Time Domain Electromagnetic Research Articles

Locally implicit discontinuous Galerkin method for time domain electromagnetics

In the recent years, there has been an increasing interest in discontinuous Galerkin time domain (DGTD) methods for the solution of the unsteady Maxwell equations modeling electromagnetic wave propagation. One of the main features of DGTD methods is their ability to deal with unstructured meshes which are particularly well suited to the discretization of the geometrical details and heterogeneous media that characterize realistic propagation problems. Such DGTD methods most often rely on explicit time integration schemes and lead to block diagonal mass matrices. However, explicit DGTD methods are also constrained by a stability condition that can be very restrictive on highly refined meshes and when the local approximation relies on high order polynomial interpolation. An implicit time integration scheme is a natural way to obtain a time domain method which is unconditionally stable but at the expense of the inversion of a global linear system at each time step. A more viable approach consists of applying an implicit time integration scheme locally in the refined regions of the mesh while preserving an explicit time scheme in the complementary part, resulting in an hybrid explicit–implicit (or locally implicit) time integration strategy. In this paper, we report on our recent efforts towards the development of such a hybrid explicit–implicit DGTD method for solving the time domain Maxwell equations on unstructured simplicial meshes. Numerical experiments for 3D propagation problems in homogeneous and heterogeneous media illustrate the possibilities of the method for simulations involving locally refined meshes.

Comparative study of local versus global methods for 1D joint inversion of direct current resistivity and time‐domain electromagnetic data

ABSTRACTDirect current resistivity and time‐domain electromagnetic (TDEM) surveys are often used in environmental, hydrological and mining evaluation. The interpretation of the data acquired with each of these geophysical methods, assuming one‐dimensional models, frequently produces ambiguous results. The joint inversion of TDEM and direct current resistivity data is considered by several authors as an efficient method to reduce the ambiguity inherent to each of these methods. This paper presents the results of a comparative study on the use of a local optimization method, iteratively reweighted least squares and global optimization methods of simulated annealing and particle swarm optimization in the joint inversion of TDEM and direct current resistivity data. The models obtained from joint inversion of synthetic data (H‐ and K‐type) and of experimental data using the three methods yield similar results. The iteratively reweighted least squares method is the fastest while the simulated annealing is the most time‐consuming. Simulated annealing and particle swarm optimization are the most efficient methods when studying equivalence problems.

Joint use of TEM and MRS methods in a complex geological setting

Transient Electromagnetic (TEM), known also as Time Domain Electromagnetic (TDEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate variations in lithology and groundwater salinity in the Nahal Hever South area (Dead Sea coast of Israel). The subsurface in this area is highly heterogeneous and composed of intercalated sand and clay layers over a salt formation, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100–225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to both the salinity of groundwater and the porosity of rocks. MRS, however, is sensitive primarily to groundwater volume, but it also allows delineating of lithological variations in water-saturated formations. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS, we found that the sandy Dead Sea aquifer filled with Dead Sea brine is characterized by a bulk resistivity of ρ x > 0.4 Ωm, whereas zones with silt and clay in the subsurface are characterized by a bulk resistivity of ρ x < 0.4 Ωm. These observations are confirmed by calibration of the TEM method performed near 18 boreholes.

Improved Monte Carlo 1D inversion of vertical electrical sounding and time‐domain electromagnetic data

ABSTRACTThis work deals with a robust and reliable global search inversion tool for vertical electrical sounding (VES) and time‐domain electromagnetic (TDEM) data, able to take into account the data quality and the dimensionality of the problem. The approach is an importance sampling method that exploits a scale property of the solution to move the random population of models closer to the solution. We obtain a more efficient sampling of the model parameter space with respect to a pure Monte Carlo method. The application of the scale properties overcomes some of the problems encountered when using optimization methods (e.g., simulated annealing), which are based on transition probability rules barely related to the specific problem or that require tuning of control parameters or training procedures (e.g., neural networks). Furthermore, the scale properties reduce the bias related to the initial definition of the boundaries of the model parameter space. A statistical test that accounts for the data uncertainties and the degrees of freedom of the problem is adopted to draw inference on the results. Synthetic and field data show that the algorithm is able to concentrate the sampling in high probability density zones of the model parameter space and to supply a reliable picture of the non‐uniqueness and equivalence problems.

