Airborne Transient Electromagnetic Survey Research Articles

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.

Inversion of airborne transient electromagnetic data based on reference point lateral constraint

The airborne transient electromagnetic method (ATEM) has become a popular tool in mineral and resource exploration due to its speed and high efficiency. The distance between two adjacent measured points is minuscule for a flight line because of the oversampling in the ATEM, which results in a sharp change in the horizontal direction. The conventional lateral constraint inversion method accumulates errors and has a significant calculation cost. Therefore, a laterally constrained, segmented inversion method based on a reference point is proposed to reduce errors and improve accuracy. Electrical information about the known logging data is employed as the lateral constraint of the reference point; alternatively, the inversion results of previous point can be used as lateral constraints, which guarantees a continuous inversion profile. The golden-section method is applied to solve the Lagrange multiplier to ensure a quick and effective convergence of inversions. Constraint inversions of a one-dimensional model show that the strategy converges quickly, and the inversion results are closer to the true resistivity of the model. Constraint inversion of a 2D model indicates that lateral constraints considerably reduce the discontinuity of a single-point inversion and smooths the inversion's resistivity pseudo-section. Finally, the effectiveness of the proposed algorithm is verified by the inversion of the airborne transient electromagnetic survey data in Xinjiang province.

Three-dimensional transient electromagnetic response of multiple grounded-wire sources

Multiple grounded-wire sources combination are proposed to improve the signal strength and resolution of the transient electromagnetic method. Multi-source excitation from the different direction was executed to identify the conductive body in the subsurface and achieve refined detection of the target body. The vector finite element method based on the rational Krylov subspace projection algorithm was used to calculate transient response of the target bodies. The voltage response calculated by our algorithm was compared with that calculated by the integral equation method and the results agreed well with them. We designed different grounded-wire combinations, including a single source, pairs of sources, and groups of three and four sources, and analyzed induced EMF and the secondary field response of one conductive body. It indicates that multi-source transient method can obtain stronger signals, which is beneficial to the discovery of deep weak anomalies. Finally, we established two adjacent conductive block bodies and compared their electromagnetic response for the different source combination. The results showed that multiple-source transient electromagnetic response anomaly amplitude and lateral resolution are superior to those of a single source. Multi-channel diagram for multiple sources have two obvious peaks, especially for three sources and four sources, but the induced EMF from a single source does not differentiate between the two conductive bodies in the horizontal direction. Multiple grounded-wire source combinations are used not only for the surface transient electromagnetic method but can also be extended to the grounded electrical-source airborne transient electromagnetic survey.

Lithology Identification of Alluvial and Lacustrine Aquifers in Baiyangdian Area by an Airborne Transient Electromagnetic Survey

An airborne transient electromagnetic (TEM) has been conducted in Baiyangdian Area to map resistivity distribution within the alluvial and lacustrine aquifers. This investigation evaluated the reliability of resistivity models of the TEM data to infer lithological distribution in alluvial and lacustrine aquifers. The collected airborne TEM data were processed and inverted using spatially constrained 1D inversion method. And the 3D electrical structure, which maps electrical resistivity trends to depths of about 160 m, can be established through interpolation. Comparison of the resistivity models to drill logs indicates resistive permeable coarse-grained sediments and conductive fine-grained sediments which are relatively impermeable. Hence, the 3D electrical structure that are related to lateral and vertical variation in lithology, can serve as baseline information for groundwater potential identification. This research indicates that groundwater detection is an ideal target for mapping with airborne TEM techniques because of the high electrical resistivity of permeable coarse-grained deposits and its contrast with impermeable fine-grained deposits.

