Transient Electromagnetic System Research Articles

Self‐Potential Tomography of a Deep‐Sea Polymetallic Sulfide Deposit on Southwest Indian Ridge

AbstractDeep‐sea polymetallic sulfides associated with hydrothermal systems are considered a potential viable resource for base and precious metals. The Southwest Indian Ridge (SWIR, Indian Ocean) hosts active and inactive hydrothermal systems. Inactive hydrothermal fields are more abundant than active fields but are difficult to remotely characterize. We report a deep‐sea self‐potential investigation to locate inactive ore deposits at the Yuhuang hydrothermal field on the ultraslow‐spreading SWIR. A horizontal array of six nonpolarizing AgCl electrodes are attached to a deep‐towed transient electromagnetic system to record the electrical field at a height of 40 m above the seafloor. The observed electrical field strength near a previously recognized sulfide deposits (verified by drilling) reaches an amplitude of 0.5 mV/m. Negative self‐potential anomalies (~ −27 mV) were observed by integrating the measured electrical field. The high electrical conductivities (up to 12 S/m) of sulfide samples measured in the laboratory and the oxidized sulfides recovered from the surface of the deposit suggest that the self‐potential anomalies are due to sulfide mineralization and corrosion of the polymetallic sulfides. Inversion of the self‐potential data reveals a localized body with a thickness of ~65 m, which is interpreted as the Yuhuang ore deposit. Our field data demonstrate that the self‐potential method is a useful exploration method to identify and image seafloor massive sulfide deposits hosted in inactive hydrothermal fields at mid‐ocean ridges.

Characterizing the diverse hydrogeology underlying rivers and estuaries using new floating transient electromagnetic methodology

The hydrogeology below large surface water features such as rivers and estuaries is universally under-informed at the long reach to basin scales (tens of km+). This challenge inhibits the accurate modeling of fresh/saline groundwater interfaces and groundwater/surface water exchange patterns at management-relevant spatial extents. Here we introduce a towed, floating transient electromagnetic (TEM) system (i.e. FloaTEM) for rapid (up to 15 km/h) high resolution electrical mapping of the subsurface below large water bodies to depths often a factor of 10 greater than other towed instruments. The novel FloaTEM system is demonstrated at a range of diverse 4th through 6th-order riverine settings across the United States including 1) the Farmington River, near Hartford, Connecticut; 2) the Upper Delaware River near Barryville, New York; 3) the Tallahatchie River near Shellmound, Mississippi; and, 4) the Eel River estuary, on Cape Cod, near Falmouth, Massachusetts. Airborne frequency-domain electromagnetic and land-based towed TEM data are also compared at the Tallahatchie River site, and streambed geologic scenarios are explored with forward modeling. A range of geologic structures and pore water salinity interfaces were identified. Process-based interpretation of the case study data indicated FloaTEM can resolve varied sediment-water interface materials, such as the accumulation of fines at the bottom of a reservoir and permeable sand/gravel riverbed sediments that focus groundwater discharge. Bedrock layers were mapped at several sites, and aquifer confining units were defined at comparable resolution to airborne methods. Terrestrial fresh groundwater discharge with flowpaths extending hundreds of meters from shore was also imaged below the Eel River estuary, improving on previous hydrogeological characterizations of that nutrient-rich coastal exchange zone. In summary, the novel FloaTEM system fills a critical gap in our ability to characterize the hydrogeology below surface water features and will support more accurate prediction of groundwater/surface water exchange dynamics and fresh-saline groundwater interfaces.

