COMPARISON OF AIRBORNE EM COIL SYSTEMS PLACED OVER A MULTILAYER CONDUCTING EARTH

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Abstract
A detailed analysis has been made on the relative effectiveness of the four types of coil arrangements used in airborne electromagnetic surveying for the mapping of subsurface inhomogeneities. By comparing the theoretical responses for the four systems over known geologic sections, it has been established that the horizontal coplanar system is the most effective for detecting inhomogeneities in single‐frequency airborne surveys and produces the largest anomaly in multifrequency work. The next largest anomaly in both the cases is obtained in the vertical coplanar arrangement. The multilayer computer program used to derive the theoretical master curves for comparing the performance of the four systems is completely general except for our assumption of the quasi‐static conditions, which is justified in all geophysical induction problems. A study of the “hidden‐layer problem” to determine the best system and conditions for detection of a thin layer sandwiched between two thicker beds has been made with the help of the program. Horizontal coplanar and vertical coplanar systems are again found to be the best and second best systems, respectively, for their detection. It is also clear that a conductive sandwiched bed is much easier to detect than a resistive one.

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