Frequency Domain Electromagnetic Method Research Articles

Integration of seasonal frequency domain electromagnetic surveys and geological data for assessing the integrity of earthen levee systems. The case study of the Panaro River (Northern Italy)

Floods rank among the most widespread and destructive natural hazards worldwide. The progressive degradation, impairment, and breach of earthen riverine levees can occur in both natural and anthropogenic environments, stemming from various scenarios or sequences of events. These may include hydraulic failure due to overtopping because of inadequate height, and structural failure occurring even prior to overtopping, due to insufficient geotechnical and hydraulic characteristics combined with external and internal erosion.Following the catastrophic flood of December 2020, caused by the collapse of a section of the levee system of the Panaro River (a tributary of the Po River, Northern Italy), local Authorities initiated a comprehensive investigation into the causes of the breach. Numerous factors, including geological, geomorphological, and ecological features, were found to have contributed to the progressive decrease of the levee integrity prior to and during the flood. This prompted a broader multidisciplinary study of the Panaro River levee system.The study expanded its focus to include the collapsed section (rebuilt in 2020), as well as an additional 30 km stretch of both the right and left levees north of Modena, totaling 60 km. Detailed geological and geophysical data were integrated into the analysis, with particular emphasis on evaluating the characteristics and integrity features of the levee system.This analysis was carried out using Frequency Domain Electromagnetic Methods (FDEM) on the top of the levees, previously calibrated using Electrical Resistivity Tomography (ERT), geological mapping, core logs, and Cone Penetration Tests (CPTs). The FDEM surveys were repeated in different environmental conditions, specifically in the dry 2021 summer season and in the wet 2023 spring season, during heavy rainfalls that caused disastrous floods in several areas of the Emilia-Romagna Region. Out of the 60 km surveyed in the study area, the comparison of the two datasets highlights an interval of about 4 km where the internal portion of the levees is characterized by relatively coarse-grained materials and higher permeability making it more prone to internal erosion phenomena. This paper describes and integrates the results of these investigations, drawing attention to the strengths and limitations of the FDEM method when applied to extensive surveys on earthen riverine levee systems. The proposed methodology contributes as well to maintenance and retrofitting efforts to reduce flood risk in the context of the present climate change scenarios.

Feasibility of using the frequency-domain electromagnetic method to detect pipeline blockages: Experimental study

The normal function of modern society requires a vast number of buried pipelines with differing specifications, which could become blocked to varying degrees during routine operation. Consequently, a major technical municipal engineering problem is how best to accurately, nondestructively, and efficiently locate blockages in underground pipelines. The frequency-domain electromagnetic (FDEM) method has been widely used in geophysical exploration because of its features of nondestructive examination and high efficiency. Considering the application of FDEM to the problem of locating blockages in underground pipelines, numerical simulations and physical experiments are conducted on different pipeline types with different blocked states, different blockage substances, and different working configurations. Results demonstrate that FDEM is most applicable for the detection of blockages in insulated pipelines, especially when the single-ended charging method is used or when the blockage has reasonable impermeability because the induction signal exhibits a particularly obvious abrupt drop at the blockage position. However, owing to the low resistivity of metals and the current propagating to the surrounding ground, which makes it easier to form more conductive paths than blockages, the FDEM displays no obvious change in signal characteristics at the position of a blockage in a metal pipeline, which means that this method has certain challenges for detecting such blockages.

Apparent Resistivity Variation Imaging Method Based on Magnetic Field Gradient by NGO-LSSVM for the Ground-Airborne Frequency-Domain Electromagnetic Method

Apparent resistivity imaging is a typical rapid imaging method in the ground-airborne frequency-domain electromagnetic method. At present, the apparent resistivity is typically calculated by the measured magnetic field, however, this imaging method exhibits limited capability in recognizing the center of three-dimensional anomalies. Therefore, this paper proposed the calculation of apparent resistivity using magnetic field gradients. To solve the problem of random artificial anomalies that existed during the calculated process, this paper presents a hybrid least square support vector machine (LSSVM) and Northern Goshawk optimization (NGO) to establish the mapping relationship between the magnetic field gradient and apparent resistivity variation. This approach enables accurate prediction of apparent resistivity variations and effectively resolves the challenge of correcting background resistivity. Furthermore, three typical theoretical models and field examples are used to predict the apparent resistivity variation, the imaging results demonstrate that the proposed NGO-LSSVM algorithm is a feasible and efficient tool for predicting the apparent resistivity variation with high accuracy. This study provides a novel and efficient imaging method, which facilitates the application of ground-airborne frequency-domain electromagnetics for high-resolution detection requirements, such as mineral exploration.

