Frequency-domain Electromagnetic Research Articles

Combined analysis of electrical and electromagnetic methods with geotechnical soundings and soil characterization as applied to a landslide study in Campos do Jordão City, Brazil

Every year, especially in summer, Brazil suffers from floods and landslides triggered by intense rainfall. These landslides are responsible for many fatalities and material losses that greatly affect the local population. The municipality of Campos do Jordão, in São Paulo state, presents such a history of mass movements that have resulted in fatalities and financial losses. A map of the municipality's risk assessment shows that about 32% of the municipality's population lives in risky areas. For this reason, Campos do Jordão is currently one of the pilot areas being used in studies for understanding landslides and mitigation techniques. In this context, this paper presents the results of an integration of geophysical surveys of, and geotechnical information on, the area. The geophysical methods applied were electrical resistivity, induced polarization, and frequency-domain electromagnetics. The geotechnical data included standard penetration test soundings and soil characterization (using particle density and X-ray fluorescence methods). The main contribution of this work is to present results from the joint application of these methodologies. Results from one of the critical areas, where a school had suffered the impact of a mass movement, indicated the displacement and burial of the retaining wall, which was built in 2002, as well as the shape of the movement, which affected the foundations of the school. It was also possible to determine the critical geological structures involved in the slope dislocation, and the potential problems with the school's foundations. The combined application of geophysical and geotechnical methods to landslide research is not very common, and so this study aimed to provide a field-based example to demonstrate the potential utility of such an integrated approach. The aim was also to foster collaboration with studies of mass movements in Brazil and help create methodologies for investigating and understanding the phenomenon.

Improving the reliability of soil EC-mapping: Robust apparent electrical conductivity (rECa) estimation in ground-based frequency domain electromagnetics

The use of frequency domain electromagnetic (FDEM) instrumentation in ground-based near-surface applications is continually expanding. In soil studies, FDEM is mainly deployed to inform on soil properties such as salinity, texture, moisture or compaction by evaluating the low induction number (LIN) apparent electrical conductivity (ECa). However, advanced FDEM applications face terrains and conditions in which these LIN approximations are no longer valid. Under such conditions, the aforementioned instrumentation output systematically underestimates ECa values; making semi-qualitative data analysis, representation and practical interpretation impossible. For this reason, methods have to be developed to relate the complex FDEM responses to a reliable ECa, beyond the limitations of the LIN approximation. In this work, we present a quadrature-phase algorithm (i.e. excluding the in-phase signal from the estimation) to obtain a robust apparent electrical conductivity at both low and high induction numbers. In addition, the algorithm classifies the ECa estimations as either robust or non-robust based on instrument noise, desired precision of the ECa estimation and the effect of subsurface magnetic variations. The algorithm accounts for electrical conductivity, instrument elevation, coil geometry, operating frequency, subsurface magnetic variation and random errors. The presented procedure, which was tested on synthetic and experimental data, consists a robust and straightforward approach to increase reliability of ECa derived soil mapping.

Tales from the outer limits: Archaeological geophysical prospection in lowland peat environments in the British Isles

AbstractIn order to systematically investigate the potential of conventional near surface geophysical techniques to locate waterlogged archaeological targets in peatlands, the authors applied four conventional geophysical methods – earth resistance, ground‐penetrating radar (GPR), magnetic gradiometry and frequency domain electromagnetics (FDEM) – to four lowland peat archaeological test sites in Great Britain. In this article we demonstrate that a Neolithic trackway was identified in the GPR data in Somerset, with likely ‘proxy’ detections of chemical changes showing up in both electrical and magnetic surveys. This was determined by a coring programme and inductively coupled plasma optical emission spectrometry (ICP‐OES) multi‐element analysis of peat samples to determine the relative concentrations of geophysically relevant chemical elements. Though no Bronze Age timbers were detected at Flag Fen, a post‐Bronze Age agricultural landscape was identified in both the GPR and gradiometer surveys. We conclude that GPR has the greatest potential for archaeological geophysical prospection in peatland environments, but that electrical and magnetic methods can usefully be employed as secondary sources of information and should not be discounted from future research. Further, this article argues that better understandings must be developed of the impacts of geochemistry on geophysical data if we are going to realistically pursue ‘whole landscape’ surveys.