Double Clamping Current Inverter With Adjustable Turn-off Time for Bucking Coil Helicopter Transient Electromagnetic Surveying

As a core component of the bucking coil helicopter transient electromagnetic (TEM) system, the current pulse inverter should agree two crucial aspects, high power and fast turn-OFF. In this article, based on the design and modeling of transmitting coil and bucking coil, a novel current inverter topology based on the double constant voltage clamping technology, which differs from the traditional single clamping technology is firstly proposed and analyzed. By using double clamping technology, both the current pulse's rising edge and falling edge are improved effectively. What's more, the turn-OFF time of the current inverter is adjustable, which is of great difference from the conventional clamping technology with nonadjustable turn-OFF time. The ground and field experiments are implemented to test the performance of the current inverter. The results of the ground test of the performance comparison with worldwide VTEM transmitter verify that it successfully converts the standard 28 V dc source from helicopter power to bipolar approximately trapezoid pulses with a peak current of 300 A and a much faster turn-OFF time than the VTEM transmitter. The results of field exploration further demonstrate that it achieves a high quality of primary field compensation and effectively improve the detection capability of the helicopter TEM surveying system.

High resolution 3D subsurface mapping using a towed transient electromagnetic system ‐ tTEM: case studies

ABSTRACTGeophysical methods are routinely applied for investigation of near surface in areas of infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. A new towed transient electromagnetic tool, called ‘tTEM’, provides rapid, efficient, high‐resolution imaging of subsurface hydrogeology, and can deliver densely spaced profiles of resistivity. These profiles can be used to map a three‐dimensional subsurface in high resolution. In this paper, we present three case studies where the towed transient electromagnetic system was used to map the subsurface at the hectare scale. In the first case, we used towed transient electromagnetic to map raw materials in the northern Jutland, Denmark. The survey was carried out to map possible sand and gravel deposits. In the towed transient electromagnetic models, the potential sand/gravel targets are identified as high resistive bodies in the top 30 m. These bodies can have significant lateral variation at scales much less than one hundred metres. In the second case study, towed transient electromagnetic was used to map the thickness of a protecting clay layer above an aquifer in Vildbjerg, a town in the central part of Denmark. Results show that the overlying clay layer has sufficient thickness (>15 m) to protect the underlying aquifer from pesticide pollution in the area. Finally, in the third case study, we used towed transient electromagnetic for mapping geology in the vicinity of a landfill. The inversion results reveal a hitherto unknown buried valley‐like feature within the top 30 m of the subsurface that was not identified by older, regional TEM surveys – a feature that can have significant impact on the water flow.

Borehole transient electromagnetic array based relative attitude determination for relief well drilling

Drilling relief well is one of the most effective way to control blowout well at present. In order to improve the connection rate, the relief well needs to trace a short distance and then gradually approaches the accident well, where the relative space attitude determination of relief well and blowout well is the key to improve the drilling efficiency. In this paper, a symmetry multi-coil array-based borehole transient electromagnetic (TEM) system for relative attitude determination of relief well drilling is presented. On the basis of the borehole TEM theory, the multi-coil array weighting is applied to improve the detection performance in the case that the blowout well is parallel to the relief well. Moreover, in order to determine the relative attitude, a symmetrical borehole TEM array structure is proposed for the short-range detection. Experiments were conducted and the results demonstrate the effectiveness of the proposed method for improving the short-range detection performance, which would offer significant information for the tracing phase of relief well drilling.

Numerical modelling analysis of multi-source semi-airborne TEM systems using a TFEM

Abstract Traditional transient electromagnetic methods use single sources for excitation and extract the characteristics of underground media by improving interpretation technology. This study focused the improvements of transient electromagnetic interpretation using complex source technology. A time-domain vector finite element method (TFEM) was applied on three-dimensional forward modelling of semi-airborne transient electromagnetic (TEM) with multiple electrical sources, and it analysed the characteristics of fields with multiple sources. The study used a model of an isolated anomalous body in a homogeneous medium as an example. The effects of different combinations of excitation sources on the distributions of the magnetic field characteristics were analysed. Numerical results showed that the magnetic field components in a specific area could be strengthened by changing the layout of the sources, which was significant for future field data collections. By comparing the transient electromagnetic fields of the vertical array dipole sources with that of the loop source, the anomaly transient electromagnetic field of multi-source was more obvious than the field with a single source. Taking a complex orebody model as an example, a cross-electric source was used to calculate the magnetic field components of the semi-airborne TEM method. The resistivity distribution characteristics of the underground medium were obtained using an apparent resistivity interpretation method of the vertical magnetic field, which fully demonstrated that a multi-source transient electromagnetic system had the ability to determine abundant resistivity information of a complex medium.

