Transmitter Loop Research Articles

Explicit observation of tertiary magnetic field in TDEM transients; plausible explanation towards induced polarization effect in geophysical prospecting

In time domain electromagnetic central loop sounding or coincidence loop measurements often a sign reversal is observed in the recorded decay transients. These negative decay transients are generally attributed to induced polarization effect. We observed a similar sign reversal in the decay transients recorded with SQUID as a receiver in central loop sounding measurements at later decay times. The magnitude of the secondary magnetic field observed in the negative decay transients is considerably high and the SQUID output never reaches its noise floor even at decay times of >155 ms. This observation stimulated us to record the current flow in the transmitter loop and therefore a small resistor is connected in series with the transmitter loop. Decay transients have been recorded simultaneously across the resistor and the SQUID which is located at the centre of the transmitter loop. To our surprise, the time at which the sign reversal occurred in the recorded decay transients with SQUID and across the resistor confirmed that the SQUID is recording the magnetic field produced by the flow of the induced current in the transmitter loop in addition to the decay of the secondary magnetic field from the ground. A novel hypothesis has been proposed to explain the flow of induced current in the transmitter loop and extensive laboratory experimental simulations have been performed to prove it. The proposed novel hypothesis explains the occurrence of sign reversal in TEM systems not only in single loop but also in coincidence loop and central loop TEM configurations. In all cases, the sign reversal is due to the self induced tertiary magnetic fields. In this work, a set of transient measurements have been carried out with different targets and different thicknesses. The experiments have been repeated by locating the targets at different depths in order observe the response of the tertiary magnetic field in the decay transients. The experimental data and subsequent analysis confirmed our hypothesis that the self induced tertiary magnetic field is responsible for the sign reversals rather than the induced polarization effects.

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

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

An information theoretic Bayesian uncertainty analysis of AEM systems over Menindee Lake, Australia

SUMMARY Long-range, active-source airborne electromagnetic (AEM) systems for near-surface conductivity imaging fall into two categories: helicopter (rotary-wing) borne or fixed-wing aircraft borne. A multitude of factors such as flying height, transmitter loop area and current, source waveforms, aerodynamic stability and data stacking times contribute to the geological resolvability of the subsurface. A comprehensive comparison of the relative merits of each system considering all such factors is difficult, but test flights over well-constrained subsurface geology with downhole induction logs are extremely useful for resolution studies. However, given the non-linear nature of the electromagnetic inverse problem, handling transmitter–receiver geometries in fixed-wing aircraft is especially challenging. As a consequence of this non-linearity, inspecting the closeness of downhole conductivities to deterministic inversion results is not sufficient for studying resolvability. A more comprehensive picture is provided by examining the variation in probability mass of the depth-wise Bayesian posterior conductivity distributions for each kind of AEM system within an information theoretic framework. For this purpose, probabilistic inversions of data must be carried out. Each acquiring system should fly over the same geology, survey noise levels must be measured and the same prior probabilities on conductivity must be used. With both synthetic models as well as real data from over the Menindee calibration range in New South Wales, Australia, we shed new light on the matter of AEM inverse model uncertainty. We do this using two information theoretic attributes derived from different Kullback–Leibler divergences—Bayesian information gain, and a strictly proper scoring rule, to assess posterior probabilities estimated by a novel Bayesian inversion scheme. The inversion marginalizes fixed-wing geometry attributes as generic nuisance parameters during Markov chain sampling. This is the first time-domain AEM study we know of, that compares nuisance marginalized subsurface posterior conductivities from a fixed-wing system, with rotary-wing derived posterior conductivities. We also compare field results with induction log data where available. Finally, we estimate the information gain in each case via a covariate shift adaptation technique that has not been used before in geophysical work. Our findings have useful implications in AEM system selection, as well as in the design of better deterministic AEM inversion algorithms.