Groundwater Protection Assessment using Frequency Domain Electromagnetic Method and Direct Current Electrical Resistivity Method in Papalanto South-West Nigeria

The aim of this work is to assess the extent of protection of the subsurface hydrogeological structures. Fieldworks were performed integrated geophysical techniques namely Frequency Domain Electromagnetic Method (FDEM) using Geonics-EM-34 to determine the vertical and lateral variations of subsurface conductivity probing depths of 20m, 40m and 60m, while subsurface profiles were obtained using Direct Current Resistivity (DCRE) with AGI Super-sting Earth Resistivity meter; current electrode spacing (AB) ranging from 1 to maximum of 100m and the potential electrodes (MN) were consequently changed from 0.25m to 5m respectively. 1141.92 mmho/m was recorded as the highest true conductivity value for the Horizontal Dipole in the second layer (Profile EMPAP5) while the highest true conductivity value for the Vertical Dipole in the first layer was 134.31 mmho/m (Profile EMPAP1). Four principal geoelectric layers inferred from the VES data where the Topsoil is partly lateritic and alluvium, Sandy Clay/Clay/Silt, Sand/Clay/Shale, and Limestone/Sandstone. Resistivity values for these layers vary from 9.78 to 1428, 1.46 to 1057, 1.46 to 451, and 15 to 10,000 Ω m with corresponding thickness of 0.5m to1.43m, 1.29m to 13m, 2.8m to 84.4m and infinity, respectively. The higher resistivity values at the surface and extremes of both edges were indication of little or no presence of leachates or contaminant plume accumulation in other area. Also, there was a noticeable general decrease of resistivity values of formation rock with depth in these investigated areas; this is an indication that the plumes must have infiltrated rapidly into the subsurface through the massive presence of weathered rock materials of average lateral distances of 15m to 167m of considerable depth and thickness of about 24.9m. The degree of leachate contamination range from 0.0007264 mho to 0.668 mho with the highest value in VESPAP2 followed by VESPAP13 and VESPAP19 while VESPAP22 exhibited the lowest conductance.

Three-dimensional magnetotelluric modeling with nontrivial anisotropy by a regularization approach

The regularization approach has been successfully applied to remove spurious solutions in the magnetotelluric (MT) forward problems of isotropic Earth media. However, spurious modes are more likely to occur in numerical solutions of anisotropic media, as electrical anisotropy introduces many more complications to electromagnetic (EM) induction in such media. This study focuses on developing the regularization approach to 3D MT forward problems of anisotropic media, especially those of nontrivial anisotropy. The governing equation is now derived with a conductivity tensor, and an accordingly adapted form of a scaled grad-div term is augmented to regularize the solutions and constrain the divergence-free condition. A new scaling scheme is proposed to cope with the complicated distribution of current densities in nontrivial anisotropy media, and an effective conductivity is approximated by the diagonal elements of the conductivity tensor to formulate the scaling factor. Numerical tests show that, for various models of electrical anisotropy, the regularization approach can effectively enforce the divergence condition and successfully suppress spurious solutions. Therefore, for nontrivial anisotropy media, this approach can also improve the efficiency of the iterative solvers while retaining the accuracy of the solutions. The derivation of the governing equation is based on the MT method. However, this strategy should be generally applicable to other frequency-domain EM methods.

Comparative analysis of divergence of tipper imaging with natural and orthogonal electric sources

ABSTRACT The divergence of the tipper (DT) is an important imaging method characterized by fast imaging speed and high lateral resolution. It has been recently adopted from the natural source airborne electromagnetic method to the orthogonal electric sources ground-airborne frequency-domain electromagnetic method. However, the imaging characteristics and patterns of DT in the two methods are unclear due to significant differences in the sources used. The application of DT in ground-airborne frequency-domain electromagnetic method is limited. Therefore, this study first identifies the similarities and differences between the two methods from the theory. Furthermore, based on the same simulation model, the response characteristics of the DT excited by natural and orthogonal electric sources are investigated. The similarities and differences between the two methods are clarified. It will contribute to the rapid development and application of the DT imaging in ground-airborne electromagnetic detection method.

