Transient Electromagnetic Survey Research Articles

Transient electromagnetic data denoising based on cluster analysis and locally weighted linear regression

AbstractIn transient electromagnetic surveys, the collected data inevitably contain noise originating from both natural and cultural sources. This noise has the potential to mask transient electromagnetic responses linked to geological features, thereby posing challenges in accurately interpreting subsurface structures. Hence, the implementation of effective noise reduction techniques is crucial in ensuring the accuracy and reliability of inversion outcomes in transient electromagnetic surveys. This study introduces a novel approach that merges k‐means clustering with locally weighted linear regression to denoise transient electromagnetic data. The results from synthetic examples illustrate that the k‐means locally weighted linear regression method can predict transient electromagnetic data closely resembling true values, similar to the long short‐term memory autoencoder. Occam's inversion results derived from denoised data using both the k‐means locally weighted linear regression and long short‐term memory–autoencoder methods can well reflect the true model. Notably, a key advantage of the k‐means locally weighted linear regression method is its independence from labelled data as the sample set. The k‐means locally weighted linear regression method was applied to field data collected at the Narenbaolige coalfield in Inner Mongolia, China. Occam's inversion models generated from the denoised field data delineate the boundary between the basaltic body and sedimentary rocks, aligning with drilling data. The inversion models derived from the noisy field data also can capture this boundary, but deep section views reveal the presence of numerous intricate high‐resistivity anomalous bodies. These observations highlight the effectiveness of the k‐means locally weighted linear regression method in denoising transient electromagnetic data.

Quantitative suppression of unsteady powerline noise in transient electromagnetic surveys: Adjustment of base-frequency and optimal choice of stacking-times.

Powerline noise is a severe interference source in urban or mine transient electromagnetic (TEM) surveys. TEM systems generally adopt the synchronous detection scheme to suppress the powerline noise. However, when the powerline frequency fluctuates differing from its nominal value (50/60Hz), the considerable powerline noise residue still remains even after synchronous detection. To overcome this problem, this paper proposes the quantitative suppression method for powerline noise taking into account the instability of powerline frequency. The method is based on the adjustment of base-frequency and optimal choice of stacking-times. We represent mathematically the sufficient condition for powerline noise suppression by synchronous detection. It consists of two equations, one with respect to base-frequency and other with respect to stacking-times. The base-frequency is adjusted according to powerline frequency estimate. We first derive the mathematical relationship between frequency estimation accuracy, residual noise amplitude, and stacking-times. Based on it, we develop an efficient algorithm to determine the optimal stacking-times. The algorithm takes as an input the powerline noise estimates and the noise tolerance limit. The tolerance limit is set to a certain value desired by the user, such as 10, 4, and 1 µV. By adjusting base-frequency and determining optimal stacking-times, the powerline noise residue after synchronous detection is reduced to below the desired tolerance limit. We verify the effectiveness of the proposed method for both simulated and actual noise. Experimental results show that the method achieves quantitative suppression of unsteady powerline noise without any damage of effective signal and prevents the measurement time loss due to excessive stacking.

Thickness and Structure of Permafrost in Oil and Gas Fields of the Yamal Peninsula: Evidence from Shallow Transient Electromagnetic (sTEM) Survey

The Yamal-Nenets Autonomous District, especially the Yamal Peninsula located in the permafrost zone, stores Russia’s largest oil and gas resources. However, development in the area is challenging because of its harsh climate and engineering–geological features. Drilling in oil and gas fields in permafrost faces problems that are fraught with serious accident risks: soil heaving leading to the collapse of wellheads and hole walls, deformation and breakage of casing strings, gas seeps or explosive emissions, etc. In this respect, knowledge of the permafrost’s structure is indispensable to ensure safe geological exploration and petroleum production in high-latitude regions. The extent and structure of permafrost in West Siberia, especially in its northern part (Yamal and Gydan Peninsulas), remain poorly studied. More insights into the permafrost’s structure have been obtained by a precise sTEM survey in the northern Yamal Peninsula. The sTEM soundings were performed in a large oil and gas field where permafrost is subject to natural and anthropogenic impacts, and its degradation, with freezing–thawing fluctuations and frost deformation, poses risks to exploration and development operations, as well as to production infrastructure. The results show that permafrost in the western part of the Yamal geocryological province is continuous laterally but encloses subriver and sublake unfrozen zones (taliks) and lenses of saline liquid material (cryopegs). The total thickness of perennially frozen rocks is 200 m. The rocks below 200 m have negative temperatures but are free from pore ice. Conductive features (<10 Ohm﮲m) traceable to the permafrost base may represent faults that act as pathways for water and gas fluids and, thus, can cause a geohazard in the oil and gas fields (explosion of frost mounds, gas blow during shallow drilling, etc.).