Interpreting Surface Large-Loop Time-Domain Electromagnetic Data for Deep Mineral Exploration Using 3D Forward Modeling and Inversion

Surface transient electromagnetic (TEM) data with large transmitter loops for deep mineral exploration are often complicated by the non-trivial coupling between extended sources and arbitrarily oriented geological targets. This case study reports a TEM field data set acquired across terranes with strong lateral inhomogeneity, which is responsible for the high inconsistency in TEM data patterns along the survey line, as well as for the negative TEM transients (sign reversal) at some near-central loop stations. 3D forward modeling and inversion, as maturing tools in recent years, offer unique opportunities to extract as much geological information from such data as possible. 3D forward simulations of representative synthetic models found that the phenomenon of sign reversal at some TEM stations is associated with compact conductors enclosed by the transmitter loop and receivers that are in the loop, but off the conductor–a situation that is common in large-loop TEM and can only be explained by 3D models. However, 3D inversion of the field data with a uniform subspace as the initial and reference model fails to converge, another point of evidence that 3D inversions of large-loop TEM data are more likely to be subject to stability issues. Our solution is to warm-start the inversion with the representative model in the forward simulation experiments as the initial model, so the ill-posed 3D inversion can escape from local minima. Finally, the vertical contact structure in our 3D-inversion model is verified by a resistivity cross section of the CSAMT method. Our case study demonstrates the demand and capability of 3D electromagnetic modeling and inversion for high-resolution deep mineral exploration. It also provides an easy-to-follow template for carrying out 3D interpretation for complex geology in practice.

Highly-Accurate Temperature-Compensated Pneumatic Measurement Using Piezoresistive Pressure Sensor

Pneumatic cylinder is known as one of the major transmission power sources in either machinery or automation operation. However, the sensor used in the pneumatic measurement is sensitive to harsh temperature environments. For this reason, a high precision pneumatic measurement using piezoresistive pressure sensor with temperature compensation is proposed in this paper. The pneumatic signal is read and amplified via A/D converter of the microprocessor. The relationship of pneumatic signal maximum level over temperature influence is also built up for a compensation purpose. A human-machine interface (HMI) with the computer is designed for real-time accurate calibration via RS-485. Based on a current loop transmitter, the measured pneumatic signal can be converted into an industrial standard (i.e., 4-20mA output). The experimental results verify that the proposed system can achieve an accurate measurement outcome below 0.5% error in both HMI and output current under a variety of temperature conditions.

Optimal design of classic RCD absorption circuit in transient electromagnetic transmitter

Abstract The classic RCD absorption circuit has been applied to shorten the turn-off time of transmitters. The current way to determine the resistor and capacitor values of the classic RCD circuit is inefficient. In this paper, an intelligent optimization method to design the classic RCD absorption circuit is proposed. First, based on Kirchhoff's law, the working principles of the RCD absorption circuit at three phases are mathematically analyzed. The formulations of current and voltage at each phase are derived. Next, the shortest turn-off time and its corresponding capacitance and resistance are determined using modeling. Finally, tests are carried out with the transmitting board and antenna to verify the simulation results. Compared with existing method, this new method is easier to operate, and it is more efficient at getting a accurate minimum turn-off time for a given transmitter loop.

Development of SQUID based TDEM system and its utilization for field survey at Tumallapalle, Andhra Pradesh, India

This paper describes the development of low Tc SQUID (Superconducting Quantum Interference Device) based TDEM (Time Domain Electro-Magnetic) system for geophysical applications. The SQUID system has been used to perform central loop TDEM sounding measurements near Tumallapalle Uranium mine, Cuddapah (dt), AP, India, where the terrain consists of a thin conducting layer below which low grade Uraniferous Phosphatic Siliceous Dolostone is present. Locating the thin conducting layer which is covered by a thick and highly resistive layer is a challenging task due to the induction of weak eddy currents. In order to enhance the resolution of the target located at larger depth, it is essential to apply larger transmitter moment and use a sensitive receiver. In this work, we combine the use of sensitive SQUID sensor and transmitter loop with larger size to achieve better target resolution. The survey has been performed using transmitter loop sizes of 100 m × 100 m and 400 m × 400 m. In order to compare the performance of the SQUID based TDEM system, the same experiments have also been repeated using induction coil as receiver which has been built for this work. In this article, we describe in detail the results obtained from the central loop TDEM measurements.