On electric fields produced by inductive sources on the seafloor

The transient electromagnetic (TEM) method has recently been proposed as a tool for mineral exploration on the seafloor. Similar to airborne TEM surveys conducted on land, marine TEM systems can use a concentric or coincident wire-loop transmitter and receiver towed behind a ship. Such towed-loop TEM surveys can be further augmented by placing additional stationary receivers on the seafloor throughout the survey area. We examine the electric fields measured by remote receivers from an inductive source transmitter within a 1D layered earth model. At sea, it is conceivable to deploy either a horizontal transmitter (such as the analogous standard airborne configuration) or a more exotic vertical transmitter. Therefore, we study and compare the sensitivity of the vertical and horizontal towed-loop systems with a variety of seafloor conductivity structures. Our results indicate that the horizontal loop system is more sensitive to the thickness of a buried conductive layer and would be advantageous over the vertical loop system in characterizing the size of a shallowly buried mineralized zone. The vertical loop system is more sensitive to a resistive layer than the horizontal loop system. The vertical electric field produced by the vertical loop transmitter is sensitive to greater depths than the horizontal fields, and measuring the vertical field at the receivers would therefore be advantageous. We also conducted a novel test of a towed horizontal loop system with remote dipole receivers in a marine setting. The system was tested at the Palinuro volcanic complex in the Tyrrhenian Sea, a site of known massive sulfide mineralization. Preliminary results are consistent with shallowly buried material in the seafloor of conductivities [Formula: see text].

Compiling a national resistivity atlas of Denmark based on airborne and ground-based transient electromagnetic data

We present a large-scale study of the petrophysical relationship of resistivities obtained from densely sampled ground-based and airborne transient electromagnetic surveys and lithological information from boreholes. The overriding aim of this study is to develop a framework for examining the resistivity-lithology relationship in a statistical manner and apply this framework to gain a better description of the large-scale resistivity structures of the subsurface.In Denmark very large and extensive datasets are available through the national geophysical and borehole databases, GERDA and JUPITER respectively. In a 10 by 10km grid, these data are compiled into histograms of resistivity versus lithology. To do this, the geophysical data are interpolated to the position of the boreholes, which allows for a lithological categorization of the interpolated resistivity values, yielding different histograms for a set of desired lithological categories. By applying the proposed algorithm to all available boreholes and airborne and ground-based transient electromagnetic data we build nation-wide maps of the resistivity-lithology relationships in Denmark.The presented Resistivity Atlas reveals varying patterns in the large-scale resistivity-lithology relations, reflecting geological details such as available source material for tills. The resistivity maps also reveal a clear ambiguity in the resistivity values for different lithologies. The Resistivity Atlas is highly useful when geophysical data are to be used for geological or hydrological modeling.

Artificial neural networks for efficient removal of coupled airborne transient electromagnetic data

Modern airborne transient electromagnetic (ATEM) surveys typically span thousands of line kilometres requireing careful data processing. When surveys are flown in populated areas data processing becomes particularly time consuming, since the acquired data is contaminated by couplings to man made conductors (power lines, fences, pipes, etc). Coupled soundings must be removed from the dataset prior to inversion, but since the signature of couplings can be subtle and difficult to describe in general terms it has so far remained mostly a manual task. Here we train an artificial neural network (ANN) to recognize coupled soundings in previously processed data and use this network to identify couplings in other data. The approach provides a dramatic reduction in the time required for data processing, since one can directly apply the network to the raw data. We describe the neural network we use and present the inputs and normalizations required for maximizing the effectiveness of the network. We present the training state and performance of the network and finally compare inversions based on manually processed data and ANN processed data. The results show that a well trained network can produce a high quality processing of ATEM data, which is either ready for inversion or in need of minimal manual processing. The results are very promising and can significantly reduce the processing time and cost of large ATEM surveys.

Macro-parallelisation for controlled source electromagnetic applications

Many geophysical computational problems can be referred to as “embarrassingly parallel”. Parallel computing utilises linked CPU cores to solve computational problems. We create a “macro” parallelisation method that rapidly recovers solutions to large scale electromagnetic forward and inverse modelling problems. The method involves software operating above a generic electromagnetic data structure. Two examples are provided.The first example quantifies the reduction in computational time where macro-parallelisation is applied to forward modelling of data generated during synthetic marine controlled source electromagnetic surveys. In the second numerical experiment we apply macro-parallelisation to recover the subsurface conductivity distribution from a large airborne transient electromagnetic survey spanning more than 2000km2. The computation time for inverting 98 thousand soundings with a serial batch approach on an i7 with a single thread was 65h. Computational time from inverting 98 thousand soundings on a single thread of a standard i7 processor was 65h. A 1700 times improvement in computation time was achieved through macro-parallelisation across just 350 cores of a Cray XC30 Supercomputer. Inversion of data for the full AEM survey took just 135s.Parallel computing is rapidly becoming an essential for geophysicists. We provide description, sequence diagrams, pseudo-code and examples to illustrate its implementation. In summary we present applied parallelisation for the masses.

