Thin Sheet Conductor Research Articles

Quantifying Airborne Induced Polarization effects in helicopter time domain electromagnetics

This paper derives the Airborne Induced Polarization (AIP) response of an airborne electromagnetic (AEM) system to a horizontal, thin sheet conductor. A vertical component double-dipole approximates helicopter systems with towed concentric horizontal transmitter and receiver loops in frequency- or time-domain. In time domain, the AIP effect typically shows up as late-time negative data with amplitude 4 to 5 orders of magnitude smaller than the early-time peak of the positive AEM responses. Because of limited bandwidth from the short sample time after the decay of inductive responses, accurate extraction of intrinsic AIP parameters other than a minimum chargeability is almost impossible. Modelling further suggests that AIP effects in double-dipole AEM systems can only be reliably detected from polarizable material in the top few tens of metres. A titanium mineral exploration case history from the Lac Brûlé area, Quebec, Canada illustrates strong spatial coherence of AIP minimum chargeability estimates and their independence from other effects such as conductivity and magnetic susceptibility.

Rapid detection and classification of airborne time-domain electromagnetic anomalies using weighted multi-linear regression

We propose a rapid and efficient methodology for the detection and interpretation of airborne time-domain electromagnetic anomalies generated by thin sheet-like volcanogenic massive sulphides (VMS) deposits in a resistive environment, which are representative of VMS deposits in the Canadian Shield.In the first step of the approach, we use high-order statistics for the detection and the recognition of a MEGATEM anomaly as indicating a thin sheet-like VMS deposit with respect to three criteria of detection: the minimum level of detection, the length of detection, and the coherence of detection over time. We adapt these criteria in order to optimise the detection of thin sheet-like VMS deposits against geological noise models. Once the anomaly is detected and recognised as the response to a thin sheet conductor, we interpret the model geometry and physical property using attributes calculated from the MEGATEM anomaly. We develop a system of weighted multi-linear regression to find the most significant attributes to estimate the dip, depth, conductance, and dimensions of a thin sheet-like VMS deposit. Stepwise regression suggests that shape attributes are most significant to estimate dip while depth is most strongly estimated by size attributes. The most significant attribute to estimate the conductance is the time constant. The size is best estimated by attributes related to the size of the anomaly. We test the regression system on thin sheet models with excellent performance. Most of the parameters of the thin sheet models were estimated within an interval of confidence about the initial property. We further test the system by estimating properties of three VMS deposits in the Abitibi Greenstone Belt, Québec, Canada, for which the geometries and geological properties are known. Most parameters are estimated within the interval of confidence for ISO, a thin sheet body, while the estimates for New-Insco and Gallen show more variability caused by departure from the reference thin sheet model.

Depth sounding by means of a coincident coil frequency‐domain electromagnetic system

The concept of electromagnetic depth sounding by means of a coincident‐coil frequency‐domain electromagnetic system is developed in theory and demonstrated by means of physical scale modeling. The concept is based on the use of distance from the target as the sounding variable. The theoretical developments are confined to soundings conducted in free‐space with respect to either a homogeneous half‐space or a thin sheet conductor in conditions that approach the resistive limit. The use of distance from the target as the sounding variable becomes practical when the sounding system is a single compact unit of the type that a coincident coil concept inherently provides. In this method of sounding, the distance from the target is determined by taking the ratios of the fields measured at a variety of distances from the target conductor. This permits not only the distance to the target to be determined but also the direction to that target as may be of interest in soundings conducted in mines.

