Time Domain Electromagnetic Research Articles

Integrated application of heliborne and ground electromagnetic surveys for mapping EM conductor for uranium exploration and its subsurface validation, North Delhi Fold Belt, Rajasthan, India: A case study

A fracture-controlled uranium deposit was identified in Proterozoic Ajabgarh metasediments of the North Delhi Fold Belt within the Khetri subbasin at Rohil, Sikar district, Rajasthan, India. Uranium mineralization in the area is associated with geologic structures, albitization, and pyroxenization of metasediments and conductors such as metallic sulfides and carbonaceous phyllites/graphitic schists. To locate uranium mineralization akin to Rohil in nearby thick soil covered areas, this association was targeted through heliborne geophysical surveys. High-resolution heliborne magnetic and time domain electromagnetic (TEM) surveys were conducted around Rohil. The survey delineated several targets with favorable geologic structures and conductors such as graphitic schist for further uranium exploration. One favorable target near Chappar village was taken up for follow-up exploration work. The EM conductor mapped from heliborne survey was subsequently validated through ground time-domain electromagnetic surveys and subsurface exploration. Modeling of heliborne and ground-based electromagnetic data revealed the presence of subsurface conducting bodies with comparable model parameters. Drilling established the presence of a subsurface conductor up to a depth of 300 m, which was attributed to the presence of graphite and sulfides (pyrrhotite) along foliation plane of carbon phyllite/graphitic schist/quartz-biotite schist and calc-silicate rock. Further detailed laboratory investigations (petrology/X-ray diffraction) of selected core samples from the conductive zones confirmed the presence of pyrrhotite and graphite responsible for EM signature. This study, carried out by using multiparameter data sets, proved the efficacy of heliborne surveys in locating favorable targets for uranium exploration in Ajabgarh group of rocks.

Application and evaluation of electromagnetic methods for imaging saltwater intrusion in coastal aquifers: Seaside Groundwater Basin, California

Developing effective resource management strategies to limit or prevent saltwater intrusion as a result of increasing demands on coastal groundwater resources requires reliable information about the geologic structure and hydrologic state of an aquifer system. A common strategy for acquiring such information is to drill sentinel wells near the coast to monitor changes in water salinity with time. However, installation and operation of sentinel wells is costly and provides limited spatial coverage. We studied the use of noninvasive electromagnetic (EM) geophysical methods as an alternative to installation of monitoring wells for characterizing coastal aquifers. We tested the feasibility of using EM methods at a field site in northern California to identify the potential for and/or presence of hydraulic communication between an unconfined saline aquifer and a confined freshwater aquifer. One-dimensional soundings were acquired using the time-domain electromagnetic (TDEM) and audiomagnetotelluric (AMT) methods. We compared inverted resistivity models of TDEM and AMT data obtained from several inversion algorithms. We found that multiple interpretations of inverted models can be supported by the same data set, but that there were consistencies between all data sets and inversion algorithms. Results from all collected data sets suggested that EM methods are capable of reliably identifying a saltwater-saturated zone in the unconfined aquifer. Geophysical data indicated that the impermeable clay between aquifers may be more continuous than is supported by current models.

Three dimensional inversion of multisource time domain electromagnetic data

We present a 3D inversion methodology for multisource time-domain electromagnetic data. The forward model consists of Maxwell’s equations in time where the permeability is fixed but electrical conductivity can be highly discontinuous. The goal of the inversion is to recover the conductivity-given measurements of the electric and/or magnetic fields. The availability of matrix-factorization software and high-performance computing has allowed us to solve the 3D time domain EM problem using direct solvers. This is particularly advantageous when data from many transmitters and over many decades are available. We first formulate Maxwell’s equations in terms of the magnetic field, [Formula: see text]. The problem is then discretized using a finite volume technique in space and backward Euler in time. The forward operator is symmetric positive definite and a Cholesky decomposition can be performed with the work distributed over an array of processors. The forward modeling is quickly carried out using the factored operator. Time savings are considerable and they make 3D inversion of large ground or airborne data sets feasible. This is illustrated by using synthetic examples and by inverting a multisource UTEM field data set acquired at San Nicolás, which is a massive sulfide deposit in Mexico.

