Search

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.

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.

A novel dual-phase-dominated fine-grained (5.5 μm) Mg-6.4Li-3.6Zn-0.37Al-0.36Y alloy was fabricated by multidirectional forging and rolling (MDFR); its mechanical properties and microstructural evolution were investigated. The ultimate tensile strength of 286 MPa and elongation of 31.8% were demonstrated in this alloy. The Portevin–Le Chatelier effect was observed in cold-rolled alloy. During MDF, the dominant grain refinement mechanisms of the α-Mg phase are mechanically fragmented grain refinement and deformation-induced in-situ dynamic recrystallization (DRX) grain refinement. However, the dominant grain refinement mechanism of the β-Li phase is deformation-induced in-situ DRX grain refinement. In later stage, thermally activated grain coarsening occurs in both phases. X-ray diffraction analyzes reveal the existence of α-Mg and β-Li phases, Al2Y, and MgLi2Al intermetallic compounds in the MDF state and the existence of α-Mg and β-Li phases, Al2Y, Al12Mg17, and MgLiZn intermetallic compounds in the cold-rolled state. Transmission electron microscopy analyzes show that high-density dislocation pile-up, dislocation tangle, subgrains, and MgLiZn particles exist in the cold-rolled alloy and raise the tensile strength. Texture analyzes reveal that strong {0001} basal texture or basal fiber of α-Mg phase exists, but the texture intensities in this alloy are not high. Annealing does not produce new texture component. A Hall-Petch model was established in this alloy: yield strength = 11.83 + 351.89 times minus square root of grain size. The relationship between critical resolved shear stress and resistance to deformation was modeled. Static recrystallization and grain coarsening during annealing were discussed.

It is widely accepted that large-scale strike-slip faults play an important role in accommodating the India‐Eurasia convergence, but the relationships between surface displacements and deep deformation along the faults remain largely uncertain. To address this issue, new magnetotelluric (MT) images were obtained along two profiles across the Xiaojiang strike-slip fault system (XJFS), one of the most tectonically active areas in southwest China. Although different in details, these two profiles show similar overall crustal structures. The upper crust is generally resistive but separated by several narrow, subvertical conductors that broadly coincide with the surface traces of the XJFS. These fault zone conductors (FZCs) are interpreted to represent fault damage zones that formed by strike-slip faulting/shearing along the faults and filled with deeply-sourced magmatic and/or metamorphic fluids. The lower crust is broadly characterized by enhanced conductivity and likely associated with partial melting at high temperatures. These observations are consistent with other geophysical measurements in this area and in favor of the development of ductile flow along the XJFS, although the pattern of flow is more complex than previously suggested. Based on these observations, we propose that active deformation along the XJFS can be better described, in the brittle upper crust, by strike-slip motion along the fluid-weakened faults, and by shear-enhanced ductile flow in the partially molten lower crust. These two mechanisms are not mutually exclusive but rather can be complementary to each other.

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.

Skywave refers to the electromagnetic wave reflected or refracted from the ionosphere and propagated in the form of a guided wave between the ionosphere and the Earth's surface. Since the skywave can propagate over large distances, it has been widely used in long-distance communication. This paper explores and demonstrates the feasibility of skywave for deep resource and energy exploration at depths of up to 10 km. Theoretical and technical advancements were accomplished in furthering skywave applications. A new solution method based on Green's function has been developed to study skywave propagation in a fully coupled lithosphere-air-ionosphere full space model. For the first time, the model allows one to study skywave distribution characteristics in the lithosphere containing inhomogeneity such as ore deposits or oil and gas reservoirs. This model also lays a foundation for skywave data processing and interpretation. On a parallel line, we have developed a multi-channel, broadband, low-noise, portable data acquisition system suitable for receiving skywave signals. Using the skywave field excited by a high-power fixed source located in central China, actual field surveys have been carried out in some areas in China including the Biyang depression of Henan Province. The initial results appear encouraging—the interpreted resistivity models prove to be consistent with those of seismic exploration and known geological information, and the exploration cost is only ∼1/4 to 1/10 that of seismic surveys. These initial successful applications of the skywave theory lay a solid foundation for further verification of the new method., This paper explores and demonstrates the feasibility of skywave for deep resource and energy exploration at depths of up to 10 km. Advancements were accomplished in furthering skywave applications.

