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Using multigeophysical exploration techniques is a common way for deep targets to be explored in complex survey areas. How to locate an unknown underground target using multiple datasets is a great challenge. The useful information in the multisource geophysical model can be extracted and fused with the help of data fusion, which also works well to correct the interpretation divergence brought on by expert experience, with image feature extraction being the key step in the fusion of the geophysical models. Traditionally, this method is often used for these kinds of geophysical images, but it significantly reduces the efficiency of feature extraction. As a result, we propose a novel method based on a transfer learning method to extract the features of multisource images. First, the ResNet50 network is used to extract the initial features of the images. Owing to the problems of feature redundancy and fuzzy features in initial features, Spearman and zero phase component analysis can be used to achieve feature reduction and enhancement, which can further improve the computational efficiency and fusion accuracy in fusion. Finally, the fusion image is obtained using fusion rules that we designed based on the current state. The algorithm's reliability is tested using field data from the Iliamna Volcano. The case study demonstrates the effectiveness of the proposed strategy, which also offers a novel way to locate subsurface targets.

This study focuses on the integrated application of the frequency-domain terrain conductivity (TC) and transient electromagnetic (TEM) methods to map an ancient riverine harbour (port) at the Egyptian archaeological site of Tell el-Rub'a (prehistoric Mendes). Fifty-one TC profiles were measured using six EM frequencies and 16 TEM soundings were performed at the selected site. The apparent conductivity maps derived from the TC survey enabled defining the lateral extension of the harbour basin and its edges to penetration depths ranging between 12 and 30 m. The vertical extension of the harbour basin was determined using the TEM inversion. Three layers were defined: the topsoil, the conductive silty clay infill and the resistive sand bedrock. The bedrock is found at depths ranging from 19 to 31 m. The TC and TEM results were combined to form a 3D image of the harbour basin. This study confirms the existence of Mendes harbour and demonstrates the effectiveness of the TC and TEM methods for such archaeological studies.

Geologic and geochemical results from the Ni-Cu deposits of the Central Asian orogenic belt show that carbon from carbonaceous sediments was incorporated in the magma that formed the Ni-Cu deposit. Thus, the presence of an Ni-Cu deposit may be hypothesized at localities where carbonaceous sediments are in contact with mafic intrusions. However, the widely developed carbonaceous layer in the Central Asian orogenic belt has low-resistivity and high-chargeability characteristics similar to those of massive and semimassive orebodies, which makes it difficult to detect Ni-Cu deposits by conventional geophysical methods. To test whether the frequency-domain controlled-source electromagnetic (FDCSEM) method can circumvent this problem, a survey was conducted using horizontal electric fields to map the spatial distribution of rock types around gabbro intrusions containing an Ni-Cu sulfide deposit and barren gabbro intrusions of the Central Asian orogenic belt. The recovered resistivity structure resolved two low-resistivity layers: one was located at an elevation range of 850 to 950 m (the elevation of the surface is about 1,000 m) and widespread along the survey lines, whereas the other was present between 300 and 600 m and was limited to only part of the survey line. These structures are interpreted to represent carbonaceous layers. Gabbro intrusions are present as high-resistivity formations. Based on the drilling results, in the area where the deep carbonaceous layer exists, the Ni-Cu orebodies are associated with gabbro intrusions, whereas where the gabbro intrusions are not associated with a carbonaceous layer, no Ni-Cu orebodies are present. These associations can be detected using FDCSEM with a horizontal electric field and could be used to explore Ni-Cu deposits.

The Bayesian method is a powerful tool to estimate the resistivity distribution and associated uncertainty from time-domain electromagnetic (TDEM) data. Because the forward simulation of the TDEM method is computationally expensive and a large number of samples are needed to globally explore the model space, the full Bayesian inversion of the TDEM data is limited to layered models. To make the high-dimensional Bayesian inversion tractable, we have adopted a divide-and-conquer strategy to speed up the Bayesian inversion of TDEM data. First, the full data sets and model spaces are divided into disjoint batches based on the coverage of the sources so that the independent and highly efficient Bayesian subsampling can be conducted. Then, the samples from each subsampling procedure are combined to get the full posterior. To obtain an asymptotically unbiased approximation to the full posterior, a kernel density product method is used to reintegrate samples from each subposterior. The model parameters and their uncertainty are estimated from the full posterior. Our method is tested on synthetic examples and applied to a field data set acquired with a large fixed-loop configuration. The 2D section from the Bayesian inversion revealed several mineralized zones, one of which matches well with the information from a nearby drillhole. The field example indicates the ability of the Bayesian inversion to infer reliable resistivity and uncertainty.