Impacts of Terrain Conductivity Survey for Groundwater Investigation in a Typical Sedimentary Formation; A Case Study of Itori, South-West Nigeria

A geophysical investigation for groundwater development involving conductivity measurement using Electromagnetic profiling was conducted to locate fractured and fissured zones and associated groundwater containing media at Itori communities, Southwestern Nigeria. The area is underlain by the typical sedimentary rocks of South-West, Nigeria with the local geology predominantly clay, shale, clay sand, limestone and Sandstone. Measurements of the ground conductivity were carried out with Geonics EM 34-3 along 5 traverses whose profile lengths varied between 160 and 200 m. Intensive geophysical fieldworks were performed utilizing Frequency Domain Electromagnetic Method (FDEM) using Geonics-EM-34 to determine the vertical and lateral variations of subsurface conductivity probing depths of 20 m, 40 m and 60 m. The EM data were acquired at 500 m intervals along 10 profiles. The Vertical Dipole Moment in the first layer exhibited the highest true conductivity and lowest true conductivity of 42.65 mS/m and 29.1 mS/m for EMITO1 and EMITO2 respectively and the corresponding Horizontal Dipole Moment exhibited the highest true conductivity of 36.0 mS/m and 25.41 mS/m for EMITO1 alongside EMITO2 and EMITO8 respectively while the Vertical Dipole Moment in the second layer exhibited the highest true conductivity of 45.62 mS/m and lowest true conductivity of 34.76 mS/m for EMITO1 and EMITO8 respectively and the corresponding Horizontal Dipole Moment exhibited the highest true conductivity of 44.0 mS/m and lowest true conductivity of 36.3 mS/m for EMITO3 and EMITO8 respectively. The qualitative interpretation of EM results identified areas of hydrogeologic importance and forms a predictive and suggestive basis for Vertical Electrical Sounding (VES) investigation; points of positive EM anomalies were considered as priority area for electrical resistivity sounding and prospective groundwater development, since they suggest lithological variations within the unconsolidated overburden and/or water–filled fissures in the bedrock. The identified major geological interfaces were suspected to be of weathered zones

Digital Signal Compensation and Sounding Depth Analysis of Portable Frequency-Domain Electromagnetic Exploration System.

Low-resistivity objects produce eddy currents when excited with electromagnetic waves of a certain frequency and then generate an eddy electromagnetic field. A portable frequency-domain electromagnetic exploration system can be used to identify this eddy electromagnetic field, and then the low-resistivity objects can be positioned. At present, portable frequency-domain electromagnetic method (FEM) exploration systems use analog signal compensation, and the sounding depth is generally calculated using empirical formulas. In order to improve the rationality of signal compensation, this paper puts forward a digital signal compensation technology, including a device design, an information extraction method, and a primary field calibration method, and makes an exploration prototype based on the digital signal compensation technology. Using 10 nV as the minimum potential detection capability, the sounding depth of the portable FEM was analyzed, and it was found that when investigating a target with the same depth, a lower frequency required a larger emission current. If this could not be met, the sounding depth became smaller, and a phenomenon appeared in which the lower the operating frequency, the smaller the sounding depth. Through the detection of known underground garages, the apparent conductivity and normalized secondary field anomalies with higher sensitivity were obtained, which indicates that the detection system based on the digital signal compensation technology is effective in practical exploration. Via long-distance detection experiments on cars, it was confirmed that the sounding depth of the portable multi-frequency FEM in practical work indeed decreases with a decrease in the operating frequency.

A surface-tunnel frequency domain electromagnetic method for mineral exploration in Tajikistan area

For old mines, numerous mining tunnels exist, which can potentially bring us closer to the deep ore-bearing structures. Therefore, we propose a surface-tunnel frequency domain electromagnetic (EM) method to utilize these mining tunnels for geophysical exploration. In this method, the transmitter is placed on the Earth’s surface, while the receiver is positioned within the tunnel space, providing higher resolution due to its proximity to the target body. This paper presents the derived analytical solution for the electric field in the surface-tunnel configuration. We also propose a survey system for our surface-tunnel frequency domain electromagnetic method and provide a field example of lead-zinc deposits in Tajikistan. Synthetic cases demonstrate a significant enhancement of the EM signal when the receiver is moved into the tunnel. The field application validates the practicality of our surface-tunnel frequency domain EM method for deep mineral exploration in active mines.