Semi-airborne electromagnetic method (SAEM) is a geophysical electromagnetic detection method which places the transmitting subsystem on the ground and the receiver subsystem on the flight platform. This transceiver configuration has been found to be more efficient than that of the traditional ground electromagnetic methods while it can get much deeper penetration than the traditional airborne electromagnetic methods. Therefore, the SAEM has been widely used in mineral, groundwater, geothermal, and other resources exploration fields. In this paper, we would like to take the most key problems encountered in the SAEM system design and application as the starting point, and then we focus mainly on reviewing the progress of the transmitter, sensor, and receiver technology, as well as data processing, modeling, and imaging methods of the SAEM in China. In particular, the research progress of multi-source SAEMs will be introduced. Beyond that, two prospecting examples with different SAEM systems will be introduced. Finally, we will discuss the future of the SAEM in terms of equipment developing, data processing, modeling and inversion, to promote the research and application in this field to a higher level.

During the past 20 years, due to the rapid development of China's economy, the amount of coal mining activities has rapidly increased, which has led to a large increase in the number of mined-out areas. Therefore, the role of geophysical exploration in coal fields has changed from the detection of coal resources to engineering geological environmental investigation. The safety, efficiency, and green production of coal mines require more advanced geophysical detection technologies. In particular, the accurate detections of the locations of water-rich mined-out areas, as well as concealed water diversion channels, have become the main focuses of coal electrical methods in recent years. Therefore, electromagnetic methods have become the main means of hydrogeological investigations and deep explorations in mine-out coal areas. In this study, a common DC method was briefly introduced, along with an induced polarization method. Then, three representative electromagnetic sounding methods, including a surface frequency-domain electromagnetic method, time-domain electromagnetic method, surface to air electromagnetic method, and mine electromagnetic method, were respectively introduced.

The surface transient electromagnetic method (TEM) is a geophysical technology normally used in detecting water-enriched zones. However, with an increase of mining depth, the accuracy and reliability of surface detection is gradually reduced, and surface TEM cannot meet the requirements of high-precision detection for coal mine production safety. In this study, surface-to-coal mine roadway TEM is proposed to detect water-enriched zones in coal mines. The resolution of this method in detecting targets, however, is still unknown. Based on a 1-D layered model, the surface-to-coal mine roadway TEM response is numerically simulated using the finite-difference time-domain method (FDTD). The results show that the surface-to-coal mine roadway TEM has higher resolution of the target above the underground receiving point and weaker resolution of the target below the receiving point. 3-D geo-electric models for typical water-enriched zones such as a water-filled mining goaf, a water-filled fault and a water-filled collapse column are established. The surface-to-coal mine roadway TEM responses are numerically simulated and compared with the surface TEM responses. The results show that for the goaf model, the surface-to-coal mine roadway TEM response is more sensitive than the surface TEM response. For fault and collapse column models, the surface-to-coal mine roadway response is more sensitive than surface response in the early delay stage but less sensitive in the late delay stage. The present study provides theoretical support for device selection and for data processing interpretation in actual work.

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.

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.

Sedimentary bauxite usually occurs in gently dipping or monoclinic folded formations. The roof of syncline bauxite tends to be flat and the bottom appears to be similar to a concave bucket. Until recently, mainly non-seismic geophysical exploration methods, such as gravity, magnetic, and electrical methods, have been used in the exploration of syncline bauxite deposits. However, both the thickness and depth of bauxite deposits can vary significantly. Therefore, gravity, magnetic, electrical, and other non-seismic methods have technical limitations, especially when planning exploration drill targets. By contrast, seismic methods have advantages in the exploration of concealed bauxite deposits due to their high vertical resolution. In this study, reflection seismics are adopted to detect bauxite layers in China’s Henan Province. It is observed that strong reflection interfaces (positive phases) appeared on the top surfaces of the bauxite layers. Meanwhile, weak reflection interfaces (both positive and negative phases are recorded) appeared on the upper surface of the lower overlying limestone layer. Seismic reflection methods cannot only define the shape and structure of the bauxite layers, but also indicating their relative depth and thickness, respectively. Importantly, subsequent exploration drilling confirmed our seismic interpretation.

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.

The Liaodong Peninsula in northern China has undergone multi-stage magmatic and tectonic modifications, similar to those recorded by the Jiaodong gold province that is the largest gold-producing district in China, indicating that Liaodong Peninsula has great potential for new discoveries. However, no significant gold deposit has been discovered thus far at depths below 1.5 km, the typical depth of the gold-bearing system in the comparable Jiaodong gold province. This study focused on the Qingchengzi orefield on the northern Liaodong Peninsula. Qingchengzi recorded multiple post-magmatic hydrothermal gold-mineralizing events along the Jianshanzi fault zone, the main ore-controlling structure in the orefield. Given the potential for undiscovered, deep-seated gold mineralization along the Jianshanzi fault zone, we performed a magnetotelluric (MT) study designed to identify any deeper exploration targets. The surface electromagnetic prospecting (SEP) system, a portable instrument suitable for use in electromagnetically mountain environments, was employed for the purpose of acquiring the MT data. Shallow conductive anomalies identified in the vicinity of the Jianshanzi fault zone were tested by known drilling recorder. The survey not only helped to generate a detailed map of the deep geological architecture, but also allowed us to estimate the vertical extent of the known gold orebody to a depth of more than 2 km.