SUMMARY The lack of a systematic comparison of field surveys on land makes it challenging to select a suitable grounded-wire transient electromagnetic (TEM) source. Therefore, in this study, we compare land-based TEM methods with single- and double-line grounded-wire sources in the broadside direction. The comparison results indicate that the amplitude of the electric field from the double-line source is one or two orders weaker than that of the single-line source in the same time range. Further, we compare the detectability of horizontal electric fields for resistive or conductive targets by analysing the root mean square misfit and decay curves. According to the decay curve analysis, the double-line source shows better detectability for resistive targets, whereas the single-line source has a larger depth of investigation in the broadside direction. In the presence of a resistive target, sign reversal is more likely in the broadside direction for the double-line source than for the single-line source. In the field survey over Kalatongke Ni-Cu sulphide deposits in Xinjiang, China, both single- and double-line configurations can locate the resistive target; however, the latter configuration outperforms the former in discriminating the different values of the resistive rocks in the broadside direction. The double-line source will be the preferred choice for exploring resistive targets in the broadside direction.

Transient electromagnetic method (TEM) is one of the major tools to image the subsurface resistivity. The gradient-based inversion of TEM data only provides a unique solution using a subjectively defined regularization penalty, leaving the uncertainty of the solution unaddressed. The Bayesian method can be used to estimate the model parameters, as well as quantify their uncertainty. However, it requires far higher computational costs than gradient-based inversion, which limits the Bayesian inversion of TEM data to 1-D assumptions. We propose an accelerated Bayesian method based on Markov chain Monte Carlo (MCMC) subposteriors to perform full 2-D inversion of TEM data. A robust scheme is designed to divide the model space of a TEM profile into subspaces so that independent MCMC chains can be used to update the parameters in each subspace in parallel. The division is based on the coverage of the source loops using a cumulative sensitivity matrix. Then, the subposteriors obtained at each subspace are merged to approximate the full posterior of the model space using a weighting strategy. A numerical test of a 2-D valley model is used to validate the proposed method. The Bayesian inversion successfully obtained posterior that converges to the true model. The median model makes a good inference of the model parameters, while the probability density function gives their uncertainty estimates. The statistical model of interest can be further extracted from the model ensemble. The proposed method provides an effective framework for Bayesian inversion of TEM data with a multidimensional forward operator.

The separation of P- and S-wavefields is considered to be an effective approach for eliminating wave-mode crosstalk in elastic reverse time migration (RTM). At present, the Helmholtz decomposition method is widely used for isotropic media. However, it tends to change the amplitudes and phases of the separated wavefields compared with the original wavefields. Other methods used to obtain pure P- and S-wavefields include the application of the elastic wave equations of the decoupled wavefields. To achieve high computational accuracy, staggered-grid finite-difference (FD) schemes are usually used to numerically solve the equations by introducing an additional stress variable. However, the computational cost of this method is high because a conventional hybrid wavefield (the P- and S-wavefields are mixed together) simulation must be created before the P- and S-wavefields can be calculated. We have developed the first-order particle velocity equations to reduce the computational cost. The equations can describe four types of particle-velocity wavefields: the vector P-wavefield, the scalar P-wavefield, the vector S-wavefield, and the vector S-wavefield rotated in the direction of the curl factor. Without introducing the stress variable, only the four types of particle velocity variables are used to construct the staggered-grid FD schemes; therefore, the computational cost is reduced. We also develop an algorithm to calculate the P and S propagation vectors using the four particle velocities, which is simpler than the Poynting vector. Finally, we apply the velocity equations and propagation vectors to elastic RTM and angle-domain common-image gather computations. These numerical examples illustrate the efficiency of our methods.

