Time Domain Electromagnetic Method Research Articles

UAV Time-Domain Electromagnetic System and a Workflow for Subsurface Targets Detection

The time-domain electromagnetic (TDEM) method is acknowledged for its simplicity in setup and non-intrusive detection capabilities, particularly within shallow subsurface detection methodologies. However, extant TDEM systems encounter constraints when detecting intricate topographies and hazardous zones. The rapid evolution in unmanned aerial vehicle (UAV) technology has engendered the inception of UAV-based time-domain electromagnetic systems, thereby augmenting detection efficiency while mitigating potential risks associated with human casualties. This study introduces the UAV-TDEM system designed explicitly for discerning shallow subsurface targets. The system comprises a UAV platform, a host system, and sensors that capture the electromagnetic response of the area while concurrently recording real-time positional data. This study also proposes a processing technique rooted in robust local mean decomposition (RLMD) and approximate entropy (ApEn) methodology to address noise within the original data. Initially, the RLMD decomposes the original data to extract residuals alongside multiple product functions (PFs). Subsequently, the residual is combined with various PFs to yield several cumulative sums, wherein the approximate entropy of these cumulative sums is computed, and the resulting output signals are filtered using a predetermined threshold. Ultimately, the YOLOv8 (You Only Look Once version 8) network is employed to extract anomalous regions. The proposed denoising method can process data within one second, and the trained YOLOv8 network achieves an accuracy rate of 99.0% in the test set. Empirical validation through multiple flight tests substantiates the efficiency of UAV-TDEM in detecting targets situated up to 1 m below the surface. Both simulated and measured data corroborate the proposed workflow’s effectiveness in mitigating noise and identifying targets.

Geophysical techniques and geomatics-based mapping for groundwater exploration and sustainable development at Sidi Barrani Area, Egypt

The exploration of groundwater on the northwestern coast of Egypt holds significant importance for urbanization and sustainable development. This is due to a deficiency in water resources, particularly in the arid regions, and the increasing reliance on groundwater as an alternative water source in response to the declining rainfall in the region. Therefore, this study adopts a comprehensive approach that combines the Geographic Information System (GIS) and remote sensing techniques with a geophysical survey. The Time Domain Electromagnetic (TDEM) and Vertical Electrical Sounding (VES) methods are used in the survey to investigate and characterize the fractured limestone aquifer that dominates the study area. A pedological investigation, including both in-situ and laboratory testing, is conducted on the soil. The findings reveal multiple layers—such as the top fill layer, sandstone layer, upper limestone layer, clayey layer, middle gypsiferous limestone layer, and lower limestone layer—within the soil profile. The fill layer is typically composed of fine- to medium-grained sand. A total of 47.7 acres were lost and 28.6 acres were gained according to the results of the shoreline change over several spatial and temporal scales. Sea level rise-related shoreline changes result in saltwater intrusion, which has a substantial effect on the groundwater quality in coastal aquifers. The integration of geophysical tools indicates the presence of two geoelectrical layers in the subsurface succession. The second layer corresponds to the Marmarica limestone of the Middle Miocene, which represents a shallow water-bearing formation in the study area. This layer exhibits moderate and low resistivity, with depths ranging from 1.58 to 52.8 m and a thickness from 23.6 to 139.12 m.

Comparison of short-offset and long-offset grounded-wire transient electromagnetic responses based on the 3D model

In this study, a recently developed time-domain electromagnetic method called the short-offset grounded-wire transient electromagnetic (SOTEM) method, which is a near-source observation method, was adopted to obtain strong signals and great detection depths. The responses of the SOTEM and long-offset transient electromagnetic (LOTEM) methods were compared to further guide and promote the SOTEM method. Currently, the comparison between SOTEM and LOTEM methods is primarily based on one-dimensional (1D) models. However, most geological bodies are three-dimensional (3D) structures. We investigated the responses of a grounded-wire transient electromagnetic method based on 3D models using the 3D finite-difference time-domain method. In addition, the signal strengths, detection sensitivities and detection depths of the SOTEM and LOTEM methods were compared. The results revealed that the field amplitudes of Ex (electrical component parallel to the transmitting source) and ∂By/∂t (magnetic component perpendicular to the transmitting source horizontally) were higher at the short offsets than at the long offsets. For the ∂Bx/∂t (magnetic component parallel to the transmitting source) and the vertical magnetic component ∂Bz/∂t, at initial times, the responses received would be stronger when closer to the transmitting source, whereas at later times, the responses would be stronger when farther from the source. Ex detection sensitivity increased with an increase in the offset at initial times, and increased with a decrease in the offset at later times. The detection sensitivities of the three magnetic field components at short offsets were higher than those at long offsets. The ∂By/∂t effective detection depth was the greatest. Generally, the effective detection depths of the three magnetic field components increased with decreasing offset. The range of the ratio of the horizontal distance, r, between the transmitting source and the target body to the effective detection depth H was 0.5-1.1.

