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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.

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 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.

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.

Transient electromagnetic method (TEM) is a geophysical tool to obtain resistivity distribution in the subsurface. Combining with the resistivity property of typical rocks, the TEM method can make inferences of the geological maps underground. The inversion method is the main technique to extract resistivity form the recorded TEM data. There are various inversion methods that have been applied to TEM data, each of which favors different model structures. It is essential to choose the optimal inversion algorithm for a TEM survey in a given geological setting. Thus, this article presents a systematic summary of recent developments of inversion methods for TEM data. We first summarize the basic concept of the TEM inversion theory. Then, the recent developments TEM inverse method are divided into deterministic inversion and stochastic inversion. For the deterministic method, we present the development of constrained inversion and joint inversion. For the stochastic method, we analyze the particle swarm optimization, Bayesian inversion, and TEM pseudo-seismic imaging. Thereafter, we prospect the future research direction of the TEM method.

Over the past decade, helicopter-borne transient electromagnetic (HTEM) systems have been rapidly developed. A new HTEM prototype (referred to as a CAS-HTEM) has been developed by the Chinese Academy of Sciences. In terms of hardware, the CAS-HTEM system uses an inflatable structure to carry the transmitting loop, which significantly reduces the weight of the system and makes it easier to transport. A dual gain receiver was innovated to extend the dynamic range of the system. In addition, an observation circuit for transmitting voltage waveform is introduced, so that the derivative waveform of transmitting current with higher SNR could be calculated. In terms of data processing, more reliable early time data could be obtained by band-limited effect removal; a weighted stacking algorithm is introduced to reduce the narrow band noise more effectively and increase the sensitivity of data to the anomaly location; a method based on τ-domain transform is used for late time signal processing. The results of the field test which was carried out in Inner Mongolia were found to be consistent with the drill data, which effectively verified the performance of this HTEM prototype.

Powerline noise is one of the most common contaminating types of the observed transient electromagnetic signal. For the conventional TEM method using the bipolar square wave as transmitter waveform, synchronous sampling is the main technology for powerline noise suppression, and notching filtering is sometimes used also. When the transmitter wave has been encoded based on pseudo-random binary sequence, it has proven difficult to achieve the effect by using the above-mentioned conventional methods for the suppression of powerline noise. This is due to the fact that the duration of each logic states of the pseudo-coded transmitter waveform is normally inconsistent. In this study, a method for the suppression of powerline noise is proposed, which is based on the independent component analysis method (ICA). In order to introduce the observation and processing details of this method more clearly, we attempt to apply this method to the time-domain electromagnetic method with coded source researched and developed by the institute of geology and geophysics of Chinese academy of sciences on the basis of the MTEM method. In terms of specific processes, the electrical measurements need to be simultaneously observed in the inline and crossline directions firstly, and then the data will be input into the processing procedures based on ICA method to realize the effective separation of the powerline noise and the useful signals. The processing results of the simulation data and field data show that the method proposed in this paper can suppress the powerline noise effectively, and the processing results build a good data foundation for the subsequent processing.

We have developed a scheme for decoupling the induced polarization (IP) effect from time-domain electromagnetic (TDEM) data. This scheme is achieved by simultaneously sampling the resistivity and pseudochargeability in a Bayesian framework. The TDEM and IP responses are simulated separately with the sampled model parameters and then are stacked to fit the IP-affected TDEM data. Thus, the influence of the IP phenomenon is eliminated in the process of recovering the resistivity. To reduce the computational cost brought by the Bayesian sampling, we use a 2D parametrization instead of sampling the full 3D space and we use a linear perturbation approximation for calculating the IP response. The linearized inversion results are used as the initial model, and a multiple proposed points algorithm is used to accelerate the sampling. We validate the proposed method with synthetic and field examples showing that it restores accurate estimates of electrical structures from the TDEM data that are significantly affected by the IP phenomenon. Our method could advance the application of the TDEM method to the scenario in which the IP may affect the TDEM data and mask the underlying geologic targets.