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 concept of a smoke-ring in electromagnetic fields can be used to describe the spatial distribution and diffusion of electromagnetic fields with either frequency or time. This provides an intuitive basis for qualitative interpretation of artificial source electromagnetic measurement results. The existing discussions regarding this field of study have mainly focused on the smoke rings of a magnetic dipole or loop source. The smoke ring of the other commonly used grounded-wire sources in the time domain was investigated. The results show that the smoke ring of grounded-wire sources is more complex than that of magnetic sources. A grounded-wire source can generate induced current in both the horizontal and vertical directions. The horizontal induced current consists of two parts. At the early stage, when the source has just been turned off, there is a low-amplitude and fast-diffuse opposite return current. With time it becomes positive, and reaches a maximum, then decreases. The horizontal induced current maximum diffuses downward perpendicular to the surface, and the vertical induced current maximum diffuses at 45 degrees from the earth’s surface with a faster diffusion velocity, which is roughly 5 times that of the horizontal induced current. The smoke ring of the horizontal induced current more closely resembles the shape of a basin, while that of the vertical induced current is more like two separate jars. Finally, the respective imaging depths estimated based on the horizontal and vertical induced currents are 0.92 and 1.25 times the diffusion depth.

A near-source electromagnetic method known as the short-offset transient electromagnetic (SOTEM) method was proposed. Compared with the traditional far-source methods, the offset of the SOTEM method is greatly reduced, to only 0.3~2 times of the maximum depth. Therefore, the SOTEM can obtain the signal with higher signal-to-noise ratio and sensitivity. Application in the Xiaoshan Mine of Henan Province showed that the SOTEM method is an effective method for deep ore exploration, especially in mountainous areas.

Transient electromagnetic method (TEM) is an important tool for mapping the subsurface resistivity. As the electromagnetic fields with different frequencies reach different depth of the Earth, the resolution of TEM method is closely related to the spectrum properties of its source signal. The pseudorandom binary sequence (PRBS) is a recently applied source signal and is thought to be superior to the conventional square wave for its adjustable frequency range and density. However, to invert the response from this optimized source, the pure secondary field needs to be extracted from the full waveform response in which parts of useful information is lost inevitably. We thus propose to directly invert the full waveform TEM response from the excitation of PRBS using a transdimensional framework. We use the reverse-jump Markov chain Monte Carlo (RJ-MCMC) sampler to get the model ensemble so that the posterior of the model can be obtained. The PRBS source is convolved with the impulse response of the Earth to fit the recorded full waveform response. The 2-D model space is discretized using a two-stage binary tree and updates in parallel, which dramatically improve the sampling efficiency. The synthetic examples compare the results from the conventional time-domain step response and the full waveform response and demonstrate that the proposed method obtains higher resolution and can better resolve the interface in the shallow and moderate depth. Furthermore, the real data example recovers the model structure that is consistent with the geologic inference, providing alternate evidence for the conjecture of the Sanshandao fault.

Existing noise removal processes for airborne electromagnetic (AEM) data generally consist of several steps, with each using a specific method to remove a specific type of noise. To improve the efficiency of AEM denoising and reduce the impact of the subjective judgment of the operators on the processing results, we have adopted a deep learning method based on a denoising autoencoder (DAE), which enables in one single processing step the removal of multisource noise. The most common noise sources in AEM data, including motion-induced noise, nearby or moderately distant sferics noise, power-line noise, and background electromagnetic noise, will be combined with a large number of simulation responses to build a training set. The data in the training set will be used to train the deep learning DAE neural network so that the neural network could fully learn the respective characteristics of the signal and noise and further effectively distinguish the AEM response signal (useful signal) from the above noise. The field data were processed using this method, and the processing results were compared with those obtained using traditional methods. The comparison test revealed that this method is helpful to reduce the influence of subjective factors on the quality of data results and compress the entire AEM data processing time.

The water burst from the Ordovician limestone underlain by the Permo-Carboniferous coal seams have potential to trigger coalmine hazards in Northern China. Therefore, it is crucial to identify and accurately map the water enrichment zones and delineate coal seams using an integrated approach based on surface TEM and subsurface wireline log information to avoid water-inrush hazard and ensure safe production of coal. We inverted surface based TEM data using 1-D Occam inversion to identify the conductive anomaly and then further quantified the zone of interest by gamma and resistivity logs. 1-D Occam inversion results show conductive zone around 370 m while higher resistivity and lower gamma ray log signatures were observed against coal seams. Groundwater inrush zone falls within the mid-range gamma ray and resistivity interval as shown on the petrophysical logs. The distinct log signatures (low gamma-ray and high resistivity values) clearly indicated coal seams at depth of 410 and 470 m and subsequently the log trends were used to distinguish between coal units and more permeable sands. The magnitude and the variability of these parameters in the borehole are attributed to the subsurface stratigraphic heterogeneity. They can be key clues for interpretation of depositional facies of coal-bearing sequence and may also be used as a constraint in characterization of groundwater enrichment zone.