3D deformation velocity field analysis and TEM method to detect the tectonic influence on the land subsidence in Zamora, Mexico

The city of Zamora is situated in a tectonic basin and, according to previous interferometric studies, has been experiencing sinking up to 13 cm/yr (2007-2011). Although the reported subsidence pattern (WNW-ESE) is similar to the orientation of the regional fault system, there is a lack of detailed studies to establish the connection between this phenomenon, the thickness of sediments, and the basement geometry. Therefore, this paper presents a 3D deformation velocity field analysis for the period of 2014–2021, using 130 images from the Sentinel-2 satellite to calculate the vertical and east-west components of the subsidence. Likewise, a campaign of 28 Transient Electromagnetic soundings was carried out around the city to determine the thickness of sediments and bedrock geometry. The results of the interferometric analysis reveal a WNW-ESE sinking pattern with the maximum vertical velocities observed near the Zamora Museum, amounting to 108 mm/yr (2014–2017) and 102 mm/yr (2018–2021). Meanwhile, the horizontal component has a heterogeneous behavior in both periods with rates reaching up to 1 cm/year. The Transient Electromagnetic survey results show the presence of two grabens, a horst, and a series of half-grabens. The thickness of sediments varies between 94 and 314 m, with the latter corresponding to the main graben located beneath the city. The axis of this structure and the largest sediment deposits are linked to the area of maximum sinking and the spatial pattern of subsidence.

Use of remote sensing, spatial and geophysical modeling, and real recharging capabilities to identify suitable areas for groundwater exploitation in dry coastal areas

Accurate identification of groundwater potential areas in arid regions is an important task for groundwater management and sustainability. As a result, this study used the innovative integration of remote sensing (RS), geographic information system (GIS), watershed modeling system (WMS), geophysical survey, and water mass balance equation to identify potential groundwater areas in the W. Dara, Eastern Desert, Egypt. A weighted spatial probability model (WSPM) of groundwater potential based on eight regulatory factors was implemented within ArcGIS software. Drainage density (DD), precipitation (P), net groundwater recharge (NGR), terrain slope (TS), lineament density (LD), lithologic group (LG), water quality (TDS), and depth to groundwater level (DGW) are the aspects considered. The Analytical hierarchy process (AHP) method was used to assign weights to these parameters, and their accuracy was estimated using the consistency ratio (CR). The resulting groundwater potential map classified W. Dara study area into five categories, ranging from very low to very high potential. A geophysical survey, in the form of Vertical Electrical Sounding (VES) and Transient Electromagnetic (TEM), was conducted along W. Dara to validate the results of the WSPM, which identified areas of high groundwater potential. The 1D inversion of VES/TEM shows that the central and western parts of W. Dara are considered the most promising areas for groundwater occurrence, and are located in areas of high and very high potential classes derived from WSPM. Moreover, the results of VES and TEM surveys showed that the proposed aquifers (Nubian Sandstone, Miocene, and Quaternary) in the study area are horizontally and vertically connected through a set of normal faults traversing NW-SE. Ten sites have been proposed for drilling additional exploitative wells in W. Dara area based on the WSPM and geophysical survey with the aim of sustainable development. Thus, the integrated techniques applied in this study proved effective in accurately determining the development strategy for arid and semi-arid coastal areas, especially those that suffer from scarcity of rainfall and increased agricultural reclamation requirements in remote areas.

Exploring the challenges of interpreting near-surface towed transient electromagnetic data on saline permafrost