Modeling and Inversion of Airborne and Semi-Airborne Transient Electromagnetic Data with Inexact Transmitter and Receiver Geometries

Airborne and semi-airborne transient electromagnetic (TEM) surveys have high efficiency but may suffer from systematic errors due to the inexact shape, position, and orientation of the transmitter and receiver, which can deviate from the nominal design because of complex terrain, platform instability, or external forces. Without considering actual survey geometry, modeling and inversion can bias the interpretation of results. We develop a universal approach to layered earth capable of modeling arbitrarily complex transmitter and receiver geometry used in airborne and semi-airborne surveys. Our algorithm decomposes an airborne loop or grounded wire source to a set of x-, y-, or z-oriented electric dipoles. An arbitrarily oriented receiver coil is simulated by projecting three-component data to the actual direction of receiving. In airborne TEM, the transmitter loop and receiver coil are often bound together on a rigid frame and tilt during the flight. Our simulations and synthetic inversion show that such a tilt may reduce responses relative to the data obtained with the nominal geometry; an inversion without considering the tilt can underestimate near-surface conductivity. In semi-airborne TEM, the transmitter wire on the surface can be crooked, and the airborne receiver coil can also tilt. Our modeling shows that the simulated data can change significantly if the actual transmitter and receiver geometry does not exactly follow the nominal survey design; if not appropriately accounted for, such an error may distort the recovered conductivity model. Finally, the benefit of our algorithm is demonstrated by an airborne TEM field data inversion of groundwater problems with the tilt angle of the transmitter–receiver frame accurately modeled. Our work provides a tool for improving the resolution of airborne and semi-airborne TEM in near-surface conductivity characterization.

Drone-based electromagnetic survey system for geophysical applications

Geophysical electromagnetic (EM) methods are used in geological mapping, mineral exploration, groundwater studies and geotechnical investigations. Airborne EM methods have the benefit of avoiding terrain obstacles such as lakes, rivers, swamps, and ravines. Compared to manned aircrafts, drones or unmanned aerial vehicles (UAVs) have benefits of their own. Drone-based surveys are versatile, fast to deploy, economical and ecologically more friendly. Presently, magnetic surveying is the only geophysical method that is routinely conducted with drones. The modest maximum payload limit of drones imposes severe restrictions on the applicability of other methods including EM and radiometric methods, for example. Finnish company, Radai Ltd has been developing Louhi, a novel drone-based frequency-domain EM survey system, in an EU funded Horizon 2020 project NEXT – New Exploration Technologies. The EM system has two operation options – the first uses a large loop on the ground as an EM source and the other uses a small portable EM transmitter loop. Both systems utilize a stand-alone and light-weight three-component EM receiver that can be towed by a drone. This article presents the theoretical background of the EM methods, the solution developed by Radai Ltd, the current version of the EM device, and results from field and flight tests that demonstrate the applicability of the drone-based EM system under development.

A Flexible Single Loop Setup for Water-Borne Transient Electromagnetic Sounding Applications.

Water-borne transient electromagnetic (TEM) soundings provide the means necessary to investigate the geometry and electrical properties of rocks and sediments below continental water bodies, such as rivers and lakes. Most water-borne TEM systems deploy separated magnetic transmitter and receiver loop antennas—typically in a central or offset configuration. These systems mostly require separated floating devices with rigid structures for both loop antennas. Here, we present a flexible single-loop TEM system, the light-weight design of which simplifies field procedures. Our system also facilitates the use of different geometries of the loop antenna permitting to adjust the depth of investigation (DOI) and the minimum sounding depth in the field. We measure the turn-off ramp with an oscilloscope and use the DOI to assess the minimum and maximum exploration depth of our single-loop TEM system, respectively. A reduction of the loop-antenna size improves early-time TEM data due to a reduced length of the turn-off ramp, whereas an increase of the loop-antenna size enhances the signal strength at late times, which allows to investigate deeper structures below the lake bed. We illustrate the capabilities of our system with a case study carried out at Lake Langau in Austria. Our results show that our system is capable of reaching a DOI of up to 50 (with a maximum radius of the circular loop of ), while it also resolves the water layer down to a minimum thickness of (when the radius is reduced to ).

