A quantitative method for deriving salinity of subglacial water using ground-based transient electromagnetics

Unexploded ordnance (UXO) investigation is mainly carried out by the magnetic exploration. However, in this paper, the transient electromagnetic (TEM) method was used in order to detect UXO more accurately. Although the TEM method is usually used for deep sounding of mineral exploration, we used small loop TEM method to detect buried UXO. In order to understand the characteristics of the small loop TEM method, many experiments using small empty cans were carried out at Ito Campus in Kyushu University and a lot of features were discovered. For example, if the size of loop becomes smaller, the penetration depth becomes smaller. These results showed that the small loop TEM was applicable to UXO investigation. Based on these results, the test survey to detect the imitation UXOs using 180×180-cm-square loop was carried out. In this field experiment, there are various kinds of imitation UXOs, but all of the UXOs were detected. As a result, it was found that UXO detection by small loop TEM method is effective compared to magnetic method. Moreover, the small loop TEM method could narrow the range of UXO location to the size of 30×30cm-square.

Transient Electromagnetic (TEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate lithological variations in the Dead Sea coast of Israel. The subsurface in this area is heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to the salinity of groundwater, the clay content and the porosity of rocks. MRS, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS in Nahal Hever area we have identified lateral extensions of different lithological formations.

The removal of native vegetation for the development of land has caused the salinisation of surface soil and water resources in Australia. It is important to determine the existence of potential salinity hazards so that preventative measures may be taken at an early stage. The current hydrogeological conditions and the topography of a given catchment influence the movement of saline groundwater, and it is of particular importance to identify areas of high- and low-recharge to groundwater. On a catchment scale, the delineation of high-recharge areas by drilling can be prohibitively expensive, and more efficient mapping methods are required. Broadband electromagnetic (EM) sounding methods offer the possibility of obtaining the vertical conductivity distribution in the regolith, to produce three-dimensional maps of the resistivity structure. These maps could be used to interpret the distribution of features related to hydrology, such as recharge and stratification of stored salt in catchments. For salinity-related problems, a regolith depth of about 50 m is relevant. It is therefore necessary to measure the high-frequency content of the EM response to differentiate shallow conductive layers. For both ground-based and airborne EM methods, the response has been modelled in a frequency range of a few hundred Hz to 50 kHz. Salinity profile types have been identified to characterise different types of recharge. These profile types have been simulated with models consisting of horizontal layers of different resistivity. The theoretical modelling has determined that EM methods can be used to distinguish different types of salinity profiles. These results are supported by inversion of EM data collected in the field in a number of catchments. The transient electromagnetic (TEM) method is used for ground-based broadband EM measurements, and modelling shows that both the depth and thickness of the salt accumulation can be resolved when its depth is as little as 6 m and its thickness 10 m. Additionally, a relatively resistive basement below the salt accumulation may be distinguished from a conductive one. Since the former case implies the presence of a high-recharge salinity profile and the latter case a low-recharge profile, this method may be used to differentiate the two types of profiles. Down-hole conductivity logs in catchments near Collie and at East Yornaning, WA, confirm that different salinity profiles can be distinguished with EM methods. At Collie, the TEM method has detected the presence of salt accumulations and has determined the depth of their lower boundary. Inversion of these results shows the presence of a resistive electrical basement below the salt accumulations, and therefore indicates that relatively high recharge conditions are associated with these salinity profiles. Similarly, measurements along traverse lines in the East Yornaning catchment show a correlation between the inversions results and known geological features.

The airborne transient electromagnetic (TEM) method is widely used in land applications but faces challenges in marine applications due to the strong masking effects of seawater. The accuracy of the inverse Laplace transform algorithm strongly affects the applicability of the 2.5D TEM method in marine research; thus, an appropriate transmitted waveform is required. To resolve these issues, a trapezoidal wave was utilized as the waveform of the current source, owing to the substantial energy contribution of the low-frequency range. Further, the Talbot algorithm was selected for the inverse Laplace transform as it can yield an accurate response with smaller summation terms than the commonly used Gaver–Stehfest (G-S) algorithm. On this basis, a rugged seabed and a subsea tunnel were also investigated. The voltage response is obtained when the flight heights of the loop source change. The results confirmed that the Talbot algorithm with a trapezoidal waveform is more reliable and robust for complex marine geological models and is expected to provide an effective approach for marine explorations.