We analyze a continuous near-surface transient electromagnetic (TEM) survey on permafrost around Ilulissat in western Greenland, an area characterized by continuous saline permafrost. The TEM data are severely affected by induced polarization (IP) effects, causing a large range of decay shapes. We identify seven unique decay shapes: oversteepened, sign change, all negative, double sign change, no apparent IP, flat spot, and positive nonmonotonic, the last two of which have not previously been identified in the scientific literature. A clear spatial dependency of the decay shapes is observed. Inversion of the data is carried out using a Cole-Cole model that poses a highly nonunique inversion problem with an extreme starting-model dependency. A series of inversion and forward-modeling experiments demonstrate these challenges and indicate that a low-resistivity and highly chargeable layer with time constant ([Formula: see text]) values between [Formula: see text] and [Formula: see text] and frequency exponent ( C) values above 0.74 is needed to fit the data used in the inversion experiment. Forward modeling further indicates that low [Formula: see text] and high C values are needed to reproduce the observed flat-spot and positive-nonmonotonic decay shapes. Based on these observations, we attribute the IP effects to the orientational polarization of ice in the soil column. This mechanism allows for low-resistivity, high-chargeability layers due to partially frozen saline sediments, a combination that is difficult to explain by the IP mechanisms traditionally considered. Based on the forward modeling of a realistic structural model of a layered sediment pack over dipping bedrock, we are able to reproduce all the observed decay shapes. This model provides a consistent framework for qualitative interpretation of the entire data set and evidence for the presence of saline deposits in the central parts of the sedimentary basins.

The distortion and misinterpretation of TEM responses caused by the IP effect

Abstract Transient electromagnetic surveys are commonly conducted to map the distribution of resistivity, a key physical property in mineral exploration and other geological prospecting problems. However, the responses obtained in regions associated with chargeable minerals are always distorted by the induced polarization effects. In this study, the distorted responses are initially simulated in the frequency domain employing the Cole–Cole complex resistivity model and subsequently converted into the time domain through a time-frequency transformation method. A uniform half-space model is employed to validate the algorithm and illustrate the distortion characteristics of the responses in polarizable formations. A three-layer model is designed to estimate the misinterpretation of slightly complicated models. An actual misinterpretation is demonstrated by field responses containing induced polarization effects collected in the Wulong Gold Mine. The results show that the distortions under different geoelectrical conditions are consistent, enhancing the responses in the early stage and counteracting the responses in the late stage. The strong induced polarizable effects distort the responses by causing explicit sign reversals, whereas the weak induced polarizable effects only distort the decay rate of the responses. These distortions are prone to causing misinterpretations and resulting in excessively intricate geological structures.

Quasi-2D inversion of surface large fixed-loop transient electromagnetic sounding data

Abstract In many cases, 1D inversion is still an important step in transient electromagnetic data processing. Potential issues may arise in the calculation of apparent resistivity using induced electromotive force (EMF) due to overshoot and the presence of multi-valued functions. Obtaining reliable and consistent inversion results using a uniform half-space as the initial model is challenging, especially when aiming for efficient inversion. Focusing on these problems, we use the land-based transient electromagnetic (TEM) sounding data, which was acquired by using a large fixed-loop transmitter, and adopt a quasi-2D inversion scheme to generate improved images of the subsurface resistivity structure. First, we have considered directly using magnetic field data or converting induced EMF into magnetic field, and then calculating the apparent resistivity over the whole zone. Next, a resistivity profile that varies with depth is obtained through fast smoke ring imaging. This profile serves as the initial model for the subsequent optimal inversion. The inversion scheme uses a nonlinear least-squares method, incorporating lateral and vertical constraints, to produce a quasi-2D subsurface image. The potentiality of the proposed methodology has been exemplified through the interpretation of synthetic data derived from a 3D intricate resistivity model, as well as field data obtained from a TEM survey conducted in a coalmine field. In both cases, the inversion process yields quasi-2D subsurface images that exhibit a reasonable level of accuracy. These images appear to be less moulded by 3D effects and demonstrate a satisfactory level of agreement with the known target area.

Occurrence of sign reversal or negative decay transients in SQUID based TDEM measurement systems – Is it due to ground geology or something else?