Faulting and Groundwater in Arid Environments: Airborne Electromagnetics in Support of Framework Hydrology in the northern Mojave Desert, southern California

Groundwater basins within the Mojave Desert of the south-western United States are often compartmentalized by faults. As part of an effort to understand and manage groundwater resources in arid environments, the U.S. Geological Survey is investigating a number of basins within the Fort Irwin National Training Center (NTC) using multiple geophysical methods. Gravity, aeromagnetic, ground-based and airborne time-domain electromagnetic (TEM) data were collected over Leach Basin, a geologically complex, internally-drained basin bisected and flanked by Quaternary faults including the Garlock and Death Valley fault zones. The airborne TEM data show abrupt changes in earth response across faulted boundaries, reflecting the strong resistivity contrast between igneous rocks and basin sediments. The distribution of faults throughout the basin can be directly obtained from the airborne data. A resistivity stratigraphy has been developed by integrating borehole geophysical logs, lab resistivity measurements, and ground-based gravity and TEM soundings. The results are applied to the airborne resistivity models and are used to trace aquifer hydrostratigraphy throughout the basin. Interpreted parameters include the depth to basement, the depth to water, and the thickness of the primary aquifer. Together with hydrologic investigations, these results are being used to estimate groundwater storage within the basin

Practical 3D inversion of large airborne time domain electromagnetic data sets

SUMMARY In this paper we show that 3D inversion of large airborne time domain EM data, which is traditionally considered impractical, can be rapidly carried out by using a thoughtful workflow. In our 3D inversion algorithm, the number of cells in the mesh and the number of soundings are two factors that slow down the inversion. Therefore, we develop a strategy of adaptive mesh and sounding refinement to minimize the number of cells and the number of soundings required by the inversion. At the beginning, a coarse mesh and a few soundings are used to quickly build up a large-scale model. Then the mesh is refined and more soundings are added based upon their data misfit. At each iteration of the inversion, a certain number of soundings are randomly selected, and we change the data selection from iteration to iteration. This allows us to down-sample the field data without much loss of information. Once the large-scale model is obtained, we carry out some tile inversions that focus on smaller areas with a locally refined mesh to better resolve the small-scale features. The workflow is demonstrated by a synthetic example with 2121 transmitters that takes about 10 hours to be solved compared to about 150 hours if we had started the inversion on a fine mesh and used all of the transmitters. The methodology of speeding up the inversion by adaptive mesh and data refinement can also be applied to other EM surveys.

Electromagnetic methods used for the characterisation of aquifers in Timor-Leste

This work was done in the frame of a greater project aimed at assessing possible impacts of climate change on the groundwater resources in Timor-Leste. With the intention to contribute to the understanding of the area?s aquifer characteristics and groundwater quality, two lines of time domain EM soundings were acquired in different coastal locations. The acquired data where then inverted and interpreted with in a contextual framework. The geophysical component within the broader project was brought in to assist with coastal aquifer characterisation and to better understand the aquifers? architecture. The method employed has proven to be an effective way of sounding the ground in Timor-Leste. Some of the results we have produced have contributed to fill-in gaps between well logs and water chemistry and have helped identify the potential presence of a seawater wedge in proximity of some of the city?s pumping wells.

The geophysical study of Drybones kimberlite using 3D Time Domain EM Inversion and 3D ZTEM inversion algorithms

Two airborne EM surveys were conducted over the Drybones kimberlite, NWT, Canada. A VTEM helicopter time domain EM survey was flown by Geotech in 2005, followed by a ZTEM helicopter tipper EM survey flown in 2009. The data sets collected over the kimberlite were inverted and interpreted first by Geotech, Ltd using a 1D TDEM conductivity-depth transform for VTEM data and a 2D inversion for ZTEM data. The data sets were transferred to UBC-GIF for 3D inversions and comparative analysis. Both the VTEM and ZTEM data have now been inverted in 3D and the results compared with the drill data over the kimberlite and with older results of previous geophysical interpretation. The 2D and 3D ZTEM inversions show the same general structure but the 3D inversion has greater electrical conductivity contrast and may have fewer artefacts in zones of low sensitivity. On a large scale the conductivity structure, recovered from the 3D inversion of time domain data, is comparable to that from the CDI transform, but the 3D result shows a more complicated distribution of the conductive properties. The 3D magnetic inversion had not been done previously and it shows susceptible material to depths of approximately 400 m, which is consistent with the known geometry of the kimberlite.

