Transient Electromagnetic Research Articles

BP neural network and improved differential evolution for transient electromagnetic inversion

In the transient electromagnetic (TEM) inversion, the BP neural network method has high efficiency owing to avoid the complicated forward model calculation in every iteration. The global optimization ability of the differential evolution Algorithm (DE) is adopted for amending BP's sensitive to initial parameters. A chaotic mutation and crossover with constraint factor DE (CCDE) is proposed in improving the global optimization ability. The CCDE-BP algorithm performance is validated by two typical testing functions and then by two geoelectric models inversion. The results show that the CCDE-BP method has better inversion speed, accuracy and stability yet with higher fitting degree. It is feasible in geophysical inverse applications.

Response characteristics of transient electromagnetic of fault structures in shallow stratum of underground space and its exploration

The key to detecting underground faults in shallow stratum of the city with transient electromagnetic method is the determination of the fault location and the characteristics of accurate underground parameters. In order to establish a field data acquisition device and method parameters suitable for the exploration of the fault structure in the shallow layer of the city, the corresponding numerical model is established based on the Shungeng mountain fault structure with shallow depth. The finite difference method is used to simulate and calculate the response characteristics of the transient electromagnetic field in the fault structure zone, which provides the basis for the field application of the method and technique. Then, based on the surface geological conditions of Dongshan section of the Shungeng mountain fault, by using the short-circuit transient electromagnetic method, field data acquisition observing system and relevant parameters suitable for transient electromagnetic prospecting of urban shallow fault structures were established through experiments on site technical conditions, and the geoelectric section. The structure and characteristics of shallow buried faults are judged and explained. The results show that the response characteristics of transient electromagnetic in the fault fracture zone are significant, and the location and structure of transient electromagnetic field can be easily distinguished. It can provide references for similar geological conditions.

The detection and response characteristics of transient electromagnetic signals caused by the deformation and fracture of overlying strata affected by mining of a thick coal seam

The height at which overlying strata fail is an important parameter for the prevention and control of mine water damage. High overlying strata failures during mining of a thick coal seam make testing in mines challenging. Considering the mining conditions for the thick coal seam in Ordos City, Inner Mongolia, China, this work applies ground detection using transient electromagnetics. The response characteristics of transient electromagnetic signals during strata failure were investigated using numerical simulation, and the results of that investigation will provide guidance for on-site exploration. In addition, based on the conditions of the ground and mining face, test wires were distributed within the mining work face area to obtain electromagnetic detection data using large wireframes on the ground to further construct an electrical profile of the coal mining and non-mining areas and explain the characteristics of strata failure. This method succeeded in the practical application and is provided as guidance for strata failure detection for thick coal seam mining.

A finite‐element time‐domain forward‐modelling algorithm for transient electromagnetics excited by grounded‐wire sources

ABSTRACTA three‐dimensional finite‐element time‐domain forward‐modelling algorithm is developed to simulate transient electromagnetics excited by grounded‐wire sources. The main advantage of this finite‐element time‐domain algorithm is that full transmitting‐current waveforms and complex‐shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half‐space model, a stratified‐medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half‐space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step‐off, step‐on and direct current waveforms. For the stratified‐medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non‐negligible topography, the effects of which on the shapes of the grounded‐wire sources must be taken into account when implementing the finite‐element time‐domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.

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.

Research and Application of the Transient Electromagnetic Method Inversion Technique Based on Particle Swarm Optimization Algorithm

The transient electromagnetic (TEM) method is widely used in shallow surface engineering geological surveys due to its advantages such as light weight, high efficiency, and strong resolution. However, interpretation and inversion of TEM data is a complicated process. The traditional algorithm of TEM inversion employs the “smoke ring” fast imaging method, which can only reflect the approximate morphology of the stratigraphic model, and the inversion accuracy is low. Therefore, this method cannot meet the requirements of high-precision inversion. In this article, we present the particle swarm optimization (PSO) algorithm for TEM inversion. First of all, the response of the rectangular loop source TEM based on electric dipole integration was calculated and compared with the analytical solution results of the rectangular loop source and the accuracy of the algorithm was verified. Then, we introduced the solution process of the particle swarm optimization algorithm and analyzed the influence of particle swarm optimization algorithm parameter selection on the accuracy of the inversion result and the convergence speed. Next, a layered medium model was established. The particle swarm optimization algorithm and “smoke ring” fast imaging method were used to perform inversion calculation. The results show that the PSO algorithm has the advantages of high efficiency and accuracy. Finally, we examined the effectiveness of the particle swarm optimization algorithm for TEM data processing by inverting survey data from an Air-raid shelter on the campus of Chongqing University in China and comparing the results with those from the “smoke ring” fast imaging. The research works in this article provide new methods and techniques for TEM data processing.