3D Numerical Modeling of Induced-Polarization Grounded Electrical-Source Airborne Transient Electromagnetic Response Based on the Fictitious Wave Field Methods

The grounded electrical-source airborne transient electromagnetic (GREATEM) system is widely used in mineral exploration. Meanwhile, the induced polarization (IP) effect, which indicates the polarizability of the earth, is often found. In this paper, the Maxwell equations in the frequency domain are transformed into fictitious wave domain, where Maxwell equations are solved by the time domain finite difference method. Then, an integral transformation method is used to convert the calculation results back to the time domain. A three-dimensional (3D) numerical simulation in a polarizable medium is presented. The accuracy of this method is proven by comparing it with the analytical solution and the existing method, and the calculation efficiency is increased five-fold. The simulation results show that the GREATEM system has a higher response amplitude in the conductive region, while IP effects cannot be identified in the conductive area. The GREATEM system has a higher response amplitude in the low-resistance region, but IP effects cannot be identified in the low-resistance area, and the detection of IP effects is more suitable for the high-resistance area. Therefore, it is necessary to improve the detection ability of the GREATEM system in the low-resistance area.

First Results From Drone-Based Transient Electromagnetic Survey to Map and Detect Unexploded Ordnance

Unexploded ordnance (UXO), which causes many civilian casualties every year, has become a serious environmental problem. To deal with the problem, we have developed a new drone-based transient electromagnetic (TEM) system, which was designed for the UXO detection through very low altitude measurements. Drone-based TEM system uses the rotorcraft equipped with central loop TEM device to realize the UXO detection. The system was more safe and efficient than a ground-based TEM system in UXO detection. Compared with the airborne magnetic method or helicopter-borne TEM system, it has the advantages of low cost, flight safety, and so on. In this letter, the system characteristics are introduced in detail, providing the theoretical analysis of the system in UXO detection and verification of model data. Finally, through the UXO detection in the former weapon test base, the results show that the system is effective and successful in UXO detection.

Location and Characterization of Unexploded Ordnance-Like Targets With a Portable Transient Electromagnetic System

Location and characterization of a buried target from its electromagnetic induction response is a key problem for unexploded ordnance (UXO) detection. This article focuses on the influence of target size, depth, and orientation on its location and characterization on the basis of single dipole model with differential evolution (DE) algorithm against data collected by a portable transient electromagnetic (TEM) system. The sensor of the portable TEM system is composed of a single-layer transmitting coil and five three-component receiving coils. The diameter and current of transmitting coil are 0.5 m and 5 A. The length and resonant frequency of receiving coil are 5.6 cm and 180 kHz. The results of test-stand experiment show that the errors in location and characterization an underground target depend not only on the signal-to-noise ratio of the response, but also on the orientation of the target. For large target vertically (tail down) oriented, the estimated depth and characteristic response only represent the part of target close to the sensor, which are smaller than the real ones. For target flat-lying oriented, the algorithm overestimates both the depth and the characteristic response for the nonuniform primary field throughout the target. Finally, field experiment has been conducted. In general, the target can be located and characterized well even if the response of two targets is superimposed. The errors in estimated positions of about 5-7 cm support the conclusions from our test-stand experiment. These errors are especially critical when excavating targets.

