Time-domain Electromagnetic System 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.

Research on the Noise Suppression of The TEM Signal by Neural Network

The Groud-source Airborne Time-domain Electromagnetic(GATEM) system is susceptible to interference during flight, including motion noise (caused by factors such as wind direction, cable vibrations, and sensor attitude), power frequency noise, and atmospheric noise. To obtain field data, and enhance the precision of abnormal target identification, it is necessary to suppress noise to the field data. In this paper, a neural network approach is employed to reconstruct the GATEM signals. This includes the establishment of a sample sets, parallel numerical simulation method of GATEM responses based on the OpenMP, deployment and execution of parallel computing programs on cloud computing platforms, and neural networks implementation for noise suppression in noisy GATEM signals. When the signal-to-noise ratio is above 30dB, the error between the denoised signal and the original signal is very small, with an average relative error not exceeding 1%. This method can effectively improve the accuracy of interpretation and imaging of GATEM signals, opening up new research directions in noise suppression for electromagnetic signals.

Deep TDEM Study for Structural and Mining Purposes: A Case Study of the Barbastro Saline-Evaporitic Formation, Spain

The objective of this study was to obtain the deep subsurface structure of a saline-evaporitic formation affected by an anticlinal structure, defining the position and thickness for its future exploitation (potassium salts). To improve this knowledge, geophysical methods can be applied to establish detailed information on geological structures at depth. This work analyzes the results of a deep time domain electromagnetic (TDEM) survey acquired over the southern flank of the Barbastro-Balaguer Anticline present in the Ebro Basin in the vicinity of Graus (Huesca, Spain), that controls the geometry of the Barbastro saline-evaporitic formation, of interest from the mining point of view. A deep time domain electromagnetic system (TDEM) is used, providing a penetration capability down to 3.0 km depth. A parametric constrained inversion over a lithological known borehole (Monzón-1) is used to obtain a resistivity model and then applied to the rest of the survey points. The applied methodology contributes to improving the geological knowledge, revealing a new detailed geological structure of the Barbastro saline-evaporitic formation affected by the Barbastro-Balaguer Anticline. The survey shows that the Barbastro saline-evaporitic formation presents a structure with a ramp hanging wall and close to flat footwall, generally dipping to the SW, with a decreasing thickness from 1103 m to 601 m, in concordance with the previous accepted geological structural model. The new geophysical study provides essential data, allowing design and drilling optimization in future mining exploitations.

Airborne Electromagnetic Survey over the Touro Copper VMS World Class Deposit (NW Spain): Geological and Geophysical Correlation

Electromagnetic (EM) methods belong to the main geophysical techniques used in the mineral exploration of massive sulphides. For selecting EM anomalies as possible massive sulphide targets, it is important to combine the geophysical results with other geological and/or geochemical techniques. In 2015, Atalaya Mining started a new mineral exploration project in the Touro Cu deposit, combining geological, geochemical (ore over 0.2% Cu), and geophysical techniques. The geophysical survey consisted of helicopter-borne EM using the versatile time-domain electromagnetic (VTEM™) max system operated by Geotech Ltd. with full-waveform processing. In total, 509 line-km of geophysical data were acquired during the survey that was completed in 2018. The results showed the massive sulphide Touro ore to be typically of the order of 0.25 ohm·m (4S/m conductivity) and host rock in the range of 1000–30,000 ohm·m, measured directly on the drill core. This modelling agreed well with the sub-horizontal dips observed for the known Touro ore bodies. The conductance modelled by the plate estimation of the VTEM data were also in good agreement with those provided by Geotech Ltd. and the resistivity/conductivity measurements we made on the massive sulphide samples from several Touro ore bodies. The combination of flat dips, good conductance, shallow depth, and, lastly, lack of conductive overburden or noneconomic conductive stratigraphy, i.e., graphitic shales and sulphide iron formation make the Touro project an ideal target for airborne electromagnetic prospecting. This paper presents the excellent correlation observed between the EM airborne anomalies and the massive sulphide blocks of the Touro copper deposit. Favourable factors contributing to the success of the survey were the high contrast in resistivity/conductivity between the massive sulphide Touro ore and the amphibolite host rock and minimal interference from “nuisance” conductors, such as graphitic shales.