Time domain electromagnetic (TEM) measurements on a buried subglacial valley in Northern Germany by using a large transmitter size and a high current

Time domain electromagnetic (TEM) measurements on a buried subglacial valley in Northern Germany by using a large transmitter size and a high current

A modular implementation of dispersive materials for time-domain simulations with application to gold nanospheres at optical frequencies

The development of photonic nano-structures can strongly benefit from full-field electromagnetic (EM) simulations. To this end, geometrical flexibility and accurate material modelling are crucial requirements set on the simulation method. This paper introduces a modular implementation of dispersive materials for time-domain EM simulations with focus on the Finite-Volume Time-Domain (FVTD) method. The proposed treatment can handle electric and magnetic dispersive materials exhibiting multi-pole Debye, Lorentz and Drude models, which can be mixed and combined without restrictions. The presented technique is verified in several illustrative examples, where the backscattering from dispersive spheres is calculated. The amount of flexibility and freedom gained from the proposed implementation will be demonstrated in the challenging simulation of the plasmonic resonance behavior of two gold nanospheres coupled in close proximity, where the dispersive characteristic of gold is approximated by realistic values in the optical frequency range.

A high-order non-conforming discontinuous Galerkin method for time-domain electromagnetics

In this paper, we discuss the formulation, stability and validation of a high-order non-dissipative discontinuous Galerkin (DG) method for solving Maxwell’s equations on non-conforming simplex meshes. The proposed method combines a centered approximation for the numerical fluxes at inter element boundaries, with either a second-order or a fourth-order leap-frog time integration scheme. Moreover, the interpolation degree is defined at the element level and the mesh is refined locally in a non-conforming way resulting in arbitrary-level hanging nodes. The method is proved to be stable and conserves a discrete counterpart of the electromagnetic energy for metallic cavities. Numerical experiments with high-order elements show the potential of the method.

A time–domain electromagnetic sounder for detection and characterization of groundwater on Mars

A prototype time–domain electromagnetic (TDEM) sounder was developed to technical readiness level (TRL) 5 to detect and characterize deep groundwater on Mars. The TDEM method induces eddy currents in the subsurface by abrupt extinction of a steady current in a large, flat-lying loop antenna, and the subsurface response is measured using the same loop or a separate receiver. TDEM has been widely used in terrestrial groundwater exploration and is ideally suited to sense the high electrical conductivity associated with saline groundwater expected on Mars. The inductive regime of TDEM is distinct from ground-penetrating radar: the latter has higher resolution but smaller depth of investigation. Our Mars-prototype TDEM was tested in the laboratory and at a local field site before the principal test was performed on Maui, Hawaii. This location was chosen because of its analogy to Mars in electrical properties: dry, resistive basalt over saline pore water. Results compared favorably to soundings made with a commercial TDEM, clearly detecting the seawater interface at depths of 250 m. We subsequently developed a ballistic deployment system for the loop antenna suitable for robotic missions. Compressed gas launches two projectiles; each consists of two spools on a guide stick. Payout on one spool is back towards the launcher and on the other toward its twin on the other projectile. In this way a triangular loop antenna is formed. The full system was tested twice, successfully achieving a distance of ∼70 m in both. A system capable of deploying a 200 m loop antenna on Mars would have mass <6 kg (including 0.3 kg electronics) and within one sol could detect groundwater at depths up to 5 km. TDEM can probe to depths not possible for radar and answer the question: does groundwater – and a likely subsurface habitable zone – exist on Mars?

High-order accurate thin layer approximations for time-domain electromagnetics. Part I: General metal backed coatings

We present a family of high-order accurate thin layer approximations for time-domain electromagnetics. The thin layer approximations are valid for metal backed coatings of general isotropic materials and certain classes of anisotropic materials on smooth curvilinear backgrounds. Both dielectric and magnetic materials can be considered. These models are non-trivial and we discuss their properties and implementation in the context of discontinuous Galerkin methods which are particularly well suited for these models. The range of validity, accuracy, and stability of the resulting schemes is demonstrated through one- and two-dimensional examples.

Strategic groundwater resources in the Tagliamento River basin (northern Italy): hydrogeological investigation integrated with geophysical exploration

The western sector of the Tagliamento River basin (Friuli Venezia-Giulia Region; northern Italy) is characterized by important water resources, both superficial and underground. In particular, in the Quaternary deposits of the plain, up to a depth of 500 m, six artesian aquifer systems exist. A large amount of lithological, geomorphological, and hydrogeological data is presented, allowing for definition of (a) the principal aquifer system of the area to a depth of about 500 m; (b) geometrical characteristics of the aquifers (thickness, lateral extension, etc); (c) the hydraulic parameters (hydraulic conductivity, transmissivity); (d) chemico-physical characteristics of the water; and (e) vulnerability to pollution of the aquifer systems. In a test area, where many boreholes were drilled for fresh water supply, the conceptual hydrogeological model was integrated by a detailed and repeated three-dimensional (3D) resistivity survey using combined electrical resistivity tomography (ERT) and time-domain electromagnetic (TDEM) soundings. ERT investigation was mainly used to get detailed information about geometry and porosity of the overburden and to calibrate the shallowest TDEM information.