The diffusive electromagnetic field can be transformed into the wave domain by means of mathematical conversion. The transformed field can then be interpreted with the tools in seismic data processing so that the identification to the underground targets can be effectively improved. However, the conversion is typically an ill-posed problem that needs to be solved using regularization tools. Based on the conventional regularization with smooth constraints in the L2 norm, the inversion result is of low resolution, while that obtained using truncated singular value decomposition (TSVD) methods is typically accurate, but has poor stability. To obtain a stable and accurate transformed electromagnetic field value, this study proposed to combine conventional regularization tools and singular value decomposition algorithms by incorporating a set of weighting coefficients. The proposed method is validated on both synthetic and observed data. The results from the proposed method are more accurate at the early time, and at the late time are more stable compared with the other methods. Furthermore, the example of field data shows that the proposed method could potentially further improve the interpretation accuracy of future mining explorations.

Hydraulic connections between aquifers is usually studied through hydrochemical analysis or by pumping tests. However, hydrochemical analyses are usually conducted in areas of variable lithology. In addition, the hydrogeological data obtained by drilling and pumping tests are typically insufficient to get 3D distributions of hydraulic head. In this paper, the time-lapse transient electromagnetic method (TEM) is used to image groundwater migration between aquifers in Inner Mongolia, China. First, 1D geophysical models of aquifers are generalized according to the hydrogeological conditions of the region, and the feasibility of detecting the multiple aquifers by TEM is analyzed and discussed. Then, the 2D models of aquifers pre- and post- pumping test are established based on the distribution of groundwater in the aquifers, and the variation law of induced electromotive force measured on the surface is analyzed. The simulation results show that significant time-lapse electromagnetic anomalies can be observed between pre- and post- pumping test and the variation in the induced electromotive force reaches a distinguishable level between 0.7 ms and 100 ms due to the vertical change in the aquifer properties. Furthermore, the electromagnetic variation generated by hydraulic connection between aquifers is greater than 30% within the range of 3/4 of the transmitting loop. Finally, a successful case history to map hydraulic connections between aquifers is conducted using a time-lapse TEM pre- and post- a pumping experiment. This simulation and field experiment shows that time-lapse TEM could characterize and monitor the groundwater migration more effectively than pump tests or hydrogeochemical methods alone.

Controlled source audio-frequency magnetotelluric (CSAMT) is typically used to obtain data regarding Cagniard resistivity by calculating the ratio between the electric field and magnetic field. However, noise conditions tend to provide different influences on the electrical and magnetic field component data. Therefore, it has become necessary to develop alternative methods to define apparent resistivity using suitable component data. In addition to the traditional Cagniard resistivity, the conventional single-component and modified single-component apparent resistivity methods have been proposed. In this research we discuss those three different apparent resistivity definitions from the aspects of theoretical formula, applicability, and calculation results of a synthetic model. We find that the modified single-component apparent resistivity method can be successfully used to achieve higher data quality nearer the source, thereby improving the exploration precision. Finally, a case study is conducted to verify our results, which show that the modified apparent resistivity method has the ability to effectively reflect the geoelectric profiling information.

In helicopter-borne transient electromagnetic (HTEM) signal processing, removal of motion-induced noise is one of the most important steps. A special type of short-term noise, which could be classified as high-frequency motion-induced noise (HFM noise) based on its cause and time-frequency features, was observed in the field data of the Chinese Academy of Sciences-HTEM system. Because the HFM noise is an in-band noise for the HTEM response, it usually remains after the normal denoising procedure developed for the conventional motion-induced noise. To solve this problem, we have developed a three-stage workflow to remove the HFM noise using the wavelet neural network (WNN). In the first stage, the WNN training is performed, and the data segment in which the HFM noise is dominant is selected as the sample set. In the second stage, the HFM noise corresponding to the data segment in which the earth’s response coexisted with the HFM noise is predicted using the well-trained WNN. In the last stage, the predicted HFM noise is removed from the corresponding original data. As an example, we applied our workflow in the field data observed in Inner Mongolia, the HFM noise is removed effectively, and the results provide a strong data foundation for the subsequent processing procedures.