The Test Flight of iFTEM-I Fixed-Wing Airborne Time-Domain Electromagnetic System in Binxian, Heilongjiang Province, China

The fixed-wing time-domain airborne electromagnetic method (FTEM) has been widely used in metal mining exploration, groundwater mapping and other fields worldwide, and in recent decades, its use has gradually also become more prevalent in China. The first generation of the fixed-wing time-domain airborne electromagnetic system (iFTEM-I), researched and developed by the Institute of Geophysical and Geochemical Exploration (IGGE), has been demonstrated in recent years. In this article, we introduce the brief working principle and system parameters of iFTEM-I, and we show that the effective prospecting depth of iFTEM-I is up to 350 m, which is indicated by a comparison of the results between iFTEM-I and a ground TEM system (TEM-30B) carried out in Binxian.

Development of SQUID based TDEM system and its utilization for field survey at Tumallapalle, Andhra Pradesh, India

This paper describes the development of low Tc SQUID (Superconducting Quantum Interference Device) based TDEM (Time Domain Electro-Magnetic) system for geophysical applications. The SQUID system has been used to perform central loop TDEM sounding measurements near Tumallapalle Uranium mine, Cuddapah (dt), AP, India, where the terrain consists of a thin conducting layer below which low grade Uraniferous Phosphatic Siliceous Dolostone is present. Locating the thin conducting layer which is covered by a thick and highly resistive layer is a challenging task due to the induction of weak eddy currents. In order to enhance the resolution of the target located at larger depth, it is essential to apply larger transmitter moment and use a sensitive receiver. In this work, we combine the use of sensitive SQUID sensor and transmitter loop with larger size to achieve better target resolution. The survey has been performed using transmitter loop sizes of 100 m × 100 m and 400 m × 400 m. In order to compare the performance of the SQUID based TDEM system, the same experiments have also been repeated using induction coil as receiver which has been built for this work. In this article, we describe in detail the results obtained from the central loop TDEM measurements.

Interpretation Method of GATEM Data based on PID Controller Iteration Downward Continuation Method

The Ground-source Airborne Time-domain Electromagnetic (GATEM) system has advantages for high efficiency and complex areas such as mountainous zone. Because of ignoring the impact of flight height, the section interpretation method seriously affects the interpretation and imaging accuracy of shallow anomalies. The PID controller iteration downward continuation method is proposed. Based on the original iteration continuation method, the differential coefficient and integral coefficient are added. The result shows that the new method remarkably decreases the iteration number, and the accuracy are verified by comparison with the numerical integration solution. The PID controller iteration downward continuation method is applied to the interpretation of GATEM data. For synthetic data, the interpretation results of continued electromagnetic response are closer to the true model than the z = 30 m interpretation results. The method is also applied to GATEM field data in Yangquan City, Shanxi Province, China. The interpretation results perform reliability using PID controller iteration downward continuation method in a GATEM field.

The Joint UAV-Borne Magnetic Detection System and Cart-Mounted Time Domain Electromagnetic System for UXO Detection

For unexploded O=ordnance (UXO) detection, individual technology cannot achieve the best detection performance. The new detection mode of joint magnetic and electromagnetic method has attracted more and more attention. In this paper, a newly developed joint detection system is introduced, a multi-rotor UAV-based magnetic system (UAVMAG) and a cart-based time-domain electromagnetic detection system (TDEM-Cart) are combined, and the cooperative processing of magnetic field and electromagnetic data is proposed. The result of the joint inversion fuses the feature vector retrieved from the magnetic field data and the feature vector inverted from the electromagnetic data, providing more accurate positioning results and richer information, which is favorable to locate and distinguish the UXO. Two field experiments are conducted, and the results show that when the joint system works in the full-coverage survey mode, both ferromagnetic and non-ferromagnetic metal targets can be detected, avoiding missed detections. In addition, when the joint system works in the cued survey mode, the detection efficiency is improved, the positioning accuracy of joint interpretation is less than 10 cm, and it shows satisfactory performance in the recognition of targets.

Early time data processing in shallow AEM exploration

For shallow exploration, high-frequency pseudo-random binary sequence is used in the time-domain airborne electromagnetic system. This system is working in the on-time and the earth response could be derived from system identification, which is different from the previous time-domain electromagnetic systems. The early time data is crucial for obtaining a detailed resolution. It could be severely disturbed by the primary field, the receiver response and noise. Here we present an approach for removing the primary field by matching subtraction in transformation of correlation calculation. The noise is suppressed as not correlated with the pseudo-random binary sequence. This approach is proved efficient by applying to simulation data based on forward modeling. The receiver influence is analyzed through the response of the equivalent electronic circuit. The 1D inversion results indicate that critical damping state and high cut-off frequency could reduce the adverse effect on the resolution of shallow exploration.

