends on multiple factors, including the spatiotemporal signature of the geomagnetic disturbance, the geometry and specifications of the power grid, and the electrical conductivity structure of the Earth’s subsurface. Strong GICs are hazardous to power grids and other infrastructure; for example, they can severely damage transformers and thereby cause extensive blackouts. Extreme space weather is therefore hazardous to man-made technologies. The phenomena of extreme geomagnetic disturbances, including storms and substorms, and their effects on human activity are commonly referred to as geomagnetic hazards. Here, we provide a review of relevant GIC studies from around the world and describe their common and unique features, while focusing especially on the effects that the Earth’s electrical conductivity has on the GICs flowing in the electric power grids.

The terrain conditions and geological structure of the mountainous areas in western China are complex. To meet the design requirements for the expressway, the route adopts the design of deep-buried super-long tunnels in hilly areas. If the highway tunnel passes through faulty structures, such as improper selection of structure and foundation form, it can cause collapse and structural instability during construction. Therefore, basic characteristics of faults and information that is more accurate should be provided during the survey process to avoid subsequent problems. The frequency and magnitude of earthquakes in this study area are high and available data show that the Xiaojiang fault was still active during the Quaternary Holocene period, which is a high-risk area for earthquakes. Therefore, a set of comprehensive technologies, such as geophysical prospecting, geological survey, and drilling, and was used to identify faults and their characteristics and to visually display underground information within a certain depth range. Here, the magnetotelluric method was used to analyze the electrical characteristics and structure of the highway tunnel. In addition, the electrical characteristics and structure of the Xiaojiang fault were discussed. Geological surveys and drilling agree well with the analysis and interpretation of the research results and further proposed a processing and interpretation method for correcting the dip angle of the fault using the statistical results from the long axis dip angle of the tensor polarization map for the wave impedance. Thus, a large amount of information in the tunnel site can be effectively obtained for predicting and evaluating the Xiaojiang fault. Furthermore, our results provide strong technical support for designing and operating highway tunnels, with meaningful experience and reference for future geophysical processing and interpretation.

The polar regions are host to fundamental unresolved challenges in Earth studies. The nature of these regions necessitates the use of geophysics to address these issues, with electromagnetic and, in particular, magnetotelluric studies finding favour and being applied over a number of different scales. The unique geography and climatic conditions of the polar regions means collecting magnetotelluric data at high latitudes, which presents challenges not typically encountered and may result in significant measurement errors. (1) The very high contact resistance between electrodes and the surficial snow and ice cover (commonly MΩ) can interfere with the electric field measurement. This is overcome by using custom-designed amplifiers placed at the active electrodes to buffer their high impedance contacts. (2) The proximity to the geomagnetic poles requires verification of the fundamental assumption in magnetotellurics that the magnetic source field is a vertically propagating, horizontally polarised plane wave. Behaviour of the polar electro-jet must be assessed to identify increased activity (high energy periods) that create strong current systems and may generate non-planar contributions. (3) The generation of ‘blizstatic’, localised random electric fields caused by the spin drift of moving charged snow and ice particles that produce significant noise in the electric fields during periods of strong winds. At wind speeds above ~ 10 m s−1, the effect of the distortion created by the moving snow is broad-band. Station occupation times need to be of sufficient length to ensure data are collected when wind speed is low. (4) Working on glaciated terrain introduces additional safety challenges, e.g., weather, crevasse hazards, etc. Inclusion of a mountaineer in the team, both during the site location planning and onsite operations, allows these hazards to be properly managed. Examples spanning studies covering development and application of novel electromagnetic approaches for the polar regions as well as results from studies addressing a variety of differing geologic questions are presented. Electromagnetic studies focusing on near-surface hydrologic systems, glacial and ice sheet dynamics, as well as large-scale volcanic and tectonic problems are discussed providing an overview of the use of electromagnetic methods to investigate fundamental questions in solid earth studies that have both been completed and are currently ongoing in polar regions.