Soil geochemical methods in archaeo‐geophysics: Exploring a combined approach at sites in Scotland

AbstractThis article explores the application of geophysical and soil geochemical methods to detect archaeological features in three traditionally ‘difficult’ survey environments in Scotland: wind‐blown sands (Bay of Skaill, Orkney), clay (Chesterhall Parks Farm, Lanarkshire) and glacial drift deposits (Forteviot, Perthshire). The results presented here are part of the first research project that systematically tested a combined approach using geophysical and soil characterisation to understand the proxy responses of known archaeological features. First, a range of geophysical techniques [earth resistance, magnetometry, frequency domain electromagnetics (FDEM) and ground‐penetrating radar (GPR)] was employed over archaeological targets. Second, the different geophysical results were considered with respect to soil chemical concentrations (total phosphate and multi‐element analysis), texture, pH, conductivity, organic matter content and magnetic susceptibility from archaeological deposits, topsoil and subsoil samples. This study demonstrates that, by focusing on the responses of single archaeological features and assessing their physical and chemical signatures, soil composition and processes involved in the ‘history’ of buried features are of importance in improving our understanding of the reasons behind their detection with geophysical means. For example, at the cropmark site at Forteviot, chemical transformations can be triggered by organic matter accumulation and increased water retention within prehistoric ditch deposits and can have an effect on the type of magnetic contrast. In addition, chemical concentrations revealing anthropogenic organic materials can explain the enhanced conductivity of theoretically impervious features, as illustrated at the Bay of Skaill site. The study also provide insights into how a particular soil environment may affect different geophysical techniques such as the masking effects of heterogeneous glacial drift deposits or deep windblown sands. These types of survey environments characterise many archaeologically rich areas in Scotland, where integrated strategies, such as the one used in this investigation, are the best option to maximise detection of subsurface features and provide confident and augmented interpretations.

Mapping seafloor massive sulfides with the Golden Eye frequency-domain EM profiler

Since the early discovery of a black-smoker complex in 1978 on the East Pacific Rise at 21°N, speculations and expectations have been driven about the potential and perspectives of mining seafloor massive sulfide (SMS) deposits in the deep-ocean. With a worldwide accelerating industrialization, emerging markets, increased commodity prices and metal demand, and advance¬ments in deep-water mining and extraction technologies, mining of SMS may become economically feasible in the near future (Kowalczyk, 2008). However, we still know little about the resource potential of SMS deposits, and the development of geophysical methods for an assessment of their spatial extent, composition, and inner structure is crucial to derive a proper assessment of their economic value. Novel geophysical mapping techniques and exploration strategies are required to locate extinct and buried clusters of SMS deposits, away from the active vent fields and of larger economic potential, but are difficult to find and sample by conventional methods. In 2015 the International Seabed Authority (ISA) assigned an exploration license for polymetallic sulfide deposits to the German Federal Institute for Geosciences and Natural Resources (BGR) in a specified area comprising 100 patches, each 10 . 10 km in size, distributed along the Central and Southeastern Indian Ridge. The challenge to acquire high resolution near-surface electromagnetic (EM) data in such geologically and morphologically complex mid-ocean ridge environments has been addressed by our recent development of the deep-sea profiler Golden Eye that utilizes a frequency-domain electromagnetic (FDEM) central loop sensor, of 3.3 m diameter (Muller et al., 2016). This system has been used in 2015 and 2017 to map active and relict hydrothermal vent fields in the SMS licensing areas. Aside from technological developments, this paper discusses new data processing routines and methods to unravel the conductivity-depth-distribution, induced polarization and magnetic susceptibility, and joint interpretation with geochem¬istry as key elements to map and evaluate SMS deposits.