Three-Dimensional Dynamics of Towed Underslung Systems Using Geometrically Exact Beam Theory

Three-dimensional transient dynamics of a towed system is investigated in this paper. The tow cable is modeled as a geometrically exact (GE) beam, and the payload is a rigid body. The GE beam model incorporates large displacements and rotations of the cable, including cross-sectional shear, which is impossible in current models. Time-domain electromagnetic (TDEM) systems, used in mineral surveying, are selected as a specific application, and so the payload is taken as a rigid ring attached to the cable through rigid, massless links. Aerodynamic loads, including von Kármán forces, are computed by adding loads experienced by small cylindrical elements constituting the cable and the payload. The heavy aircraft remains unaffected by the underslung system’s motion. The governing equations are derived and solved using a nonlinear finite element method, which requires computationally coupling the GE beam (cable) with the rigid payload. The dynamics of a model TDEM system under several flight conditions is then simulated to investigate 1) transient oscillations driven by von Kármán effect, 2) steady-state payload configurations, and 3) von Mises stress in the cable at its point of attachment to the aircraft. Our general formulation, with straightforward modifications of the payload model, will also be useful for other such cable-towed systems.

Characterization of TDEM System With SQUID and Fluxgate Magnetometers for Geophysical Applications

This article describes the characterization of the SQUID-based time domain electromagnetic (TDEM) system for use in geophysical applications. This system has been characterized near the laboratory where the background noise, especially the cultural noise, is relatively low. A transmitter loop in the form of a square (2 × 2 m) with the excitation current of 0.212 A in the form of modified square pulses with a frequency of 2.5 Hz has been used to induce eddy currents in a nearby conducting target. A three-axes low Tc dc SQUID magnetometer and its associated fast readout electronics have been used to record the transient response of the target. Meanwhile, as part of this developmental work and to get the field experience, the TDEM measurements have been performed in the field with transmitter loop size of 100 × 100 m and fluxgate magnetometer as a receiver. Here, we observed that the decay of the secondary magnetic field has always unexpectedly reached a negative value even at the earlier time windows. Subsequently, the TDEM experiments with fluxgate as the receiver have been repeated with the same configuration as done in the past with SQUID, and the performance of the magnetometers, such as fluxgate and SQUID, have been compared. The experimental results were analyzed in detail and show that the fluxgate magnetometer behaves as the combination of magnetic field sensor and induction coil at earlier decay times, and magnetic field sensor only at later decay times.

The identification and mitigation of geohazards using shallow airborne engineering geophysics and land-based geophysics for brown- and greenfield road investigations

South Africa is a mineral-rich country with a diverse geology and a long history of mining. The rich history of mining activities includes the extraction of coal from the Ecca Group Sediments of the Karoo Supergroup (250 Ma), gold and uranium from the Witwatersrand Supergroup (2900 Ma), as well as platinum, uranium, tin and lead from the layered Bushveld Igneous Complex (BIC) (2150 Ma). The extraction of gold, copper, tin, lead and rare earth minerals also took place in the Archean rocks of Swazium age (3500–3000 Ma). The historical mining records have either not been accurately recorded or have been lost over time. This has resulted in significant geohazard risk during infrastructure development, especially in and around historical mining towns, such as Johannesburg and Ermelo. These geohazard risks require careful appraisal and quantification prior to any infrastructure design or construction. This case study aims to set out the development aspects of the Multi-Faceted Geophysical Modelling Systems approach, which was used by the South African National Roads Agency SOC Ltd (SANRAL) during an investigation of undermined ground for the historical coal-mining town of Ermelo in South Africa. The N11/N2 ring road was planned to go around Ermelo to ensure mobility between major routes, whilst still maintaining town access. The systems approach used a combination of airborne geophysics (Versatile Time Domain Electromagnetic System (VTEMTM) and magnetics), generally used in mining exploration, land-based and borehole geophysics, borehole water testing, and ground-truthing. The approach was continuous and iterative, building on the data at hand and reducing unnecessary investigations while eliminating the possibility of anomalies being missed, as in the case of conventional discrete drilling. The investigation ensured that 100% of the route was comprehensively investigated with a high confidence in the geological and geophysical data, and concomitant mitigation of infrastructure risk. The Multi-Faceted Geophysical Modelling Systems approach was successfully used to identify a previously unknown 1 × 1 m mining stope cavity at 90 m depth and a 3 × 5 m access tunnel at 24 m depth in a timely and cost-effective manner. Seven reverse-circulation percussion boreholes confirmed the structural integrity of these underground cavities, as well as the structural geology along the centreline. Based on the great success achieved in identifying shallow anomalies, this Multi-Faceted Geophysical Modelling Systems approach is now being considered for field trails on the dolomitic formations and the Wild Coast greenfields road project where there are large historical slumps and many fault lines. Thematic collection: This article is part of the Ground-related risk to transportation infrastructure collection available at https://www.lyellcollection.org/cc/Ground-related-risk-to-transportation-infrastructure