Geophysical inversion under different stabilizers has different descriptions of the target body boundary, especially in complex geological structures. In this paper, we present an extremum boundary inversion algorithm based on different stabilizers for electrical interface recognition. Firstly, we use the smoothest and minimum-support stabilizing functional to study the applicability of adaptive regularization inversion algorithm. Then, an electrical interface recognition method based on different stabilizers is developed by introducing extremum boundary inversion algorithm. The testing shows that the adaptive regularization inversion method does work for different stabilizers and has a low dependence on the initial models. The ratio of the smooth and focusing upper and lower boundaries obtained using the extremum boundary inversion algorithm can clearly demarcate electrical interfaces. We apply the inversion algorithm to the magnetotelluric (MT) data collected from a preselected area of a high-level-waste clay-rock repository site in the Tamusu area. We recognized regional structures with smooth inversion and the local details with focusing inversion and determined the thickness of the target layer combined with the geological and drilling information, which meets the requirement for the site of the high-level waste clay-rock repository.

An accurate tensor estimation function is a guarantee for acquiring a reliable underground electrical structure model. The past studies regarding transmission functions aim at obtaining smooth and continuous sounding curves through the time domain by data screening means such as least square, robust and remote reference processing methods as well as digital filtering techniques such as wavelet denoising, empirical mode decomposition and Hilbert–Huang transform denoising. As SNRs in different field measurement sites are different, if different data processing algorithms like single-site processing, remote reference processing, magnetic field correlation-based remote reference processing and magnetic channel sharing are used for data processing at the same measuring site, 3N-2 groups of sounding curves with different qualities will be obtained. Manual screening method has been mainly used in the past for screening of the abovementioned 3N-2 groups of sounding curves. If the quantity of sounding site is large, this manual screening method is of low efficiency without quantitative screening indexes. In this paper, discrete Frechet distance is mainly utilized to carry out quantitative screening of 3N-2 groups of sounding curves processed through the abovementioned four processing methods in order to obtain smooth, continuous and reliable sounding curves, thus avoiding subjectivity and low efficiency problems in the past manual screening. A quantitative sorting of all kinds of data processing results can be conducted, and the optimal sounding curve is finally picked out for data inversion. The above sounding curve selection result has improved processing accuracy of magnetotelluric data.

The development of Tongchuan City, Shaanxi Province, located in the northwestern region of China, is restricted by water resources. The direct current resistivity and induced polarization sounding methods are typically applied in finding urban groundwater. These methods, however, are not effective due to their complicated topography and geological conditions. The application practice shows that the audio magnetotelluric (AMT) method has a large depth of exploration, high work efficiency, and high lateral resolution. To investigate the distribution of groundwater resources, we deployed three audio-frequency magnetotelluric profiles in the city area. The impedance tensor information of AMT data is obtained using SSMT2000. AMT data dimension analysis reveals that the two-dimensional structural features of the observation area are obvious. The main structure of the observation area is about 45° northeast, as indicated by structural trend analysis. A shallow two-dimensional electrical profile of 1 km in Tongchuan City is obtained by two-dimensional nonlinear conjugate gradient inversion. Finally, combined with regional geological information, the geological structure characteristics reflected by the electrical profile were obtained along with the detailed characteristics of water-rich structures in the area. The influence of the structure on the groundwater distribution was analyzed, and the water-rich areas were identified. This work contributes to the prospective development of Tongchuan City.

We applied the technique of the genetic algorithms and a local methodology integrating the Gauss–Newton and Conjugate Gradient (GNCG) techniques to test one-dimensional inverse modeling of synthetic magnetotelluric data. The result of this modeling applied to a homogeneous and isotropic five-layer model led to the development a hybrid algorithm (GAGNCG), combining the aforementioned techniques, for inverse modeling of one-dimensional magnetotelluric data. The GAGNCG modeling of the synthetic data performs more efficiently than the local methodology in terms of both procedure and results. This showed that the hybridization procedure maximized the advantages of using the global search methodology and minimized the disadvantages of the local technique. Based on these results, we developed another hybrid methodology (GA2D), built from some characteristics of the genetic algorithm and the simulated annealing method, for the inverse modeling of two-dimensional magnetotelluric data. The results were satisfactory, and the GA2D algorithm was a good starting point for the inverse modeling of two-dimensional data.

