Time Domain Electromagnetic Method Research Articles

Generalized Tikhonov's algorithm for accurate calculation of one‐dimensional transient responses directly in time domain

ABSTRACTIn the last two decades, forward modelling for the time domain (transient) electromagnetic method has concentrated almost entirely on multi‐dimensional models and algorithms. At the same time, the interpretation of real field data is still mainly one dimensional. This is caused by the lack of an efficient multi‐dimensional acquisition procedure supported by sufficiently fast and reliable inversion software, on the one hand, and by the great efficiency of one‐dimensional field set up and interpretation of the data on the other hand. The latter is particularly true for the short offset transient electromagnetic method, which is much less sensitive to multi‐dimensional effects, compared to long offset methods. The most commonly used one‐dimensional forward modelling algorithms are based on the spectral method, which requires calculating rapidly oscillating Fourier–Bessel (Hankel) integrals. Due to the very fast decay of short offset responses, the integrals become computationally unstable at late times of the transient process. Although this problem has been successfully solved for practically feasible measurement times of conventional short offset systems using transverse electric and mixed transverse electric and transverse magnetic fields, it turned out crucial for novel methods based on the use of unimodal transverse magnetic fields. These methods are much more sensitive to geoelectric parameters of the Earth in general and those of resistive targets, in particular, but they generate responses, which drop at late times significantly faster than those of conventional methods. Such behaviour of transverse magnetic fields represents severe computational problem for the spectral method, but is successfully solved by direct time domain algorithms. This article describes a generalization of the well‐known Tikhonov's solution to a boundary value problem directly in time domain, which is applied to an arbitrary one‐dimensional earth model excited by an arbitrary source. Contrary to existing spectral algorithms, the described method allows accurate calculations of both transverse electric and transverse magnetic transient responses at arbitrarily late times. On the other hand, it is more time efficient than finite‐difference/finite element direct time domain algorithms and provides analytical late‐stage asymptotic solutions.

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.

Assessment groundwater occurrences using VES/TEM techniques at North Galala plateau, NW Gulf of Suez, Egypt

Water resources assessment is important for urbanization and industrial projects in remote areas. As the area is far from the Nile and characterized by rarity of rainfall, the main source of freshwater is groundwater. Vertical Electrical Sounding (VES) and Time Domain Electromagnetic (TEM) methods are chosen. To overcome the ambiguity, TEM station no.1 was conducted close to a borehole. The information from the borehole includes lithology and producing aquifer zones. The processing started with the inversion of VES data then use the models obtained as the primary model for TEM inversion. The last models from VES and TEM were further used to construct 3 geoelectrical profiles and 8 contour maps illustrating subsurface water-bearing aquifers electric resistivity. From the results, two aquifers are found divided by a highly conductive clay unit. The shallow one is consisting of coarse sand with silt. It displays moderate to low resistivity, depth ranging from 6 m to 33 m underground surface and a thickness of 15–35m. On the other hand, the deep one shows low resistivity values that may show brackish to fresh water with depths to the upper surface ranges from 58m to 118m and thickness distribution of 26–80m. In general, the groundwater flow is expected to be from southwest to northeast towards Suez Gulf as indicated by the elevation trending of the aquifers. According to the resistivity distribution, the direction of seawater intrusion is the northeast in the directions of Suez Gulf.

Comparison of Very Fast Simulated Annealing and Modified Particle Swarm Optimizaton Inversion Method for 1-D TDEM Data Modelling