Artificial neural networks for removal of couplings in airborne transient electromagnetic data

ABSTRACTModern airborne transient electromagnetic surveys typically produce datasets of thousands of line kilometres, requiring careful data processing in order to extract as much and as reliable information as possible. When surveys are flown in populated areas, data processing becomes particularly time consuming since the acquired data are contaminated by couplings to man‐made conductors (power lines, fences, pipes, etc.). Coupled soundings must be removed from the dataset prior to inversion, and this is a process that is difficult to automate. The signature of couplings can be both subtle and difficult to describe in mathematical terms, rendering removal of couplings mostly an expensive manual task for an experienced geophysicist.Here, we try to automate the process of removing couplings by means of an artificial neural network. We train an artificial neural network to recognize coupled soundings in manually processed reference data, and we use this network to identify couplings in other data. The approach provides a significant reduction in the time required for data processing since one can directly apply the network to the raw data. We describe the neural network put to use and present the inputs and normalizations required for maximizing its effectiveness. We further demonstrate and assess the training state and performance of the network before finally comparing inversions based on unprocessed data, manually processed data, and artificial neural network automatically processed data. The results show that a well‐trained network can produce high‐quality processing of airborne transient electromagnetic data, which is either ready for inversion or in need of minimal manual processing. We conclude that the use of artificial neural network scan significantly reduce the processing time and its costs by as much as 50%.

Geophysical imaging of Silurian carbonates by use of ground and airborne electromagnetic and radiometric methods on the island of Gotland, Sweden

The island of Gotland in the Baltic Sea exhibits rock types dominated by limestone and marlstone, deposited in a Silurian carbonate platform environment. The strata are extensively exposed in outcrops, quarries, and coastal cliff sections. Low-lying marlstone-dominated areas are separated by highs dominated by limestone. The surface bedrock is well known, but the subsurface composition down to a 100-m depth has, until recent deployment of modern geophysical measurements, been largely unknown. Our geophysical surveys and methods were performed with the aim to evaluate their usefulness in characterizing the upper 100 m of the bedrock in a carbonate-dominated bedrock setting. The surveys were performed in connection to a bedrock mapping project including geologic and geophysical investigations. The airborne techniques complemented by ground measurements and correlation with outcrops and boreholes resulted in a definition of bedrock boundaries and especially subsurface characterization of bedding sequences. The airborne electromagnetic (very low frequency) survey was accompanied by radio magnetotelluric (RMT) measurement on the ground along several profiles. The difference in resistivity between high-resistive limestone overlaying a more conductive marlstone gave favorable conditions for this type of survey. The results have so far given new and valuable information on the thickness of the limestone as well as the distinction of lateral changes of the bedrock related to the depositional setting. The RMT measurements have made it possible in a new way to remotely map out bedding sequences in carbonates and to detect saline groundwater and fracture zones. In addition, a recently performed airborne transient electromagnetic survey contributed to the interpretation of the RMT measurements that we evaluated. The applied geophysical methodology and correlation with outcrops revealed carbonate bedrock with geophysical properties, enabling a better understanding concerning the groundwater conditions, limestone resources, and radon situation on the island of Gotland.

Deep groundwater and potential subsurface habitats beneath an Antarctic dry valley.

The occurrence of groundwater in Antarctica, particularly in the ice-free regions and along the coastal margins is poorly understood. Here we use an airborne transient electromagnetic (AEM) sensor to produce extensive imagery of resistivity beneath Taylor Valley. Regional-scale zones of low subsurface resistivity were detected that are inconsistent with the high resistivity of glacier ice or dry permafrost in this region. We interpret these results as an indication that liquid, with sufficiently high solute content, exists at temperatures well below freezing and considered within the range suitable for microbial life. These inferred brines are widespread within permafrost and extend below glaciers and lakes. One system emanates from below Taylor Glacier into Lake Bonney and a second system connects the ocean with the eastern 18 km of the valley. A connection between these two basins was not detected to the depth limitation of the AEM survey (∼350 m).