EMFEM: A parallel 3D modeling code for frequency-domain electromagnetic method using goal-oriented adaptive finite element method

We have developed an open-source parallel 3D forward modeling code called EMFEM for the frequency-domain electromagnetic method (FDEM). This code is based on the adaptive finite element method and uses the total-field approach to solve the time-harmonic Maxwell equations. The use of tetrahedral meshes enables modeling of problems with complex geometry and topography. We utilize an iterative solver and an optimal block-diagonal preconditioner and auxiliary Maxwell solver to accelerate the convergence rate. We also use goal-oriented error estimation to adaptively refine the mesh and improve the accuracy of the solution. In addition, we develop a parallel algorithm based on the mesh partitioning approach. The effectiveness of the adaptive mesh refinement is demonstrated through a 1D layered model for marine controlled-source electromagnetic (CSEM) problems. We further present two 3D models for CSEM and magnetotelluric (MT) problems to study the scalability of the parallel algorithm, and the accuracy of the solutions is cross-validated with other open-source codes. The code is implemented in C++ and follows the modular design principle, making it easy to extend and maintain. The code is capable of modeling large-scale 3D problems and utilizing high-performance computing platforms. Furthermore, as the code is freely available, it can serve as a cross-validation tool for other 3D modeling codes and can also be utilized to implement 3D inversion routines.

Optimized Weight Low-Frequency Search Coil Magnetometer for Ground-Airborne Frequency Domain Electromagnetic Method.

The vertical component magnetic field signal in the ground-airborne frequency domain electromagnetic (GAFDEM) method is detected by the air coil sensor, which is parallel to the ground. Unfortunately, the air coil sensor has low sensitivity in the low-frequency band, making it challenging to detect effective low-frequency signals and causing low accuracy and large error for interpreted deep apparent resistivity in actual detection. This work develops an optimized weight magnetic core coil sensor for GAFDEM. The cupped flux concentrator is used in the sensor to reduce the weight of the sensor while maintaining the magnetic gathering capacity of the core coil. The winding of the core coil is optimized to resemble the shape of a rugby ball, taking full advantage of the magnetic gathering capacity at the core center. Laboratory and field experiment results show that the developed optimized weight magnetic core coil sensor for the GAFDEM method is highly sensitive in the low-frequency band. Therefore, the detection results at depth are more accurate compared with those obtained using existing air coil sensors.

Shadow effect and source overprint effect of short-offset transient electromagnetic method

AbstractIn artificial-source electromagnetic explorations, the shadow effects and source overprint effects are two of the key factors to affect the detection reliability. Compared with frequency-domain electromagnetic methods, the transient electromagnetic (TEM) shadow effect and source overprint effect are more complex, especially for the short-offset TEM (SOTEM) observed in the near-source region. However, there is less relevant research. Therefore, in this paper, we first realize 3D SOTEM simulations based on the vector time-domain finite element method, then further analyze the generation of SOTEM shadow effect and source overprint effect from view of the diffused electromagnetic fields, as well as their influence on ground horizontal electric field Ex and vertical induced voltage dBz/dt responses. Results show that the attraction or repulsion of the electrical inhomogeneity would affect the diffusion speed of the electromagnetic fields, which is the essential in the two source effects. Besides, from view of the ground Ex and dBz/dt responses, the relative anomaly of SOTEM shadow effect is almost twice those of the SOTEM source overprint effect, i.e. the SOTEM shadow effect is easier to record, especially for dBz/dt responses. In addition, owing to the shadow effect, SOTEM may fail to identify the real target anomaly, whereas the influence of the source overprint effect for recognizing the target anomaly mainly occurs on all-time dBz/dt responses and early-time Ex responses (before about 10 ms), that is, observing late-time SOTEM Ex responses can effectively avoid such influence. Hence, to fully ensure avoiding the influence of the source overprint effect, we suggest adopting high-power transmission to increase the late-time signal-to-noise ratio in field measurements.

Synthesizing magnetotelluric time series based on forward modeling

The validity of magnetotelluric time-series processing methods has been lacking reasonable testing criteria. Since the time series synthesized by existing techniques are not fully derived from a given model, they are not reliable. In this paper, we present a novel approach to synthesize magnetotelluric time series based on forward modeling and the correspondence between frequency and time domain electromagnetic fields. In this approach, we obtain the electromagnetic response of two orthogonal polarization sources for a given model by magnetotelluric forward modeling, and simulate the randomness of the polarization of the natural field source by a linear combination of the two polarization sources. Based on the correspondence between the frequency and time domain electromagnetic fields, the electromagnetic fields obtained by forward modeling in the frequency domain are transformed into the time domain, and finally the time series are synthesized. The test results on 1D and 3D models validate the effectiveness of the proposed method and the correctness of the procedure. After adding noise to the synthesized time series, we can test the performance of each method by comparing the results of the time series processing methods with the response of the given model. Therefore, the method presented in this paper can be used to construct standard magnetotelluric time series, which can be used as a carrier to construct synthetic data satisfying various noise distributions, and for the study of related methods. This method can also be used to synthesize time series of other frequency-domain electromagnetic methods.