Application of Occam's inversion to airborne time-domain electromagnetics

Airborne time-domain electromagnetics (ATDEM) methods are regularly used for mining, hydrocarbon, and groundwater exploration. A large quantity of data is collected along survey lines from an aircraft, and there is an incentive to interpret these data in a systematic way. When the geology is appropriate, the use of 1D inversion methods is justified. Among these methods are: conductivity-depth transform (CDT) (Wolfgram and Karlik, 1995), layered-earth inversion (Sattel, 1998), Zohdy's method (Sattel, 2005), and Occam's inversion (Constable et al., 1987; Sattel, 2005). These methods either require considerable tuning to get realistic results, are limited to step response data, or require considerable experimentation with the initial guess to ensure a reasonable result. The advantage of the Occam's algorithm is that it can be easily adapted to different ATDEM methods and is not strongly dependent on the initial guess. Furthermore, there are not a lot of parameters to tune in order to get a reasonable result....

The application of airborne geophysics in the exploration for iron ore in the Zouerate area of Mauritania

An airborne time domain electromagnetic and magnetic survey (Geotech VTEM system; VTEM is a helicopter-borne Time Domain Electromagnetic (TDEM) system with concentric loop transmitter receiver geometry.) was flown over the Kedia mountain and Mhaoudat Intrusive in Mauritania. Hematite has been mined in both these areas for many years. The objectives of this survey were to determine if additional ore bodies existed below old mined areas and to find and delineate possible new ore bodies. A detailed structural interpretation, using the magnetic and EM data, was done with the assumption that major faults could be a controlling factor on the occurrence of iron ore. Decay curve analysis was done of the VTEM data as a fast, first pass delineation of conductive areas. Good correlation with existing mines was found. CDI?s were consequently calculated for all flight lines. These were used to calculate conductivity contour maps at various depth intervals below the surface. Conductive areas were correlated with the magnetic data as well as the known geology. In several areas known iron ore was associated with the edges of observed anomalies. These anomalies continued at depth and distance, indicating possible extensions of the ore. Further anomalies occurred in a sand covered area south of Mhaoudat, leading to the possibility of new iron ore deposits.

The geophysical response of the Tusker gold deposit, Lake Victoria gold fields, Tanzania

The Tusker gold deposit is located in the Lake Victoria Goldfields of Tanzania. The deposit contains total estimated resources of 123.27 Mt at 1.15g/t Au, as at 5th September, 2006. Geophysical techniques trialled over the deposit include down hole measurements, airborne time domain electromagnetics, dipole-dipole induced polarisation and resistivity and airborne magnetics. Petrophysical measurements suggest the deposit is associated with a conductive and chargeable signature. This is confirmed by airborne EM and dipole-dipole IP and resistivity data. Magnetic data map the unmineralised magnetic mudstone package overlying the deposit, and show that stratigraphy is deformed.

Airborne - EM hydrocarbon mapping in Mozambique

We lay out a case history of a multi-method airborne geophysical survey to assist a hydrocarbon exploration project in central Mozambique. The dedicated targets of the survey are near surface resistivity anomalies caused by hydrocarbon seepage plumes. The geophysical survey concept includes the use of time-domain electromagnetics, magnetometry and gamma-spectrometry applied from rotary and fixed wing aircraft. Preliminary data interpretation, closely tied to 2D seismic information and the regional geological understanding, indicates a successful delineation of a seepage alteration zone just one month after survey completion.

Spatial delineation of groundwater salinity using deep time domain electromagnetic geophysical measurements: A feasibility study

The objective of this study was to delineate the spatial distribution of fresh, brackish, and saline groundwater bodies, as well as brine, beneath the Judea Desert, Israel. We used the deep time domain electromagnetic (TDEM) geophysical system, Cycle‐5M, which provides the maximum exploration capability, to depths ∼1.5–2 km below land surface. The study was conducted at 21 locations at the desert plateau, where fresh groundwater flows through the upper aquifers from the replenishment area at the Judea Mountains toward the Dead Sea springs. In deeper aquifers, brackish and saline groundwater bodies exist, which originated from lakes and lagoons that existed at the Dead Sea Rift valley during the Pleistocene and Pliocene, respectively. It was found that within carbonate formations, fresh groundwater (C &lt; 103 ppm total dissolved solids (TDS)) is characterized by a resistivity of more than 15 Ω m, brackish groundwater (103 &lt; C &lt; 104 ppm TDS) is characterized by 5–15 Ω m, saline groundwater (104 &lt; C &lt; 105 ppm TDS) is characterized by 0.5–5 Ω m, and brine (105 &lt; C ppm TDS) is characterized by less than 0.5 Ω m.