For controlled source audio frequency magnetotellurics (CSAMT) which supported by traditional dipole hypothesis theory, the observations point should be located in far source zone. However, during field measurements, due to limited transmitter power, the distance between transmitter and receiver is usually not too large. Sometimes, field measurements are carried out in the near source zone, and the traditional dipole hypothesis will produce large errors under such condition. So, the errors between the approximate solution of traditional dipole approximation and the exact solution of the long grounded-wire source are firstly studied in this paper. Moreover, relative error distribution between exact solution of the long grounded-wire source and electric dipole source variations with different frequencies has been analyzed. It finds that the traditional dipole hypothesis bring large errors in the near source zone and no longer be satisfied to such condition. As a further study, a new ‘harmonic point charge’ solution for CSAMT, instead of the traditional dipole hypothesis, is proposed in this paper, which lay a foundation to realize near source prospecting and obtain more accurate near source responses in frequency domain electromagnetic.

Multi-channel transient electromagnetic (M-TEM) system, which is a time-domain control source electromagnetic (CSEM) system, is mainly used in hydrocarbon exploration . In this paper, we experimentally applied the method to the investigation of mineral resources. Firstly, we introduced the basic principles and imaging technique of a typical M-TEM survey. Then, we presented a field example which was conducted over a molybdenum mine with the newly developed instruments. The high power transmitter and the pseudo random binary sequences (PRBS) are employed in the field work . The result of the field test validates the feasibility of the M-TEM system in identify orebody.

In northern China, thick loess strata with low resistivity are widely present and can shield the deep induced electromagnetic field signal and influence the accuracy of deep targets while electromagnetic prospecting is being performed. To achieve deep target exploration with high resolution, a new technique known as the short offset transient electromagnetic method (SOTEM) is applied in this research area. This method can be used to perform near-source configuration surveys, but shows no obvious difference between the deep magnetite deposit and low-resistivity layer in this survey area. Therefore, it cannot explore the magnetite deposit directly using the traditional magnetic component (Hz) of SOTEM. The Ex component, which can more effectively distinguish the high-resistivity surrounding rock and overlying low-resistivity layer than the Hz component, is used for exploration. First, 2D forward modeling and sensitivity analysis of the high-resistivity body are carried out to determine the different responses of the Ex and Hz components, respectively. Moreover, to better determine the interface between the surrounding rock and low-resistivity layer, 1D Occam inversion and the generalized inverse matrix inversion method are used. Furthermore, aiming at the static effect of the Ex component in field exploration, joint inversion of the Ex and Hz components is carried out for comprehensive interpretation. Finally, a field test is conducted, and the drilling results are used for verification. The research results can provide an effective geophysical model for the exploration of contact metasomatic magnetite deposits.

At present in China, the use of the transient electromagnetic method (TEM) is emerging as a leading geophysical technique for exploration of water-filled zones in coal mines. These zones are more conductive than the host coal and are easy targets to map. However, there is a growing interest for the investigations of double-layered or multi-layered mined-out zones. Therefore, it is necessary to study the feasibility of TEM's abilities to detect double-layered, water-filled voids. In this study, the basic hydrogeological conditions of a survey area, located in the northern China, are described. The corresponding geophysical models of the single- and double-layered water-filled zones are developed from borehole logging data. Then, forward calculations of different models are carried out with 1D numerical simulations. The modeling results show that it is feasible for TEM to identify these types of targets under certain conditions, including instrument sensitivity, low resistivity for the water-filled zones, and shallower depths. Moreover, the field survey for locating double-layered water-filled zones in coal mines in the Datong region of Shanxi Province is verified by well drilling.