EMD-Based Borehole TEM Array Signal Denoising and Baseline Wander Correction for NDT of Downhole Casings

Transient electromagnetic (TEM) techniques have been proven to be efficient for nondestructive testing (NDT) operations due to their pulsed eddy-current properties. However, in the field of downhole measurements, harsh environments may significantly influence the NDT performance in downhole casings. In this paper, an empirical mode decomposition (EMD) method based on borehole TEM array signal denoising and baseline wander (BW) correction is proposed to compensate for the bad measurement conditions that affect downhole NDT. Based on the borehole TEM signal model, we investigated the principle of the EMD approach for the borehole TEM response, where the background magnetic noise and temperature drift effects were analyzed by considering the motion measurement and effective permeability. It was found out that although the BW can be effectively removed with the EMD approach, the performance of the signal denoising is closely related to the measurement speed of the downhole NDT sensors. To solve this problem, we proposed an array-based ensemble EMD method to improve the denoising performance of the borehole TEM signals by formulating a three-dimensional borehole TEM data structure, where the generation of the noise-aided data can be more efficient by employing the borehole TEM array. The performance of the proposed method was verified by applying it to a borehole TEM system for the NDTs of oil-well casings. In addition, field experiments were conducted, and the results demonstrated the effectiveness of the proposed method.

Comprehensive Ahead Prospecting of Tunnels in Severely Weathered Rock Mass Environments with High Water Inrush Risk: A Case Study in Shaanxi Province

The ahead geological prospecting of severely weathered surrounding rock in a tunnel is challenging due to complex geological conditions. Hence, the best approach is to use a variety of geophysical exploration methods. In this study, we have applied seismic, electrical resistivity (ER), and transient electromagnetic (TEM) methodologies for ahead prospecting of the highly weathered and complex surrounding rock at the diversion tunnel of the Hongyan River to Stone River Water Transfer Project in Shaanxi Province, China. The seismic method was employed to detect structural information for an area of 100 m from the front of the tunnel, the TEM method was used to obtain the resistivity information within 60 m, and the ER method was conducted to obtain detailed resistivity information over a range of 30 m. The integrated results generated a comprehensive interpretation of the geological body located within 100 m of the front of the tunnel. We divided this geological area into intact, severely weathered, and slightly weathered sections. Information on the water content of each section was also produced. The results of subsequent excavations are consistent with our results, proving the effectiveness and feasibility of a comprehensive approach for analyzing highly weathered and complex surrounding rock.

A Simple and Robust Total Variation-Based Inversion Scheme for Transient Electromagnetic Data

Transient electromagnetic (TEM) data are conveniently inverted to visualized underground structure with the widely used Occam’s inversion. However, in sedimentary environments, Occam’s inversion performs poorly in reproducing the sharp boundaries as it produces smoothed results of subsurface geophysical properties. Here, we develop a simple and robust total variation (TV) based inversion scheme for TEM data to preserve sharp boundaries and improve the accuracy of recovered underground structure. To solve the standard L2-TV optimization problem, we propose a reconstructing minimization (RM) approach. The TV-based inversion scheme is divided into two phases. In Phase I the misfit is brought down to a desired level utilizing Occam’s inversion. In Phase II we utilize RM approach to implement TV regularization until the convergent result is achieved. Taking advantage of the excellent stability of Occam’s inversion, the TV-based inversion scheme has the characteristics of robustness to different initial models. The switch process between Occam’s inversion and the RM approach is rather simple to realize due to their similar solution forms. The synthetic and field data examples validate the efficiency and accuracy of the proposed TV-based inversion scheme. We further present a novel analysis based on reconstruction of stabilizer to illustrate the reason for their distinctive behaviors of TV-based inversion and Occam’s inversion.

Application of FCTEM60-1 Instrument in Invert Rebar Testing for Highway Tunnel

The invert of a highway tunnel is the foundation for the entire tunnel structure. Its quality has a direct influence on the stability of tunnel structure. However, in practice the tunnel invert has various defects for a variety of reasons. FCTEM60-1 instrument is able to rapidly cut off the current using constant voltage clamping technique, thus significantly reducing the shallow dead-zone effect of transient electromagnetic (TEM). In this study the instrument is applied to testing invert rebar execution in a highway tunnel. The study result shows FCTEM60-1 instrument successfully detected the invert rebar, demonstrating its feasibility and reference value to tunnel quality test.

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.

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.