A Novel 3-D Imaging Method for Subsurface Targets Based on Time-Domain Electromagnetic Induction System

In recent years, the time-domain electromagnetic (TEM) method has played an important role in the detection and identification of subsurface targets, but the imaging of underground limited volume targets has always been a major problem. In this letter, we propose a novel 3-D imaging method to image subsurface targets using the TEM induction (TEMI) system. The method is based on the discretization of the underground into several grids which represent the independent magnetic dipoles contribution to the measured responses collectively. The strength of every dipole is obtained by minimizing the mismatch between computed and observed secondary response from the targets using the iterative optimization method. Then, the simulation and field experiments are done to verify the feasibility of this method. The results show that the method proposed herein has a small amount of computation and strong robustness, which help visualize the underground targets in three dimensions accurately and rapidly.

SkyTEM system calibration: Two systems, one dataset

An extensive AEM survey recently commissioned by Geoscience Australia involved the use of two separate SkyTEM helicopter airborne electromagnetic (AEM) systems collecting data simultaneously. In order to ensure data consistency between the two systems, we follow the Danish example (conceived by the hydrogeophysics group from Aarhus University) of using a hover test site to calibrate the AEM data to a known reference. Since 2001, Denmark has employed a national test site for all electromagnetic (EM) instruments that are used there, including the SkyTEM system. The Lyngby test-site is recognised as a well-understood site with a well-described layered-earth structure of 5 layers. The accepted electrical structure model of the site acts as the reference model, and all instruments are brought to it in order to produce consistent results from all EM systems. Using a ground-based time-domain electromagnetic (TEM) system which has been calibrated at the Lyngby test site, we take EM measurements at a site selected here in Australia. With sufficient information of the instrument, we produce a layered-earth model that becomes the reference model for the two AEM systems used in the survey. We then bring the SkyTEM systems to the hover site and take soundings at multiple altitudes. From the hover test data and the ground based model, we calculate an optimal time shift and amplitude scale factor to ensure that both systems are able reproduce the accepted reference model. Conductivity sections produced with and without calibration factors show noticeably different profiles.

LOUPE - A Portable EM Profiling System

Loupe is a new, portable, time-domain electromagnetic system specifically designed for rapid reconnaissance and near-surface conductivity measurements. The receiver and transmitter are each carried by one person on a backpack. Loupe incorporates a 3-component coil sensor with 100 kHz bandwidth, a fast-switching transmitter loop, a simple user-interface and the ability to navigate and recover position using RTK GPS. By virtue of intelligent signal processing, carried out in real-time and in post-acquisition processing, Loupe will operate effectively in the presence of interference from power transmission lines and other sources of noise that traditionally degrade the performance of EM systems in urban areas.A completely portable time-domain EM system of this nature is unique. This paper will discuss the design and features of Loupe and likely applications for it.Here, we present an example of data from a geotechnical investigation. By the time of the conference, we expect to be able to present results from other work done with Loupe in exploration and mine projects. This system, and its results, will be compared against other techniques to illustrate the increase in near-surface resolution and speed of broadband data collection.