Time-Domain Electromagnetics for Subsea Permafrost Mapping in the Arctic: The Synthetic Response Analyses and Uncertainty Estimates from Numerical Modelling Data

Subsea permafrost stability is the key to whether pre-performed methane sequestered in hydrate deposits escapes to the overlying strata. By making use of the 1D numerical modeling and field data, we analyze the capabilities of the time-domain (transient) electromagnetic method (TDEM) when being applied for subsea permafrost mapping, and study the effect of the background resistivity structure on the inversion models’ accuracy for a series of settings typical for the East Siberian Arctic Shelf—the broadest and shallowest shelf in the world ocean, which represents more than 70% of the subsea permafrost. The synthetic response analysis included the construction of a series of resistivity models corresponding to different settings (presence/absence of ice-bonded permafrost layer, different position of its top and bottom boundaries, different width and thickness of thawed bodies or taliks, variable seawater depth and its resistivity), and calculation of synthetic apparent resistivity responses used to assess their sensitivity to changes in the target parameters of the resistivity structure. This was followed by regularized inversion of synthetic responses and comparing resulting models with original (true) ones, which allowed us to understand the possible uncertainties in the geometry and resistivity of the reconstructed permafrost layer, depending on seawater depth and unfrozen layer thickness, as well as confirm the overall efficacy of TDEM technology for the subsea permafrost imaging. That is crucially important for understanding the current state of the subsea permafrost-hydrate system and possible future dynamics.

Carbonate-rock aquifers in arid environments (central Tunisia, Western Mediterranean province): gravity and time domain electromagnetic methods investigations

Gravity and time domain electromagnetic (TDEM) methods were used in this study to investigate the subsurface hydrogeology of the carbonate-rock aquifer in arid environments (central Tunisia, Western Mediterranean province). The El Houdh basin can be considered an appropriate case for evaluating the vulnerability of groundwater resources under present-day climate change. The identification of structural context and the discontinuities affecting the Eocene or Campanian to Maastrichtian carbonates is an important key to recognize water recharge and discharge pathways and to develop effective and long-term groundwater exploration strategies. First, we have produced an available residual gravity field using a specific Gaussian filter. Then, we generated derivative maps, Euler deconvolution solutions map and a 3D gravity model to delineate different anomalies and to estimate the depth-to-basement parameter and the subsurface density contrasts. Positive and negative anomalies mapping and 3D gravity modelling showed that the El Houdh basin is associated with an asymmetric ‘perched syncline’ with a segmentation into NE–SW sub-basins (local negative residual anomalies) separated by NW–SE lineaments. The TDEM survey was calibrated using boreholes to image the karst saturated and unsaturated zone and to deduce the epikarst and endokarst relationship. Finally, the proposed method provides a fast and valuable approach for better management of hydrogeological exploitation. Thematic collection: This article is part of the Climate change and resilience in Engineering Geology and Hydrogeology collection available at: https://www.lyellcollection.org/cc/climate-change-and-resilience-in-engineering-geology-and-hydrogeology

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.

3-D Full-Waveform Modeling and Analysis of Induced Polarization and Magnetic Viscosity Effect in Time-Domain Electromagnetic Method

3-D Full-Waveform Modeling and Analysis of Induced Polarization and Magnetic Viscosity Effect in Time-Domain Electromagnetic Method

Detection of shallow and deep conductive zones using direct current resistivity and time-domain electromagnetic methods, west El-Minia, Egypt