The ground‐airborne transient electromagnetic method with a galvanic source has such advantages as high efficiency, great exploration depth, high signal to noise ratio and good performance in mountainous areas. However, so far its interpretation system has not been established, which restricts the development of this method. This paper aims at building up a complete ground‐airborne electromagnetic detection system to enrich the theory of this method. This paper consists of three parts: (1) full field apparent resistivity definition; (2) Kirchhoff immigration of TEM virtual wave field; and (3) inverse synthetic aperture imaging. Magnetic field intensity is selected for full field apparent resistivity definition. The results of theoretical models show that the first and last part of the apparent resistivity curve can reach the resistivity of the first and the last layer, respectively. Thus, full space and full time apparent resistivity definition is achieved. Based on previous study, Kirchhoff migration of ground‐airborne transient electromagnetic method with a long wire source is realized. A correlation method is adopted for multiple survey point synthesis, which pushes the traditional single point processing forward point‐by‐point for multi‐coverage inverse synthetic aperture imaging.The results of layered models show that full field apparent resistivity can reveal the underground electric change smoothly, completely and gradually. When the resistivity of the middle layer changes, the differentiation of full field apparent resistivity is obvious, which can easily and intuitively identify the electric layer. As receiver altitude, offset and time are considered simultaneously, full field apparent resistivity can be calculated in full space and full time. The results of the complex model with a water‐bearing gob shows that the further the offset from the gob is, the weaker the resistivity anomaly on full field apparent resistivity section is. There are two interfaces from the Kirchhoff immigration results, of which the upper one indicates surface. As the electrical difference between earth and air is large, the wave field signal of this interface is strong, which spreads over the whole area. While the anomalies of the lower interface are mainly distributed in the middle area and weaken outwards, which indicate the water bearing gob. The upper interface changes little after inverse synthetic aperture imaging, while the gob anomaly narrows and has a clear boundary. The anomaly boundary matches well with the designed model. Inverse synthetic aperture imaging of ground‐airborne transient electromagnetic survey with a long wire source is realized on the basis of inverse synthetic aperture radar (ISAR). Full field apparent resistivity is put forward with inverse function and iteration, which can realize calculation of this variable in the full zone and full time. Pseudo‐seismic migration of transient electromagnetism is used in 3D transient field imaging. Inverse synthetic aperture radar is introduced in inverse synthetic aperture imaging of the ground‐airborne transient electromagnetic method with a long wire source, which can help enhance resolution. The results of designed models show that correlation can strengthen useful signal and improve signal to noise ratio and resolution, which prove the efficiency of inverse synthetic aperture imaging of the ground‐airborne transient electromagnetic method with a long wire source.

Joint use of TEM and MRS methods in a complex geological setting

In this research study, the landslide events of Huangshan City in China's Anhui Province were monitored through the integration of two geophysical techniques: the magnetic resonance sounding (MRS) and transient electromagnetic (TEM) methods. The JLMRS-III system was used to determine the distributions of the free water in the strata and the MiniTEM system was utilized to image the subsurface electrical resistivity structures. Data acquisition was carried out on two planned survey lines during a wet season. A set of 15 measurements were completed using MRS array detection; a set of 44 measurements were completed using TEM method. The monitoring activities were repeatedly implemented before and after rainfall occurred. The results of water content information, relaxation time, and resistivity distributions were determined through joint inversion. Plots of the results clearly show the aquifer and resistivity structure of the study area. The monitoring data of displacement, pore-water pressure, and rainfall was collected to help differentiate the slip blocks, slip beds, and slip belts in the accumulation bodies of the landslides. We have delineated the potential landslide area, and determined the sliding depth range at 4 to 8 m. We analyzed the stability of the landslide based on the comparison of water content distribution and the relevant geological information in the sliding body to reveal the possibility of future destruction. The results of this study demonstrated that the combined techniques of MRS and TEM have the capability to effectively monitor potential landslides for key parameters needed for risk assessment.