Transient electromagnetic (TEM) data can be seriously distorted by induced polarization (IP) phenomena when a polarizable body is present. The TEM field generated by a grounded-wire source contains transverse electric (TE) and transverse magnetic (TM) modes. The IP effect is most commonly studied with the TEM total field, rather than considering the difference between TE and TM fields. To investigate the effect of IP phenomena on the TE and TM fields, we have performed a detailed analysis on IP-distorted TEM data based on numerical and field examples. We first compare the IP effect on the TE and TM fields when polarizable bodies with different polarizable parameters are present. The TM field is more severely affected by the IP effect than the TE field. Compared to a single grounded-wire source, a double-line grounded-wire source can generate a larger TM field in the horizontal electric field. We compare the IP effect on TEM data from single- and double-line grounded-wire TEM configurations, and find that the data from the double-line configuration have a higher TM/TE ratio and are more severely affected by IP phenomena than in the single-line case. Thus, it would be easier to identify and extract the IP response from field data acquired with a double-line grounded-wire source configuration. These results have been verified by a field survey of the Kalatongke copper-nickel ore district, which has predominantly layered geology, in Xinjiang, China.

The surface-to-borehole transient electromagnetic (TEM) method, which uses a rectangular loop as its source, has shown great potential in mineral exploration at greater depth. However, recent studies have demonstrated that a grounded-wire source can resolve the resistive targets and provide detection at greater depths than the loop source. Thus, we propose a surface-to-borehole TEM method using a grounded wire as a transmitter at short offsets. We conducted numerical experiments to explain the variation of different parameters and its comparable effects on the TEM transients. Similarly, a systematic comparison of the cross-sectional electric field distribution maps, borehole profiles of the time derivatives of the magnetic field, qualitatively reveal that the proposed method induces detectable responses for conductive and resistive targets at greater depths. In the present study, unlike the loop source configuration, the time derivative of an underground magnetic field in horizontal direction showed high sensitivity to resistive bodies. Furthermore, 1-D inversion was performed to provide quantitative interpretation of the borehole data. The 1-D inversion of the surface and borehole data was realized using an Occam-type inversion scheme. The results of both synthetic and field data indicate that inverting the surface or borehole data separately resulted in degraded resolution in both shallow and deep earth. Thus, we also compared the results from the inversions of surface and borehole data in order to explore the merits of joint inversion in resolving shallow and deep structures.

China is facing many challenges in the area of high-precision exploration for deep mineral resources, and also in environmental and engineering fields. It has become an urgent task to realize geophysical explorations with high precision and high efficiency in complex environments. It has been found that the traditional transient electromagnetic method (TEM) had a series of problems related to theory, method, and instrument, as well as data interpretation. Based on the reviews of present TEM achievements, three new improved measurements are introduced. The central-loop system was improved by a large fixed-loop system; the long-offset system was changed to the short-offset one; the resistivity profile was transformed into a virtual wave-field profile. Finally, this paper points out that the future research direction of the TEM method is in terms of equipment, data processing, and big data information.

An ideal metallogenic and prospecting model has important guiding significance for aluminum ore development and geophysical exploration. Previous research in this field only focused on ore body evaluations and metallogenic belts. Selection of reasonable geophysical methods for exploration of bauxite deposits is still challenging. To solve these problems, a new metallogenic model based on the new geological information is established firstly. The ore-bearing rock and ore bodies of sedimentary bauxite deposits are strictly controlled by the paleo-topographic conditions. The undulating shapes of the paleo-topography control the scales and shapes of ore-bearing rock series and bauxite ore bodies. Then, different geophysical methods applied for exploration of bauxite deposit are analyzed based on the petrophysical measurement. For the areas with thick overburden and large buried depths, it was found that fixed-loop transient electromagnetic (TEM), high-precision gravity, and seismic exploration methods were most effective. In regard to the areas with thin overburden and small buried depths, direct current (DC) sounding, high-precision gravity, seismic exploration, and TEM methods were considered to be effective. However, for the areas with very shallow buried depths, DC sounding and seismic exploration methods were deemed to be more suitable. This study selected the actual exploration processes of the Wanggudong bauxite mining area in western Henan as an example, and the application of a DC sounding method for sedimentary-type bauxite was successfully introduced.