Numerical estimation of the sea effect on the distribution of induction arrows in the Japanese island arc

Anomalous distribution of induction arrows in the Japanese island arc has been supposed to be due, at least partly, to induced electric currents flowing in the surrounding seas. None the less, no reliable estimation has been made of this regional problem. Here we attempt to solve the problem, using the algorithm of bimodal electromagnetic induction in a thin-sheet conductor. The non-uniform conductance of the thin sheet corresponds to seawater depths in the surrounding seas. It is found out that induction arrows are controlled strongly by the open seas such as the Pacific Ocean. The effect of the semi-closed Japan Sea is somewhat weaker, and is dependent on period. A detailed comparison between the calculated and the observed induction arrows over the Japanese island arc indicates, however, that the sea effects alone cannot fully account for the observations. The differences between the observed and the calculated responses delineate lateral inhomogeneities in the subsurface conductivity structure which are not included in the present model.

Interfacing the general 3D A- psi method with a thin sheet conductor model

A finite-element scheme for modeling 3-D eddy currents in general thick conductors and thin sheet conductors which can be in electrical contact is described. Volume eddy currents are modeled in terms of the magnetic scalar potential A and sheet eddy currents are modeled in terms of a scalar stream function T. Nonconducting regions are modeled using magnetic scalar potentials. The interface between the general thick conductors and the thin sheet conductors is described. The method has been implemented in the 2- and 3-D electromagnetics software package MEGA. The method yields good results for all cases tried in which the assumption of uniform distribution across the thickness of the sheet is reasonable. Results are presented for tokamak poloidal field coils. >

Transient 3D eddy currents in thin sheet conductors

A method for solving problems involving time-transient eddy-current flow in thin sheet conductors is described. The sheets can have arbitrary shape and varying thickness and conductivity, but their thickness must be small compared to the skin depth of the material. A 3D differential-equation-based finite-element scheme is used. Nonconducting regions are modeled in terms of magnetic scalar potentials, and thin conducting sheets are modeled in terms of a scalar stream function.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Finite element method for 3D eddy current flow in thin conducting sheets

A finite element method for modelling power frequency three-dimensional eddy current distributions in thin-sheet conductors is introduced. The thin sheets may be of arbitrary shape and spatially varying in thickness and conductivity, but must be thin compared to the skin depth of the material at the excitation frequency. A symmetric sparse set of equations is obtained and solved using the preconditioned, biconjugate gradient technique.

3D eddy currents in multiply connected thin sheet conductors

A method for calculating 3D eddy current distributions in thin sheet conductors of arbitrary shape and of small thickness relative to the skin depth of the material at the excitation frequency is described. The regions surrounding the conducting sheets are modelled using magnetic scalar potentials and the sheet current is represented by a scalar stream function. The field equations are in differential form and are modelled numerically using finite elements. The resulting sparse symmetric set of equations is solved using the pre-conditioned bi-conjugate gradient technique. Discontinuities in sheet conductivity and thickness can be modelled. This is used to simulate the effect of holes in the sheets.

INTERPRETATION OF PULSE EM ANOMALIES OVER GANI CONDUCTORS, KURNOOL, INDIA*

AbstractA survey using Crone pulse electromagnetic (PEM) equipment was conducted near the village of Gani in Andhra Pradesh, India. The anomalies obtained had a shape typical for an inclined sheet conductor. From the multichannel TEM response, the multifrequency Slingram response was calculated.A comparison of the Slingram response of the Gani conductors with the free‐air model response of a thin sheet conductor shows phase rotation and attenuation of the anomaly vector, thereby confirming the presence of a conductive overburden. Interpretation of the Gani results using free‐air phasor diagrams causes an overestimation of depth and conductance of the target conductor unless data obtained at a sufficiently low frequency are used for the interpretation. For areas like Gani this frequency can be as high as 336 Hz.

Some thin sheet responses to a model dipole—dipole prospecting system

The theory for scaling electromagnetic systems is applied in the construction of a model for the Slingram dipole—dipole prospecting technique. The horizontal and vertical components for responses of vertical and horizontal thin-sheet conductors are obtained. The effect of conductor size on the response is studied. Examples of profiles are presented and results from a range of response parameters and depths are condensed into phasor diagrams. Some qualitative and semi-qualitative comments are made on these results.