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.

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.

The semi-airborne transient electromagnetic method (TEM) has been widely used in resource exploration and other fields due to its high efficiency, strong adaptability to the complex environment and high resolution. However, the conventional semi-airborne electromagnetic method continues to use the survey area of the grounded-wire TEM method. Compared with the existing ground-based TEM method, the semi-airborne TEM method is more sensitive to signal intensity because observed data is easily affected by noise for receiving position in the air. Therefore, it's unreasonable to consider only signal intensity or sensitivity related to offset in survey area. To obtain more suitable area, we discuss the distribution of the transient electromagnetic field with electrical source as well as the attenuation characteristics of the signal at different flight altitude through the forward modeling. Then, we obtain the sensitive area and flight altitude applied by the error analysis method in typical models. Finally, we describe a method what is herein dubbed the fuzzy comprehensive evaluation, which greatly reflects the membership degree of the evaluation object from different single factors to each level of fuzzy subset. Therefore, we use this method to get the optimal survey area and flight altitude under different noise levels. In low noise level, the signal intensity and sensitivity account for 33.3% and 66.7% respectively in selecting the optimum area and altitude, resulting in that the optimal area and flight altitude are coincides with the sensitive area and altitude better. While in the area filled with noise, the most area of strongest signal intensity is considered the optimal area and altitude. A field example shows that the aforementioned method can be well resolved in identifying survey lines and improving data quality.

Several magmatic Ni-Cu deposits associated with Permian-Triassic mafic-ultramafic intrusions have been discovered during the last three decades in the southern part of the Central Asian Orogenic Belt, the northeastern margin of Tarim, and the margin of the North China Craton. The accurate delineations of the large resistive rock masses are crucial for locating potential ore bodies when they are located beneath rock masse. The controlled-source electromagnetic method (CSEM) is an important tool for investigating resistivity contrast in earth subsurface. In this study, we present a comprehensive example of using frequency domain CSEM survey to delineate the resistive rock masses that are related to the forming of orebody. The horizontal electric field, which is more sensitive to resistive target than the other components, is used in our study. We first built a geophysical model of the survey area based on prior confirmed geophysical and geological information. Then, both forward and inverse modeling were performed in order to assess the detectability of the resistive ore-forming rock masses in the model. Finally, this study acquired field data in the Kalatongke Ni-Cu Deposit, which is located in the Kalatongke District of the southern section of the Central Asian Orogenic Belt, and is specifically part of China’s southern Altai Orogenic Belt. The resistive ore-forming rock masses were successfully identified during the survey process and further verified by the drilling results. It was observed that both the synthetic and field examples indicated that the adopted frequency domain CSEM method successfully measured the electrical fields, and could be utilized to explore deeply buried resistive targets in future prospecting activities.

This paper presents a method for converting the full waveform electromagnetic response excited by the pseudo-random binary sequences (PRBS) into subsurface resistivity cross section, based on the recovery of the transient electromagnetic diffusion field. The impulse response of the earth is extracted from the full waveform data efficiently using the Wiener-Hopf equation. Then, the backward accumulation is used to obtain the step off response from the extracted earth impulse response. An OCCAM inversion routine, which incorporates the band limitation of the derived transient, is adopted to produce the geoelectric models. Finally, the method was applied to the experimental field data set, and the consistency between the recovered models and the geological condition verifies the validity of this method.

Although the transient electromagnetic method (TEM) has been used in geo-exploration for many years, the inversion precision of TEM data is still very limited and new techniques are needed to improve TEM data interpretation. Mathematically, TEM data can be converted into a series of virtual waves. Multi-aperture electromagnetic modelling shows that the coherence of multi-aperture TEM and echo waves measured at adjacent positions from the same geological body is high. Therefore, it is feasible to apply multi-aperture synthesis to TEM data. Based on the idea of synthetic aperture radar, a new data-processing method has been developed that uses superposition to realise multi-aperture data integration as well as Kirchhoff migration and imaging. After the pseudo-wavelet extraction from TEM data, the traditional approach of profile-based multi-aperture synthesis has been developed for each survey station. Furthermore, the traditional single point approach was applied for multiple point coverage. The technology ...