Time domain electromagnetic method (TDEM) is a geophysical method that can be applied to a wide range of problem. TDEM data requires non-linear inversion in its modelling because of the complex relationship between data and model. A global approach is an approach in nonlinear inversion calculation which is not sensitive to the initial model and capable of obtaining global minimums. Previous studies proved that one type of inversion method will not give the most satisfactory results for all different types of inversion problems. For this reason, this study was conducted to discuss the performance of global non-linear inversion methods for 1D TDEM data modelling so that future inversion process can be carried out more optimally. In this study, the performance of VFSA and MPSO inversion method in the modelling of synthetic and field 1D TDEM data is compared. The inversion modelling is done multiple times in order to consider the random factor present in both methods and also to compare the result’s variability of each method. From the result of synthetic data inversion, it can be seen that there is a variation of inversion performance between models which is caused by the ambiguity in 1D TDEM. Based on the comparison of the two methods, MPSO performs better in inversion of data with lower level of ambiguity, and vice versa. Overall, it is concluded that the VFSA method performs better than MPSO because it has lower variability and gives better inversion results for data with high ambiguity which is usually the case for real 1D TDEM data.

3-D Modeling of Time Domain Electromagnetic (TDEM) Method to Analyze the Layered Earth Structure in the Geothermal Systems

3-D Time Domain Electromagnetic (TDEM) using Finite-Difference Time-Domain (FDTD) method has been successfully conducted to illustrate the electromagnetic wave behavior in the Earth’s surface. In this modeling, we consider the attenuation of electromagnetic wave energy which is one of the physical properties. This is very important to determine the reservoir potential area of geothermal systems as one of renewable energy. FDTD modeling has also some advantages in solving EM field interaction problem on complex geometry and analyzing transient problems. TDEM method is used to model the value of electric and magnetic fields as a function of the time, distance, and depth. Medium conductivity in the subsurface which is used in this modeling are adjusted to the real condition on the geothermal system. The results of this modeling can be applied to describe the behavior of electromagnetic wave in the which is possible to determine the reservoir potential of geothermal systems.

Ground-source Airborne Time-domain ElectroMagnetic (GATEM) modelling and interpretation method for a rough medium based on fractional diffusion

Ground-source Airborne Time-domain ElectroMagnetic (GATEM) modelling and interpretation method for a rough medium based on fractional diffusion

Extraction of Multimode S-Parameters by Using a Hybrid FDTD/SVD Technique

In this communication, a method for extracting multimode S-parameters in guided-wave structures is presented based on the finite-difference time-domain electromagnetic method in conjunction with the frequency domain (FD) singular value decomposition scheme. The present method can calculate the multimode S-parameters directly from the field distribution in the FD. It is found that the proposed modal analysis and multimode S-parameter extraction agree excellently with the analytic value and other available simulation techniques. Also, the method for multimode S-parameter extraction is validated by various microstrip discontinuities.

Three-dimensional modeling of frequency- and time-domain electromagnetic methods with induced polarization effects

Electrical conduction describes the ability of porous media to conduct electrical charges and induced polarization (IP) describes their ability to reversibly store electrical charges. An effective conductivity can be defined as a complex number with frequency-dependent components (i.e., the conductivity is also dispersive). Although IP effects have been observed in frequency- and time-domain electromagnetic (FDEM and TDEM, respectively) data for years, most FDEM and TDEM studies still treat the earth using the conductivity alone (therefore neglecting IP effects). Electromagnetic field data inversion and interpretation require a quantitative three-dimensional modeling with dispersive conductivities, which is still a challenging problem. Using the generic partial-differential-equation solver Comsol Multiphysics's application program interface (API) with Matlab, we have successfully developed three-dimensional FDEM and TDEM modeling with IP effects. Benchmarks are performed using analytical solutions and other numerical techniques. Results with and without IP effects are also compared and analyzed to illustrate the importance of taking IP effects into account. Our modeling could be of great importance in quantitatively studying IP effects in the FDEM and TDEM methods, in developing new field configurations, and also in educational purposes.