Hydrogeophysical characterization using the transient electromagnetic method (TEM) and the DC resistivity sounding (VES) method was implemented in the central part of Azraq Basin (Qa Basin), Jordan, to identify and map the spatial distribution of shallow fresh and saline groundwater in the upper aquifer systems. The alluvium (Al) and chert limestone (URC) shallow aquifers show different degrees of groundwater salinization. The range of groundwater resistivity varies from 0.06 to 10.8 ohm-m. Saline groundwater was detected at depths between 5 to 30 m where the aquifers have a wide spectrum of resistivity values from 0.14 to 120 ohm-m. The integrated geophysical and hydrogeologic models are significantly correlated in chloride concentration, groundwater resistivity, and aquifer resistivity. Using 1D inversion results from the TEM and VES soundings in addition to quasi-3D modeling (1D spatially constrained inversion) at selected TEM sites, groundwater resistivity variation was attributed to two different salinization mechanisms. First, the spatial distribution of the salt content in mud flat deposits had a significant effect on the groundwater salinity. Second, in situ dissolution of near-surface rock-forming salts occurred at areas away from the mud flat deposits. The proposed hydrogeophysical models revealed the potential effect of both mechanisms in the study area.

ABSTRACTThe transient electromagnetic method (TEM) is a prominent geophysical technique, and the TEM inversion for resistivity models is a crucial aspect of physical exploration. However, TEM inversion faces challenges such as nonlinearity, multiple solutions and ill‐conditioning, which can lead to inaccurate results. In response to these challenges, metaheuristic algorithms have been extensively studied for their innovative approaches to solving inverse problems. Despite this, many existing metaheuristic inversion algorithms exhibit limitations, including premature convergence, slow convergence speed and inadequate computational accuracy. To address these issues, an improved bat algorithm (IBA) that incorporates logistic chaotic mapping and a spiral flight strategy (Logistic Chaotic Mapping and Spiral Flight Strategy‐Based Bat Algorithm, LSBA) has been proposed. The logistic chaotic mapping strategy is utilized to initialize the population of the bat algorithm to enhance the initial convergence rate. Moreover, the spiral flight strategy facilitates the bats’ escape from local optima, thereby improving the algorithm's local exploration capabilities and solution accuracy. Numerical simulations, synthetic models and field experiments have demonstrated that the LSBA significantly enhances solution precision (the degree of closeness between the algorithm's inverted parameters and the true values), convergence speed and anti‐noise performance. The LSBA effectively retrieves the stratigraphic parameters of the true model and accurately represents the geological information of actual mining areas, thereby validating the efficacy and feasibility of the proposed approach in TEM inversion.

The Kedong foreland thrust zone is located in the Tarim basin, western China. It shows high potential as an oil resource, but available seismic data are poor due to the serious static shift that results from the uneven, thick loess in the near-surface layer. In an effort to learn more about oil reserves in this basin, 3-D MT and TEM (Transient Electromagnetic Method) surveys were conducted during the 3-D seismic acquisition. The TEM method is relatively inexpensive, and has high resolution in the detection of shallow structures. We programmed the software of TEM data inversion and obtained the sectional resistivity data. By means of comparison with deep microlog data, we correlate the rock’s lithology with its resistivity. The distribution of the loess layer is described in detail, and the thickness is also predicted to have an error less than 10 m compared with the microlog data. The near-surface lithology model is built accordingly, and it provides reference for the building of near-surface velocity models and the static corrections of seismic data. Because TEM have no static shift, they can help the static corrections of 3-D MT data; in other words, they limit the distortion during the 3D MT datainversion.