Three-dimensional Land FD-CSEM Regularized Inversion Based on Edge Finite-element Method

There is a desire to obtain rapid and stable inversion results and clearly reconstruct subsurface resistivity structure in frequency domain (FD) electromagnetics. Three-dimensional modeling of land FD controlled-source electromagnetic (CSEM) data is vital to improve the understanding of electromagnetic responses collected in increasingly complex geologic settings. Three-dimensional inversion of land FD-CSEM data is a mathematically non-unique problem with instability, due to the noise contained in the data and its inherent incompleteness. The main difference between our method and those from previous work is that the edge finite-element approach is applied to solve the three-dimensional FD-CSEM generated by a horizontal electric dipole source. Firstly, we formulate the edge finite-element equation through the Galerkin method, based on the Helmholtz equation of the electric fields. Secondly, in order to check the validity of the modeling code, we compare the numerical results with the analytical solutions for a homogeneous half-space model. For further tests, we calculate the electromagnetic responses for another two models with more practical structures. Finally, the three-dimensional inversion is carried out based on a regularization method with smoothness-constraints to obtain stable solutions.

Low signal-to-noise FDEM in-phase data: Practical potential for magnetic susceptibility modelling

In this paper, we consider the use of land-based frequency-domain electromagnetics (FDEM) for magnetic susceptibility modelling. FDEM data comprises both out-of-phase and in-phase components, which can be related to the electrical conductivity and magnetic susceptibility of the subsurface. Though applying the FDEM method to obtain information on the subsurface conductivity is well established in various domains (e.g. through the low induction number approximation of subsurface apparent conductivity), the potential for susceptibility mapping is often overlooked. Especially given a subsurface with a low magnetite and maghemite content (e.g. most sedimentary environments), it is generally assumed that susceptibility is negligible. Nonetheless, the heterogeneity of the near surface and the impact of anthropogenic disturbances on the soil can cause sufficient variation in susceptibility for it to be detectable in a repeatable way. Unfortunately, it can be challenging to study the potential for susceptibility mapping due to systematic errors, an often poor low signal-to-noise ratio, and the intricacy of correlating in-phase responses with subsurface susceptibility and conductivity. Alongside use of an accurate forward model – accounting for out-of-phase/in-phase coupling – any attempt at relating the in-phase response with subsurface susceptibility requires overcoming instrument-specific limitations that burden the real-world application of FDEM susceptibility mapping. Firstly, the often erratic and drift-sensitive nature of in-phase responses calls for relative data levelling. In addition, a correction for absolute levelling offsets may be equally necessary: ancillary (subsurface) susceptibility data can be used to assess the importance of absolute in-phase calibration though hereby accurate in-situ data is required. To allow assessing the (importance of) in-phase calibration alongside the potential of FDEM data for susceptibility modelling, we consider an experimental test case whereby the in-phase responses of a multi-receiver FDEM instrument are calibrated through downhole susceptibility data. Our results show that, while it is possible to derive approximate susceptibility profiles from FDEM data, robust quantitative analysis hinges on appropriate calibration of the responses.

Joint inversion of seismic traveltime and frequency-domain airborne electromagnetic data for hydrocarbon exploration

Geophysical joint inversion requires the setting of a few parameters for optimum performance of the process. However, there are yet no known detailed procedures for selecting the various parameters for performing the joint inversion. Previous works on the joint inversion of electromagnetic (EM) and seismic data have reported parameter applications for data sets acquired from the same dimensional geometry (either in two dimensions or three dimensions) and few on variant geometry. But none has discussed the parameter selections for the joint inversion of methods from variant geometry (for example, a 2D seismic travel and pseudo-2D frequency-domain EM data). With the advantage of affordable computational cost and the sufficient approximation of a 1D EM model in a horizontally layered sedimentary environment, we are able to set optimum joint inversion parameters to perform structurally constrained joint 2D seismic traveltime and pseudo-2D EM data for hydrocarbon exploration. From the synthetic experiments, even in the presence of noise, we are able to prescribe the rules for optimum parameter setting for the joint inversion, including the choice of initial model and the cross-gradient weighting. We apply these rules on field data to reconstruct a more reliable subsurface velocity model than the one obtained by the traveltime inversions alone. We expect that this approach will be useful for performing joint inversion of the seismic traveltime and frequency-domain EM data for the production of hydrocarbon.