A Review of Electrical and Electromagnetic Methods for Coal Mine Exploration in China

During the past 20 years, due to the rapid development of China’s economy, the amount of coal mining activities has rapidly increased, which has led to a large increase in the number of mined-out areas. Therefore, the role of geophysical exploration in coal fields has changed from the detection of coal resources to engineering geological environmental investigation. The safety, efficiency, and green production of coal mines require more advanced geophysical detection technologies. In particular, the accurate detections of the locations of water-rich mined-out areas, as well as concealed water diversion channels, have become the main focuses of coal electrical methods in recent years. Therefore, electromagnetic methods have become the main means of hydrogeological investigations and deep explorations in mine-out coal areas. In this study, a common DC method was briefly introduced, along with an induced polarization method. Then, three representative electromagnetic sounding methods, including a surface frequency-domain electromagnetic method, time-domain electromagnetic method, surface to air electromagnetic method, and mine electromagnetic method, were respectively introduced.

Application of the Time Domain Electro-Magnetic Method to surveying of tunnels with significant overburden and swellable ground

Application of the Time Domain Electro-Magnetic Method to surveying of tunnels with significant overburden and swellable ground

Combined System of Magnetic Resonance Sounding and Time-Domain Electromagnetic Method for Water-Induced Disaster Detection in Tunnels.

Underground construction projects such as tunnel construction are at high risk of water-induced disasters. Because this type of disaster poses a serious threat to worker safety and productivity, instruments and methods that can accurately detect the water source are critical. In this study, a water detection instrument that combines Magnetic Resonance Sounding (MRS) and Time-domain Electromagnetic Method (TEM) techniques to yield a joint MRS-TEM interpretation method was developed for narrow underground spaces such as tunnels. Joint modules including a transmitter and receiver were developed based on a dual-purpose and modular design concept to minimize the size and weight of the instrument and consequently facilitate transportation and measurement. Additionally, wireless control and communication technology was implemented to enable inter-module cooperation and simplify instrument wiring, and wireless synchronization was accomplished by implementing a Global Positioning System (GPS)-based timing scheme. The effectiveness and reliability of the instrument were verified via indoor laboratory tests and field measurement signal tests. Furthermore, the practicability of the combined instrument and its interpretation method was verified via a field case performed in a tunnel in Hubei, China.

Research study on the short offset time-domain electromagnetic method for deep exploration

As a kind of geophysical method, the principle of transient electromagnetic method (TEM) observes the secondary vortex field in the gap of the primary field, which is applied to the investigation of resources and environmental engineering survey. In order to realize the rapid and precise detection of the deep targets (due to its relative great detection depth and high accuracy), the short offset time-domain electromagnetic method (SOTEM) is proposed in this paper. In addition, its feasibility, observation and interpretation techniques are researched. The results show that SOTEM has the same near source detection capability as the central loop configuration when choosing the step waveform, and as a result, the primary and secondary fields can be separated theoretically. The application has shown the effectiveness and superiority of SOTEM in the deep target exploration.

Delineation of Subsea Freshwater Extension by Marine Geoelectromagnetic Soundings (SE Mediterranean Sea)

This study presents a novel application of marine geoelectromagnetic technique, in an attempt to delineate freshwater extension of the coastal sub-aquifers beneath the SE Mediterranean Sea, along central Israel. The novel marine Ex-Bz time domain electromagnetic (TDEM) geophysical method was applied, demonstrating high sensitivity to the presence of sub-seafloor electrically resistive structures up to several kilometers offshore (shallow marine environment). The study included 19 marine measurements located offshore between Ashdod in the south to Tel-Aviv in the north, where a previous onshore study detected fresh groundwater below seawater intrusion at the coast line. The offshore measurements were conducted to distances of up to 3 km from the shoreline. It was found that the lower sub-aquifer contains fresh groundwater (resistivity >10 Ω-m) to a distance of 2.8 km offshore along the 30 km strip, and is probably constrained by lateral geo-facial changes.