Geomagnetic and FDEM Methods in the Roman Archaeological Site of Bocca Delle Menate (Comacchio, Italy)

The increasing use of geophysical investigations for archaeological purposes is now provided also by Italian reforms about preventive archaeology. They allow not only the discovery or the spatial definition of possible buried archaeological evidence, but they are also able to define the state of preservation of ancient structures. The Bocca delle Menate archaeological site is in Comacchio village territory, situated in Ferrara provence (Emilia Romagna region, Italy). The archeological site provides important evidence of the Roman presence in the Po Delta (Italy). The Roman villa was excavated between 1958 and 1959, during the reclaiming works in the Mezzano Valley (Comacchio, Ferrara). An archaeological preliminary survey and a geophysical field trip using Geomagnetic and Frequency Domain Electromagnetic Methods were carried out, following the aim to identify the planimetry of the villa previously excavated and eventually newly discovered archaeological remains. The geomagnetic results detected the archaeological buried structures, even if the original disposition of them is not completely highlighted. The electromagnetic method was able to depict the geological and geomorphological background surrounding the Roman villa. The obtained results highlighted that the applied geophysical methods are excellent tools for the preservation, protection, and monitoring of archaeological heritage previously excavated, adding to their already known importance as best tools for new archaeological buried remains detections.

Application of Helicopter-Borne and Ground–Airborne Electromagnetic Detection to Tunnel Engineering Investigation

The Cedaya-S340 Holgutu Highway Construction Project is located in the Mongolian Autonomous Prefecture of Bayingolin in the Xinjiang Uygur Autonomous Region. It is an important traffic channel that connects Luntai County and Hejing County at the southern foot of Tianshan Mountains. As the major component of the highway project, the Huola Mountain Tunnel has a sharp topographic relief, and, therefore, commonly used land geophysical detection instruments cannot work on it. Therefore, we conducted a qualitative survey on the ground-to-air and airborne electromagnetic detection methods used at the Huola Mountain Tunnel site to provide basic data for the design of highway tunnels. The geophysical survey summarized the ground–airborne frequency-domain electromagnetic method (GAFEM) and the helicopter-borne time-domain electromagnetic method (HTEM) developed by Jilin University, and measured 15 measuring lines. Apparent resistivity imaging was performed for each section, and the results were consistent. This study comprehensively analyzed the apparent resistivity profile and geological mapping data. Then, the study inferred the major stratigraphic boundaries, fault fracture zones, rock fragmentation, weakness, karst development, and water content in accordance with background value, low-resistivity anomaly shape, low-resistivity anomaly value, and gradient value in the apparent resistivity profile. Finally, the study identified the scope of two main low-resistivity anomalies, located at the tunnel entrance and exit, respectively, which are basically consistent with the known fault location. The results of this study show that on the basis of the apparent resistivity maps of GAFEM and HTEM, the overall distribution law is basically consistent with site landform, hydrogeology, tectonic geology, and aerial image data. The results provide guidance for the construction of the Huola Mountain Tunnel and ensure the construction safety and progress of the tunnel.

Design of Depth-Focused Electromagnetic Transmitting Scheme Based on MFSPWM Method

Aiming at the frequency-domain electromagnetic method (FEM) detection system, a depth-focused transmitting scheme based on multi-frequency sinusoidal pulse width modulation (MFSPWM) method is proposed to solve the problems of low exploration efficiency in conventional single-frequency transmitting scheme and low vertical resolution in 2n multi-frequency pseudo-random (MFPR) transmitting scheme. Depending on the target depth range and required measurement accuracy, a depth-focused waveform is designed to focus the excitation field energy on a narrow frequency band. In this article, the principle of MFSPWM method is theoretically analyzed and the expression of depth-focused waveform is derived by Fourier analysis. Then, the parameter optimization of the main frequencies is performed for the desired spectrum. Further, simulations and experimental tests were carried out to verify the feasibility and effectiveness of the proposed scheme. The results show that the proposed scheme has better spectral features with multiple closely spaced frequencies and less undesired harmonics, which ensures high vertical resolution for the specific depth target. After parameter optimization, the peak-to-peak value of the current waveform was reduced by about 40% and the effective signal of the high-frequency component was increased by about 20%. The forward simulation and field test results show that the proposed scheme has higher detection accuracy and interference resistance for the specific depth target compared to the MFPR scheme.