Carbon-bearing stratum normally features low resistance and high polarization. If the lithostratigraphy of the exploration area contains large amounts of carbon, the induced polarization anomaly caused by metal sulfide ore bodies will be inundated by the high polarization of carbon-containing wall rock. In this work, we adopted time-domain induced polarization (TDIP) and controlled-source audio-frequency magnetotellurics (CSAMT) on deep prospecting of the carbon-bearing stratum of the Ar Horqin Banner, Inner Mongolia. The underground medium is divided into target geologic bodies according to the geological information within the known exploration line borehole, and the physical properties of various target geologic bodies are calculated using weighted averages to build a geologic-geophysical model that can fit the observation data. Consequently, we can determine the range and morphological characteristics of the electrical properties of the ore-bearing geologic bodies in the inversion results in the study area. Then we can use the characteristics summarized from the known exploration line to interpret unknown exploration line. Results indicated that, when the difference in physical properties between the ore body and interference wall rock is not clear, the geologic body can be classifi ed via the paragenetic (associated) assemblage relations of the underground medium. Geological interpretation is guided by the comprehensive physical properties of ore-bearing geologic bodies to avoid interferences.

We present an interpretation of newly acquired audio-magnetotelluric data to reveal the subsurface geometry of Cimandiri Fault Zone, one of the major active faults in the western part of Java. The line section is 25 km long in a nearly north–south direction across the axes of CFZ with 24 stations of 750–1200 m spacing intervals. The 2D AMT inversion model shows two conductive zones in the southern part that may be associated to the Miocene rocks of the Southern Mountains, and a conductive zone in the northern part that is likely to be associated with Gunung Walat fold-belt. The subsurface structures of the Southern Mountains are dominated by south-dipping thrusts that may uplift the shallow marine sediments. Shallow seismicity occurred around CFZ indicating the activity of these blind thrusts.

In recent decades, diapirs are frequently used for CO2 and natural gas storage due to their extremely impermeable and non-reactive characteristics. Among various approaches, we use an integrated interpretation approach to resolve the diapir no. 4 belonging to the Shurab diapirs (SD). The SD is a group of diapirs that have pierced to the surface of the Qom basin of Central Iran, which is a candidate for natural gas storages. The complex geology of the SD is the main cause that previous 2D seismic surveys across the diapir could not provide required information to propose any location for any exploration borehole. Consequentially, 28 magnetotelluric (MT) and 1 audio-magnetotelluric station were measured along a SW-NE profile. Dimensionality and strike analysis for all stations is done by the use of phase tensor analysis. We used the nonlinear conjugate gradient algorithm to invert the TE- and TM-modes data simultaneously in 2D. The resistivity model was compared with the interpreted results of the post-stack depth migration model using seismic attributes. In order to extract the determinative geological information from the low-quality seismic section, envelope, variance, sweetness and instantaneous frequencies attributes were used. The integrated interpretation of the seismic and MT data resolves a precise geometry of the salt body, location of the dense part of the diapir as well as the tectonics around the diapir. The integrated interpretation of seismic and MT data of diapir no. 4 resulted in an exploration drilling program.

Noise interference, especially from human noise, seriously affects the quality of magnetotelluric (MT) data. Strong human noise distorts the apparent resistivity curve, known as the near-source effect, causing poor reliability of MT data inversion. Based on analyzing the frequency characteristics of human noise resulting from the surrounding environment, a new wavelet-based denoising method is proposed for both synthetic and real MT data in this paper. The new technique combines multi-resolution analysis with a wavelet threshold algorithm based on Bayes estimation and has a remarkable effect on denoising at all band frequencies. The multi-resolution analysis method was employed to reduce long-period noise, and a wavelet threshold algorithm was used to eliminate strong high-frequency noise. In this research, the improved algorithm was assessed via simulated experiments and field measurements with regard to the reduction in human noises. This study demonstrates that the new denoising technique can increase the signal-to-noise ratio by at least 112% and provides an extensive analysis method for mineral resource exploration.