Mapping Buried Aquifers with HTEM in the Fort McMurray, Alberta Region

Heliborne time-domain electromagnetic systems (HTEM) have proven to be effective tools in mapping sub-surface resistivity. As these systems are able to measure increasingly earlier time channels after the waveform's turn-off, they improve their ability to resolve resistivity contrasts in near-surface geology. This is demonstrated with the VTEM system over the Aspen property, near Fort McMurray, Alberta. This area provides a useful setting in mapping near-surface resistivity variations, specifically the location and geometry of the Pemmican Valley aquifer. This begins with the a Tau constant analysis of the earliest time channels of the VTEM decay curve that laterally mapped the Pemmican Valley aquifer and the existence of a shallower, previously unmapped, east-west trending aquifer. This is confirmed through 1D inversions of the dataset. The 1D inversion models accurately resolve the main near-surface geo-electrical units within the Aspen property when compared to resistivity well logs and a ground DC resistivity survey. Analysis of the stitched 1D inversion section maps provides useful resistivity maps that reflect the geological model.

HELITEM detects the Lalor VMS deposit

CGG deployed HELITEM, a helicopter-borne time domain electromagnetic (TDEM) system over the Lalor Deposit and other proximal deep conductors of Hudbay Minerals in northern Canada. The Lalor Deposit is ~570 m deep and is a difficult target for airborne EM systems. The system was configured with a dipole moment of up to 1.9 MAm2 at base frequencies of 15 Hz and 30 Hz. The results of this survey have been used to characterise the TDEM response from deep volcanogenic massive sulphide (VMS) deposits within the region. HELITEM is the only airborne controlled source TDEM system to have detected the Lalor Deposit.

Achieving accurate interpretation results from full-waveform streamed data AEM surveys

Successful interpretation of airborne electromagnetic (AEM) data depends strongly, but not exclusively, on the quality of data. The ability to accurately describe the system, knowledge of the data processing procedures as well as an understanding of the geological targets all contribute to derive accurate models. Xcite™ is a recently developed helicopter time domain electromagnetic system featuring an inflatable frame and full-waveform recording of streamed data. Working with the streamed data allows for restructuring of traditional data processing routines to customize data sets for specific applications. It also allows for a better understanding of the corrections that are required to isolate the secondary field response from measurements acquired on a helicopter platform. An accurate system description as well as transparency in the data processing phase allows interpreters to get the most value from airborne electromagnetic data.

Shallow groundwater investigation using time-domain electromagnetic (TEM) method at Itay El-Baroud, Nile Delta, Egypt

The Nile Delta is one of the oldest known ancient delta, largest and most important depositional complex in the Mediterranean sedimentary basin. Furthermore, it is a unique site in Egypt that is suitable for accumulation and preservation of the Quaternary sediments. In this work we applied time-domain electromagnetic (TEM) method to investigate the Quaternary sediments sequence as well as detecting the groundwater aquifer in the area of study.A suite of 232 TEM sounding at 43 stations were carried out using a “SIROTEM MK-3” time-domain electromagnetic system. A simple coincident loop configuration, in which the same loop transmits and receives signals, was employed with loop side length of 25m. The 1-D modeling technique was applied to estimate the depth and the apparent resistivity of the interpreted geoelectrical data.Based on the interpretation of the acquired geophysical data, four geoelectric cross-sections were constructed. These sections show that the Upper Quaternary sequence consists of three geoelectric layers. The Holocene Nile mud is separated into two layers: the agricultural root zone (Layer 1) and thick water saturated mud (Layer 2). The Upper Pleistocene sandy aquifer (Layer 3) is very complicated non-linear boundary. This aquifer is the most important unit since it is considered as the main water bearing unit in the study area.

Development of a low noise induction magnetic sensor using magnetic flux negative feedback in the time domain.

Time-domain electromagnetic system can implement great depth detection. As for the electromagnetic system, the receiver utilized an air coil sensor, and the matching mode of the sensor employed the resistance matching method. By using the resistance matching method, the vibration of the coil in the time domain can be effectively controlled. However, the noise of the sensor, especially the noise at the resonance frequency, will be increased as well. In this paper, a novel design of a low noise induction coil sensor is proposed, and the experimental data and noise characteristics are provided. The sensor is designed based on the principle that the amplified voltage will be converted to current under the influence of the feedback resistance of the coil. The feedback loop around the induction coil exerts a magnetic field and sends the negative feedback signal to the sensor. The paper analyses the influence of the closed magnetic feedback loop on both the bandwidth and the noise of the sensor. The signal-to-noise ratio is improved dramatically.