TEM measurement in a low resistivity overburden performed by using low temperature SQUID

• A sandwich-layered earth is modeled to obtain a relationship between the resolution of the B field and the exploration depth.
• A transmitter and receiver system in TEM was designed for the exploration of areas with thick low resistivity overburden.
• TEM measurement used by SQUID can detect both shallow and deep regions.

Exploration of areas with thick low resistivity overburden is still a challenge for time domain transient electromagnetic method (TEM). We report modeling of a sandwich-layered earth by simulating the B field response with different conductive target layer thicknesses, thus obtaining a relationship between the resolution of the B field and the exploration depth. A low temperature Superconducting Quantum Interference Device (SQUID) is an ideal sensor for measuring the secondary magnetic field B in TEM measurements, because its sensitivity of several fT/√Hz is independent of frequency. In our TEM experiments, we utilized two different coils as receivers, a simple SQUID system, and a large transmitter loop of 200 × 200 m2 to compare the detected decay curves. At some measurement points, a decay signal of more than 300 ms duration was obtained by using the SQUID. Apparent resistivity profiles of about 9 km length are presented.