The detection accuracy of the grounded-wire transient electromagnetic method is affected by the incorrect geoelectric structure and burial depth due to shadow and source overprint effects. To better understand these effects, the three conditions under which these effects are present are analyzed separately. First, the analytical expressions are derived for computing the response of the formative wave and the surface wave in homogenous earth. The effects of the formative wave at the anomaly’s location and at the receiving site are analyzed. Then, the response variation due to the presence of an anomaly between the transmitter and the receiver is quantitatively analyzed using the finite-difference method. Finally, the effects of shadow and source overprint on the data inversion are investigated, which is verified by a case study. The wrong geoelectric information is caused by the shadow and source overprint effects. To avoid this, multisource observation can be used to weaken and utilize the shadow and source overprint effects.

The grounded-wire transient electromagnetic method (TEM) has been widely used for near-surface metalliferous prospecting, oil and gas exploration, and hydrogeological surveying in the subsurface. However, it is commonly observed that such TEM signal is contaminated by the self-transient process occurred at the early stage of data acquisition. Correspondingly, there exists a minimum depth of investigation, above which the observed signal is not applicable for reliable data processing and interpretation. Therefore, for achieving a more comprehensive understanding of the TEM method, it is necessary to perform research on the self-transient process and moreover develop an approach for quantifying the minimum detection depth. In this paper, we first analyze the temporal procedure of the equivalent circuit of the TEM method and present a theoretical equation for estimating the self-induction voltage based on the inductor of the transmitting wire. Then, numerical modeling is applied for building the relationship between the minimum depth of investigation and various properties, including resistivity of the earth, offset, and source length. It is guide for the design of survey parameters when the grounded-wire TEM is applied to the shallow detection. Finally, it is verified through applications to a coal field in China.

The grounded-wire transient electromagnetic (TEM) survey has been successfully applied in the mining industry to detect resources in the subsurface. Recently, research efforts have been devoted to developing both the theory and application of the short-offset TEM (SOTEM), which requires the layout of a line source for transmitting the electromagnetic signals. In practice, however, the terrain complexities of exploration areas, such as mountains and construction areas, often limit the layout of such linear geometry and moreover cause the source to be folded several times, which undesirably increases the computational differences especially when the line-source model is used for field data processing and interpretation. Therefore, to achieve high resolution TEM detection, it is now necessary to adapt the source layout to the local terrain. For describing the effect of source geometry on the SOTEM surveying, this study first presents a superposing-dipole approach for estimating the SOTEM field from arbitrarily-shaped sources and then qualifying the relative differences in the horizontal electric field due to varying source geometry. It is expected that such approach could be implemented for improving the TEM detection accuracy by taking into account all the turning points in a non-linear source layout, especially in the restricted areas. [Figure: see text]

Long-offset transient electromagnetic (LOTEM) has received great attention in mineral, hydrocarbon and hydrogeological investigations for the last several years. Conventionally, TEM soundings have been presented as apparent resistivity curves as function of time. With development of sophisticated computational algorithms, it became possible to extract more realistic geoelectric information by applying inversion programs to 1-D and 3-D problems. Here, we analyze LOTEM data by carrying out analysis in time, frequency and pseudo-seismic domain supported by borehole information. At first, H, K, A & Q type geoelectric models are processed using a proven inversion program (1-D Occam inversion). Second, time-to-frequency transformation is conducted from TEM ρa( f) curves for the same models based on all-time apparent resistivity curves. Third, 1-D Bostick's algorithm was applied to the transformed resistivity. Finally, EM diffusion field is transformed into propagating wave field and constructed pseudo-seismic section. The transformed seismic-like wave indicates that some reflection and refraction phenomena appear when the EM wave field interacts with a geoelectric interface at different depth intervals due to contrast in resistivity. In all three cases, synthetic tests showed that conductive anomaly in resistive host environment can be retrieved more clearly than resistive target. A case study illustrates the successful application of proposed approach in recovering a water-filled mined-out area in a coal field located in Ye county, Henan province, China. The results support the introduction of pseudo-seismic imaging technology in long-offset version of TEM which can also be an useful aid if integrated with seismic reflection technique to explore possibilities for high resolution EM imaging in future. [Figure: see text]