State‐of‐the‐art geophysics for metal exploration

AbstractThe new discovery of the economical metal deposit is getting more difficult year by year because the unexplored areas tend to be more remote areas than ever, and the metal prospective zones are deepening in the recent decades. Therefore, the advent of the state‐of‐the‐art or advanced geophysics technology to peer the deeper parts of the ground with higher accuracy and spatial resolution has been aspired in metal exploration industry. The author reports and introduces the drone‐based magnetic survey, the three‐dimensional time‐domain IP inversion algorithm using Cole–Cole parameter, the time domain electromagnetic method using superconducting quantum interference device (SQUID), and passive seismic survey as the state‐of‐the‐art or advanced geophysics technology for metal exploration.In the early stage of metal exploration, airborne magnetic survey has been used in a wide variety of exploration programs for large areas to map basement structures and to delineate other magnetic structures. The drone‐based magnetic survey is recently getting prevailing in the metal exploration because it can be conducted at much lower cost with simpler clearance for its flight by the governmental authority, and with higher spatial resolution than the conventional fixed‐wing or heliborne magnetic survey.In the target identification stage, induced polarization (IP) method has been recognized as one of the extremely powerful tools in the discovery of many new metal deposits. The historical various studies on complex resistivity measurements for metal sulphides, graphite, magnetite and pyrrhotite have revealed that there are significant differences in the spectral responses among metal sulphides, graphite, magnetite and pyrrhotite which is capable of being parametrized using the empirical Cole–Cole parameter model. JOGMEC developed the three‐dimensional time‐domain IP inversion code using Cole–Cole parameter model so as to interpret the difference of IP sources using Cole–Cole parameters, and has applied this code to its exploration activities. Transient electromagnetic method (TEM) also has a well‐established place in metal exploration. Since recent exploration targets are generally concealed by thick conductive overburdens such as weathered layers and black shale, these geological covers preclude induction currents generated by the TEM transmitter loop on the surface from diffusing into the exploration targets plentifully beneath the conductive covers. B‐field measurement using SQUID magnetic sensor is one of the technologies which enable TEM to investigate deeper than dB/dt measurement by the conventional induction coil system, because B‐field decays slower than dB/dt and time decay of B‐field reaches the system noise floor at later time than dB/dt if dynamic ranges of the both systems are almost identical. JOGMEC had developed the TEM system using a high‐temperature superconductor (HTS) SQUID, and has been using it in its metal exploration.The other current trend of mining geophysics is application of seismic method to metal exploration. In the target identification stage, deep geochemistry using aircore drilling is much utilized over the areas covered by transported cover. In order to design the aircore drilling survey, passive seismic survey has been utilized to evaluate thickness of transported cover in order to make plans of specification (length) of the aircore drilling survey, which is also a new trend in mining geophysics.

Full‐wave modelling of early‐time measurements in time‐domain electromagnetics: Consideration of coupling between Tx–Rx antennas and the ground

ABSTRACTEarly‐time measurements of time‐domain electromagnetics carry information of the near surface, and therefore, the interpretation of such measurements is important, particularly when utilizing small‐loop acquisition devices. However, these early‐time gates are often distorted by responses from the acquisition system itself, primarily caused by self‐transients associated with the transmission link composed of the transmitter and receiver loops and with the investigated medium electrical properties. Estimating all such potential couplings is crucial for interpreting early‐time measurements and gathering information on the near surface. In this study, a receptor loop made of 20 turns was built for this purpose. Field results and simulations were then compared, which shows how full‐wave finite‐difference method in the time‐domain simulations can be used to accurately calculate the impedance characteristics of time‐domain electromagnetics systems. All simulations were performed using finite‐difference time‐domain software based on a fully explicit three‐dimensional solver. First and second sets of simulations, respectively, were carried out in free space to compare the results to an alternate simulation method with equivalent lumped electrical circuits and on a homogeneous half‐space with varying electrical resistivities. The results were then used to improve the inversion of field data acquired on a test site in Garchy, France.

A self-decoupled 32-channel receive array for human-brain MRI at 10.5 T.