This paper seeks to explain the origin of sign reversal or zero-crossover effect observed at late decay times in the experimental data involving the application of SQUID in geophysical explorations based on TDEM technique. A complete SQUID based TDEM system has been developed and used for field survey after extensive characterization of the system in the laboratory. When the TDEM system was tested in a relatively noisy environment of the laboratory, the sign reversal effects was not observed but the effect was observed conspicuously in the experimental data recorded during an actual field survey at a site far away from the urban electromagnetic noise. The transient electromagnetic data recorded during the field survey showed that the decay transients surprisingly reached negative values in a terrain where a thin conducting layer is sandwiched between thick upper and thin lower resistive layers. The major advantage of using the SQUID sensor in TDEM applications is its ability to achieve extremely high sensitivity even at very low frequencies, enabling one to investigate the conductivity of the layers buried deep below the ground surface. While the decay transients recorded with the use of SQUID magnetometer were successful in providing information pertaining to the buried conducting layer, an unexpected zero crossover was found at later decay times as seen also by a few research groups in the past. The origin of this peculiar zero crossover has not been clearly established so far, and has been speculatively attributed to several parasitic phenomena such as instrumental drift, remnant responses of the previous transmitter pulses, induced polarization effect, poor offset removal, etc., Extensive analysis of our experimental data obtained in the field has, however, shown that the genuine reasons for the observation of zero crossover effect in the data recorded in transient electromagnetic survey using SQUID are actually attributable to the flow of the reverse current and its associated magnetic field generated by the transmitter loop. In this paper, we discuss the possible reasons for the observation of this zero crossover by various research groups, and present a detailed analysis of our own experimental data.

Semi-Airborne UAV-TEM System Data Inversion with S-Plane Method—Case Study over Lake Baikal

The article presents the results of transient electromagnetic (TEM) prospecting surveys using an unmanned aerial system carried out at Lake Baikal, which is a unique geoelectrical setting where low-resistivity lacustrine sediments are located under a relatively isotropic water body. The task was to investigate the possibility of using a drone-based TEM survey to delineate the electrical stratigraphy of the subsurface at depths between 50 and 300 m, separated into layers and blocks. A new version of the SibGIS UAV-TEM unmanned system was used, significantly improved compared to the prototype previously described in the literature. The current switch providing bipolar current pulses connected to a grounded electrical line was the source of the electromagnetic field in the geological environment. The hexacopter carrying a measuring system consisting of 18-bit ADC and sensor—analog of 50 × 50 loop, was the receiving system. We measured survey data of 16 traverses over the Baikal going from the shore to the depths. Significant attention is being paid to a new approach to data inversion. For fast interpretation of the TEM data, we used the Sτ-method, which allows for tracing the change in the apparent longitudinal conductivity with depth. It is shown that thanks to the new sensor and current switch, the data quality has increased significantly; now, the UAV system can register sounding curves up to 1 ms. As a result, new data on the geological structure of the shelf zone of Lake Baikal were obtained. They had a good fundamental agreement with the predecessor data obtained from terrestrial measurements (from ice cover), allowing us to conclude that the UAV-TEM technology can already replace conventional ground-based electromagnetic surveys.

The effects of relative position of anomalous body and topography on the semi-airborne transient electromagnetic survey

The semi-airborne transient electromagnetic method using short-offset has the characteristics of high signal-to-noise ratio and large detection depth, which can adapt to environmental monitoring, groundwater mapping and metal survey in complex terrain. The terrain effects will modulate semi-airborne transient electromagnetic method response. Most terrains effects studies assume that the anomalous body is located below the centre of topography or under the slope, however, in the direction of electromagnetic propagation, several scenarios exist regarding the position of an anomalous body relative to a topography, and different situations will have different anomalous characteristics. In this study, the time-domain finite-element modelling using unstructured mesh is used to investigate the distribution of the electromagnetic response of different time channels of different anomalous bodies in different positions relative to a topography under two conditions: mountains and valleys. The results show that (1) The influence of the relative position of the anomalous body and the topography is similar in the mountain and valley. When the anomalous body is located on the side of the topography far from the transmitter, it is more affected by the topography than when the anomalous body is located on the side of the topography near the transmitter, and (2) in the early stage, the distribution of the electromagnetic response is primarily determined by the topography, in the middle stage, the coupling between anomalous bodies at different locations and the topography results in an extremely complex response distribution, in the late stage, the response distribution primarily originates from the eddy current of the anomalous body, and the effect of the topography is moderate. When using semi-airborne transient electromagnetic method for environmental monitoring and shallow resources survey in complex terrain, data processing and interpretation should be performed based on the position of the anomalous body relative to the topography.

