Grounded-wire Source Research Articles

Comparative analysis of computational schemes for FEM modeling of 3D time-domain geoelectromagnetic fields excited by a horizontal grounded-wire source

Two mathematical formulations are considered for calculating the time-domain electromagnetic fields excited by a horizontal grounded-wire source in a 3D geological medium. The first of them is formulated for a vector potential (A-formulation), the second one is formulated for the electric field strength (E-formulation). Both formulations are also considered when using the primary-secondary field approach (PSFA), where the primary field is calculated in a horizontally layered medium. The calculation of 3D non-stationary fields is performed using the vector finite element method (FEM). For time discretization, both two-point and three-point implicit schemes are considered. As the initial field, we used the stationary field of the direct current, the corresponding mathematical formulations are also presented using the PSFA. To verify computational schemes, a comparison with the results of other authors is given. For the problems of marine electrical exploration, we present a comparison of two formulations, analyze the effectiveness of PSFA and the use of a three-point implicit scheme instead of a two-point one. The results of computational experiments have shown that when using both formulations, the use of PSFA and the three-point implicit scheme “with backward overstepping” (instead of the two-point implicit scheme) can reduce computational costs by more than an order. In this case, both formulations give solutions that are close to each other in accuracy and computational costs. However, the implementation of the E-formulation using the PSFA requires certain thoroughness and in some cases can lead to a sharp deterioration in the numerical solution accuracy at late times. This is due to the fact that when using the vector FEM, the numerical solution can lead to a nonzero constant field, which is the gradient of some scalar function.

Application of surface-to-roadway TEMs for detecting deep aquifers in coal fields: An example from North China

Transient electromagnetic methods (TEMs) are effective tools for hydrogeological investigations of coal mines. However, traditional ground-based and underground-based TEMs typically fail to detect deeply buried water-bearing bodies in the majority of coal mines in northern China, where the Quaternary overburden is generally thick and many aquifers exist. In the present study, a surface-to-roadway TEM configuration was proposed that utilized grounded-wire sources to transmit signals with large moments on the surface and required receivers that had the ability to record signals with high resolution in underground mining roadways. First, the proposed device was introduced. Subsequently, a synthetic model was established based on the existing information of the Zhuxianzhuang coal field in Anhui Province, China, along with the analysis of forward and inversion results. Finally, the acquisition and processing of field data were conducted. Both the synthetic modeling and field results demonstrated the feasibility and superiority of the proposed method. The deeply buried aquifers were better characterized by the surface-to-roadway configuration than by ground-based surveys.

Investigation of urban active faults using the short-offset transient electromagnetic method: an example from Shunping, Hebei Province, China

Abstract The presence of an urban active fault carries a geological hazard risk that threatens urban construction. Therefore, detecting such faults is crucial for mitigating potential risks and ensuring safe urban development. The grounded-wire source short-offset transient electromagnetic method (SOTEM) offers an opportunity to identify faults with high-conductivity. The results of a SOTEM survey conducted in Shunping, Hebei Province, were presented to detect buried faults and locate the industrial park. Data were analysed using the one-dimensional inversion method with SOTEMsoft software, which was denoised using the short-time sliding window adaptive singular value decomposition algorithm based on the wavelet threshold. Despite the challenges posed by urban features, particularly traffic, the survey results located the fault and hold significant potential to reveal its extent and depth. The study highlights the importance of avoiding fault sites during urban construction, even though the area is currently tectonically stable. This paper reveals an unusual opportunity to study the effectiveness of electromagnetic methods for detecting buried faults in cities and reveal geological structures covered by Quaternary formations.

Transient Electromagnetic Smoke Ring Due to a Grounded-Wire Source

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 the 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. In the present work, smoke rings of other commonly used grounded-wire sources in the time domain are 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 an induced current in both horizontal and vertical directions. The horizontal induced current includes positive and negative parts. Both parts of the horizontal induced current maximum diffuse downward perpendicular to the surface, but the diffusion velocity of the negative part is faster, approximately 5.6 times that of the positive part. The vertical induced current maximum diffuses at 45° from the grounded surface with a velocity approximately 5 times that of the positive horizontal induced current. The smoke ring of the horizontal induced current closely resembles a basin, while that of the vertical induced current is more similar to two separate cylinders. Finally, the imaging depth based on horizontal and vertical induced currents is obtained by the statistical relationship between the induced current maximums and the diffusion depth, which are equal to 1.4 times and 1.25 times the diffusion depth, respectively.