The geometry of source and receiver is one of the most crucial factors for the success of a controlled-source electromagnetic (CSEM) investigation. The inline electric-source configuration, widely used in the marine CSEM method, is not commonly employed for land CSEM surveys. To examine the feasibility of the inline electric-source time domain electromagnetic method (TEM), we first analyzed the optimal observing area for the land-based electric-dipole source by imaging the distribution of each component and the anomalies caused by a resistive or conductive target layer. Then, we conducted an error-space analysis to evaluate the sensitivity of each inline component based on the result of the optimal observation area. Finally, we collected field data from a typical inline dipole-dipole configuration and inverted the data with an algorithm based on Occam's principle. [Figure: see text]

The migration of the transient electromagnetic method (TEM) has been a research topic for decades. The method, which uses the mathematical q transform to perform imaging in the wave domain, has proved its validity in some circumstances. One of the crucial steps of this kind of method is the inverse q transform, which extracts the pseudo-wave field that satisfies the wave equation from the TEM data. Conventionally, the time range of the q integral is divided into several windows and the transformation result is recovered using a regularization method, in which each window is summed up to get the final wave field. Although the recovered wave field demonstrates the key features as is indicated in the wave equation and the corresponding imaging techniques have proved to be useful, the accuracy of the extraction of these wave fields hampered the development of more accurate migration techniques. This paper will focus on the extraction of the wave field from the TEM data. A novel correlation stacking method was proposed and compared with singular value decomposition (SVD) and the pre-conditioned regularization conjugate gradient method. The comparison of the results from these methods demonstrates that the extraction results of the proposed correlation stacking method were more accurate and stable. The pseudo-seismic wave field extraction results of the actual measured TEM data showed that the correlation stacking method was the optimal method for the pseudo-seismic wave field extraction of TEM data. [Figure: see text]

Near-source electromagnetic technology has been developed and applied in the exploration of petroleum, metallic ore, coal, and engineering geology due to its high efficiency, high detection accuracy, and deep depth of investigation. In this paper, research and applications of the frequency-domain electromagnetic sounding method (FDEM), wide-field electromagnetic method (WFEM), modified central-loop transient electromagnetic method (TEM), and short-offset grounded-wire TEM (SOTEM) with obvious near-source characteristics, were reviewed and analyzed. From the 1960s to 1990s, the FDEM method and equipment were extensively developed in China. These methods have played important roles in the exploration of coal resources. Based on controlled source audio-frequency magnetotelluric (CSAMT) and FDEM methods, a new method has been developed by deriving a new expression to calculate apparent resistivity. This method, which is referred to as WFEM, has been studied, applied, and received great attention in China. To increase work efficiency and reduce the influence of local transverse anisotropy on the detection processes, a modified central-loop TEM detection technology based on the central loop transient electromagnetic method was developed in China. The advantages of SOTEM in near-source surveys with high resolution and increased depth detection stimulated academic research interest to further develop grounded-wire TEM techniques. [Figure: see text]

Ground-based, electrical-source, and UAV-borne receiver TEM configurations have previously been used to map mines in Jiangsu Province, China. In this study, the EM responses of air-filled mine tunnels were simulated by using a three-dimensional (3D) vector finite element method. A new apparent resistivity formula has been proposed for the ground-airborne TEM configuration. In the study area, field measurements were carried out along 36 profiles for the ground-airborne TEM and 16 profiles for ground TEM. The ground-airborne TEM results were determined to be in good agreement with the ground TEM survey results using a surface central loop, and were also consistent with the known geologic conditions. The experiment was successful and showed that a ground-ground-airborne TEM method could potentially become a novel alternative for both future civilian and military applications. [Figure: see text]