Influence of dispersive soil electromagnetic properties on hand‐held time domain electromagnetic sensors

ABSTRACTAlthough metal detectors remain the workhorses of humanitarian demining, it is well established that the performance of both continuous wave (frequency domain) and pulsed induction (time domain) detectors can be severely compromised by so‐called ‘soil‐effects’. Generally, problem soils reduce the signal‐to‐noise ratio and increase the false‐detection rate. In certain locations, the soil‐effect is so severe as to render the detector practically inoperable. The current study is part of an ongoing international effort to establish and quantify the influence of soil electromagnetic properties on the operation of metal detectors and related sensor technologies. In particular, we examine the relative influence of soil electrical conductivity, magnetic susceptibility and associated frequency dependence on the time domain electromagnetic (TDEM) response of pulsed induction metal detectors and related small‐scale TDEM sensors.

Induced polarization in airborne EM

A major impediment in the path toward airborne induced polarization (IP) is an effective method to quantify data from inductive sources, such as those used in airborne electromagnetic systems. We modeled inductive IP using a combination of Warburg and exponential decay models as a basis for fitting electromagnetic data from ground time-domain electromagnetic (TEM) and airborne versatile TEM (VTEM) surveys. Observed decays were deconvolved into electromagnetic and IP constituents by constrained least-squares fitting of basis functions modified to account for transmitter waveforms. The method was confirmed through synthetic modeling of 2D and 3D structures, and when applied to ground TEM or airborne TEM data, obtained an estimate of apparent chargeability at each station or fiducial. In the case of a VTEM survey in Africa, the apparent chargeabilities mapped graphitic sediments and provided spatially consistent indications of clay concentrations. A limitation on this airborne IP for airborne applications is motion noise, which places a lower limit on usable base frequency and begins to significantly affect the signal at the later delay times, when IP effects are most visible.

Calibrating a Salt Water Intrusion Model with Time‐Domain Electromagnetic Data

Salt water intrusion models are commonly used to support groundwater resource management in coastal aquifers. Concentration data used for model calibration are often sparse and limited in spatial extent. With airborne and ground-based electromagnetic surveys, electrical resistivity models can be obtained to provide high-resolution three-dimensional models of subsurface resistivity variations that can be related to geology and salt concentrations on a regional scale. Several previous studies have calibrated salt water intrusion models with geophysical data, but are typically limited to the use of the inverted electrical resistivity models without considering the measured geophysical data directly. This induces a number of errors related to inconsistent scales between the geophysical and hydrologic models and the applied regularization constraints in the geophysical inversion. To overcome these errors, we perform a coupled hydrogeophysical inversion (CHI) in which we use a salt water intrusion model to interpret the geophysical data and guide the geophysical inversion. We refer to this methodology as a Coupled Hydrogeophysical Inversion-State (CHI-S), in which simulated salt concentrations are transformed to an electrical resistivity model, after which a geophysical forward response is calculated and compared with the measured geophysical data. This approach was applied for a field site in Santa Cruz County, California, where a time-domain electromagnetic (TDEM) dataset was collected. For this location, a simple two-dimensional cross-sectional salt water intrusion model was developed, for which we estimated five uniform aquifer properties, incorporating the porosity that was also part of the employed petrophysical relationship. In addition, one geophysical parameter was estimated. The six parameters could be resolved well by fitting more than 300 apparent resistivities that were comprised by the TDEM dataset. Except for three sounding locations, all the TDEM data could be fitted close to a root-mean-square error of 1. Possible explanations for the poor fit of these soundings are the assumption of spatial uniformity, fixed boundary conditions and the neglecting of 3D effects in the groundwater model and the TDEM forward responses.