Receive array layout, noise mitigation, and B0 field strength are crucial contributors to SNR and parallel-imaging performance. Here, we investigate SNR and parallel-imaging gains at 10.5 T compared with 7 T using 32-channel receive arrays at both fields. A self-decoupled 32-channel receive array for human brain imaging at 10.5 T (10.5T-32Rx), consisting of 31 loops and one cloverleaf element, was co-designed and built in tandem with a 16-channel dual-row loop transmitter. Novel receive array design and self-decoupling techniques were implemented. Parallel imaging performance, in terms of SNR and noise amplification (g-factor), of the 10.5T-32Rx was compared with the performance of an industry-standard 32-channel receiver at 7 T (7T-32Rx) through experimental phantom measurements. Compared with the 7T-32Rx, the 10.5T-32Rx provided 1.46 times the central SNR and 2.08 times the peripheral SNR. Minimum inverse g-factor value of the 10.5T-32Rx (min[1/g] = 0.56) was 51% higher than that of the 7T-32Rx (min[1/g] = 0.37) with R = 4 × 4 2D acceleration, resulting in significantly enhanced parallel-imaging performance at 10.5 T compared with 7 T. The g-factor values of 10.5 T-32 Rx were on par with those of a 64-channel receiver at 7 T (eg, 1.8 vs 1.9, respectively, with R = 4 × 4 axial acceleration). Experimental measurements demonstrated effective self-decoupling of the receive array as well as substantial gains in SNR and parallel-imaging performance at 10.5 T compared with 7 T.

Three-dimensional finite-difference modeling of time-domain electromagnetic responses for a large-loop source

The time-domain/transient electromagnetic (TEM) method employs an alternating magnetic field that is established by passing a bidirectional pulsed current through a transmitter loop. A large wire loop on or above the earth is one of the most widely used sources in TEM survey systems. To simulate TEM responses of the large loop system, the effect of the loop size must be considered in a numerical modeling scheme. We have developed an efficient three-dimensional (3D) TEM modeling algorithm using a staggered-grid finite-difference time-domain (FDTD) method. In this paper, we first derive FDTD TEM equations using fictitious displacement currents based on an explicit mid-point method. Then, we employed a conversion method to efficiently obtain stepoff TEM responses from step-on responses. Further, following the concept of the conversion method, the charging and discharging processes of current waveform in loop does not need to be identical, implying we can apply the simulation scheme for more general source waveforms. The developed algorithm has been verified through comparison of numerical results for a homogeneous halfspace medium. Finally, we analyze TEM responses generated from a large loop source for a 3D conductive body buried in a homogeneous half space.

Detection Capability and Application Analysis of the Small-Loop Transient Electromagnetic Method

When working in the areas with strong interference and limited sites, such as cities, dams, and mountainous areas, using the conventional transient electromagnetic (TEM) devices is difficult to lay out large transmitter loops for conventional surveys. The small-loop transient electromagnetic method, which has the advantages of lightweight and high efficiency, is a new technology of current researches. First of all, A one-dimensional (1-D) layered model and a three-dimensional (3-D) homogeneous half-space model were established by using numerical simulation method. The research results show that the small-loop TEM devices can not only accurately reflect the anomaly information of deep targets, but also be more sensitive to the detection of shallow targets. By using high-current technology, the signal intensity and detection depth of multi-turn small-loop TEM devices and that of large loop device are the same. Finally, the accuracy and effectiveness of small-loop devices were verified through analysing the results of dam karst investigation. The research work provides a reference for the qualitative analysis of response characteristics and data interpretation of small-loop TEM.

Inductively Induced Polarization and Its Overall Assessment Using a Normalized Transient Response

Abstract —The paper considers the basic regularities of inductively induced polarization (IIP) manifestation in the transient electromagnetic method. When calculating inductive transient responses of a polarizable ground, we assumed that the electrical conductivity of geologic materials is described by the Cole-Cole model. To present and analyze the results of computer-based simulation of the IIP phenomenon, we used a normalized transient response. It is defined as the ratio of the transient electromotive force (EMF) response of a polarizable ground to the EMF response of a ground that differs from the polarizable one only in that it has zero chargeability. The coordinates of the minimum of the normalized transient response are useful for an overall assessment of IIP manifestation. We show, by the example of a homogeneous polarizable half-space, how the inductive transient response is affected by the Cole-Cole model parameters and the size of transmitter and receiver loops. For a two-layer ground, IIP manifestation depends also on whether the base or upper layer is polarizable, as well as on the layer thickness. Inductively induced polarization is most pronounced when a conductive polarizable layer overlies a resistive nonpolarizable base. In this case, at a certain thickness of the layer, the IIP effect far exceeds that observed in the presence of a thick polarizable layer and even of a homogeneous polarizable half-space.