The water-rich zones in the coal fields of Northern China have seriously threatened the safe production of coal mines. It is necessary to accurately locate the deep seated and complex water bodies which are usually located in Ordovician limestone. Due to the relatively high cost of seismic exploration, the development effective electromagnetic detection methods and corresponding data processing techniques for the detection of these water-rich areas becomes a topic for the EM geophysicist. In this study, a method of rapidly estimating the burial depth and undulating morphology of coal bed foundations by employing frequency domain electromagnetic sounding data was proposed. The method uses the overshoot/undershoot phenomenon of the apparent resistivity for the frequency domain electromagnetic measurement to quantitatively determine the burial depth of the northern China coal structure bases. First, the featured frequency points were confirmed, where the overshoot and undershoot effects appeared in the electromagnetic sounding curves beside the borehole drilling sites. Then, the correlation coefficient between the confirmed frequency points and the burial depth of coal seam bed was obtained according to the drilling data. Finally, the burial depth of the coal seam beds in the entire survey area was evaluated based on the correlation coefficient and electromagnetic data. This method was applied in the northern areas of Taiyuan City in Shanxi Province, and achieved good results. [Figure: see text]

The in-loop transient electromagnetic (TEM) method has been widely used for reliable investigation of mined-out areas of the subsurface. However, the method has limited applications in mountainous coalfields that are often covered with tall vegetation: first, it requires extra work to clear the surface and set up the TEM equipment; more importantly, dense vegetation restricts the required layout of a standard rectangular loop, and such geometry irregularity can decrease the detection accuracy. This study proposes using short-offset grounded-wire TEM (SOTEM) for enhanced detection of the mined-out areas in vegetation-covered mountainous fields. The distance between the SOTEM source and receivers is flexible and thereby the receiving location can be freely adjusted according to the environmental conditions of the surveying area, without reducing the detection resolution. Properties of the proposed SOTEM method are well tested before real applications, and our test indicates that SOTEM has an axis-symmetric ...

Increases in the mining depth of coal pose a significant challenge to the conventional loop source time-domain electromagnetic (TEM) method that requires significant enlargement of the loop size and transmitting current to realize the deeper sounding results required. As an alternative, TEM devices based on a grounded wire source are generally used to solve detections deeper than several hundred meters. To map the water-enriched zones buried underneath approximately 1000 m at a coal mine in Shaanxi, China, loop TEM and short-offset transient electromagnetic (SOTEM) measurements were conducted. We carried out 1D forward modeling and inversion constrained by drilling informa-tion, and the results reveal that the resolution capability of loop TEM and SOTEM is almost the same in detecting a conductive layer in the absence of any noise. However, for a given noise level and decay time, the SOTEM method provides a deeper investigation than loop TEM without compromising sensitivity. The field examples validated the synthetic results. The loop TEM with dimensions of [Formula: see text] realized a maximum depth of 1000 m, whereas the reliable detection depth of 1500 m was achieved by using a 723 m long grounded wire source using the SOTEM method. Moreover, the labor required is significantly reduced, and the efficiency is dramatically raised using the SOTEM method. Our results predict that the SOTEM method should play a more important role in deep hydrogeophysical investigations.

The grounded-wire transient electromagnetic (TEM) surveying is often performed along the equatorial direction with its observation lines paralleling to the transmitting wire with a certain transmitter–receiver distance. However, such method takes into account only the equatorial component of the electromagnetic field, and a little effort has been made on incorporating the other major component along the transmitting wire, here denoted as axial field. To obtain a comprehensive understanding of its fundamental characteristics and guide the designing of the corresponding observation system for reliable anomaly detection, this study for the first time investigates the axial electric field from three crucial aspects, including its decay curve, plane distribution, and anomaly sensitivity, through both synthetic modeling and real application to one major coal field in China. The results demonstrate a higher sensitivity to both high- and low-resistivity anomalies by the electric field in axial direction and confirm its great potentials for robust anomaly detection in the subsurface.

The inline dipole–dipole time-domain electromagnetic method is a useful supplementary tool for the exploration of a resistive target such as a hydrocarbon resource. Currently, the interpretation of the data from this configuration is based on the fitting of the modeled response and the recorded response in a least square sense. In the paper, we propose an imaging method based on a mathematical transformation named the q-transform, which could make the subsequent imaging of TEM data using the interpretation technique in seismic method possible. The imaging strategy is based on analytical derivation and analysis of the transformed wave-field from which pseudo-velocity and resistivity can be extracted. A numerical test is employed to get the transformed wave-field from the time-domain electromagnetic response with the selected regularization method. The proposed imaging method is tested on the synthetic data obtained from a homogeneous and layered half space models. The results reveal that it is a useful method to recover the resistivity of an underground medium and can provide an additional tool for fast and qualitative imaging of the subsurface resistivity.