3D inversion of an integrated ground-based and waterborne transient electromagnetic survey

Recent instrument advancements in the transient electromagnetic (TEM) method enable waterborne applications as well as traditional ground-based surveys. We investigate a common framework to handle combined data sets from ground-based and waterborne TEM surveys under one model domain. The modeling complexity increases for two main reasons: (1) multidimensionality effects are unavoidable in data from settings with strong conductivity contrasts and (2) different systems have different sensitivity footprints which are challenging to integrate into a common domain. We address these challenges using a previously developed 3D inversion scheme: first, octree-based forward modeling is used to describe the multidimensional environment for more accurate field simulations. Second, a decoupling between the forward and inversion mesh offers the flexibility of modeling individual soundings to minimize computational costs, while allowing a commonly shared model domain for the inversion. We validate the method through synthetic and field case studies. The synthetic studies indicate that: (1) a careful forward mesh refinement is required for models with thin and highly conductive top layers; (2) compared with 3D forward responses, 1D modeling has an approximate 300% error directly over the coastline decreasing to 10% error 50 m away; and (3) the 3D inversion outperforms the 1D inversion by a lower data misfit and more accurate model reconstruction. The field case further underlines the better consistency of the 3D inversion, which delineates the lithologic transition from sand to clay and is verified by a better agreement with existing borehole data. Based on these experiments, we conclude that (1) 3D inversion is preferred over strong resistivity contrasts arising along a coastline, (2) careful mesh refinement and decoupling of the forward and inversion mesh is an efficient approach to handling computational challenges on forward while maintaining a common inversion mesh, and (3) more focus on optimization is required to realize a full-scale 3D inversion for integrated surveys in a coastal area.

Quasi-2-D Bayesian inversion of central loop transient electromagnetic data using an adaptive Voronoi parametrization

SUMMARYCentral loop transient electromagnetic (TEM) data are often interpreted by conventional 1-D or quasi-2-D inversion techniques. For example, the lateral constrained inversion (LCI) is a powerful technique for quick interpretation of central loop TEM data, and can produce spatially consistent resistivity images for profile data by assuming spatial correlation between adjacent model parameters. Such quasi-2-D techniques are very powerful in cases multidimensional effects are small or negligible. However, the inverse solution of conventional LCI methods strongly depends on subjective interpreter choices such as the model regularization and the imposed lateral constraints. Due to inherent non-linearity and nonuniqueness of the TEM inverse problems, this can result in biased model parameters and their estimated model uncertainties. We present a transdimensional Markov chain Monte Carlo method for the quasi-2-D inversion of TEM data using a Bayesian inference framework. We term the approach quasi-2-D, since the model is parametrized in 2-D with unstructured Voronoi cells, whereas the TEM response at each station is predicted using a 1-D forward solution to make the problem computationally affordable. During the inversion, the number of Voronoi cells as well as their positions and resistivities are variable. Accordingly, the level of model complexity is automatically determined by the framework and adapted to the spatial resolution of the data, thus avoiding the need for subjective model regularization or spatial constraints. The approach is validated using synthetic data and compared to 1-D Bayesian and conventional Gauss Newton inversion techniques. The application to dense field data from a floating TEM survey leads to a consistent subsurface image with unbiased uncertainty estimates and a plausible depth of investigation. The quantitative uncertainty information provided by the Bayesian framework is beneficial in identifying resolution.

Combined geophysical methods to investigate seawater intrusion in the Souss-Massa coastal area, Morocco

Groundwater are strategic water resources for irrigation and drinking purposes in the Souss-Massa semi-arid region (Morocco). These resources have been threatened during the last decades by different factors such as drought, over-pumping and marine intrusion. The latter is responsible for the degradation of the groundwater quality along the coastal area, which is reducing its usability. Therefore, the localisation of the freshwater/saline water interface is essential for better sustainable management of groundwater resources in the area. The objective of the present study is to assess the temporal changes, the extent, and the depth of the seawater intrusion in the Souss-Massa coastal area. To fulfil this objective, two monitoring sites were equipped with water conductivity measurement devices (Subsurface Monitoring Device – SMD) to observe the temporal changes of the salinity at a local scale, as well as to support the interpretation of a transient electromagnetic (TEM) survey. This survey consisted of 83 TEM soundings, along thirteen profiles located perpendicularly to the ocean and the two rivers delineating the study area. The data from the SMD stations highlight the temporal changes of electrical conductivity (EC) with depth, clearly illustrating the freshwater/saltwater interface location in the central part of the study area (−35.55 m NGM at the Tifnit location, ca. 1 km from the coast). In addition, the results of the TEM measurements show that the marine intrusion is already advanced towards the land, over a maximum distance of 2500 m. Finally, some new monitoring sites have been proposed to improve the current monitoring network for seawater intrusion and assess whether mitigation solutions lead to a slower progression of the seawater intrusion. Decision-makers can use these results as a basis for the development and implementation of effective policies to ensure a sustainable and integrated water resources management in this region.