Magnetovariation methods, which are applicable to study the mantle conductivity, require long lasting registration of natural magnetic field variations. Such data usually can be recorded in stationary conditions on geomagnetic observatories. The magnetotelluric and magnetovariation measurements have been carried out at seven semi-permanent sites on Ukrainian territory for approximately 10–15 years by the Ukrainian State Geological Research Institute as well as the S.I. Subbotin Institute of Geophysics of the National Academy of Sciences of Ukraine for monitoring the effects and phenomena related to electromagnetic field. Since the quality of records was acceptable, we used these data to study the Earth’s mantle conductivity peculiarities to obtain the information on sites remote from geomagnetic observatories. The Z-H method was applied to collected data to estimate the magnetovariaton transfer functions, which were concatenated with magnetotelluric ones and further inverted by the 1D OCCAM algorithm to obtain the geoelectrical cross sections for the measuring sites. To extend the region under study and to obtain more reliable results, similar data from three Ukrainian geomagnetic observatories in Kyiv, Lviv and Odessa were included in this study. Obtained conductivity models were related to the known geological structure in the upper and middle mantle beneath the Ukrainian territory.

The Changbaishan volcano is an active and considerably hazardous volcano located on the border of China and North Korea. This paper summarizes a series of geophysical surveys as well as seismological and volcano-observational networks around the Changbaishan volcanic area. We characterize deep structures related to the Changbaishan volcanic area. The prominent low-velocity anomalies and low-resistance bodies associated with the magma system under the Changbaishan volcano were detected in the crust and upper mantle, and high-velocity anomalies were imaged within the mantle transition zone, suggesting that the origin of the Changbaishan volcano is related to the subducted Pacific slab. However, there exist a few major obstacles for comprehensively elucidating the deep structure of the Changbaishan volcano as well as for the preparedness for and response toward future volcanic unrest and activity. It is essential to collect data from both China and Korean Peninsula to image the deep structure beneath the Changbaishan volcanic area. A multi-disciplinary approach comprising seismological investigations, deformation information from GNSS and InSar, and gravity and magnetotelluric surveying is a reliable manner for imaging high-resolution structures and fluid movement for the spatial distribution and variation of the volcanic magma chamber. An effective volcano-monitoring network system is considerably important to improve hazard assessments and characterize the potential future eruption of the Changbaishan volcano.

The geophysical electromagnetic (EM) theories are commonly based on the assumption that the conductivity of underground media is frequency independent. However, due to the existence of induced polarization (IP) effect, many earth materials are dispersive, and their electrical conductivity varies significantly with frequency. Therefore, the conventional numerical techniques are not proper for EM forward modeling in general dispersive medium. We present a new algorithm for modeling three-dimensional (3D) EM data containing IP phenomena in frequency domain by using an edge-based finite element algorithm. In this research, we describe the dispersion behavior of earth media by using a Cole–Cole complex conductivity model. Our algorithm not only models land and airborne EM surveys but also provides more flexibility in describing the surface topography with irregular hexahedral grids. We have validated the developed algorithm using an analytic solution over a half-space model with and without IP effect. The capabilities of our code were demonstrated by modeling coupled EM induction and IP responses in controlled-source audio magnetotelluric (CSAMT) and airborne electromagnetic (AEM) examples. This algorithm will have important guiding significance for survey planning in the dispersive areas, and it could be taken as a forward solver for practical 3D inversion incorporated IP parameters.

A magnetotelluric (MT) survey has been conducted in the eastern part of the Kachchh region of Gujarat State (crossing Little Rann) starting from Adesar village in the north to Enjar village in the south, where limited geophysical data is available. The profile has been acquired in the N–S direction having a total length of 52 km with the interstation spacing of 3–4 km. From two-dimensional (2D) inversion of the MT data, four conductive zones are identified. The analysis reveals that the depth of the basement varies from 1.2 km (in the south) to 1.8 km (in the north). The first conductive zone is found at a distance of ~ 3 km in the south of Adesar and is identified at the location of the Kanmer fault [KF; the eastern extension of South Wagad fault (SWF)] and might be the KF. The second conductive zone is found at a distance of ~ 42 km in the south of Adesar and ~ 13 km north of Enjar village and is located at the contact zone of Kachchh and Saurashtra peninsula at the location of the North Kathiawar fault (NKF). It is inferred as the NKF. The third conductive zone (at a distance of ~ 27 km in the south of Adesar) is found between the first and second conductor. This is interpreted as a step fault, formed during the rifting process of Kachchh. The lower crust (below 20 km) is found conductive (~ 120 Ω m) and might indicate the presence of fluid-filled mafic/ultramafic rocks in the lower crust.

