A rapid imaging approach for transient electromagnetic data based on gradient features

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.

Very low-frequency (VLF) communication signals of 15–25 kHz contaminate the transient electromagnetic (TEM) data. The notch filter or Butterworth filter is commonly used to suppress VLF noise in addition to synchronous detection of TEM data such as gating and stacking, while also introducing the TEM signal distortion around the VLF band. We propose a B-spline gating, which suppresses the VLF noise while integrating TEM data in the gate interval without any extra filter. B-spline gating is designed to ensure that the zeros of its amplitude frequency response (AFR) are located around VLF frequencies by optimizing the knot vector of the B-spline. It is available for various VLF frequencies. We apply B-spline gating to synthetic TEM data compared with rectangular gating and full-tapered gating, the AFRs of B-spline and full-tapered gating are 25 dB lower at high frequencies than rectangular gating, and the amplitude of B-spline is more reduced at VLF frequency than full-tapered gating; the standard error of the B-spline gates is reduced by >15% compared to the full-tapered gates. The field example from a TEM survey in the Inner Mongolia Autonomous Region of China shows that the amplitude of B-spline gating frequency response in late gates is reduced by about 20 dB at 24.1 kHz VLF than that of full-tapered gating, and the standard error is lower. It is concluded that B-spline gating can suppress VLF noise while following the normal decay of TEM signal without distortion.

A fast approximate three dimensional inversion scheme for interpreting ground transient electromagnetic (TEM) data is presented. The scheme relies on linear superposition of magnetic moments. The moment transform of TEM data is a time-weighted integral of the impulse response. In effect, the moment transformation converts the 3-D TEM inversion problem into a 3-D magnetic inversion problem, with accuracy traded for speed. A starting model is generated from conductivity-depth sections, and the inversion is conditioned using standard potential field inversion devices such as depth weighting.

Transient electromagnetic (TEM) data can be seriously distorted by induced polarization (IP) phenomena when a polarizable body is present. The TEM field generated by a grounded-wire source contains transverse electric (TE) and transverse magnetic (TM) modes. The IP effect is most commonly studied with the TEM total field, rather than considering the difference between TE and TM fields. To investigate the effect of IP phenomena on the TE and TM fields, we have performed a detailed analysis on IP-distorted TEM data based on numerical and field examples. We first compare the IP effect on the TE and TM fields when polarizable bodies with different polarizable parameters are present. The TM field is more severely affected by the IP effect than the TE field. Compared to a single grounded-wire source, a double-line grounded-wire source can generate a larger TM field in the horizontal electric field. We compare the IP effect on TEM data from single- and double-line grounded-wire TEM configurations, and find that the data from the double-line configuration have a higher TM/TE ratio and are more severely affected by IP phenomena than in the single-line case. Thus, it would be easier to identify and extract the IP response from field data acquired with a double-line grounded-wire source configuration. These results have been verified by a field survey of the Kalatongke copper-nickel ore district, which has predominantly layered geology, in Xinjiang, China.

The transient electromagnetic (TEM) responses measured in areas with highly polarizable media commonly show induced polarization (IP) effects, which poses difficulties to the TEM data interpretation and inversion with conventional methods. When present, the IP effects break the monotony in TEM decay curves and can even cause sign reversals, and lead to the singularity and non-monotony of inversion method. It is therefore important to take the IP effects into account when processing those TEM data. In this paper, a new inversion method is developed to invert TEM data with IP effects. The Cole-Cole model was introduced to the forward modeling solver so that the IP effects can be accurately calculated. The regularization terms were adapted to the objective function and a global optimization method, the particle swarm optimization algorithm, was used to solve the optimization problem, which weakened the singularity and non-monotony of the inversion greatly. Field data were collected in Wulong gold mine where IP effects were detected. The data were inverted using the method developed in this paper, and the inversion results were able to identify the depth extension of NO. 163 ore-bearing quartz veins and their associated fine-grained diorite veins. The global inversion method in this paper primarily solved the inversion problem of IP-affected TEM responses, and recovered the polarization parameters, which can be used as an aid to the geological interpretation.