Three-Dimensional Transient Electromagnetic Forward Modeling for Simulating Arbitrary Source Waveform Using Convolution Approach

The transient electromagnetic (TEM) method utilizes artificial transmitters and measures electromagnetic responses to reveal the resistivity information of the subsurface. The current waveform of transmitters has non-negligible effects on induced fields. Therefore, 3-D TEM forward modelling algorithms need the capability of simulating arbitrary waveforms to obtain accurate responses. In time-stepping-based 3-D TEM forward modelling, the source term approach is frequently used, which employs the source current density to model the waveform variation during time-stepping. The source term approach, however, requires fine-time discretization to describe complex waveforms, which could significantly raise the computational cost. We present a robust convolution approach that computes the convolution between the time derivative of the waveform and the step-off response to incorporate the waveform effects in 3-D TEM modelling. The convolution approach does not discretize the waveform using time steps. Hence, it is advantageous when modelling full-waveform cases. The developed algorithm is based on the finite-element method using unstructured grids and the implicit backward Euler approach. Both galvanic and inductive transmitters are incorporated. Ground and airborne TEM surveys are tested using an actual airborne TEM waveform, a full waveform of the 2( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>n</i></sup> )-sequence pseudo-random signal, and various synthetic waveforms. Accuracy is validated against the 1-D and 3-D solutions of published studies. The source term and convolution approaches are compared. Synthetic examples show that the latter approach simplifies the waveform incorporation in TEM modelling and substantially improves time-stepping efficiency without sacrificing accuracy.

Extraction of Multiple Electrical Parameters From IP-Affected Transient Electromagnetic Data Based on LSTM-ResNet

The induced polarization (IP) effect due to a polarizable body distorts transient electromagnetic (TEM) data, thereby potentially triggering sign reversal phenomena in the measured response. The measured horizontal electric field associated with a grounded-wire TEM is more strongly affected by IP effects than the measured vertical field, meaning that data inversion is more problematic for its component. The traditional inversion method, which assumes a frequency independent resistivity, is complex to extract the chargeability. Yet, the chargeability provides critical information, so it is important to extract the chargeability in addition to the resistivity from IP-affected TEM data. Thus, we proposed a data-driven method based on deep learning to recover the resistivity and chargeability of IP-affected horizontal electric fields. This method, named LSTM-ResNet, combines long short-term memory (LSTM) and a residual network (ResNet) to estimate subsurface electrical properties. Synthetic tests showed that LSTM-ResNet is computationally efficient and accurate for inversion problems. Based on the inverse results with data added noise, we found that a well-trained neural network was not sensitive to noise. A case study was performed by applying LSTM-ResNet to field data collected by a grounded-wire TEM survey at the Kalatongke copper-nickel ore deposit. LSTM-ResNet recovered the simultaneous resistivity and chargeability distributions of subsurface structures from the IP-affected horizontal electric TEM field. The results show a high-chargeability and low-resistivity layer, which was consistent with the lithologic profiles based on drilling cores, indicating the accuracy and robustness of the proposed framework for multi-parameter inversion.

Electromagnetic Surveys for Petroleum Exploration: Challenges and Prospects

Transient electromagnetic (TEM) surveys constitute an important element in exploration projects and can be successfully used in the search for oil and gas. Different modifications of the method include shallow (sTEM), 2D, 3D, and 4D (time-lapse) soundings. TEM data allow for solving a large scope of problems for estimating resources and reserves of hydrocarbons, discriminating reservoir rocks, detecting tectonic features, and characterizing drilling conditions. TEM surveys are applicable at all stages, from initial prospecting to production, and are especially efficient when combined with seismic surveys. Each stage has its specific objectives: estimation of net pay thickness, porosity, and fluid type during prospecting, optimization of well placement and prediction of drilling conditions in exploration, and monitoring of flooding during production. Electromagnetic soundings resolve permafrost features well and thus have a high potentiality for exploration in the Arctic petroleum province. At the first reconnaissance stage of regional prospecting in East Siberia, electromagnetic and seismic data were used jointly to map the junction of the Aldan basin (part of the Aldan-Maya foredeep) with the eastern slope of the Aldan uplift and to constrain the limits of Neoproterozoic sediments. The TEM-based images revealed reservoir rocks in the Upper and Middle Neoproterozoic strata. TEM data have implications for the amount of in-place oil and gas resources in prospects, leads, and plays (Russian categories D1–3) at the prospecting and exploration stages and contingent recoverable reserves (C2) during exploration (latest stage). The contribution of the TEM survey to oil and gas evaluation is quantified via economic variables, such as the value of information (VOI) and expected monetary value (EMV).

