Transient electromagnetic sounding using a 5 m square loop^*

Abstract

Our study of the diffusion of transient electromagnetic (TEM) fields into a two-dimensional (2D) earth, simulated by a finite-difference time-domain (FDTD) algorithm, proves that the depth of investigation depends primarily on the measurement time and the ground resistivity. Sufficient signal strength can be gained by a small central-loop configuration equipped with a high-power transmitter and a large equivalent area receiver coil. When receiver sensitivity is 1 mV, average resistivity of the overlying section is around 50 Ω.m, and the noise level is 3 to 20 nV/m², a depth of exploration of 150 m can be achieved with a 5 m × 5 m loop with 100 A of transmitter current; for a noise level of 0.1 nV/m², the depth of exploration is up to 800 m with 1200 A of transmitter current. The results have been compared with results from the electrical resistivity method and from large loop techniques, and verified by drilling. We show that using a small loop can attain not only a greater depth of investigation but also higher resolution. The small loop configuration with a high-power transmitter is a convenient combination, and useful for exploration in mountainous terrains and restricted sites.