Quartz-pebble conglomerate (QPC) in the Dhanjori basin, a rifted greenstone belt, is known for its gold mineralization. Audio-magnetotelluric (AMT) soundings have been acquired for 32 sites spread in grid fashion for frequency 10kHz to 10 Hz along four parallel profiles. Due to noise conditions in the data only 26 sites have been used for further studies. The site spacing between adjacent AMT sites were ~250 m. The grid spreads over QPCD and Mayurbhang Granite (MG) of rocks. The dimensionality and the directionality analysis hinted towards a complex structure which tempted to perform a 3D inversion of AMT data for full impedance data. To obtain the optimum value of initial uniform half space resistivity, 3D inversion of off-diagonal component of inversion data was performed with various values of resistivity. The optimized value of resistivity was then used to perform 3D inversion for full impedance component with different set of error floor for the impedance component. The final 3D model showed the presence of a continuous conducting feature all along the three profile. Whereas, the southernmost profile showed three well developed conductors. The model corroborates well with electrical resistivity tomography (ERT) sections and GSI borehole which lies within the same geological domain. The borehole intercepted sulphide mineralization with varying concentration from the near surface to ~ 100m. The near surface conducting heterogeneity thus is attributed to sulphide mineralization.

Magnetotelluric (MT) data has been collected along 32 stations along E–W profile in northern part and eight LMT (long period MT) stations in north-central part of Saurashtra region. Dimensionality analysis is carried out prior to MT modelling for obtaining the subsurface dimension as well as the direction of the underlying substructures. To estimate the subsurface dimensionality from MT data, different techniques Swift skew, Bhar’s skew, normalized weights, phase tensor (PT) analysis and Wall’s rotational invariant approach have been applied. These results suggest 1D structure for lower periods (0.01–1 s) and 3D structure for higher periods (1–10000 s) along two different profiles indicating that the study area is highly heterogeneous. Regional strike has been estimated through phase tensor (PT) and Groom–Bailey (GB) techniques suggests N40°E regional strike direction that correlates well with the Delhi–Aravalli tectonic trend. 2D modelling of MT/LMT data sets brings out different resistivity and conductivity blocks. Basaltic magmatic intrusion and its recrystallization have resulted in resistivity blocks with conductivity anomalies (trapped fluids) in between them. It has been reflected as 3D structures at higher periods. Different sedimentary basins at shallow depth are observed as 1D structure in dimensionality analysis.

The Delhi Seismic Zone (DSZ) is a seismically highly active intraplate region that keeps experiencing recurrent seismicity of minor to moderate nature. The seismic activity in the DSZ in recent times has evoked concerns about the vulnerability of the region due to potential earthquakes. Although the spatial distribution of recorded earthquakes gives a fairly good idea about the zone of the seismicity, the information about the underlying crustal structure and seismogenic faults is largely missing. In the present work, we have revisited the MT data acquired at various locations across Haryana to understand whether there are any anomalous features in the crust that might correlate with the seismicity in the western part of the DSZ. The MT data recorded at six groups of sites yielded good impedance tensors in the period range of 0.001–1000 s. A significant result obtained by 2-D inversion of distortion corrected impedance tensors is the presence of a northward dipping electrical conductor (

In western and southwestern Anatolia, the geothermal gradient cannot be accurately measured in many wells due to the complex horizontal water cycle. The aim of this study is to calculate the geothermal gradients by using a new approach for such wells. In this case, the first step is to calculate the geothermal gradients in order to estimate the heat flow values around the wells. Naturally, a direct heat flow estimate cannot be made for wells without a geothermal gradient. However, the heat flow values can be indirectly obtained by different methods. Curie point depth, silica geothermometer, and magnetotelluric conductive layer depth are some of these indirect methods. In this study, we used the heat flow data based on the Curie depth. In this approach, Curie temperature gradients are calculated using the Curie heat flow values and thermal conductivities for problematic wells. An empirical relationship between the Curie temperature gradient and the correctly measured temperature gradients in the region was established. With this relationship, it is possible to make an approach from the current Curie temperature gradient to the normal temperature gradient. In this way, heat flow can also be calculated for areas where no geothermal gradient can be obtained, and heat flow distribution over the whole area can be given. This proposed approximation, to obtain thermal gradient and heat flow, can also be extended to any other region.