Validation of Advanced EM Models for UXO Discrimination

The work reported here details basic validation of our advanced physics-based electromagnetic induction forward and inverse models against data collected by the Naval Research Laboratory Time-Domain Electromagnetic Multisensor Towed Array Detection System platform. The data were collected under laboratory-type conditions using both artificial spheroidal targets and real unexploded ordnance (UXO). The artificial target models are essentially exact and enable detailed comparison of theory and data in support of measurement platform characterization and target identification. Real UXO targets cannot be treated exactly, but it is demonstrated that quantitative comparisons of the data with the spheroid models nevertheless aid in extracting key target discrimination information, such as target geometry and hollow target shell thickness.

Regularizing dipole polarizabilities in time-domain electromagnetic inversion

Abstract Recent advances in time-domain electromagnetic (TEM) sensors have dramatically improved discrimination of buried unexploded ordnance (UXO). In contrast to commercial standard mono-static sensors, the multi-static, multi-component geometries of next generation TEM sensors provide diverse excitations of a detected target. Inversion of observed data using the parametric TEM dipole model typically produces well-constrained estimates that can subsequently be inputted into a discrimination algorithm. In particular, the principal dipole polarizabilities provide information about target size and shape. Shape is represented by two transverse polarizabilities orthogonal to a target's axis of symmetry. Equality of transverse polarizabilities is diagnostic of an axisymmetric body of revolution and so has been proposed as a useful feature to discriminate between axisymmetric UXO and non-axisymmetric metallic clutter. Here we show that estimated transverse polarizabilities can sometimes be poorly constrained in an inversion of multi-static TEM data. This motivates our development of a regularized inversion algorithm that penalizes the deviation between transverse polarizabilities. We then develop an extension of the support vector machine (SVM) classifier that uses all models obtained via regularized inversion to make discrimination decisions. This approach achieves the best performance of all candidate discrimination algorithms applied to a number of real data sets.

Joint diagonalization applied to the detection and discrimination of unexploded ordnance

Efforts to discriminate buried unexploded ordnance from harmless surrounding clutter are often hampered by the uncertainty in the number of buried targets that produce a given detected signal. We present a technique that helps determine that number with no need for data inversion. The procedure is based on the joint diagonalization of a set of multistatic response (MSR) matrices measured at different time gates by a time-domain electromagnetic induction sensor. In particular, we consider the Naval Research Laboratory’s Time-Domain Electromagnetic Multisensor Towed Array Detection System (TEMTADS), which consists of a [Formula: see text] square grid of concentric transmitter/receiver pairs. The diagonalization process itself generalizes one of the standard procedures for extracting the eigenvalues of a single matrix; in terms of execution time, it is comparable to diagonalizing the matrices one by one. We present the method, discuss and illustrate its mathematical basis and physical meaning, and apply it to several actual measurements carried out with TEMTADS at a test stand and in the field at the former Camp Butner in North Carolina. We find that each target in a measurement is associated with a set of nonzero time-dependent MSR eigenvalues (usually three), which enables estimation of the number of targets interrogated. These eigenvalues have a characteristic shape as a function of time that does not change with the location and orientation of the target relative to the sensor. We justify analytically and empirically that symmetric targets have pairs of eigenvalues with constant ratios between them.

The quantitative advantages of using B-field sensors in time-domain EM measurement for mineral exploration and unexploded ordnance search

The use of simple models for decay of conductive targets under conductive overburden and for the decay of magnetically permeable conductive steel objects allows quantitative consideration of the advantages of the use of magnetic-field detectors in time-domain electromagnetic (TEM) measurements, or more generally, the advantage of step response over impulse response TEM systems. We identified eight advantages of the step response versus impulse-response systems. The first two advantages relate to the inductive limit (early time) decay behavior, in which a target response amplitude is largely dependent on geometrical rather than conductivity parameters. Five further advantages occur when measuring response of a target in a conductive host or under conductive overburden; the maximum target-to-overburden response occurs 25%–30% earlier in time, the earliest target detection time occurs a factor 2–4 earlier, and the amplitude advantage of target-to-overburden response is a factor in the range of 1–10 for the step versus impulse-response systems, respectively. These advantages agree quantitatively with field observations on a chalcopyrite orebody under conductive cover. We used a model response for a conductive permeable sphere to derive mathematically consistent approximations for the power-law and exponential decay behaviors for step and impulse responses of metal objects, from which the onset of late-time exponential decay of EM responses of unexploded ordnance occurs about a factor of two earlier in time for the step response. This earlier-time transition together with the higher signal-to-noise ratio available from the step-response measurement makes measurement of the fundamental time-constant of unexploded ordnance (UXO) possible for medium and large UXO where the time constant is in the range of tens of milliseconds. This time-constant thus becomes accessible as an additional parameter for UXO characterization and discrimination.