On the time-domain electromagnetic response of a conductive permeable sphere

The time-domain EM response of a conducting sphere has been subject of past studies with a particular goal of understanding the response of steel unexploded ordnance items. A change in formulas and parameter definition between authors has created inconsistency between published results and analytic formulas. Correction of formulas is required for further studies based on the conducting permeable sphere.

A novel converter topology for TEM applications

Time-domain Electro Magnetic (TEM) systems, are used for remote sensing of conductive mineral deposits buried under the surface of the earth. A low frequency trapezoidal current excitation set-up in an exciter coil loop causes a flux that penetrates deep into the earth. When the excitation is cut off sharply, conductive deposits in the earth carrying flux react by causing eddy current flows within them. The flux produced by such eddy currents is picked up by a sensor coil. By comparing the emf induced in the sensor coil with a priori known emf patterns for mineral deposits, the presence of mineral deposits can be ascertained. The voltage, current and energy levels of the TEM system, require special type of excitation technique.Power converters for such non-standard requirements are not reported in the literature, particularly for TEM applications. This paper introduces TEM systems to the reader and presents the requirements for excitation. A converter topology to meet the requirements, it’s analysis, control and performance are presented. Among other alternatives that the authors have attempted, the topology presented features reduced number of passive elements, high voltage gain and low losses. These features enable the sensor head to be operated from the normal low level battery.

Near-surface P-velocity modelling by integrated seismic, EM, and gravity data: examples from the Middle East

Onshore seismic data from the Middle East and around the world are adversely affected by near-surface velocity anomalies. Shallow-section velocity characterization represents a key issue for land data processing in both time and depth domains. In the Middle East, near-surface complexity can be attributed to sand dunes, wadies, sabkha environment, shallow high-velocity layers (carbonate outcrops), karsting, and fractured layers. The correct estimate of the near-surface velocity field is fundamental to obtain reliable seismic images. Conventional workflows for near-surface model building often involve interpreting and inverting refracted arrival information. These methods may fail to appropriately reconstruct the shallow velocity complexities in the presence of large velocity inversions, rapid lateral velocity changes, and noisy refracted arrival data. The combination of additional geophysical measurements (i.e., non-seismic), not influenced by the problems affecting seismic propagation, are needed to address the shallow velocity modelling problem. This paper describes three different approaches recently applied in distinctly different environments around the Middle East to solve near-surface modelling problems. The first example from the United Arab Emirates (UAE) demonstrates the integration of seismic and time-domain electromagnetics (TDEM) for sand dune statics application. The second example from Saudi Arabia also addresses sand dune statics, but this time by simultaneous joint inversion (JI) of seismic and gravity measurements. The final example applies the simultaneous JI technique for the near-surface velocity component of a depth migration model.

Quasi-3D modeling of airborne TEM data by spatially constrained inversion

We present a new methodology, spatially constrained inversion (SCI), that produces quasi-3D conductivity modeling of electromagnetic (EM) data using a 1D forward solution. Spatial constraints are set between the model parameters of nearest neighboring soundings. Data sets, models, and spatial constraints are inverted as one system. The constraints are built using Delaunay triangulation, which ensures automatic adaptation to data density variations. Model parameter information migrates horizontally through spatial constraints, increasing the resolution of layers that would be poorly resolved locally. SCI produces laterally smooth results with sharp layer boundaries that respect the 3D geological variations of sedimentary settings. SCI also suppresses the elongated artifacts commonly seen in interpretation results of profile-oriented data sets. In this study, SCI is applied to airborne time-domain EM data, but it can also be implemented with other ground-based or airborne data types.

Resistivity structure of Unzen Volcano derived from time domain electromagnetic (TDEM) survey

Time-domain electromagnetic (TDEM) surveys were carried out around Unzen Volcano, Shimabara Peninsula, South-west Japan in 2001 and 2002 in the eastern part of the peninsula. The surveys were a completion of the previously interpreted TDEM data collected from surveys in 1995 in the western part of the peninsula. Interpretation of the recorded transient magnetic fields resulted in a general feature of the resistivity structure which are composed of three main layers: a resistive surface layer, a conductive layer (below 10 Ωm) from about sea level to 2.5 km below the surface, and a third resistive layer. The conductive layer is considered to be a complex of water-saturated and altered layers which spreads widely beneath Shimabara Peninsula. The spatial conductance distribution shows a W–E trending pattern of high conductance from Tachibana Bay to the summit of Unzen (Fugen–Dake). This evidence indicates that volcanic gas was supplied to shallow layer in the western part of the peninsula, which is in accordance with the path of magma migration derived from seismic and deformation studies. Another high conductance zone was also found near Mayu–Yama in the eastern part of Shimabara Peninsula, indicating magma degassing process due to the possibility of the presence of a magma system beneath it.