MODELS IN APPLIED GEOELECTROMAGNETICS

Abstract

"Theoretical analysis of the electromagnetic induction in conducting models of simple geometry and scale-model experimental investigation over relatively more realistic and analytically intractable models have been carried out. Whilst the former is useful in elucidating the physical principles, the latter additionally provides a quantitative basis for the interpretation in Exploration Geoelectromagnetics. At the outset, the principles of induction prospecting and procedures of data interpretation are briefly stated. Some problems which provided incitement for the present work are also mentioned. In order to gain a useful physical insight into the induction processes in extended objects, an analysis of quasi-static induced currents in a two-layer spherical model has been made. The numerical computation of their spatial distribution shows that the amplitude of the current density in the core decreases due to the presence of a conducting cover. An apparent enhancement of the in-phase component is, however, caused because of the phase rotation of the current vector by the cover. Current density maxima in the core and the cover are found

to occur for some characteristic frequencies. A reduction of the current density in the less conducting cover with increase in the conductivity of the core is also noticed. This apparent redistribution of the induced currents within the composite system is of greater significance in the case of elongated conductors as evinced by the results of scale-model experiments. Effect of the inhomogeneity in the conductivity has also been examined in some cases. The rnaximuin value as well as the variation of the vertical component of the scattered field along a profile over a similar spherical model has also been calculated. Besides conforming to the analysis of the induced currents, investigations on the scattered field bring out an apparently paradoxical phenomenon that for certain induction number of the cover the in-phase component of the response reduces with the increase of the core conductivity. The anomaly profiles also reveal that for a frequency band, whose value is governed by the conductivity values of the system, the electromagnetic response of a homogeneous sphere may be greater than that of a layered one of same size but enclosing a concentric core of higher conductivity. A multi-frequency response analysis is found to help delineate the layering of the spherical system. Analytical expressions for the response of a highly conducting vein embedded in a partially conducting half-space and overlain by a conducting overburden have been obtained using the Wifcner-Hopf technique. The responses of more realistic and complex model set-ups have been investigated through experimental simulation. The principle of physical simulation is discussed in brief. A possibility of simulating anisotropic media is also outlined. Salient features of the experimental set-up including the transmitting and receiving equipment have been described. A fast and continuous measurement of the anomaly along a profile over a conducting target has been achieved through the method of sampling the signal induced in the receiving coil. Investigations of the multi-frequency electromagnetic response of targets of various shapes, sizes and conductivities have been done. The effect of conductive as well as nonconductive contact between the target and the surrounding medium on the response of the former has been studied. The response of a target is appreciably modified due to its conductive contact with the solution. In the case of elongated targets a pronounced enhancement of the response is observed when the target is immersed in a conducting solution. The effect may be ascribed to the 4 - collection of more current lines by the better conducting target. However under similar conditions, the response of symmetrical bodies like spheres do not change significantly. In the case of horizontal sheet-type models even the shape of the anomaly is changed due to conductive contact with the surrounding medium making it appear thinner and of greater depth extent. The existence of maxima in the curves which depict the variation of the enhancement ratio with the conductivity contrast between the target and the surrounding medium indicates the zones of greater effect of the surrounding medium. In cases when the conducting overburden is insulated from the target, a reduction of the response vector and its rotation in time-phase is obtained. The dependence of the variation of response on the shape of the target and the type of contact with the surrounding medium and/or the overburden introduces an ambiguity in the interpretation. Furthermore, because of the phase-change in the anomaly vector, the in-phase and quadrature components of the response are modified in different proportions causing a substantial change in their ratio i.e. the induction index. The induction index is found to vary with change in depth of burial also. A first order classification of anomalies which is usually

done on the basis of induction indices will, therefore, no more be valid in such cases. Finally, the response of closely spaced conductors which are known to occur in nature frequently, has also been investigated. A minimum distance between neighbouring bodies has been identified for which their composite response is almost a geometrical superposition of the individual responses. At closer distances between them the anomaly profiles appear like that of a single body owing to the mutual inductive interaction which precludes their resolution. At such a close spacing of the bodies as this, their combined response may even be less than the response of the either and hence the estimated conductivities in such cases will be on the lower side. The anomaly profiles and the anomaly index diagrams presented in the thesis are expected to provide a quantitative basis for the interpretation of the induction prospecting data over some pertinent geological situations."