Near-surface electromagnetic induction — Introduction

There has been explosive growth in near-surface electromagnetic (EM) induction geophysics in the past several years (Everett, 2012). New and experienced practitioners are achieving great success in applying the method to an increasing variety of problems. Moreover, theorists are becoming better able to exploit the rich information content that is available in electromagnetic induction data sets. The EM induction method, with its broad opportunities to design new transmitters, receivers, and interpretation tools, continues to offer wide avenues to capture the spatial complexity of the subsurface. This special Geophysics issue brings forward the latest achievements, which should stimulate interest across a broad spectrum of geophysicists, as well as set the tone for continuing developments in this field. This special issue contains advances in theory, instrumentation, data processing and interpretation, and innovative applications of near-surface applied EM induction geophysics. The range of topics is varied and, as described below, includes modeling and inversion, airborne electromagnetics, hydrogeophysics, soil science, audiomagnetotellurics, unexploded ordnance (UXO) discrimination, archaeology, geothermal mapping, and joint inversion of EM and magnetic resonance sounding data. Vrbancich analyzes helicopter time-domain EM data acquired over shallow seawater overlying reefs and sediment-filled paleovalleys. A favorable comparison is shown of water depths and sediment thickness derived from 1D inversion with known bathymetry and independent marine seismic …

On the value of including x-component data in 1D modeling of electromagnetic data from helicopterborne time domain systems in horizontally layered environments

Helicopter borne time domain EM systems historically measure only the Z-component of the secondary field, whereas fixed wing systems often measure all field components. For the latter systems the X-component is often used to map discrete conductors, whereas it finds little use in the mapping of layered settings. Measuring the horizontal X-component with an offset loop helicopter system probes the earth with a complementary sensitivity function that is very different from that of the Z-component, and could potentially be used for improving resolution of layered structures in one dimensional modeling. This area is largely unexplored in terms of quantitative results in the literature, since measuring and inverting X-component data from a helicopter system is not straightforward: The signal strength is low, the noise level is high, the signal is very sensitive to the instrument pitch and the sensitivity function also has a complex lateral behavior. The basis of our study is a state of the art inversion scheme, using a local 1D forward model description, in combination with experiences gathered from extending the SkyTEM system to measure the X component. By means of a 1D sensitivity analysis we motivate that in principle resolution of layered structures can be improved by including an X-component signal in a 1D inversion, given the prerequisite that a low-pass filter of suitably low cut-off frequency can be employed. In presenting our practical experiences with modifying the SkyTEM system we discuss why this prerequisite unfortunately can be very difficult to fulfill in practice. Having discussed instrumental limitations we show what can be obtained in practice using actual field data. Here, we demonstrate how the issue of high sensitivity towards instrument pitch can be overcome by including the pitch angle as an inversion parameter and how joint inversion of the Z- and X-components produces virtually the same model result as for the Z-component alone. We conclude that adding helicopter system X-component to a 1D inversion can be used to facilitate higher confidence in the layered result, as the requirements for fitting the data into a 1D model envelope becomes more stringent and the model result thus less prone to misinterpretation.

Is the fast Hankel transform faster than quadrature?