Using Combined Close-Range Active and Passive- Remote Sensing Methods to Detect Sinkholes

In the Dead Sea region of Israel, sinkholes collapse can be observed easily due to the large number of sites. The continuous decrease in Dead Sea level over the last 30 years, caused a substantial increases the sinkhole activity (more than 3,000 sinkholes upper layer collapse). Sinkholes of up to 50 m diameter are found to be clustered in sites with variable characteristics. In this research, we have developed methods for prediction of sinkholes appearance by using mapping and monitoring methods based on active and passive remote-sensing means. These methods are based on measurements from several instruments including field spectrometry, geophysical ground-penetration radar (GPR) and a frequency domain electromagnetic (FDEM) instrument. Field spectrometry was used to compare the spectral signatures of soil samples collected near progressing sinkholes and those taken in regions with no visible occurrence of sinkholes. Active remote sensing showed higher electrical conductivity and soil moisture in the former regions. Measurements were taken at different time points to monitor the progress of an embryonic sinkhole. The research steps included (i) review of previous published literature, (ii) mapping of regions with an abundance of sinkholes in various stages, and areas that are vulnerable to them, (iii) data analysis and development of warning indicators, accessible information to the scientific community. The result derived from this research indicates the possibility to build a pre-warning tool to detect the formation of sinkholes.

Integrated Geophysical Study to Delineate Mineralised Zones in Southwest Ashanti Belt of Ghana

Ground Frequency Domain Electromagnetic (FDEM) and Magnetic surveys were conducted in Southwest of the Ashanti belt in the Western Region of Ghana. The aim was to delineate potential gold mineralisation zones that may occur within the area. The magnetic data processing steps involved analytic signal, first vertical derivative, upward continuation, CET grid analysis as well as depth to basement model (Werner deconvolution). For the electromagnetic survey using the two coil separations (100 m and 150 m) and the multi frequencies (222 Hz, 888 Hz, 3555 Hz), the data in-phase component was processed to produce EM component maps showing the potential mineralisation zones. Lineament analysis using the rose diagram showed that the area is dominated by N-S and NNW-SSE lineaments. The upward continuation and depth to the magnetic source bodies indicated that the magnetic bodies in the study area can be located at depth less than 200 m.

Numerical electromagnetic frequency domain analysis with discrete exterior calculus

In this paper, we perform a numerical analysis in frequency domain for various electromagnetic problems based on discrete exterior calculus (DEC) with an arbitrary 2-D triangular or 3-D tetrahedral mesh. We formulate the governing equations in terms of DEC for 3-D and 2-D inhomogeneous structures, and also show that the charge continuity relation is naturally satisfied. Then we introduce a general construction for signed dual volume to incorporate material information and take into account the case when circumcenters fall outside triangles or tetrahedrons, which may lead to negative dual volume without Delaunay triangulation. Then we examine the boundary terms induced by the dual mesh and provide a systematical treatment of various boundary conditions, including perfect magnetic conductor (PMC), perfect electric conductor (PEC), Dirichlet, periodic, and absorbing boundary conditions (ABC) within this method. An excellent agreement is achieved through the numerical calculation of several problems, including homogeneous waveguides, microstructured fibers, photonic crystals, scattering by a 2-D PEC, and resonant cavities.

Transforming a time-domain electromagnetic signal to a frequency-domain electromagnetic response using regularization inversion