As the previous study known, based on surfacelevel observational analysis using GPS data from Light Detection and Ranging (LiDAR) satellite imagery with a resolution of 0.9 m and Interferometric Synthetic Aperture Radar (IFSAR), it is presumed that The Lembang Fault is an active sinistral strike-slip fault located approximately $\mathbf{1 0 ~ k m}$ north of Bandung, extends for about 29 km and is divided into six segments. Previous palaeoseismological studies indicate a slip rate of $3-5.5 \mathrm{~mm} /$ year, with the potential to generate earthquakes of magnitude 6.5-7 on the Richter Scale. This highlights the importance of detailed investigations to support disaster mitigation strategies. One approach involves examining subsurface geometric patterns and resistivity distributions using Electrical Resistivity Tomography (ERT), Vertical Electrical Sounding (VES), and Transient Electromagnetic (TEM) methods. Four sites suspected of active tectonic deformation were analyzed: Batu Lonceng (Line 1), Gunung Batu (Line 2), Jambudipa (Line 3), and Cimeta (Line 4). Refers to Geological map and the Resistivity table by Telford, at Batu Lonceng, an SW-NE ERT profile revealed a SE-NW thrust fault caused by volcanic uplift ($2.195-13.611$ ohm.m-1 at 32.5 m depth). At Gunung Batu, VES data indicated volcanic uplift with high resistivity values ($857-5.143 \mathrm{ohm}. \mathrm{m}-1$) and $\mathrm{SE}-\mathrm{NW}$ thrust fault at $\mathbf{1 5 ~ m}$ depth, while TEM analysis suggested an anticline of alluvial and sandy deposits, with resistivity values of 300-700 ohm.m-1 at $\mathbf{5 0 ~ m}$ depth. In Jambudipa, interpolation of an ERT West-East profile and TEM data along the North-South direction revealed a SE-NW thrust fault in Muril Hamlet at 40 m depth ($300-400$ ohm.m-1) within volcanic lithology. Finally, at Cimeta, a North-South cross-section identified a NW-SE trending normal fault at 10 m depth ($54-113$ ohm.m-1) composed by volcanic sandstones and limestones. Overall, inversion results across all sites achieved RMS errors below 5%, demonstrating consistency with geological mapping and confirming the reliability of the integrated ERT, VES, and TEM approaches in characterizing active tectonic structures along the Lembang Fault.

In 2011, the Summer of Applied Geophysical Experience (SAGE) students acquired transient electromagnetic (TEM) sounding data at 20 sites in the Caja del Rio (CDR) area, west of Santa Fe, New Mexico to assess groundwater conditions and the potential for future geophysical work. The CDR area is reported to have geothermal potential, so TEM data were collected to constrain the structure and water quality of the local aquifer. Unfortunately, the CDR survey area also has multiple noise sources present including a power transmission substation, a series of high‐voltage transmission lines, and gas and water pipelines. It is necessary to assess the impact of the noise sources on TEM data from study areas like the CDR before inverting and interpreting the data. We analyzed the maximum amplitude of background noise relative to the measured transient decay and used the deviation of the sounding response from a power‐law decay to empirically estimate the extent of electromagnetic (EM) coupling to anthropogenic structures at each site along the profile. The theoretical effects of EM coupling as a function of distance were also calculated for an infinite line source. A correlation between potential noise sources and measured background noise was found, but no evidence of EM coupling was observed in the data, which we attribute to the conductive environment and relatively small transmitter loop size. In a more resistive environment the background noise would have been more problematic and a larger transmitter moment would increase coupling with the infrastructure. Our analysis indicates that the TEM data are valid, and for this data set the estimated signal‐to‐noise ratios are sufficient for reliable interpretation of TEM inverse models. The electrical resistivity of the CDR subsurface was modeled to a depth of approximately 150 meters. Results of two‐ and three‐layered one‐dimensional TEM inverse models are in agreement with well‐log data in the CDR area. Porosity variations along the top of the aquifer were not resolvable due to high clay content in the sediments. The results of this study leave room for further investigations of the local aquifer structure and chemistry.

Addressing problems related to water quality and quantity (e.g. irrigation, contaminant transport and vulnerability to climate change), requires detailed information about the hydrogeological framework over large areas. Airborne transient electromagnetic (TEM) is a geophysical technique capable of constraining subsurface resistivity to several hundred meters depth; the resulting sections can be interpreted in terms of the hydrogeologic framework which constrains groundwater flow. The aim of this research is to define the hydrostratigraphy of the High Plains Aquifer over a range of different geological settings, in Nebraska, USA. A challenge is to accurately characterize the airborne TEM systems in order to recover subsurface resistivity models consistent with independent ground truth information, especially in areas where targets exceed 100m depth. Several airborne TEM systems are being evaluated for their applicability to these problems. Processed data are inverted in 1D with lateral constraints to obtain a pseudo‐2D resistivity section. Recently developed model assessment techniques are applied and extended to investigate regions of the model that are most sensitive to the data. These tools provide estimates of the depth of investigation (DOI), the top of investigation (TOI) and determine the uncertainty of the resulting model parameters. The DOI, TOI and start model selection problems are solved using a sensitivity‐matrix analysis method. DOI and TOI information is important because it is possible that the measured data do not have sensitivity to certain depth intervals within the target aquifer. One result is a map showing the configuration of the top‐ and base‐of‐aquifer surfaces, in relation to DOI and TOI surfaces derived from the TEM data, together with estimates from model parameter uncertainty. Results are verified with ground‐based TEM soundings and lithologic/geophysical bore hole logs.