The Progress of the Helicopter-Borne Transient Electromagnetic Method and Technology in China

The central and western regions of China have appropriate conditions for mining development, but there are many deserts, swamps and forest-covered areas in this vast land. As a result, the traditional on-ground methods are difficult to carry out efficiently in these areas. The airborne transient electromagnetic method can implement detection over large areas rapidly and efficiently, and play an important role in terms of resource and environment detection. In recent years, two Helicopter-borne Transient electromagnetic (HTEM) systems, CHTEM-I and CAS-HTEM, were developed in China. In this paper, the difficult issues in the development of HTEM hardware and software will be analyzed, firstly. By tracing the specific technical paths to solve these difficult issues, the latest research achievements in the development process of CHTEM-I and CAS-HTEM will be expounded. Finally, we would like to briefly discuss the research and development direction of HTEM technology in China to promote further development of the research and application of this technology.

Multiple-Transmit Focusing for the Nondestructive Testing of Downhole Casings Based on Borehole Transient Electromagnetic Systems

Borehole transient electromagnetic (TEM) array has proven to be efficient for the downhole nondestructive testing (NDT) of metal casings through the eddy-current property. However, restricted by bad downhole conditions, the simple increase of the receiving array is not sufficient enough for improving the downhole NDT performance. In this paper, we present a multiple-transmit focusing (MTF)-based borehole TEM system for the NDT of downhole casings. On the basis of the borehole TEM signal model, the response of the multiple-transmit array is derived by employing the matrix form of the borehole TEM response. It was shown that the excited magnetic field by the multiple-transmit array can be focused by weighting the current of each transmitter. Using this property, a modified linear constrained minimum variance-based multiple-transmitting array weighting method was proposed to realize the MTF. Moreover, a subarray partition approach was proposed to simplify the MTF realization, where the subarray weighting and mean square error were also analyzed. The MTF method performance was verified by applying it to a borehole TEM system for the NDTs of downhole casings. Finally, simulations and experiments were conducted, and the results demonstrated the effectiveness of the proposed method.

The Development and Applications of the Helicopter-borne Transient Electromagnetic System CAS-HTEM

Over the past decade, helicopter-borne transient electromagnetic (HTEM) systems have been rapidly developed. A new HTEM prototype (referred to as a CAS-HTEM) has been developed by the Chinese Academy of Sciences. In terms of hardware, the CAS-HTEM system uses an inflatable structure to carry the transmitting loop, which significantly reduces the weight of the system and makes it easier to transport. A dual gain receiver was innovated to extend the dynamic range of the system. In addition, an observation circuit for transmitting voltage waveform is introduced, so that the derivative waveform of transmitting current with higher SNR could be calculated. In terms of data processing, more reliable early time data could be obtained by band-limited effect removal; a weighted stacking algorithm is introduced to reduce the narrow band noise more effectively and increase the sensitivity of data to the anomaly location; a method based on τ-domain transform is used for late time signal processing. The results of the field test which was carried out in Inner Mongolia were found to be consistent with the drill data, which effectively verified the performance of this HTEM prototype.

Classification of unexploded ordnance-like targets with characteristic response in transient electromagnetic sensing

Characteristic response parameterized in time-dependent target intrinsic polarizabilities can be used to discriminate unexploded ordnance (UXO) from harmless items. This study explains the measurement, analysis, and classification of UXO-like targets against data collected under laboratory-type conditions with a specially designed transient electromagnetic (TEM) system. After a brief description of the single dipole model and TEM system, the characteristic responses of all UXO-like targets have been measured with an improved long straight solenoid. The influence of the target shape and size on characteristic response is analyzed. Results show that targets of the same shape share similar power law function decay rate at an early time. The larger the target size is, the greater the response will be. The characteristic response for harmless targets decays faster than the UXOs, leading to a considerable difference in a later time, which can be used for target classification. When the characteristic response is parameterized as kt−βe−γt in 100 ms, targets can be effectively separated with the fitted parameters k, β and γ. However, when fitted in 10 ms, the fitted parameter γ may be thousands of times than the one fitted in 100 ms, resulting in classification failure. The normalized response combined with the fitting parameter β can be used for target classification to overcome the instability of fitted parameter γ in 10 ms. Finally, all UXOs are completely separated from harmless targets within 10 ms.