Wide applications of time-domain electromagnetic (TEM) data require 3D inversion. A possible strategy is to use the developed 3D inversion algorithms in frequency-domain (FD) electromagnetic (EM) methods. Thus, the key of the strategy is how to transform the time-domain ([Formula: see text]-[Formula: see text]) EM signal into the FD. An inversion algorithm has been developed to transform the [Formula: see text]-[Formula: see text] signal into a corresponding FD response. In this method, a step-off current is presumed. Under this assumption, the Fourier transform relating the EM FD response to the [Formula: see text]-[Formula: see text] signal becomes a sine or cosine transformation. Using the polynomial approximation method, the transformation turns into a linear equation. From a set of [Formula: see text]-[Formula: see text] signals, FD responses could be obtained by solving these linear equations in the least-squares sense. To reduce the nonuniqueness of the solution, and enhance the solution stability, an additional smoothness constraint on the FD response is imposed, thus converting the minimization problem into a regularization inversion problem. The algorithm is applied to synthetic and field vertical magnetic data in the in-loop TEM surveying mode. The numerical results show that in the entire audio-frequency range, the relative errors between the inversed and theoretical FD responses of the real and imaginary parts are almost all less than 1%, with the largest discrepancy of 5% occurring at high frequencies. There are two significances behind our work: First, the possibility of accurately transforming [Formula: see text]-[Formula: see text] response into FD response in audio-frequency range is coming into true, thereby (from the mathematical perspective) implementing the equivalence between the responses of the EM method in the time domain and the FD. Second, the algorithm provides a new approach to interpret TEM data in 3D mode by using developed 3D FEM inversion techniques.

The optical trapezoid model: A novel approach to remote sensing of soil moisture applied to Sentinel-2 and Landsat-8 observations

The “trapezoid” or “triangle” model constitutes the most popular approach to remote sensing (RS) of surface soil moisture based on coupled thermal (i.e., land surface temperature) and optical RS observations. The model, hereinafter referred to as Thermal-Optical TRAapezoid Model (TOTRAM), is based on interpretation of the pixel distribution within the land surface temperature - vegetation index (LST-VI) space. TOTRAM suffers from two inherent limitations. It is not applicable to satellites that do not provide thermal data (e.g., Sentinel-2) and it requires parameterization for each individual observation date. To overcome these restrictions we propose a novel OPtical TRApezoid Model (OPTRAM), which is based on the linear physical relationship between soil moisture and shortwave infrared transformed reflectance (STR) and is parameterized based on the pixel distribution within the STR-VI space. The OPTRAM-based surface soil moisture estimates derived from Sentinel-2 and Landsat-8 observations for the Walnut Gulch and Little Washita watersheds were compared with ground truth soil moisture data. Results indicate that the prediction accuracies of OPTRAM and TOTRAM are comparable, with OPTRAM only requiring observations in the optical electromagnetic frequency domain. The volumetric moisture content estimation errors of both models were below 0.04cm3cm−3 with local calibration and about 0.04–0.05cm3cm−3 without calibration. We also demonstrate that OPTRAM only requires a single universal parameterization for a given location, which is a significant advancement that opens a new avenue for remote sensing of soil moisture.

Correcting for static shift of magnetotelluric data with airborne electromagnetic measurements: a case study from Rathlin Basin, Northern Ireland

Abstract. Galvanic distortions of magnetotelluric (MT) data, such as the static-shift effect, are a known problem that can lead to incorrect estimation of resistivities and erroneous modelling of geometries with resulting misinterpretation of subsurface electrical resistivity structure. A wide variety of approaches have been proposed to account for these galvanic distortions, some depending on the target area, with varying degrees of success. The natural laboratory for our study is a hydraulically permeable volume of conductive sediment at depth, the internal resistivity structure of which can be used to estimate reservoir viability for geothermal purposes; however, static-shift correction is required in order to ensure robust and precise modelling accuracy.We present here a possible method to employ frequency–domain electromagnetic data in order to correct static-shift effects, illustrated by a case study from Northern Ireland. In our survey area, airborne frequency domain electromagnetic (FDEM) data are regionally available with high spatial density. The spatial distributions of the derived static-shift corrections are analysed and applied to the uncorrected MT data prior to inversion. Two comparative inversion models are derived, one with and one without static-shift corrections, with instructive results. As expected from the one-dimensional analogy of static-shift correction, at shallow model depths, where the structure is controlled by a single local MT site, the correction of static-shift effects leads to vertical scaling of resistivity–thickness products in the model, with the corrected model showing improved correlation to existing borehole wireline resistivity data. In turn, as these vertical scalings are effectively independent of adjacent sites, lateral resistivity distributions are also affected, with up to half a decade of resistivity variation between the models estimated at depths down to 2000 m. Simple estimation of differences in bulk porosity, derived using Archie's Law, between the two models reinforces our conclusion that the suborder of magnitude resistivity contrasts induced by the correction of static shifts correspond to similar contrasts in estimated porosities, and hence, for purposes of reservoir investigation or similar cases requiring accurate absolute resistivity estimates, galvanic distortion correction, especially static-shift correction, is essential.

