DEEP LONG-OFFSET TRANSIENT ELECTROMAGNETIC SURVEYS FOR CRUSTAL STUDIES IN THE USA

Controlled-source electromagnetic methods are often thought of as being used for detailed studies of Earth conductivity at shallow depths, such as is appropriate for minerals exploration. However, for the past quarter of a century, the Colorado School of Mines has placed considerable effort into the development of similar techniques for exploration deeper into the Earth's crust. The main difference between these techniques and shallow methods is primarily that the source moment is generally one to two orders of magnitude greater for the former. These deep electromagnetic sounding studies allow for a considerable flexibility in survey design so that optimum results may be obtained in diverse geological settings. Four time domain electromagnetic sounding surveys in different geological settings across continental U.S.A. are reviewed. The surveys on the U.S. West Coast, in the Pacific Northwest, New York state, and southern Colorado, provide insight into not only the diversity of the technique but also its ability to gather knowledge of deep electrical structures.