Direct current (DC) resistivity and time-domain electromagnetic (TEM) methods were used to distinguish between saturated and dry zones and in an evaluation of groundwater resources in the western desert of Egypt. DC-resistivity method was used to evaluate the shallow section while TEM method was used for the shallow and deep aquifer sections. In the decay (voltage) curves, the last parts of the early stage and the intermediate stage refer to the shallow aquifer while the late stage indicates the deep aquifer. The effect of ramp-off time was studied. The shallow unrecorded depths of the high current diffusion speed and first and last diffusion depths were determined. The resistivity and thickness of surface deposits and loop side length are the prime influences on diffusion, investigation, and shallow unrecorded depths. By combining the two methods, the subsurface layers were classified into eight conductive and resistive zones. The third (15-75.5 O m), sixth (55-124 O m), and seventh (35-54 O m) are high conductively zones that corresponds to the shallow and deep aquifers, respectively. The TEM method delineated the sedimentary basin and the maximum penetration depth was ∼1300 m. The complementary of two methods confirmed that the resistivity method is sensitive to shallow conductive and resistive zones while the TEM method is sensitive to shallow and deep conductive and resistive zones. The TEM method assisted in identifying the shallow unrecorded depths, the effect of ramp-off time, diffusion depths, and the effect of shallow resistive depths on diffusion depths.

The Use of Electrical Resistivity Tomography and Time Domain Electromagnetic Methods to Investigate the Superficial Deposits at Al al-Bayt University, Jordan as a Case Study

DOI:10.17014/ijog.9.3.355-369In this study, an integration of Electrical Resistivity Tomography (ERT) and Time-Domain Electromagnetic (TDEM) methods have been used to investigate the superficial deposit characterization at Al al-Bayt University area. ERT and TDEM results helped to delineate the subsurface geology, and to map soil and basalt flow thicknesses as well as subsurface geological structures. Superficial deposit thicknesses were found in the range of 9 to 16 m, whereas the underlying basalt flow thickness was found to vary from a few meters in the western part to more than 60 m in the most eastern part of the studied area. The ERT results permitted a subsurface lithology characterization of the upper 35 m below ground surface (mbgs), the soil/superficial deposit resistivity was found in the range of 5 ̶40 Ohm.m, and thickness within 12 ̶ 15 mbgs. The TDEM results permitted mapping and delineating the subsurface geology up to 80 m, and allowed mapping the main subsurface structures. The soil/superficial deposits have resistivity in the range of 10 to 90 Ohm.m and the thickness in the range of a few meters up to 15 m. The study recommends a detailed geophysical study before starting any type of geo-engineering construction.

Three-dimensional forward calculation of time domain electromagnetic response stimulated by a pseudo-Gaussian pulse source

The calculation of three-dimensional (3D) electromagnetic responses using the controlled source time domain electromagnetic method suffers from low accuracy of the early response calculation and difficulty in the convergence of the later response for large-scale models. To address these problems, we propose a method of decomposing the time domain electromagnetic response into a primary field and a secondary field. This approach can easily address the above issues. In this paper, we use a time domain finite difference method based on a non-uniform step size and staggered grid and use an iterative algorithm with a non-uniform step size to calculate the late time domain electromagnetic response under high power pulse source excitation. Several example simulations were compared with analytical and spectral Lanczos decomposition solutions and demonstrate the correctness of the 3D forward modeling method. The response of a 3D anomaly excited by an electric dipole source pulse current was analyzed. The calculated time domain response revealed that the field excited by the high power pulse source diffused in the earth over time and exhibited a secondary diffusion process of the anomalous field generated by the 3D anomaly. The numerical simulation results showed that the time domain electromagnetic responses excited by the pulse source were stable and effective. The amplitude of the anomalous response was much greater than that of the step source, the signal-to-noise ratio was greater, and the detectability was better. The method may be used in inversion of the time domain electromagnetic response if the calculation speed can be further improved.

Combining geophysical methods (DC, IP, TDEM and GPR) to characterise mining waste in the Linares-La Carolina district (southern Spain)

The accumulation of waste materials in old, abandoned mining districts without prior sealing of the substrate currently poses a significant risk of contamination to soils and surrounding waters. Some of these mining dams have undergone a reclamation and sealing process in recent years to alleviate this problem. The current study is an analysis of the effectiveness of using different geophysical techniques for characterising these structures and monitoring the isolation performed. This study was carried out in the old mining district of Linares-La Carolina (southern Spain). The following techniques were investigated in this study: direct current (DC) resistivity, induced polarization (IP), ground-penetrating radar (GPR) and the time domain electromagnetic method (TDEM).Combining DC and IP made it possible to characterise the internal structure of the abandoned mining dams, whereby the geometry of these deposits was determined and the corresponding potential risks were assessed. In addition, percolation zones of mining leachates were detected that indicate defects in the sealing stage.TDEM was a good complementary technique for calculating the depth of the bedrock on which the mining dam is located. GPR could only be used to investigate shallow depths at all of the antenna frequencies used (100, 250 and 500 MHz) but was useful for obtained detailed information about the last stages of filling and characterising the encapsulation performed during sealing.