Multi-scale Target Detection for Underground Spaces with Transient Electromagnetics Based on Differential Pulse Scanning

Improvements in detecting underground spaces in urban areas require improvements to the radiation efficiency and resolving power of the artificial source. Due to the use of bipolar pulse and the low efficiency of the artificial source, the existing artificial sources have difficulty in accurately imaging complex urban underground spaces of varying depths and sizes, to complete the task of “transparent detection” of urban underground spaces to depths of two hundred meters. In this paper, a new type of transient electromagnetic radiation based on a high-performance artificial source and differential pulse is proposed. At the same time, multi-resolution theory and multi-scale information extraction technology are combined to further improve the resolution of the radiation field in imaging complex urban underground structures of varying sizes. With this new method, we are able to achieve our goal of transparent detection of ‘two hundred meters' depths of urban underground space.

3D inversion of MT data from northern Mexico for geothermal exploration using TEM data as constraints

San Felipe's area has geothermal surface manifestations such as hot springs and high temperature in drinking water wells. As a first stage of the geothermal exploration program, a regional geophysical survey has been carried out where 17 MT (Magnetotelluric) and TEM (Transient Electromagnetic) soundings were recorded along 4 different profiles. Both techniques were applied in the same spots and the TEM data are used to correct for static shift in MT data. Transient responses were measured from 1 to 70 ms, using a 50 x 50m2 single-loop configuration. MT signals were recorded with Metronix systems in the period range from 0.001 to 100 s. A robust processing scheme is applied to the MT time series and estimated Transfer Functions are appraised. The dimensionality analysis of MT data suggests a 1D and 3D subsurface structure. Thereby, a three-dimensional inversion of MT data is later carried out. The information obtained from the inversion modeling of TEM data is incorporated into the 3D MT inversion scheme. Thus, unconstrained and constrained inverse models are achieved and appraised. The preferred 3D inversion model is correlated to geological information of the survey area and a conductive structure is interpreted as a sedimentary basin at its shallow part and as a fault zone with geothermal fluids at depths >1.5 km.

A new method for absolute underground positioning based on transient electromagnetics

SUMMARY We present a procedure for localizing underground positions using a time-domain inductive electromagnetic (EM) method. The position to be localized is associated with an EM receiver placed inside the Earth. An EM field is generated by one or more transmitters located at known positions at the Earth’s surface. We then invert the EM field data for the receiver positions using a trust-region algorithm. For any given time regime and source–receiver geometry, the propagation of the electromagnetic fields is determined by the electrical conductivity distribution within the Earth. We show that it is sufficient to use a simple 1-D model to recover the receiver positions with reasonable accuracy. Generally, we demonstrate the robustness of the presented approach. Using confidence ellipses and confidence intervals we assess the accuracy of the recovered location data. The proposed method has been extensively tested against synthetic data obtained by numerical experiments. Furthermore, we have successfully carried out a location recovery using field data. The field data were recorded within a borehole in Alberta (Canada) at 101.4 m depth. The recovered location of the borehole receiver differs from the actual location by 0.70 m in the horizontal plane and by 0.82 m in depth.

A fast approximation for 1-D inversion of transient electromagnetic data by using a back propagation neural network and improved particle swarm optimization

Abstract. As one of the most active nonlinear inversion methods in transient electromagnetic (TEM) inversion, the back propagation (BP) neural network has high efficiency because the complicated forward model calculation is unnecessary in iteration. The global optimization ability of the particle swarm optimization (PSO) is adopted for amending the BP's sensitivity to its initial parameters, which avoids it falling into a local optimum. A chaotic-oscillation inertia weight PSO (COPSO) is proposed for accelerating convergence. The COPSO-BP algorithm performance is validated by two typical testing functions, two geoelectric models inversions and a field example. The results show that the COPSO-BP method is more accurate, stable and needs relatively less training time. The proposed algorithm has a higher fitting degree for the data inversion, and it is feasible to use it in geophysical inverse applications.

Formation resistivity measurement based on a transient electromagnetic field excited by an acoustic probe in an open-hole

AbstractThe monopole probe commonly used in acoustic logging can generate vibrations in its natural frequency under the excitation of the pulse signal, and excite transient electromagnetic (TEM) signals of the same frequency. The acoustic probe receives both acoustic and TEM field signals. The TEM field propagates and attenuates in conductive well fluid and formations, carrying formation resistivity information that could be used for formation evaluation. Based on the axisymmetry of the TEM field in an open-hole, theoretical calculation of the TEM field components in an open-hole model was performed. The results revealed that TEM response signals decay rapidly with time, and electric field intensity along the well axis Ez is approximately linear with formation resistivity. On this basis, we proposed a new method for measuring formation resistivity in an open-hole, which could supplement conventional logging methods. Also, it does not affect signal processing of acoustic logging and only applies the TEM signal to determine formation resistivity. The new method could accomplish a more comprehensive petroleum formation evaluation, which is of great significance to the integrated design of the well logging instrument.