System stability and calibrations for hand-held electromagnetic frequency domain instruments

There are a few multiple-frequency domain electromagnetic induction (EMI) hand-held rigid boom systems available for shallow geophysical resistivity investigations. They basically measure secondary field real and imaginary components after the system calibrations. One multiple-frequency system, the EMP-400 Profiler from Geophysical Survey Systems Inc., was tested for system calibrations, stability and various effects present in normal measurements like height variation, tilting, signal stacking and time stability.Results indicated that in test conditions, repeatable high-accuracy imaginary component values can be recorded for near-surface frequency soundings. In test conditions, real components are also stable but vary strongly in normal surveying measurements. However, certain calibration issues related to the combination of user influence and measurement system height were recognised as an important factor in reducing for data errors and for further processing like static offset corrections.

Water saturated sand and a shallow bay: Combining coastal geophysics and underwater archaeology in the south bay of Tel Dor

The south bay of Tel Dor was examined during a detailed underwater archaeological expedition and through an onland, coastal Frequency Domain Electromagnetic (FDEM) geophysical survey. The aim was to find out if the bay situated adjacent to the southern edge of the tel, was used for maritime activity when the tel was inhabited. Results indicate that the FDEM method provides useful data in waterlogged coastal environments, where other more conventional methods are not satisfactory in terms of identifying archaeological remains. A number of linear features were detected on the FDEM maps, the most visible being a rectangular area facing the sea on the shoreline and an NW-SE trending near-linear feature. Distribution of underwater archaeological evidence for marine activity such as anchors and mooring stones also seems to be aligned in an NW-SE direction, parallel to the latter feature on shore. If indeed related to archaeology, the features detected by the FDEM method are probably located on the clay substrate, buried underneath thousands of years of sand accumulation. Given the sedimentary history of the bay, and the ages of the nearby coastal structures, it is possible that this is a maritime structure dating to the Bronze Age or even earlier.

Geophysical characterization of saltwater intrusion in a coastal aquifer: The case of Martil-Alila plain (North Morocco)

Several factors can affect the quantity and the quality of groundwater resources, but in coastal aquifers seawater intrusion is often the most significant issue regarding freshwater supply. Further, saltwater intrusion is a worldwide issue because about seventy percent of the world's population lives in coastal regions.Generally, fresh groundwater not affected by saltwater intrusion is characterized by low salinity and therefore low electrical conductivity (EC) values. Consequently, high values of EC in groundwater along the coastline are usually associated to seawater intrusion. This effect is amplified if the coastal aquifer is overexploited with a subsequent gradual displacement of the freshwater-saltwater interface towards the continent.Delineation of marine intrusion in coastal aquifers has traditionally relied upon observation wells and collection of water samples. This approach may miss important hydrologic features related to saltwater intrusion in areas where access is difficult and where wells are widely spaced. Consequently, the scarcity of sampling points and sometimes their total absence makes the number of data available limited and most of the time not representative for mapping the spatial and temporal variability of groundwater salinity.In this study, we use a series of geophysical methods for characterizing the aquifer geometry and the extension of saltwater intrusion in the Martil-Alila coastal region (Morocco) as a complement to geological and hydrogeochemical data. For this reason, we carried out three geophysical surveys: Gravity, Electrical Resistivity and Frequency Domain Electromagnetic. The geometry of the basin has been determined from the interpretation of a detailed gravity survey. Electrical resistivity models derived from vertical electrical soundings allowed to characterize the vertical and the lateral extensions of aquifer formations. Finally, frequency domain electromagnetic methods allowed delineating the extension of the saltwater intrusion.