Applied Geophysics for Managed Aquifer Recharge.

Increasing water stress and decreasing supplies caused by growth and climate variability have expanded demand for managed aquifer recharge (MAR) projects to provide water supply resilience. Some of the most important factors in determining the performance of a MAR project include site selection, subsurface hydrogeologic characteristics and associated properties of the storage zone. Costs for invasive subsurface investigations to address these factors have slowly increased over the past two decades, with drilling costs increasing dramatically by as much as 30% or more since COVID-19 hit, a result of supply chain issues, steel prices, and manpower challenges. This paper provides a high-level review of major geophysical methods that have become more mainstream over the past decade or two to supplement invasive subsurface investigations and are very cost effective when compared to drilling boreholes and installing wells, which provide only point data. The more commonly used surface geophysical methods include ground-based and airborne time-domain electromagnetic methods (TEM), electrical resistivity, and seismic reflection. Airborne TEM methods (AEM) collect data very quickly, avoiding ground-based access constraints, and land-based methods are especially efficient using towed arrays. Electrical resistivity measurements provide resolution comparable to TEM but require more time than towed methods. Seismic reflection surveys are more expensive than other methods but typically have a much greater depth of penetration and can provide high resolution information on aquifer geometry, geology, and faults. Borehole geophysics is one of the more common methods used in MAR, providing near hole formation data and ground truths surface geophysics.

Hydrogeophysical characterization of the Southern Red Sea coastal aquifer, Saudi Arabia, using Time-Domain electromagnetic method

The research aims to characterize the shallow coastal aquifer of the Red Sea in terms of lateral and vertical extensions and to determine the factors controlling the quality of groundwater using the Time-Domain Electromagnetic sounding method. The aquifer consists of alluvial sediments with an increasing thickness toward NE. Groundwater resistivity is variable from less than 5 Ω.m at the coastal strip to 20 Ω.m at 5 km toward NE. Furthermore, resistivity reached up to 100 Ω.m in other parts of the area. Variations in aquifer resistivity are attributed to lithological changes and seawater intrusion. Aquifer resistivity and the total dissolved solid values from wells indicate that the aquifer is adequate for agricultural usage except along the coastal areas.

Denoising algorithm of ground-airborne time-domain electromagnetic method based on Variational Bayesian-based adaptive Kalman filter (VBAKF)

The ground-airborne time-domain electromagnetic (GATEM) method, as a new type of electromagnetic method, offers detection efficiency and large investigation depth. It is an efficient electromagnetic detection technique for geological exploration and groundwater exploration. The field data are disturbed by motion noise and mixed noises, such as white noise and electromagnetic noises, which will reduce the data quality and accuracy of data interpretation. The characteristics of GATEM responses are analysed, and a denoising algorithm based on variational Bayesian-based adaptive Kalman filter (VBAKF) is proposed. Considering the attenuation characteristics of the GATEM response, the exponentially fitted data input into the VBAKF. Then the VBAKF with inverse Wishart priors is used to denoise GATEM field data, and the unknown process noise covariance matrix (PNCM) and measurement noise covariance matrix (MNCM) are estimated by variational Bayesian. The VBAKF algorithm is tested on the responses with different noises. It is verified on a layered geological model and compared with the adaptive Kalman filter (AKF) algorithm and the combined wavelet filter (CWF) algorithm. It is also tested on synthetic data of an anomalous model. Through synthetic data, the VBAKF algorithm can effectively suppress noise of GATEM data. Finally, the algorithm is applied to field data in Leshan, Jilin province, China. It effectively improves the quality of field data.

Subsurface Modeling using Gravity and TDEM in Jiken, Blora Regency

Geophysics plays an essential role in imaging the subsurface condition of the earth. Gravity and time-domain electromagnetic method is geophysical method that can be used to visualize the subsurface condition. In this research, gravity and TDEM surveys have been carried out in Jiken, Blora Regency. Modeling technique is needed to obtain the subsurface model that bring information related to subsurface condition. Inversion modeling is applied separately to the gravity data and TDEM data, respectively. The density interface inversion modeling is used to gravity data.Subsurface condition is approached by three layers model with different density contrast values in each layer. In these schemes, the modeling process will determine the thickness for every layer. While for TDEM modeling,differential evolution technique is applied to model the TDEM data. The subsurface condition is approached by four layers model. In these schemes, the modeling process will determine the resistivity and thickness for every layer. Finally, the model obtained from gravity inversion modeling is integrated into the model from TDEM inversion data by overlaying the model.