The first semi-airborne transient electromagnetic survey for tunnel investigation in very complex terrain areas

We present an unmanned aerial vehicle (UAV) based semi-airborne transient electromagnetic method (SATEM) for tunnel investigation in tunnels constructed in complex terrains. To the authors’ best knowledge, this is the first report on such application. Tunnel investigation must be carried out before its construction, but in some areas, it is extremely challenging due to the complex terrains. To this end, we aim to develop a semi-airborne geophysical method for such cases. Specifically, a grounded-wire source is placed on the surface transmitting an electromagnetic field into the underground. An induction coil is carried by a small UAV in the air to acquire data on top of the tunnel. Low resistivity targets near the tunnel are induced by the transmitter and pure secondary fields are produced with relation to the size and location of the targets. The information is collected by the coil to invert the subsurface resistivity profile. We conducted our first experiment in the Damo Tunnel in Guangxi, China. Here we present the design of this method and the workflow for tunnel investigation. The prospecting results are compared with the actual results exposed by excavations, showing a good agreement. This first attempt of SATEM shows great potential for tunnel investigation in complex terrain areas.

Joint Interpretation of Marine Self‐Potential and Transient Electromagnetic Survey for Seafloor Massive Sulfide (SMS) Deposits: Application at TAG Hydrothermal Mound, Mid‐Atlantic Ridge

AbstractThe self‐potential (SP) method is expected to play an important role in the exploration of seafloor massive sulfide (SMS) resources. Since the redox potential changes with depth below the seafloor, SMS deposits are the source of an electrical (source) current density, generating in turn a recordable electrical field. This electrical field can be remotely measured at and above the seafloor. By integrating this electrical field, we obtained a so‐called SP profile or map. The electrical conductivity distribution of SMS deposits is an important ingredient in the inversion of these SP data in order to distinguish between causative primary sources (i.e., associated with the SMS deposits themselves) and secondary current sources associated with conductivity contrasts below the seafloor. Such ingredient is therefore necessary to properly localize SMS deposits and eliminating ghosts associated with high conductivity contrasts. The conductivity structure of the seafloor is however unknown in most SP surveys. This study presents the first joint interpretation of transient electromagnetic and SP surveys. The field data were acquired at the Trans‐Atlantic Geotraverse (TAG) hydrothermal mound. The conductivity structure of TAG mound was obtained by inverting transient electromagnetic data. This electrical conductivity distribution is then used to invert the SP data. The obtained (primary) source current distribution delineates the geometry of the SMS deposits. The deposits are located in the center of the mound as well as at its edges, in a way that is consistent with water column data. Combining the two geophysical methods improves our ability to better decipher the fine geometry of SMS deposits below the seafloor.

Imaging Arctic Permafrost: Modeling for Choice of Geophysical Methods

Knowledge of permafrost structure, with accumulations of free natural gas and gas hydrates, is indispensable for coping with spontaneous gas emission and other problems related to exploration and production drilling in Arctic petroleum provinces. The existing geophysical methods have different potentialities for imaging the permafrost base and geometry, vertical fluid conduits (permeable zones), taliks, gas pockets, and gas hydrate accumulations in the continental Arctic areas. The synthesis of data on cryological and geological conditions was the basis for a geophysical–geological model of northern West Siberia to a depth of 400 m, which includes modern permafrost, lenses of relict permafrost with hypothetical gas hydrates, and a permeable zone that may be a path for the migration of gas–water fluids. The model was used to model synthetic seismic, electrical resistivity tomography (ERT), and transient electromagnetic (TEM) data, thus testing the advantages and drawbacks of the three methods. Electrical resistivity tomography has insufficient penetration to resolve all features and can run only in the summer season. Seismic surveys have limitations in mapping fluid conduits, though they can image a horizontally layered structure in any season. Shallow transient electromagnetic (sTEM) soundings can image any type of features included into the geological model and work all year round. Thus, the best strategy is to use TEM surveys as the main method, combined with seismic and ERT data. Each specific method is chosen proceeding from economic viability and feasibility in the specific physiographic conditions of mountain and river systems.