Airborne transient electromagnetic (TEM) methods initially developed for the detection of deep conductive targets are continuously being upgraded to improve near-surface resolution of both vertical and lateral conductivity distributions suitable for salinity mapping. However, since it is a combination of location, proximity to the surface, as well as the mobility of saline groundwater that constitutes the problem, it is important to the hydrogeologist to map the complete subsurface structure responsible for the aquifer framework. A QUESTEM survey was flown over the Jemalong-Wyldes Plains area, New South Wales, by World Geoscience Corporation with the help of funding by the National Soil Conservation Program. The data have been re-evaluated with respect to ground TEM, DC resistivity, GEONICS EM 34-3, shallow drill hole sampling and a comprehensive hydrogeological investigation. Apparent conductance maps were used to guide the location of specific ground surveys, and conductivity-depth sections calculated from the airborne data were used for semi-quantitative comparison with the ground methods. Comparison with TEM and DC resistivity results showed correlation with true conductivity trends down to ~140 m. Comparison with GEONICS EM 34-3 data and electrical conductivity (EC) measurements on drill cutting samples to a depth of 10 m also showed good correlation with the near-surface features of the airborne TEM conductivity-depth section. The significant correlation with both shallow and deep conductivity trends observed in ground surveys suggests that the presentation of complete conductivity-depth sections calculated from airborne TEM, should become routine procedure in hydrogeological investigation.

Transient electromagnetic (TEM) method is a reliable means of non‐destructive testing for fault diagnosis of grounding grid in substation. In view of the small transmitter loop of the TEM system and many measuring points and survey lines of grounding grid detection in substation. This study presents a fast solution to TEM apparent resistivity imaging for grounding grid detection using artificial neural networks. The input ‐output mapping relation of neural network is established based on TEM response characteristics of the grounding grid. The built network could map the recorded TEM data of grounding grid detection and quickly obtain the resistivity image. The proposed method offers accuracy and fast computation for the resistivity imaging of grounding grid. Feasibility and technical attractiveness of the proposed method in fast imaging of apparent resistivity is investigated with the measurement of grounding grids in the actual substations. It can be used in real time so that the recorded TEM data in substation can be calculated without re‐training, which avoids time‐consuming inversion computation. The rapid processing of massive data and submitting the detection results of the grounding grid to customers rapidly and in real time, which is expectation for a modern power industry.

Over the past five years, the use of the transient electromagnetic (TEM) method for both lateral and vertical profiling of polluted groundwater and saline soil has been investigated, and the results have been compared with dc sounding and frequency domain methods. In the groundwater pollution studies, it has been found that with either two-layer or three-layer inversions of the TEM data a subtle low-resistivity feature which correlates with the presence of a pollution plume in the aquifer near a landfill site can be detected. Data obtained since 1985 at approximately six-month intervals have shown a continuing development of the resistivity low. In the soil salinity studies, investigations have concentrated on the use of TEM and dc sounding methods to determine the thickness and resistivity of the surface leached layer and of salt accumulations. Modelling of the response obtained with a ground-based TEM system shows that measurements at delay times earlier than 1 ms are required to resolve the relevant parameters of a layered-earth model used to approximate a saline soil environment.

The transient electromagnetic (TEM) method is a geophysical exploration technique used in a wide range of applications such as mineral exploration, hydrogeology surveys, and underground geological mapping. In recent years, it has been applied in tunnel construction to forecast geological anomalies in front of the excavation face. However, the TEM response can be affected by metallic objects such as excavation carts and metallic support structures, which makes interpretation more difficult. The purpose of this study is twofold: to introduce a new method for extracting and eliminating TEM interference from metallic bodies in a tunnel excavation environment and to gain insight regarding the influence of metallic objects on the TEM response by physical modeling in the laboratory and studies conducted in the field. The concept of resistive limits is introduced; these limits can be used as eigenvalues in the inversion process, thus achieving more accurate identification of both the location and size of ball-shaped, low-resistivity anomalies (gouge materials) in surrounding rock that has high resistivity. Both the physical laboratory simulations and the field experiments provide insight on the interference effect of metal objects such as a steel arches or excavation carts on the TEM response. Field experiments inside a tunnel indicate that the maximum distance between the excavation cart tested and the tunnel face that can influence the TEM response is 30 m, while the maximum influence distance of a steel arch is 1 m in the vertical plane of the coil and 2 m in the axial direction. The proposed inversion process was shown to be capable of extracting the interference of metallic objects from TEM responses and improve the accuracy of TEM interpretation in identifying geological anomalies.

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.