The first semi-airborne transient electromagnetic survey for tunnel investigation in very complex terrain areas

We present an unmanned aerial vehicle (UAV) based semi-airborne transient electromagnetic method (SATEM) for tunnel investigation in tunnels constructed in complex terrains. To the authors’ best knowledge, this is the first report on such application. Tunnel investigation must be carried out before its construction, but in some areas, it is extremely challenging due to the complex terrains. To this end, we aim to develop a semi-airborne geophysical method for such cases. Specifically, a grounded-wire source is placed on the surface transmitting an electromagnetic field into the underground. An induction coil is carried by a small UAV in the air to acquire data on top of the tunnel. Low resistivity targets near the tunnel are induced by the transmitter and pure secondary fields are produced with relation to the size and location of the targets. The information is collected by the coil to invert the subsurface resistivity profile. We conducted our first experiment in the Damo Tunnel in Guangxi, China. Here we present the design of this method and the workflow for tunnel investigation. The prospecting results are compared with the actual results exposed by excavations, showing a good agreement. This first attempt of SATEM shows great potential for tunnel investigation in complex terrain areas.

Modeling and Inversion of Airborne and Semi-Airborne Transient Electromagnetic Data with Inexact Transmitter and Receiver Geometries

Airborne and semi-airborne transient electromagnetic (TEM) surveys have high efficiency but may suffer from systematic errors due to the inexact shape, position, and orientation of the transmitter and receiver, which can deviate from the nominal design because of complex terrain, platform instability, or external forces. Without considering actual survey geometry, modeling and inversion can bias the interpretation of results. We develop a universal approach to layered earth capable of modeling arbitrarily complex transmitter and receiver geometry used in airborne and semi-airborne surveys. Our algorithm decomposes an airborne loop or grounded wire source to a set of x-, y-, or z-oriented electric dipoles. An arbitrarily oriented receiver coil is simulated by projecting three-component data to the actual direction of receiving. In airborne TEM, the transmitter loop and receiver coil are often bound together on a rigid frame and tilt during the flight. Our simulations and synthetic inversion show that such a tilt may reduce responses relative to the data obtained with the nominal geometry; an inversion without considering the tilt can underestimate near-surface conductivity. In semi-airborne TEM, the transmitter wire on the surface can be crooked, and the airborne receiver coil can also tilt. Our modeling shows that the simulated data can change significantly if the actual transmitter and receiver geometry does not exactly follow the nominal survey design; if not appropriately accounted for, such an error may distort the recovered conductivity model. Finally, the benefit of our algorithm is demonstrated by an airborne TEM field data inversion of groundwater problems with the tilt angle of the transmitter–receiver frame accurately modeled. Our work provides a tool for improving the resolution of airborne and semi-airborne TEM in near-surface conductivity characterization.

Comparison of transient electromagnetic fields excited by single- and double-line grounded-wire sources on land

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.

Comparison of semi-airborne transient electromagnetic data from double-line and single-line grounded-wire sources

The semi-airborne transient electromagnetic method (TEM) involves a ground-based electrical source and an airborne magnetic field component receiver and is an effective method for exploration in complex terrains. However, the ground transmitter selection for these methods is typically challenging. In the present study, we examine the magnetic field responses of double-line and single-line source devices. Response attenuation and plane distribution are utilized to analyze and compare the characteristics of the two sources. In the double-line source, the inline and cross-line components of magnetic field are axisymmetric, with distribution characteristics of six-petaled flowers. The vertical component exhibits a four-quadrant distribution comparable to that of the x component in the single-line source; the components are generated separately or jointly by current-carrying wires and grounding ends. Although the double-line source is generally characterized by weaker response intensities than the single-line source, its components (especially the cross-line and vertical components) exhibit higher resolutions for low- and high-resistivity targets. Thus, with adequate measured signal strength, the double-line source is preferable for semi-airborne TEM involving an electrical source.

The 3D Modeling of GATEM in Fractured Random Media Based on FDTD

Grounded-source airborne time-domain electromagnetic (GATEM) method is an effective detection method of geological survey. The real geological media are rough, self-similar characteristics, and the diffusion process is anomalous diffusion. This paper primarily considers the GATEM responses of a grounded wire source in random media. Von Kármán function is used to establish three-dimensional (3D) random media model and the GATEM responses are realized based on 3D finite-difference time-domain (FDTD) method. The method is verified by homogeneous half-space model. The electromagnetic responses of random abnormal body model are analyzed, and the results show that the abnormal body can be clearly identified. The electromagnetic responses of a fractured model are analyzed, and the results show that the tilt angle of the fault can be reflected.