Vers une cartographie tridimensionnelle en temps réel des aquifères superficiels à partir d’une approche de modélisation hybride

A hybrid modeling approach is proposed for near real-time three-dimensional (3D) mapping of surficial aquifers. First, airborne frequency-domain electromagnetic (FDEM) measurements are numerically inverted to obtain subsurface resistivities. Second, a machine-learning (ML) algorithm is trained using the FDEM measurements and inverted resistivity profiles, and borehole geophysical and hydrogeologic data. Third, the trained ML algorithm is used together with independent FDEM measurements to map the spatial distribution of the aquifer system. Efficacy of the hybrid approach is demonstrated for mapping a heterogeneous surficial aquifer and confining unit in northwestern Nebraska, USA. For this case, independent performance testing reveals that aquifer mapping is unbiased with a strong correlation (0.94) among numerically inverted and ML-estimated binary (clay-silt or sand-gravel) layer resistivities (5–20 ohm-m or 21–5,000 ohm-m), and an intermediate correlation (0.74) for heterogeneous (clay, silt, sand, gravel) layer resistivities (5–5,000 ohm-m). Reduced correlation for the heterogeneous model is attributed to over-estimating the under-sampled high-resistivity gravels (about 0.5 % of the training data), and when removed the correlation increases (0.87). Independent analysis of the numerically inverted and ML-estimated resistivities finds that the hybrid procedure preserves both univariate and spatial statistics for each layer. Following training, the algorithms can map 3D surficial aquifers as fast as leveled FDEM measurements are presented to the ML network.

Characterizing initial-state conductivity distribution at a CO2 injection site with airborne, surface, and borehole electromagnetic induction methods

Electromagnetic (EM) methods were used to characterize (1) the general near-surface geology and stratigraphy and (2) the initial electrical conductivity distribution at a enhanced oil recovery (EOR) site to assess and monitor possible near-surface environmental impacts of a carbon sequestration experiment. The field study was conducted at Cranfield Field, an EOR site where is being injected into a depleted oil and gas reservoir in the Cretaceous lower Tuscaloosa Formation in western Mississippi. The study focused on Tertiary and younger strata between the ground surface and maximum depths of approximately 200 m (656 ft) that host groundwater more than 3000 m (9843 ft) above the oil and gas reservoir and injection zone. It included an airborne geophysical survey collecting frequency-domain EM data, time-domain surface EM measurements, borehole logging with EM induction, natural gamma spectra, and water-level measurements. Different approaches of temperature drift corrections for the borehole EM data were compared; good results of consistent and accurate conductivity values were produced by combining both directions of a two-way (uphole and downhole) measurement. The airborne EM provided data over a large area with sufficient detail to give an overview for the subsequent surface and borehole surveys, the surface time-domain data gave insight into greater depths, and the borehole induction data provided the necessary details. These three EM methods complement each other in areal coverage, lateral and vertical resolution, and exploration depth. Together, they can provide a comprehensive near-surface characterization of the study area that is necessary to establish initial-state conditions that support future monitoring of potential migration to the near-surface environment.

Applying FDEM, ERT and GPR at a site with soil contamination: A case study

This study employed the combination of three methods, namely the Frequency Domain Electromagnetic (FDEM), Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to evaluate a heavy-metal contaminated site for both pre- and post-remediation investigations. The main goals were to verify the position and the integrity of the underground storage tanks (UST), and to determine the effectiveness of remediation to ensure no contaminants remained at the site. In general, the GPR survey was effective at locating shallowly buried objects. However, due to the highly conductive nature of the heavy-metal laden sludge, the GPR signals were attenuated severely. Thus, the first attempt to use GPR in the pre-remediation investigation did not achieve the desired results and other methods were deployed. The existence of the UST and the sludge within were confirmed by ERT and the UST shape was mapped by FDEM. The principal remediation scheme was soil replacement by replacing the contaminated soil with clean silt. Based on the distinctive property differences of the contaminated soil and the clean silt, the completion of the remediation was confirmed by the differences between pre-remediation and post-remediation in GPR, ERT and FDEM results.