Bayesian subsampling of time-domain electromagnetic data using kernel density product

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

Magnetic Viscosity Effect in Magnetic-Source Time-Domain Electromagnetic Surveys

The effect of magnetic viscosity (MV) has continued to be observed with the development and application of the time-domain electromagnetic method (TDEM). Field and laboratory data show that the MV effect is characterized by a -1±0.4 scope power-law delay in the late stage of electromagnetic response. Research on the MV effect can improve the detection accuracy of TDEM and assist in prospecting for ferromagnetic minerals. Most of the studies are based on the Chikazumi susceptibility model and the one-dimensional modeling method. However, the late-stage electromagnetic response shows a -1 power-law delay, which is inconsistent with the measured data. The log-uniform distribution of relaxation time τ in the Chikazumi model is not always appropriate. This study considers the Cole-Cole susceptibility model with a log-normal distribution of relaxation time. The three-dimensional (3D) modeling method of the MV effect is raised based on the rational function approximation algorithm and recursive convolution technique; the control equations and iterative process were adjusted based on finite-different time-domain (FDTD) method. The effectiveness was verified via half-space and layered models; the effects of susceptibility parameters on the response were clarified; moreover, the MV effect of the 3D anomalous model was analyzed. Our method can model the fractional propagation process of the MV effect more efficiently and help to improve the prospecting accuracy of the TDEM method under complex magnetic geological conditions.

Inversion Method of a Highly Generalized Neural Network Based on Rademacher Complexity for Rough Media GATEM Data

The ground-source airborne time-domain electromagnetic (GATEM) method is an effective electromagnetic exploration technology. The actual geological medium has rough characteristics; however, the current inversion methods for GATEM data are mostly based on homogeneous medium and extract only resistivity information. In this article, a neural network (NN) is served as extracting the two parameters of resistivity and roughness for rough medium. The structural parameter selection of NN has no fixed formula and is often related to experience. The NN has difficulty converging to the target accuracy if the structural parameters are not selected properly. To realize high-precision inversion of GATEM data, this article introduces Rademacher complexity to limit the generalization error and improve the generalization ability of the NN. Above all, a sample set of the GATEM response, resistivity, and roughness of the rough medium is established. In the next place, a fully connected NN structure is constructed, and a highly generalized NN is obtained by using Rademacher complexity. Then the mapping relationships are established through training, and the NN method is served as inverting the resistivity and roughness. The initial NN and the highly generalized NN are used to invert the GATEM response of rough medium for typical geological models. The results of the highly generalized NN based on Rademacher complexity are closer to the real models. The method is applied to the GATEM field data in Zhuxianzhuang, Anhui Province, China, and the results are consistent with the geological data.

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.

Groundwater aquifer detection using the time-domain electromagnetic method: A case study in Harrat Ithnayn, northwestern Saudi Arabia

Eight time-domain electromagnetic profiles were carried out in Harrat Ithnayn, northwestern Saudi Arabia, to detect potential groundwater-bearing aquifers in the area. The collected data were processed and analyzed, with the results inferred or extrapolated between one-dimensional vertical soundings to obtain two-dimensional resistivity sections of the subsurface geological setting. We found that the resistivity values change laterally and vertically throughout the area. High resistivity values (greater than 100 Ohm-m) occupy most of the sections down to depths of approximately 100–150 m, whereas the low-resistivity regions of the section appear to be shallower than 100 m toward the west in the northern part of Harrat Ithnayn. The high-resistivity regions correspond to the Harrat basalts and are underlain by low resistivity (down to a few Ohm-m) in the saturated zones. There are also localized regions of low resistivity at depths of approximately 20–30 m, indicating shallow local groundwater aquifers. Aquifer thickness varies from approximately 50 to more than 100 m. In the southern part, the resistivity sections indicate that the main aquifer is at a depth of 150–200 m (possibly near 170 m) below the basalt. A saturated zone or shallow aquifer approximately 20–30 m thick also exists at a depth of approximately 15 m. In both the shallow and deep aquifers, the resistivity values indicate that there is good quality fresh water.