Determination of Water Table Depth Using Geophysical Methods

Water table (WT) depth is an important parameter in engineering and environmental studies. This information can be easily obtained through drilling boreholes. Some geophysical methods can also contribute to indirectly determine the WT depth. The methods that are effective in achieving this goal are GPR (ground penetrating radar) and electrical resistivity (ER). Other methods, such as FDEM (frequency domain electromagnetic method), seismic refraction, and seismic reflection, can also be employed to measure WT depth. This article presents a discussion on the use of some geophysical methods to determine WT depth, based on a brief literature review and analysis of some data obtained by the author.

Mapping a hazardous abandoned gypsum mine using self-potential, electrical resistivity tomography, and Frequency Domain Electromagnetic methods

This paper is the first phase of an integrated geophysical investigation of an abandoned gypsum mine in Blackhawk, South Dakota, USA. Recent ground collapses have occurred over these tunnels, the extent of which was heretofore unknown. Summarized herein are survey results from the northern and northeastern parts of the study area. We used Electrical Resistivity Tomography (ERT) as the main method of exploration due to its high resolution, accuracy, and high data coverage. We measured nine resistivity lines using Wenner and Dipole-Dipole arrays. We also applied the Self-Potential (SP) and Frequency Domain Electromagnetic (EM31) methods in the northeastern part of the study area. Our results reveal: (1) many low-resistivity tunnel-shaped anomalies in the eastern zone, which is flooded; and (2) high-resistivity tunnel-shaped anomalies in the western zone, which is dry. The flooded and dry zones are clearly differentiated by the resistivity method. A negative SP anomaly—created by the downward seepage of CaSO4-water in the tunnels—is a key to outlining abandoned mine tunnels.

Three-dimensional forward modeling and characterization of the responses of the ground-airborne frequency-domain electromagnetic method

Investigation of three-dimensional forward modeling for the ground-airborne frequency-domain electromagnetic (GAFEM) method and the characterization of its responses are presented. The electric field Helmholtz equation in the frequency domain is discretized by the Galerkin weighted residual method. The secondary electric field is the unknown to be solved for, discretized using edge-element basis functions. The edge-element basis functions are also used as the weight functions in the Galerkin approach. The solution of the discretized system is then obtained by a direct matrix-equation solver. To verify the effectiveness of the forward-modeling method and the accuracy of the corresponding code for computing the EM fields and reproducing their physical behavior, the numerical solutions computed using the presented method are compared against the equivalent solutions calculated from other techniques for a 3-D conductivity model. The good agreement between the various numerical methods and codes for this model demonstrates the accuracy of the responses computed by the method presented here. The method is then used to analyze the characteristics of GAFEM responses by comparing the magnetic fields and transfer functions (i.e., the magnetic field components Hx, Hy and Hz, and the transfer functions Tzx = Hz / Hx and Tzy = Hz / Hy) for different source-receiver configurations, altitudes of the observations and frequencies for several 3-D earth models. The aim is to improve the understanding of GAFEM responses and to determine the preferred measurement components and survey parameters. The responses at different frequencies of a complex model that includes topography are also investigated to study the characteristics of GAFEM responses of realistic earth models.

A Design Method of the Transmitting Waveform of Electromagnetic Sounding Based on Selective Harmonic Elimination

The exploration target at different depths through the ground-airborne frequency domain electromagnetic method (GAFDEM) is detected by transmitting waveforms at different frequencies. When taking these different depths into detail, arbitrarily distributed frequencies are needed. However, the current transmitting waveforms are mostly in a fixed frequency ratio or frequency difference, which fails to meet the requirements of exploration accuracy and efficiency at the same time. Therefore, as a solution to this problem, this paper proposes a transmitting waveform design method based on selective harmonic eliminated pulse width modulation (SHEPWM) technology. In the SHEPWM method, three transmitting waveforms with the desired spectrum are obtained by directly controlling the switching angles of a binary sequence with an artificial neural network algorithm. Firstly, our study puts forward the basic theories and principles of the full-periodic asymmetric SHEPWM waveform. Secondly, the study, through simulation, realizes the pseudorandom, depth-focused, and layer-identification waveform with different detection depths. Finally, the application of the proposed SHEPWM waveform to the geological survey in Kaili City, Guizhou Province, proves the correctness and feasibility of this proposed method.