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.

ABSTRACTWe investigate the early time induced polarization (IP) phenomenon in frozen unconsolidated rocks and its association with transient electromagnetic (TEM) signals measured in northern regions. The distinguishing feature of these signals is the distortion of the monotony or sign reversals in the time range from a few tens to a few hundreds of microseconds. In simulating TEM data, the IP effects in frozen ground were attributed to the dielectric relaxation phenomenon rather than to the frequency‐dependent conductivity. This enabled us to use laboratory experimental data available in the literature on dielectric spectroscopy of frozen rocks. In our studies we focused on simulating the transient response of a coincident‐loop configuration in three simple models: (i) a homogeneous frozen earth (half‐space); (ii) a two‐layered earth with the upper layer frozen; (iii) a two‐layered earth with the upper layer unfrozen. The conductivities of both frozen and unfrozen ground were assumed to exhibit no frequency dispersion, whereas the dielectric permittivity of frozen ground was assumed to be described by the Debye model. To simplify the presentation and the comparison analysis of the synthetic data, the TEM response of a frozen polarizable earth was normalized to that of a non‐polarizable earth having the same structure and resistivities as the polarizable earth. The effect of the dielectric relaxation on a TEM signal is marked by a clearly defined minimum. Its time coordinate tmin is approximately three times larger than the dielectric relaxation time constant τ. This suggests the use of tmin for direct estimation of τ, which, in turn, is closely associated with the temperature of frozen unconsolidated rock. The ordinate of the minimum is directly proportional to the static dielectric permittivity of frozen earth. Increasing the resistivity of a frozen earth and/or decreasing the loop size results in a progressively stronger effect of the dielectric relaxation on the TEM signal. In the case of unfrozen earth, seasonal freezing is not likely to have an appreciable effect on the TEM signal. However, for the frozen earth, seasonal thawing of a near‐surface layer may result in a noticeable attenuation of the TEM signal features associated with dielectric relaxation in a frozen half‐space. Forward calculations show that the dielectric relaxation of frozen unconsolidated rocks may significantly affect the transient response of a horizontal loop laid on the ground. This conclusion is in agreement with a practical example of inversion of the TEM data measured over the permafrost.

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.

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.

Durusu Lake is the biggest and most important freshwater source supplying drinking water to the European side of Istanbul. In this study, electrical resistivity tomography (ERT) and transient electromagnetic (TEM) measurements were applied to detect a possible salt water intrusion into the lake and to delineate the subsurface structure in the north of Durusu Lake. The ERT and TEM measurements were carried out along six parallel profiles extending from the sea coast to the lake shore on the dune barrier. TEM data were interpreted using different 1-D inversion methods such as Occam, Marquardt, and laterally constrained inversion (LCI). ERT data were interpreted using 2-D inversion techniques. The inversion results of ERT and TEM data were shown as resistivity depth sections including topography. The sand layer spreading over the basin has a resistivity of 150–400 Ωm with a thickness of 5–10 m. The sandy layer with clay, silt, and gravel has a resistivity of 15–100 Ωm and a thickness of 10–40 m followed by a clay layer of a resistivity below 10 Ωm. When the inversion of these data is interpreted along with the hydrogeology of the area, it is concluded that the salt water intrusion along the dune barrier is not common and occurs at a particular area where the distance between lake and sea is very close. Using information from boreholes around the lake, it was verified that the common conductive region at depths of 30 m or more consists of clay layers and clay lenses.

Recently the development of TEM (Transient Electromagnetic) provides higher resolution and greater exploration depth than ever before and it is time to consider whether this advanced TEM technique can be applied to under-water mapping. About 250 TEM soundings have been measured within a lake of about 120m by 60m in a muddy deposits environments in southern Taiwan. For the TEM measurements the terrTEM unit (Monex GeoScope) was used with an inloop configuration, for which the receiver spacing is about 5 m and the transmitter loop is about the same size as the lake. For imaging the vertical and horizontal resistivity distribution within the deposit and the surrounding area, all sounding results were interpreted using a smooth inverse procedure. Some auger drilling and sounding were available for correlation. This led to a final model. Underneath the water bottom is mud/shale inter-bed with a depth greater than 50 m and resistivity ranging from 1 to 5 ohm-m. The depth of the clayey layer predicted by TEM data is comparable to that of drilled wells nearby, and the thick mud to the southeast suggests the lake dipping to the southeast. The success of the method depends upon the appropriate design of survey parameters; this new application is open to further refinement, both in instruments and in survey techniques.

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.