Geochemical studies in Deccan syneclise have identified anomalous concentration of light hydrocarbon gases in the surface soil from the Tapti rift region in Narmada–Son Lineament, India. We have reanalysed and modelled broadband MT data from this region to delineate basalts and underlying sediment configuration, which is favourable for hydrocarbon accumulation. We carried out 1D and 3D modelling of the MT data for this purpose. Modelling studies brought regions of thick sub-basaltic sediments towards NW and SW of Nepanagar. But very less anomalous light hydrocarbon gas concentration is observed along NW of Nepanagar. This may be due to the presence of thick basalt cover in that area, which prevents the leakage of gaseous hydrocarbons on to the surface soil. We observed a comparatively thin basalt layer and high sediment thickness with a high concentration of light hydrocarbon gases near to Burhanpur area, which strongly suggests better hydrocarbon prospects in the area. It is inferred that the relatively thin and fractured/vesicular nature of basalt in this region facilitates the upward migration of light hydrocarbon components to the surface soil.

The impedance tensor and tipper are shown to be non-causal in some classes of 2-D and 3-D magnetotelluric (MT) models, including those widely encountered off shore. At the same time, examination of a large database of onshore MT records yields only a handful of non-causal tensors, being non-causal due to the influence of nearby artificial conductors rather than some exotic geological conditions. This result implies that the actual chance to encounter a non-causal impedance tensor or tipper in terrestrial MT exploration is vanishingly small, thus securing the universal applicability of the dispersion relation technique for quality assessment of the mentioned transfer functions.

As a new kind of renewable and environmental-friendly energy to generate electricity, hot dry rocks (HDR) geothermal reservoirs have been studied, along with the enhanced geothermal system (EGS). Geophysical methods have been used for the geological characterisation in different scales. The small-scale geophysical data are required for the local geological analysis so as to provide prior information for HDR modelling. Gonghe basin is in the northeast margin of the Qinghai–Tibet Plateau. Several drilling records indicate that this basin is a potential HDR prospecting area with high geothermal gradient, high heat flow and widespread igneous rock distribution, especially the Gonghe town (Qiabuqia) along with its neighbouring area. Gravity and magnetic surveys were carried out here. To better understand the areal and vertical distribution of the HDR, the gravity and magnetic data were inverted using 2D manual inversion and 3D cross-gradient joint inversion based on the smooth $${l}^{0}$$ norm constraint of minimum support functional stabiliser. The 2D model showed the sedimentary cap with a thickness of around 1000–1500 m. Granites of different periods and intrusion process were widely distributed and underlined the sediments accompanied by some deep faults. As for the HDR delineation, 3D models showed a potential area along Gonghe town and Dongba. The density and susceptibility were estimated at over 2.6 g/cm3 and 4 × 10–3 SI separately, when referred with exiting HDR distribution along the geological profile of DR4–QR1–DR3–DR2 wells. The upper boundary of HDR was outlined at the depth of around 2000 m, and the volume of HDR was then estimated around 6100 km3 above 3500 m depth. The appearance of the density and susceptibility models was affected by the lithology, stress and hydrothermal alteration. More precise geophysical methods including the microgravity, seismic and MT (magnetotelluric) surveys would be more applicable at the HDR exploitation stage.

Knowledge of the sedimentary thickness and configuration of the basement structure are important to estimate the seismic hazards in active foreland basin. We present sedimentary thickness of the northern Indo-Gangetic plain estimated from impedance tensors of 12 magnetotelluric sites. Occam’s one dimensional inversion scheme was applied to invariant of the magnetotelluric tensors for the period range of 0.001–1 sec at each site. Inverted one dimensional model corroborates north-easterly dipping Indian basement and accordingly increased thickness of the sedimentary column towards north and east direction. The top sedimentary layers of varying thickness and resistivity are correlated with the known borewell logs and previous geophysical studies around the study area. Significant difference is observed in the resistivity of the Indian basement and thickness of the sedimentary column across the Delhi Hardwar Ridge. The difference in resistivity may be an indicative of variation in compactness and degree of saturation of the sedimentary cover and the nature of the Indian basement rock across the Delhi Haridwar Ridge.

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.