Electromagnetic and electrical methods applied to mapping coked coal – A case study from the Bowen Basin, Eastern Australia

SummaryIn coal mining it is important to know if localised areas of coal have been rendered worthless through direct contact with volcanic intrusions. To avoid dilution, accurate coked coal boundaries are traditionally mapped by drilling.BHP has made significant strides in applying geophysical methods to add value at its coal operations. With respect to the mapping of coked coal BHP have tried a suite of electromagnetic and electrical methods; enjoying success at improving the resolution of coked coal boundaries with significant implications on the amount of drilling required.Recent trials of moving loop and airborne transient electromagnetic (TEM), Sub-audio magnetic (SAM), electrical resistivity imaging (ERI) and induced polarisation (IP) have encouraged the company to ramp up the inclusion of more TEM in its 5-year operational plans.Unconstrained 1D inversion of moving loop TEM data correlated well to drilled intersections of coked coal. This enabled an improved coked coal boundary to be mapped.An interesting observation is contrary conductivity sections obtained from moving loop TEM and ERI, with the latter method failing to identify the strong conductive layer mapped in both TEM and drilling. Possible conductivity anisotropy of the coked coal or the overlying sediments is being considered as a likely explanation for the failure of ERI to map the coked coal.Inversion of SAM TFEM data partially worked to map the coked coal but only when constrained by the TEM inversion results.The signal to noise ratio in the IP data proved to render the data uninterpretable, suggesting that the coked coal is not chargeable, with the graphite being in massive rather than disseminated form.It is concluded that moving loop TEM and helicopter TEM both efficiently map coked coal sufficiently to target reduced drilling programs. Hence, where scales are appropriate, helicopter surveying is ideal to maximise efficiency without compromising resolution or results.

Mapping bedrock topography and moraine deposits by transient electromagnetic sounding: Oslo graben, Norway

ABSTRACTIn this research, we have studied the capabilities and limitations of the transient electromagnetic sounding applied to investigate the structure and composition of the moraine deposits covering a crystalline basement. It has been experimentally demonstrated that using the ungrounded antennas one can not only determine the electrical resistivity of different layers in the alluvium deposits and map the surface topography of the crystalline basement down to a depth of 70–100 m, but can also minimize the costs and the damage done to the environment. Additionally, transient electromagnetics helps to reveal the layers within the moraine strata that often have different resistivities and depths. The use of single antennas of size 12 m × 12 m–25 m × 25 m for field excitation and registration of transient responses allows for maximum localization of the targets and provides a high horizontal resolution in strongly heterogeneous media. The limitations of the method due to the induced polarization effect have also been investigated. Overall, despite some limitations, the transient electromagnetic method appears to be effective in the mapping, assessment and analysis of hazards in landslide‐prone areas, prior to geotechnical soundings in wells.

Magnetotelluric imaging of the Iguatu Basin in NE Brazil: From shallow aquifers to mid- crustal conductors

In this work, the Iguatu basin in Northeastern Brazil was imaged by 49 broadband (~0.01–1,000s) magnetotelluric (MT) and transient electromagnetic (TEM) soundings displaced on a 3D arrangement effected by a NE-SW along-basin major axis profile and three NW-SE transversally oriented profiles. The Iguatu basin is an intracratonic sedimentary basin, with genesis related to the opening of the South Atlantic Ocean (160-115 Ma). The basin was implanted over Precambrian basement of the Borborema Province. The sedimentary rocks deposited from Early Cretaceous to Quaternary include medium to coarse sandstones with conglomerate levels at the bottom, siltstones, mudstones, followed by Tertiary similar sedimentary rocks and Quaternary unconsolidated alluvium coverings. The MT responses (impedance and tipper) were robustly estimated and static-shift corrected using transient electromagnetic (TEM) measurements. Both rotational invariant and phase tensor dimensionality analyses revealed a complex subsurface structure, characterized by 2D/1D structures for short period data (<1 s), associated to sedimentary rocks within the basin, and a 3D behavior for longer periods, related to crustal depths. Three-dimensional inversion of full impedance was performed yielding a preferred 3D resistivity model with nRMS error of 1.99, which recovered important geoelectrical structures within the basin and crust. Two remarkably low resistivity anomalies (<1 Ω m) were observed in the crust of the Borborema Province, a feature also observed other MT studies elsewhere in the province. They can be ascribed to either the Mesozoic reactivation of the Aiuaba-Orós-Jaguaribe shear zone or the percolation of aqueous fluids between the zones of weakness presented in the fractured basement beneath the Iguatu basin; or even be a combination of both mechanisms. The resistivity model varies appreciably within the basin alternating low resistivity (<1 Ω m) graben-like and resistive (>300 Ω m) horst-like features, reflecting a structurally complex rift tectonic system of this ~800 m deep pull apart basin. A good correlation between resistivity values and shallow unconfined aquifers were spotted.