Three-dimensional transient electromagnetic response of multiple grounded-wire sources

Multiple grounded-wire sources combination are proposed to improve the signal strength and resolution of the transient electromagnetic method. Multi-source excitation from the different direction was executed to identify the conductive body in the subsurface and achieve refined detection of the target body. The vector finite element method based on the rational Krylov subspace projection algorithm was used to calculate transient response of the target bodies. The voltage response calculated by our algorithm was compared with that calculated by the integral equation method and the results agreed well with them. We designed different grounded-wire combinations, including a single source, pairs of sources, and groups of three and four sources, and analyzed induced EMF and the secondary field response of one conductive body. It indicates that multi-source transient method can obtain stronger signals, which is beneficial to the discovery of deep weak anomalies. Finally, we established two adjacent conductive block bodies and compared their electromagnetic response for the different source combination. The results showed that multiple-source transient electromagnetic response anomaly amplitude and lateral resolution are superior to those of a single source. Multi-channel diagram for multiple sources have two obvious peaks, especially for three sources and four sources, but the induced EMF from a single source does not differentiate between the two conductive bodies in the horizontal direction. Multiple grounded-wire source combinations are used not only for the surface transient electromagnetic method but can also be extended to the grounded electrical-source airborne transient electromagnetic survey.

Characteristic analysis and optimal survey area definition for semi-airborne transient electromagnetics

Semi-airborne transient electromagnetics (SATEM) is a versatile tool in quick geophysical surveys at a low cost. This method usually uses a grounded-wire source on the surface to transmit electromagnetic field into the earth and a receiver hanging on unmanned aerial vehicles to acquire the secondary field in the air. Compared to conventional transient electromagnetic methods, the grounded-wire system has complications caused by the combined effects of the transmitter wire and the grounding points, with complications affecting the simplicity of the secondary response. In this study, we use the electric dipole integral algorithm to simulate the SATEM responses in a layered earth model and compare the responses with different offsets, receiver heights, and geometry positions of the receiver. With these analyses, we define the optimal survey area for a layered earth to be the area with a monotonic decay rate of the secondary field. We establish an ellipse boundary distribution function to determine the suitable and unsuitable survey areas, and areas outside the ellipse are the suitable or optimal survey areas. Two examples with short and long line sources are used to test the definition and demonstrate the usefulness of the definition. The ellipse boundary function can be used in real SATEM survey designs.

Induced polarization effect on grounded-wire transient electromagnetic data from transverse electric and magnetic fields

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.

A Surface-to-Borehole TEM System Based on Grounded-wire Sources: Synthetic Modeling and Data Inversion

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.

Magnetic Viscosity Effect on Grounded-Wire TEM Responses and Its Physical Mechanism

Magnetic viscosity (MV) effect is a disturbance to the transient electromagnetic method (TEM). This effect will cause slow decay at the intermediate-late time of the TEM responses, leading to erroneous interpretation. Although there are extensive researches on the MV distortion to TEM data, they are focused on conventional loop source configuration, while little has been studied for grounded-wire source configuration that has become the essential role for relatively deep-buried targets. Hence, we present the study of MV effect on the grounded-wire source TEM data. We first derived the formulas for calculating the secondary field due to MV effect, based on the Chikazumi model. The dependence of the MV effect on model parameters, such as resistivity, susceptibility, offset and geometry of the superparamagnetic layer, is then examined to illustrate the intrinsic physical mechanism of MV effect. We find that the MV effect on the grounded-wire source TEM data can be suppressed by adjusting the parameters of the survey design. Thereafter, we propose the method to quantify the MV effect when estimates of resistivity and susceptibility are available. Finally, the method for choosing the optimal offset to suppress the MV effect is proposed, which can be used as a guide for the installation of fieldwork of grounded-wire TEM survey.

A new infinitesimal computational approach to calculating frequency-domain electromagnetic response

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.

A comparison of loop time-domain electromagnetic and short-offset transient electromagnetic methods for mapping water-enriched zones — A case history in Shaanxi, China

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.

