Abstract

Electromagnetic sounding in the frequency domain can be performed in two ways--either by changing frequency at a location (frequency sounding) or by changing the transmitter-receiver (T-R) separation using a fixed frequency (geometric sounding). These changes in frequency or separation parameters effect vertical scanning of conductivity distributions below the earth's surface. In case of thin conducting layers, there could be an optimum range of frequencies or T-R separations that provide maximum resolution of the layer parameters. Thus, for a given buried target layer, it should be possible to find ranges of frequencies or separations that yield the best focusing. The present study deals with the focused resolution of a thin conducting layer in frequency sounding with variable T-R separation for four different dipole configurations.It is observed from the inversion of the data from various dipole electromagnetic (EM) systems that different T-R separations have different resolutions for the same target layer. It is also observed that for a particular loop system, the best resolution is observed at an optimum T-R separation. The resolution becomes poorer when the T-R separation is either increased or decreased from this particular separation. Thus it has been possible to propose a 'zone of focusing' for various dipole EM configurations. The study reveals that this zone is broadest for the horizontal coplanar loops system, implying that this system yields good resolution over a wide range of T-R separations. Compared to this, the perpendicular loops system yields a very sharp peak implying that it resolves the target over a very narrow range of separations. However, the perpendicular loops system provides resolution most parsimoniously requiring the least T-R separation. This is followed by the vertical coplanar, vertical coaxial, and horizontal coplanar loops systems.

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