The fast Hankel transform (FHT) implemented with digital filters has been the algorithm of choice in EM geophysics for a few decades. However, other disciplines have predominantly relied on methods that break up the Hankel transform integral into a sum of partial integrals that are each evaluated with quadrature. The convergence of the partial sums is then accelerated through a nonlinear sequence transformation. While such a method was proposed for geophysics nearly three decades ago, it was demonstrated to be much slower than the FHT. This work revisits this problem by presenting a new algorithm named quadrature-with-extrapolation (QWE). The QWE method recasts the quadrature sum into a form conceptually similar to the FHT approach by using a fixed-point quadrature rule. The sum of partial integrals is efficiently accelerated using the Shanks transformation computed with Wynn’s [Formula: see text] algorithm. A Matlab implementation of the QWE algorithm is compared with the FHT method for accuracy and speed on a suite of relevant modeling problems including frequency-domain controlled-source EM, time-domain EM, and a large-loop magnetic source problem. Surprisingly, the QWE method is faster than the FHT for all three problems. However, when the integral needs to be evaluated at many offsets and the lagged convolution variant of the FHT is applicable, the FHT is significantly faster than the QWE method. For divergent integrals such as those encountered in the large loop problem, the QWE method can provide an accurate answer when the FHT method fails.

A weak approximation of stochastic time domain electromagnetic response of a conductive permeable sphere

Time domain electromagnetic (TDEM) response of a conductive permeable sphere is a main topic for good understanding and in developing past studies for detection and discrimination of buried metallic objects. Stochastic differential equation model is a valuable tool for stimulating real experience. In this paper, according to an Ito integral, we have obtained a weak approximation of stochastic TDEM response of permeable and nonpermeable sphere. We have used the deterministic solution of TDEM response of conductive permeable sphere by changing one of the variables that have been obtained from the boundary conditions of problem to random variable. By adding white noise to random variable and using stochastic integral, we have displayed stochastic time domain response of conductive permeable sphere. The Ito integral includes a factor that shows infirmity and intensity of noise which has been simply considered constant. Accordingly, we have showed the effect of noise for magnetic and nonmagnetic spheres. Numerical results from step and impulse response of TDEM have shown that the amount of this factor is different for the two types of spheres.

Investigation on polynomial integrators for time-domain electromagnetics using a high-order discontinuous Galerkin method

In this work, we investigate the application of polynomial integrators in a high-order discontinuous Galerkin method for solving the time-domain Maxwell equations. After the spatial discretization, we obtain a time-continuous system of ordinary differential equations of the form, ∂tY(t)=HY(t), where Y(t) is the electromagnetic field, H is a matrix containing the spatial derivatives, and t is the time variable. The formal solution is written as the exponential evolution operator, exp(tH), acting on a vector representing the initial condition of the electromagnetic field. The polynomial integrators are based on the approximation of exp(tH) by an expansion of the form ∑m=0∞gm(t)Pm(H), where gm(t) is a function of time and Pm(H) is a polynomial of order m satisfying a short recursion. We introduce a general family of expansions of exp(tH) based on Faber polynomials. This family of expansions is suitable for non-Hermitian matrices, and consequently the proposed integrators can handle absorbing media and conductive materials. We discuss the efficient implementation of this technique, and based on some test problems, we compare the virtues and shortcomings of the algorithm. We also demonstrate how this scheme provides an efficient alternative to standard explicit integrators.

Integrated seismic-electromagnetic workflow for sub-basalt exploration in northwest Saudi Arabia

A project involving magnetotellurics, shallow EM and gravity acquisition was carried out in northwest Saudi Arabia to improve the imaging of a basalt-covered section of the Al Jawf graben where seismic is particularly noisy. The Al Jawf graben extends from Jordan to Saudi Arabia. Mesozoic and Paleozoic sections contained within the structure have the potential to host multiple oil reservoirs. The lack of reliable seismic data in this area motivated the use of a multi-geophysics exploration approach. Electromagnetic (EM) techniques were identified as the optimal complementary geophysical data set. In fact, the EM signal propagates through the resistive basalt cover with minimal attenuation, providing information about the underlying conductive sedimentary section. A combination of active-source time-domain EM (TDEM) and broadband magnetotelluric (MT) techniques imaged the base of the shallow basalt cover and its internal variations as well as larger-scale faults that control the graben structure. Integration of the broadband EM and gravity data with seismic was achieved through simultaneous joint inversion (SJI) to derive an integrated velocity model that was subsequently used to reprocess the seismic data in the time and depth domains. Seismic reprocessing resulted in a substantial improvement in the seismic imaging of deep structures predicted by the structural model but not imaged by the previous seismic processing.