Rapid approximate inversion of airborne TEM

Rapid interpretation of large airborne transient electromagnetic (ATEM) datasets is highly desirable for timely decision-making in exploration. Full solution 3D inversion of entire airborne electromagnetic (AEM) surveys is often still not feasible on current day PCs. Therefore, two algorithms to perform rapid approximate 3D interpretation of AEM have been developed. The loss of rigour may be of little consequence if the objective of the AEM survey is regional reconnaissance. Data coverage is often quasi-2D rather than truly 3D in such cases, belying the need for ‘exact’ 3D inversion.Incorporation of geological constraints reduces the non-uniqueness of 3D AEM inversion. Integrated interpretation can be achieved most readily when inversion is applied to a geological model, attributed with lithology as well as conductivity. Geological models also offer several practical advantages over pure property models during inversion. In particular, they permit adjustment of geological boundaries. In addition, optimal conductivities can be determined for homogeneous units. Both algorithms described here can operate on geological models; however, they can also perform ‘unconstrained’ inversion if the geological context is unknown.VPem1D performs 1D inversion at each ATEM data location above a 3D model. Interpretation of cover thickness is a natural application; this is illustrated via application to Spectrem data from central Australia. VPem3D performs 3D inversion on time-integrated (resistive limit) data. Conversion to resistive limits delivers a massive increase in speed since the TEM inverse problem reduces to a quasi-magnetic problem. The time evolution of the decay is lost during the conversion, but the information can be largely recovered by constructing a starting model from conductivity depth images (CDIs) or 1D inversions combined with geological constraints if available. The efficacy of the approach is demonstrated on Spectrem data from Brazil.Both separately and in combination, these programs provide new options to exploration and mining companies for rapid interpretation of ATEM surveys.

Subsurface imaging reveals a confined aquifer beneath an ice-sealed Antarctic lake

Liquid water oases are rare under extreme cold desert conditions found in the Antarctic McMurdo Dry Valleys. Here we report geophysical results that indicate that Lake Vida, one of the largest lakes in the region, is nearly frozen and underlain by widespread cryoconcentrated brine. A ground penetrating radar survey profiled 20 m into lake ice and facilitated bathymetric mapping of the upper lake basin. An airborne transient electromagnetic survey revealed a low-resistivity zone 30–100 m beneath the lake surface. Based on previous knowledge of brine chemistry and local geology, we interpret this zone to be a confined aquifer situated in sediments with a porosity of 23–42%. Discovery of this aquifer suggests that subsurface liquid water may be more pervasive in regions of continuous permafrost than previously thought and may represent an extensive habitat for microbial populations.

Assessing impacts of climate change, sea level rise, and drainage canals on saltwater intrusion to coastal aquifer

Abstract. Groundwater abstraction from coastal aquifers is vulnerable to climate change and sea level rise because both may potentially impact saltwater intrusion and hence groundwater quality depending on the hydrogeological setting. In the present study the impacts of sea level rise and changes in groundwater recharge are quantified for an island located in the Western Baltic Sea. The low-lying central area of the investigated part of the island was extensively drained and reclaimed during the second half of the 19th century by a system of artificial drainage canals that significantly affects the flow dynamics of the area. The drinking water, mainly for summer cottages, is abstracted from 11 wells drilled to a depth of around 20 m into the upper 5–10 m of a confined chalk aquifer, and the total pumping is only 5–6% of the drainage pumping. Increasing chloride concentrations have been observed in several abstraction wells and in some cases the WHO drinking water standard has been exceeded. Using the modeling package MODFLOW/MT3D/SEAWAT the historical, present and future freshwater-sea water distribution is simulated. The model is calibrated against hydraulic head observations and validated against geochemical and geophysical data from new investigation wells, including borehole logs, and from an airborne transient electromagnetic survey. The impact of climate changes on saltwater intrusion is found to be sensitive to the boundary conditions of the investigated system. For the flux-controlled aquifer to the west of the drained area only changes in groundwater recharge impacts the freshwater–sea water interface whereas sea level rise does not result in increasing sea water intrusion. However, on the barrier islands to the east of the reclaimed area, below which the sea is hydraulically connected to the drainage canals, and the boundary of the flow system therefore controlled, the projected changes in sea level, groundwater recharge and stage of the drainage canals all have significant impacts on saltwater intrusion and the chloride concentrations found in abstraction wells.