Huoqiu iron deposit is a typical Precambrian banded iron-formation (BIF) field which is located in the North China Craton (NCC). To detect the deep ore bodies around Dawangzhuang Village in Yingshang County, north of the Huoqiu deposit field, electromagnetic methods were tested. As the ore bodies are buried under very thick conductive Quaternary sediments, the use of EM methods is a great challenge. Short-offset transient electromagnetic method (SOTEM) was applied in the area as we wanted to test due to its detection depth and resolution. A 2D model was first built according to the geology information and magnetic measurement results. Then, 2D forward and 1D inversion were carried out using FDTD and Occam’s algorithm, respectively. The synthetic modeling results helped us with the survey design and interpretation. Two 1400-m-long survey lines with offset of 500 and 1000 m were laid perpendicular to the BIF’s strike, and the transmitting parameters were selected by a test measurement at the vicinity of a local village. Finally, the structure of survey area and BIF bodies were determined based on the 1D inversion results of real data, and showed a consistency with the subsequent drill results. Our application of SOTEM in detecting hidden BIF buried under very thick conductive layer has shown that the method is capable of penetrating great depth more than 1000 m even in a very conductive environment and will be an effective tool for deep resources investigation.

The loop source TEM method has been widely used in the detection of a mined out area in China. In the cases that the laying of traditional rectangle or square transmitting loop is limited due to the presence of obstacle on the path of the loop, the changing of the shape of the transmitting loop to bypass the obstacle is a labor saving solution. A numeric integration scheme is proposed to calculate the response and Jacobian of the segmented loop source from that of an electric dipole source. The comparison of forward response between the segmented loop and square loop shows the effect of loop geometry on the decay curves. In order to interpret the data from an irregular source loop, this paper presents an inversion scheme that incorporate the effect of loop geometry. The proposed inversion scheme is validated on the synthetic data, and then applied to the field data. The result reveals that the developed inversion scheme is capable of interpreting the segmented loop source TEM field data.

The short-offset transient electromagnetic method (SOTEM) has been extensively used for mineral and hydrocarbon exploration and hydrogeological investigations due to its ease of use and capability to generate diagnostic subsurface information. At present, the data processing methods of SOTEM are mainly focused on one dimensional inversion. To apply the proven inversion methods of frequency domain electromagnetic methods to SOTEM data, this paper presents a new transformation relation from time to frequency based on the similarity between SOTEM all-time apparent resistivity and magnetotelluric (MT) apparent resistivity. Results show that the transformation coefficients depend on the variation trend of SOTEM all-time apparent resistivity curves. Bostick inversion and conjugate gradient inversion techniques were applied to transformed SOTEM data and the results were validated by some simulated calculations and field measured data. This study provides a novel method to SOTEM data processing and a useful aid to join inversion with MT data.

Equivalence causes non-uniqueness in interpretations of the grounded-wire transient electromagnetic method (TEM). Many different layered models may produce the same response curves practically. However, the conventional theory of equivalence is based on the three-layer geo-electric models while the earth may not be reducible to a simple three-layer structure. Therefore, it is important to perform research on equivalence based on more complex models to reduce the uncertainty the grounded-wire TEM interpretations. Here, we extend the theory of equivalence based on a multi-layer target in the grounded-wire TEM, consisting of SS-, HH-, and TT-type. The analysis of relative error is applied to investigate the effect of measured components and receiver locations on equivalence. The results show that equivalence of multi-model occurs when any of three conditions is satisfied: the longitudinal conductance is constant in the far area, the thickness of targets do not change, or the transverse conductance of a multilayer target remains unchanged. In order to largely resolve the effect of equivalence on data interpretation, a strategy combined with short-offset survey and joint inversion of the electric component and magnetic component is presented and verified by the field survey in Shanxi Province, China.

