Development of an opposing coils frequency domain electromagnetic method transmitter

Abstract

The frequency-domain electromagnetic (FDEM) method is widely applied in near-surface geophysical investigations. However, traditional bistatic FDEM sensors often exhibit low spatial resolution, while monostatic designs such as gradient and bucking coils face challenges due to structural complexity and limited anti-interference capabilities. Opposing coils, a weak-coupling configuration commonly used in transient electromagnetic (TEM) methods, have shown promising performance. Based on the equivalence between time and frequency domains, this study aims to fabricate an FDEM transmitter using opposing coils to verify its performance in frequency domain. A prototype transmitter was developed using a full-bridge circuit to generate alternating current from a battery source, with an absorption circuit added for circuit protection. To overcome high-frequency suppression caused by coil inductance, series resonance was employed to increase emission current. Two relay control schemes were tested, and the causes of abnormal pulse phenomena were analyzed. Field experiments comparing the prototype with the commercial GEM-2 system demonstrate that the prototype—featuring stronger transmission currents and an opposing-coil configuration—yields higher signal amplitudes and more distinct anomalies. These results indicate enhanced spatial resolution and sensitivity to shallow subsurface features, underscoring the effectiveness of opposing coil designs for improving near-surface FDEM imaging.