Application of Grounded Electrical Source Airborne Transient Electromagnetic (GREATEM) System in Goaf Water Detection

Since goaf water can cause water inrush, collapse or other severe geological hazards in coal mining, its detection has become a significant issue. As the grounded electrical source airborne transient electromagnetic (GREATEM) system can provide considerable prospecting depth, lateral resolution, and high detection efficiency, the method may be suitable for goaf water detection. To verify the feasibility, a field electromagnetic survey over Qinshui coal mine (Shanxi province, China) was conducted, and the system was used to detect the geoelectric characteristics of goaf water. After data acquisition, data processing, and apparent resistivity calculation, the resistivity profile of survey area is clearly presented, which is consistent with the information provided by Shanxi Coal Geology Geophysical Surveying Exploration Institute, China. The result shows that the application of GREATEM system is an effective technique for resistivity detection of goaf water, and can reduce the risks in coal mining.

Demonstration of the newly developed MTEM systems for gold detection in China

Previously, the multi‐channel transient electromagnetic (MTEM) system has been successfully developed and applied to hydrocarbon reservoirs exploration on land and offshore environments in France. In order to explore the mineral resources buried at the depths up to 2,000 m in China, we developed a new MTEM system. In this paper, we present the components of the system, methods, principles, the parameters of the system, and the data interpretation method. To validate the effectiveness of the methodology and the developed system, we carried out an MTEM survey to locate the controlling faults of a gold deposit in the east of China. The survey contains 30 receivers distributed over 12‐km survey. By comparing the MTEM interpretation results with the known geological background, we find that MTEM methods can provide a good estimation of the ore‐controlling faults and this method could be a potential tool for the future deep mineral exploration.

Applying Stray Inductance Model to Study Turn-off Current in Multi-Turn Loop of Shallow Transient Electromagnetic Systems

The multi-turn, small loop is usually used as the transmitting loop for shallow transient electromagnetic (TEM) surveying. As an inductive load of the current inverter of the TEM system, it finds that there are serious oscillations at early part of the turn-off current. However, the existed RLC equivalent model of the loop alone fails to explain this phenomenon. In this paper, a novel current inverter for shallow TEM survey is designed and analyzed. The passive constant voltage clamping technology is used to achieve fast linear turn-off current. Furthermore, a novel stray inductance model of the current inverter is first proposed and utilized to analyze the serious oscillations of turn-off current. Based on this model, the turn-off current of the current inverter is simulated, and the simulated results agree well with the measured results of the experiment, which verifies that the proposed stray inductive model is effective and reasonable. According to the TEM principle, the high-frequency oscillations of the early part turn-off current will badly influence the shallow transient electromagnetic response. In order to suppress the oscillations of the turn-off current, two effective suppressing methods are proposed and analyzed. It will effectively enhance the electromagnetic data interpretation quality for shallow TEM survey.

The study of time domain denoising for the time-frequency electromagnetic method prospecting data

The time-frequency electromagnetic prospecting system has both the characteristics of frequency sounding and transient electromagnetic sounding system, can obtain apparent resistivity both in frequency domain and time domain, therefore, thus, more accurate and credible geoelectric information can be obtained. After analyzing, there are mainly three typical types of noise in time-frequency electromagnetic observed data, they are 50Hz power frequency and its harmonics interference noise, high-frequency random impulse noise and low frequency interference noise. In view of characteristics of these three types of noise and the time-frequency electromagnetic prospecting signal, we adopted frequency domain band stop filtering method remove the 50 Hz power frequency and its harmonics interference noise, proposed a segmentation and extension median filtering method and a fitting fixed extreme EMD method to remove the high frequency random impulse noise and the low frequency interference noise separately, and proposed a median filtering window size test selection method based on variance and skewness coefficient of sample signal. Moreover, we designed a denoising processing fl ow as handle 50Hz power frequency and its harmonics interference noise at fist step, handle high frequency random impulse noise at second step, handle low frequency interference noise at last step, it can give full advantages of the denoising handling methods. By theoretical analysis and verification by practical observation data experiments, the methods what we adopted and proposed and the processing flow we designed can effectively remove the three types of interference noises effectively and avoid method error, can preserve phase and amplitude information of effective signal accurately, finally, high quality and satisfactory time domain signal waveform can obtained as a result, and lays a good foundation for extracting accurate transient electromagnetic attenuation curve in time domain for the Time-Frequency electromagnetic prospecting data.