The North China Craton (NCC) hosts abundant mineral resources, making it economically an important Precambrian block in China. The thick conductive loess strata in NCC presents a great challenge for successful application of controlled-source electromagnetic method (CSEM) by masking the deeper targets due to thick loess strata of conductive nature. In this work, we proposed a multiple-source CSEM method that addresses these weaknesses by using multiple-sources with inline and broadside configurations. The short offset was adopted to increase the S/N. Joint inversion of inline and broadside sources datasets enhance the detection accuracy. Based on an indirect detection strategy and the known ore-forming information, a deeply seated ore body at 1157 m was delineated.

Electromagnetic methods play an important role in mineral exploration and are widely used in the search for metallic resources such as copper, molybdenum, lead-zinc, bauxite, uranium, etc. In this paper, we focus on reviewing the application and development of electromagnetic methods, such as magnetotelluric (MT), audio magnetotelluric (AMT), controlled-source audio magnetotelluric (CSAMT), and transient electromagnetic (TEM), respectively. This paper also presents examples of electromagnetic methods applied to the exploration of metal deposits on land, airborne and in the marine environment. Furthermore, we discuss the future development of electromagnetic prospecting tools, the data processing, modeling and inversion, interpretation, and their application in complex geological environments. The future of successful mineral exploration using EM methods will be focused on concealed and deep target exploration, and possibly one day on resources on the seafloor.

The central-loop time-domain electromagnetic (TEM) method has been widely used in hydrogeological prospecting. Because this type of TEM configuration is relatively insensitive to lateral resistivity contrasts, the mathematical derivation of an apparent resistivity formula, valid for later recording times, is greatly simplified. However, no simple formula exists for an in-loop off-center TEM sounding. In this type of acquisition, the field response is recorded at several points surrounding the central location. In-loop TEM surveys are frequently employed to obtain high spatial resolution within engineering geophysical exploration. Although the apparent resistivity formula of a fixed-loop TEM system is also applicable for the non-central receiver points used in an in-loop TEM system, the apparent resistivity values need to be extracted using an iterative method that requires complicated integrations. Presently, no fast method exists that can give access to the apparent resistivity values in a direct manner. The main objective of this paper is to present a possible solution to this computational problem. A new solution is presented that takes as a starting point a circular transmitter loop where an analytical solution exists for non-central receiver points. This analytical solution is further approximated through the combined use of least-mean square (LSQR) determined polynomial coefficients and an equivalent circular loop to represent the rectangular loop. The new apparent resistivity formula, valid for in-loop TEM, has been tested on field data. A successful case study from central China is presented where an increased sensitivity to locate water-filled zones has been obtained.

Traditional TEM study mainly focuses on the generation and application of the TE field using a loop or grounded-wire source; but in recent decades, lots of efforts have been made for implementation of the TM field and even the integration of the TE field with the TM one into anomaly detection in the subsurface. However, no applicable principles have been proposed for selecting the optimal electromagnetic field for various subsurface targets. The transient electromagnetic (TEM) fields generated from grounded-wire source consist of the TE-mode response (current-carrying wire), the TE–TM mode response (grounding ends) and the combined TEM-mode response (current-carrying wire and grounding ends). This study performs a comparison of TE/TE–TM/TEM fields by generating them from grounded-wire source and testing their distribution characteristics, detection depth, and sensitivity to anomalies, using both synthetic 1D model and two field surveys in China. The comparisons demonstrate that, the detection depth of the TE–TM field is smaller than those of both the TE and combined TEM fields. Meanwhile, for electric field, the TE–TM response provides a better detection than the TEM one, but with an uneven distribution. Therefore, the TE–TM electric field requires well-designed arrangements of receiving positions when applied to real projects. For the magnetic field, the TEM response has the best detection capability compared to the TE and TE–TM ones, but is least sensitive to layer thickness and resistivity, especially for an embedded layer with low resistivity.