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SQUIDs in Geomagnetism and Prospecting
Superconducting Quantum Interference Device (SQUID) applications using geomagnetism were considered from the earliest days since their invention by Jim Zimmerman and Arnold Silver in 1966. A benchmark workshop chaired by Harold Weinstock and William Overton in 1980 set out the full potential of SQUID use in geophysical prospecting and identified a wide variety of applications where SQUID-based systems have the potential to make significant contributions. This was followed by John Clarke’s review where low temperature devices, which use helium cooling (LTS), were discussed. Unfortunately the exploration industry did not widely adopt this technology except for SQUID magnetometers for rock magnetism. Howver, the development of liquid nitrogen-cooled (HTS) SQUIDs during the 1990’s led to a renewed interest in their use in mineral exploration. Since then, SQUIDs have become a common choice as a receiver for various geo-prospecting techniques. Their potential, as well as the improvements in their electronics has seen both LTS and HTS systems gaining acceptance and wide usage. This paper considers the impact of SQUIDs on mineral exploration techniques and briefly describes their use in some applications. It will consider the recent development of SQUIDs for Transient Electro-Magnetics (TEM) in detail. A case study will be presented that outlines the 13 years of development that has led to a commercial system responsible for various mineral discoveries. Technical aspects of the required system and device innovations will be discussed. The renewed development of SQUID magnetic tensor gradiometery will also be reviewed including the exciting research undertaken by various research groups.