Tunnel Magnetic Resonance Tomography for 2-D Water-Bearing Structures Using Rotating Coil With Separated Loop Configuration

In tunnel construction, to prevent the occurrence of water inrush, the geological conditions of faults and underground rivers must be determined in advance. As a direct detection method of groundwater, magnetic resonance sounding (MRS) has been applied for the advanced detection of water-related hazards in tunnels and mines recently. However, the results of conventional 1-D MRS cannot correctly reflect the spatial distribution characteristics of complicated water-bearing structures. In this article, we propose a measurement scheme using a rotating coil with separated transmitter and receiver loop configuration (SEP) for the 2-D imaging of water-bearing structures, such as faults and conduits. In this scheme, the receiver coil rotates several times, while the transmitter coil, which is separated from the receiver coil by a certain distance, remains stationary. Moreover, all the observed data participate in the inversion to achieve the 2-D magnetic resonance tomography (MRT). Numerical simulations of water-bearing faults are performed, and we compare and analyze the 2-D sensitivity and inversion results of two measurement schemes for a rotating coil, i.e., an overlapping transmitter and receiver loop configuration (OVE) and SEP. The results showed that the imaging results of the SEP are better than those of the OVE because the OVE imaging has a symmetric artifact. Finally, we discuss the influence of the transceiver distance, the resistivity, and the environmental noise on the imaging results. Moreover, the imaging results of the water-bearing conduit at different locations were obtained to validate the effectiveness of the SEP rotating coil measurement scheme.

Non-remote reference noise cancellation - using reference data in the presence of surface-NMR signals

Surface-NMR measurements commonly suffer from low signal-to-noise ratios. In recent years, with the introduction of multi-channel surface-NMR instruments, the technique of remote-reference noise cancellation (RNC) was developed and significantly improved the applicability of surface-NMR. The current formulation of RNC requires a reference loop to be placed a remote distance from the transmitter loop such that no NMR signal is recorded. Reference loops placed at non-remote distances have been envisaged to provide both improved noise cancellation performance and field efficiency; however, the concept has not been previously applied because the theoretical framework was missing. In this paper, the theoretical framework is presented. It is demonstrated that reference loops placed at non-remote distances provide superior noise cancellation performance. Considerations for placing the reference loop relative to the transmitter loop are provided, and the theoretical framework is evaluated based on a semi-synthetic example using real field noise and synthetic surface-NMR data.

Improving the accuracy of 1D surface nuclear magnetic resonance surveys using the multi-central-loop configuration

Surface nuclear magnetic resonance is a near-surface geophysical method for characterizing the spatial distribution of liquid water in the top 100 m of the subsurface. The recovered water content models are obtained through the solution of an ill-posed inverse problem that is a function of acquisition parameters, including location and shape of the transmitter and receiver coils. In this paper, we introduce the multi-central-loop acquisition and inversion strategy where one or several smaller receivers coils are placed in the center of the larger transmitter loop and where all the data sets synchronously recorded through each loop are inverted simultaneously. We investigate the attributes of this acquisition and inversion strategy including the ability to provide improved resolution, accuracy and reduced uncertainty on the estimated subsurface models compared to single channel acquisition methods. Using widely-adopted inversion methods and introducing a new data interpretation technique called Bayesian Evidential Learning 1D imaging, we show that the multi-central-loop configuration provides improved recovery of synthetic models and reduced levels of inverted parameter uncertainty. A field case is also presented where the multi-central-loop results appear to better match the lithologic knowledge of the area compared with single channel configurations, again providing smaller uncertainties.

Differential Target Antenna Coupling (DTAC) EM Surveying with Stationary Transmitter Loop and Moving In-Loop Receivers

Following our previous studies of the Differential Target Antenna Coupling (DTAC) method with horizontal and vertical arrays for EM surveys, in this paper we study the application of the DTAC method to a different configuration, where a large, stationary transmitter loop is on the ground surface. We then run profile lines inside this loop. The DTAC method is effective in eliminating errors due to the large variations in the primary field along profile lines within the transmitting loop. Operational tests show that we obtain more diagnostic DTAC anomalies over buried targets than using just the Bx and By data. The DTAC method also produces smaller false-alarm targets due to background geology variations, compared with Bz measurements. The DTAC method can be used with either time- or frequency-domain data and the receiver can be moved on the ground or deployed from an airborne vehicle, such as a drone.