Accurate Measurement of Characteristic Response for Unexploded Ordnance With Transient Electromagnetic System

Characteristic response parameterized in time-dependent target intrinsic polarizabilities has been widely used to create feature libraries for unexploded ordnance (UXO) discrimination. This paper discusses the accurate measurement of characteristic response for UXOs with transient electromagnetic (TEM) system. First, a specially designed TEM system, which utilizes a modified solenoid and a pair of rectangular coils as transmitting coils to ensure the uniformity of primary field up to 96% throughout the target, is described. Then, the characteristic responses of 12 UXOs are measured from directions vertical or parallel to the axis at distances from 40 to 120 cm. The measured characteristic responses show a power law decay function at an early time and an exponential decay function at a later time. The characteristic responses parallel to the target axis are larger, and the decay is considerably slower than those vertical to the axis, leading to one to two orders of magnitude difference at a later time. Results also show that the characteristic response measured from the direction vertical to the axis of the target is more precise than that from the direction parallel to the axis at the same distance. The portion of the target that is close to the system accounts for a large proportion of the response. A target can be approximated as a single dipole only when it is located no less than twice its length from the system. Finally, the characteristic responses of 12 targets are parameterized with the Pasion–Oldenburg expression for feature library creation.

Reducing Motion-Induced Noise With Mechanically Resonant Coil Sensor in a Rigid Helicopter Transient Electromagnetic System

Motion-induced noise (MIN) level is a crucial factor in a rigid helicopter transient electromagnetic (TEM) system, which is limited by two intrinsic constraints. First, the large transient transmitting current generates a disordered electromagnetic field at the sensing area that restricts the configuration of the coil sensor. Paradoxically, the MIN's frequency range is from 10 to 40 Hz, which covers the TEM signal's base frequency (25 Hz) due to the mechanical character of the coil sensor on the platform. Filtering the MIN is very difficult because the MIN and TEM signals are cogenetic from the coil sensor and they have overlapping frequency distributions. An effective way to reduce the MIN is to separate it from the TEM signal in the frequency spectrum. Hence, a mechanically resonant coil sensor configuration is designed in this paper to change the mechanical character of the coil sensor and reconcile the two conflicting aspects. Prospectively, the frequency of the MIN is decreased to 8.6 Hz. After processing the recorded data with the same method as before, the MIN level is reduced to 10 nV/m2 from previously 60 nV/m2 in field exploration. Given the anomaly response over the ore body, the detection resolution is improved. The detection depth can be increased by dozens of meters based on the explanatory layer model.

Calculation and application of full-wave airborne transient electromagnetic data in electromagnetic detection

Airborne electromagnetic transient method enjoys the advantages of high-efficiency and the high resolution of electromagnetic anomalies, especially suitable for mining detection around goaf areas and deep exploration of minerals. In this paper, we calculated the full-wave airborne transient electromagnetic data, according to the result of numerical research, the advantage of switch-off time response in electromagnetic detection was proofed via experiments. Firstly, based on the full-wave airborne transient electromagnetic system developed by Jilin University (JLU-ATEMI), we proposed a method to compute the full-waveform electromagnetic (EM) data of 3D model using the FDTD approach and convolution algorithm, and verify the calculation by the response of homogenous half-space. Then, through comparison of switch-off-time response and off-time response, we studied the effect of ramp time on anomaly detection. Finally, we arranged two experimental electromagnetic detection, the results indicated that the switch-off-time response can reveal the shallow target more effectively, and the full-waveform airborne electromagnetic system is an effective technique for shallow target detection.