지상 송신원 항공 전자탐사 자료의 횡적 제한 역산

최근 도입된 지상 송신원 항공 전자탐사 시스템(grounded electrical-source airborne transient electromagnetic, GREATEM)은 신호가 강력하여 가탐심도 향상은 물론 잡음이 심한 지역에서도 적용 가능하다. 비록 GREATEM은 지상에 설치된 긴 전선을 송신원으로 사용하는 시간영역 전자탐사법이지만, 방대한 항공탐사 자료의 2차원 혹은 3차원 해석은 계산시간이 너무 많이 소요되어 실질적인 적용이 어렵기 때문에 GREATEM 탐사 자료는 주로 1차원 해석에 의존하고 있다. 일반적으로 방대한 항공 전자탐사 자료의 해석은 각 측점에서 얻어진 자료에 대한 1차원 역산 결과를 병합하여 전기비저항 2차원 단면을 작성하는 방법이 널리 사용되고 있다. 그러나 이러한 병합 단면은 전기비저항이 너무 급격하게 변하는 문제점을 보인다. 횡적 제한 역산법(laterally constrained inversion, LCI)은 이러한 문제점을 해결하기 위하여 개발되었으며, 연속성이 뛰어난 역산 단면을 제공하게 된다. 이 연구에서는 우선 수치 모델링을 통하여 곡선 전류원에 대한 GREATEM 탐사 자료의 특성을 분석하였다. 또한 GREATEM 탐사 자료에 대한 횡적 제한 역산법을 개발하였다. 이 방법은 각 측점에서 획득된 모든 1차원 자료와 층서 모델을 하나의 역산 시스템에 병합하여 처리하므로 수평적 연속성이 뛰어난 역산 단면을 제공하게 된다. 개발된 역산 알고리듬을 GREATEM 탐사 자료에 적용한 결과, 해당 지역의 층서를 효과적으로 반영하는 역산 영상을 얻을 수 있었다. Recently, the grounded electrical-source airborne transient electromagnetic (GREATEM) system with high power source was introduced to achieve deeper investigation depth and to overcome high noise level. Although the GREATEM is a transient electromagnetic system using a long grounded wire as the transmitter, GREATEM data have been interpreted with 1D earth models because 2D or 3D modeling and inversion of vast airborne data are complicated and expensive to calculate. Generally, 1D inversion is subsequently applied to every survey point and combining 1D images together forms the stitched conductivity-depth image. However, the stitched models often result in abrupt variations in neighboring models. To overcome this problem, laterally constrained inversion (LCI) has been developed in inversion of ATEM data, which can yield layered sections with lateral smooth transitions. In this study, we analysed the GREATEM data through 1D numerical modeling for a curved grounded wire source. Furthermore, we developed a laterally constrained inversion scheme for continuous GREATEM data based on a layered earth model. All 1D data sets and models are inverted as one system, producing layered sections with lateral smooth transitions. Applying the developed LCI technique to the GREATEM data, it was confirmed that the laterally constrained inversion can provide laterally smooth model sections that reflect the layering of the survey area effectively.

Semi-airborne Model for Radiation From Large Grounded Wire Antenna

This letter proposes a semi-airborne model, which is used to calculate the three magnetic field components generated by a large grounded wire antenna. In the model, the antenna is an arbitrary-length grounded wire while the observing points are placed in the air. In the experiment, the magnetic field was generated by a large grounded wire source antenna excited by bipolar square wave. Optically pumped magnetometer suspended below a helicopter was employed to measure the magnetic field. The numerical results based on the proposed model fit to the experiment data very well. This will provide great benefits for semi-airborne prospecting or other flight cases.

A Comparison of Different-Mode Fields Generated from Grounded-Wire Source Based on the 1D Model

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.

Appraisal of an Array TEM Method in Detecting a Mined-Out Area Beneath a Conductive Layer

The transient electromagnetic method has been extensively used for the detection of mined-out area in China for the past few years. In the cases that the mined-out area is overlain by a conductive layer, the detection of the target layer is difficult with a traditional loop source TEM method. In order to detect the target layer in this condition, this paper presents a newly developed array TEM method, which uses a grounded wire source. The underground current density distribution and the responses of the grounded wire source TEM configuration are modeled to demonstrate that the target layer is detectable in this condition. The 1D OCCAM inversion routine is applied to the synthetic single station data and common middle point gather. The result reveals that the electric source TEM method is capable of recovering the resistive target layer beneath the conductive overburden. By contrast, the conductive target layer cannot be recovered unless the distance between the target layer and the conductive overburden is large. Compared with inversion result of the single station data, the inversion of common middle point gather can better recover the resistivity of the target layer. Finally, a case study illustrates that the array TEM method is successfully applied in recovering a water-filled mined-out area beneath a conductive overburden.