Investigating subsurface structures of Gachsaran oil field in Iran using 2D inversion of magnetotelluric data

The magnetotelluric method is most often considered in hydrocarbon exploration in cases which are difficult for seismic imaging. In 2012, the National Iranian Oil Co. (NIOC) conducted an electromagnetic survey which included magnetotelluric and time domain electromagnetic (TEM) methods in the Gachsaran oil field in the south-west of Iran to delineate the reservoir formation of the region. Magnetotelluric data were collected at 5215 sites and a regular site spacing of 200 m was utilised. We chose a 16.5 km profile perpendicular to the main geological strike direction in the study area. The Bahr’s skew and Mohr diagrams were used to perform the dimensionality analysis of the magnetotelluric (MT) data and indicated that the subsurface structures are one dimensional or two dimensional at shallow depths, whereas they are mainly three dimensional at lower depths. The phase tensor showed that the dominant geoelectrical strike in the survey area is in the NW–SE direction. Two dimensional inversion was utilised to acquire a realistic resistivity model that was compromise between the spatial smoothness of the inversion model and the MT data fit. Apparent resistivity and phase data were modelled using the smoothness-constrained least-squares method. Models obtained of the TM, TE and TM+TE mode data were examined to have the best possible interpretation. The resulting 2D model revealed the main anticline and overthrust zone in the region. The near surface layer in the model which has a low resistivity, was identified as the cover rock of the region. The formation of the top of the reservoir in the region is estimated to be located at the depth of 1400–1900 m below sea level. The resistivity model is in good correlation with the geological features and the results of well drilling.

Continuous TDEM for monitoring shale hydraulic fracturing

Monitoring and delineating the spatial distribution of shale fracturing is fundamentally important to shale gas production. Standard monitoring methods, such as time-lapse seismic, cross-well seismic and micro-seismic methods, are expensive, timeconsuming, and do not show the changes in the formation with time. The resistivities of hydraulic fracturing fluid and reservoir rocks were measured. The results suggest that the injection fluid and consequently the injected reservoir are characterized by very low resistivity and high chargeability. This allows using of the controlled-source electromagnetic method (CSEM) to monitor shale gas hydraulic fracturing. Based on the geoelectrical model which was proposed according to the well-log and seismic data in the test area the change rule of the reacted electrical field was studied to account for the change of shale resistivity, and then the normalized residual resistivity method for time lapse processing was given. The time-domain electromagnetic method (TDEM) was used to continuously monitor the shale gas fracturing at the Fulin shale gas field in southern China. A high-power transmitter and multi-channel transient electromagnetic receiver array were adopted. 9 h time series of Ex component of 224 sites which were laid out on the surface and over three fracturing stages of a horizontal well at 2800 m depth was recorded. After data processing and calculation of the normalized resistivity residuals, the changes in the Ex signal were determined and a dynamic 3D image of the change in resistivity was constructed. This allows modeling the spatial distribution of the fracturing fluid. The model results suggest that TDEM is promising for monitoring hydraulic fracturing of shale.

Numerical Analysis of Land-Based Inline-Source Configuration for the Controlled-Source Electromagnetic Method

The geometry of source and receiver is one of the most crucial factors for the success of a controlled-source electromagnetic (CSEM) investigation. The inline electric-source configuration, widely used in the marine CSEM method, is not commonly employed for land CSEM surveys. To examine the feasibility of the inline electric-source time domain electromagnetic method (TEM), we first analyzed the optimal observing area for the land-based electric-dipole source by imaging the distribution of each component and the anomalies caused by a resistive or conductive target layer. Then, we conducted an error-space analysis to evaluate the sensitivity of each inline component based on the result of the optimal observation area. Finally, we collected field data from a typical inline dipole-dipole configuration and inverted the data with an algorithm based on Occam's principle. [Figure: see text]

Electrical Resistivity Tomography and TDEM Applied to Hydrogeological Study in Taubaté Basin, Brazil

