High-Performance Dual-Channel Squid-Based TEM System and Its Application

Transient electromagnetic method (TEM) is an effective way for mineral resource prospecting if an induction coil is used as the traditional sensor. Superconducting quantum interference device (SQUID) obtains excellent sensitivity at low frequency as compared to coil. Thus, the exploration depth and the accuracy of TEM can be greatly improved. However, an ultrahigh slew rate (>10 mT/s) is needed for transient primary field recording, which is a great challenge for a single high-sensitivity SQUID receiver. In this paper, two SQUIDs with different sensitivities (1.5 nT/Φ
₀
and 99 nT/Φ
₀
) are employed to record the whole TEM decaying signal. The high-sensitivity SQUID (1.5 nT/Φ
₀
) is in reset status when transmitting and is locked after tens of microseconds after transmitting has been cut off. The low-sensitivity SQUID (99 nT/Φ
₀
) remains locked during the whole procedure with high slew rate (~198 mT/s) to record the fast-switching transmitting field. The two channel SQUID outputs are combined together to form a complete decaying curve. A field test was carried out. The decaying curve with a noise level of 100 fT@DC-25 kHz and a fast-switching field of ~630 nT cutting off in tens of microseconds (slew rate up to ~56 mT/s) was recorded after stacking. This dual-channel SQUID-based TEM system obtains high stability, which is useful for small transmitting loop (<;50 m) configuration.