OPPOSING COILS TRANSIENT ELECTROMAGNETIC METHOD FOR SHALLOW SUBSURFACE DETECTION

Abstract

Transient electromagnetic method (TEM) is a commonly-used geophysical method of measuring the secondary electromagnetic field induced by transient pulse sources. Because of the inherent mutual-induction between TX and RX coils, the measured field always contains the mutual-induction field, which becomes stronger when TX coil becomes smaller and closer to RX coil. It influences the near-surface TEM systems, which utilize small TX coils to effectively resolve the near surface explorations. The mixture of the mutual-induction field with the secondary field at early period leads to a blind region in the shallow surface in TEM exploration. To solve this problem, we applied two concentric and parallel current coils in the TEM system. They are opposing coils, which means that the two coils are physically same, but the currents in them are of equal value and in reverse directions. The RX coil is equidistant to the two opposing coils. By means of this special arrangement, the mutual-induction field received at the RX coil is zero, which means that the mutual induction effect is efficiently eliminated, and pure secondary field of the conductive underground will be measured. It is proved by theory and model calculation that opposing coils transient electromagnetic method has advantages in lateral resolution. It is quite suitable for small coil TEM system aiming at shallow subsurface detection.