This research applies Electrical Resistivity Tomography (ERT) and Time Domain Electromagnetic Method (TDEM) to study the hydrogeology of the Taubaté basin, which is characterized by half-grabens with about 850 m of maximum sediments thickness. The study area is in Taubaté city, São Paulo State, Brazil, where the Taubaté aquifer is an important water source. The Taubaté Group is the main sedimentary package of the basin; it is formed mainly by shales that form aquicludes, and thin layers of sandstones that form the aquifer. There are 40 groundwater exploration wells in Taubaté city that provide important information. The study purpose is to characterize the geoelectrical stratigraphy of the subsurface to locate the contact between the Quaternary and Tertiary sediments and to identify the Taubaté aquifer. The ERT is used for shallow investigations (tens of meters) and the TDEM can reach a great investigation depth (hundreds of meters). Therefore, these geophysical methods are complementary. The ERT data were acquired with the pole-dipole array with 20 m of electrodes spacing and 400 m length, and the TDEM data with the central-loop array with a 200 × 200 m transmitter loop. The results permit to define the contact between the Quaternary and Tertiary sediments around 15 m depth, the Pindamonhangaba Formation between 15 m and 30 m depth and the Taubate Group between 30 m and 300 m depth. The TDEM method defined the Taubaté Group as a single geoelectric layer because the shale and the sandstone layers are all very conductive. The basement is formed by gneiss, which is a very resistive rock. The TDEM method is not able to identify a high conductor/resistor contrast. Overall, the results are consistent with the known geology and the wells information.

Applicability of the time-domain electromagnetic method to investigation of ground classification along tunnel routes

Applicability of the time-domain electromagnetic method to investigation of ground classification along tunnel routes

Use of time-domain electromagnetic (TDEM) method to investigate seawater intrusion in the Lebna coastal aquifer of eastern Cap Bon, Tunisia

A multi-method geophysical survey, using time-domain electromagnetic (TDEM) fields, is conducted on the north-eastern coastal aquifer of Cap Bon, Tunisia. The aim of the study is to investigate the inland extent saltwater intrusion, to estimate the fresh-saltwater transition zone, and to characterize the lithologic spatial variability of the underground layers. Moreover, the TDEM is used to determine both fluid resistivity and bulk resistivity which are required to calculate the porosity by applying the Archie’s equation. The survey of TDEM profiles is carried out along the left and right overbanks of the Lebna river from the coast until the Lebna dam. The interpretation and the analysis of these profiles allow us to better identify the flux exchanges between the river and the underlying aquifer. On the basis of the 1D and 2D longitudinal and transversal resistivity profiles as well as the existing boreholes data and groundwater quality, it seems that the freshwater-saltwater interface is moving forward inland. Furthermore, electromagnetic results combined to the Archie’s law show that the porosity values are at the ranges of 12–36 and 5–31% for the Quaternary outcropping and Pliocene formation, respectively.

A comparison of loop time-domain electromagnetic and short-offset transient electromagnetic methods for mapping water-enriched zones — A case history in Shaanxi, China

Increases in the mining depth of coal pose a significant challenge to the conventional loop source time-domain electromagnetic (TEM) method that requires significant enlargement of the loop size and transmitting current to realize the deeper sounding results required. As an alternative, TEM devices based on a grounded wire source are generally used to solve detections deeper than several hundred meters. To map the water-enriched zones buried underneath approximately 1000 m at a coal mine in Shaanxi, China, loop TEM and short-offset transient electromagnetic (SOTEM) measurements were conducted. We carried out 1D forward modeling and inversion constrained by drilling informa-tion, and the results reveal that the resolution capability of loop TEM and SOTEM is almost the same in detecting a conductive layer in the absence of any noise. However, for a given noise level and decay time, the SOTEM method provides a deeper investigation than loop TEM without compromising sensitivity. The field examples validated the synthetic results. The loop TEM with dimensions of [Formula: see text] realized a maximum depth of 1000 m, whereas the reliable detection depth of 1500 m was achieved by using a 723 m long grounded wire source using the SOTEM method. Moreover, the labor required is significantly reduced, and the efficiency is dramatically raised using the SOTEM method. Our results predict that the SOTEM method should play a more important role in deep hydrogeophysical investigations.