Electromagnetic Induction Studies

This report constitutes an attempt to review the major developments and identify important trends in the broad field of geophysical electromagnetic nduction and related phenomena over the past four years. Following in the spirit of previous reports of this type [e.g., Filloux, 1979; Hermance, 1983b], the work of US researchers will be emphasized, although we will cover foreign research when appropriate. Many of the recent theoretical developments and the largest EM field program ever (EMSLAB) are the direct result of' international cooperation, and strict adherence to the concept of national boundaries would result in an uninformative and incomplete review. Due to the fact that readers of this paper have diverse interests ranging from theory through field to laboratory studies, we have attempted to treat a variety of topics in EM induction and electrical geophysics. We begin by reviewing the state-of-the-art in data collection, including new instrumentation. We continue by examining data analysis methods, with an emphasis on noise and bias reduction in the computation of' the magnetotelluric and magnetic variations response functions. We then treat forward modelling developments, especially for twoand three-dimensional induction problems. Recent progress has been made in EM induction inverse problems, and we assess the impact of this on the field. An overview of field measurements in North America is given, including the recent EMSLAB experiment which was carried out in 1985--1986 in the northwest US, southwest Canada, and contiguous offshore regions. This is followed by a review of developments in oceanic applications of EM principles. The reference list is believed to be complete through June 1986. In the interest of brevity, only refereed publications or works in press are included, and meeting abstracts or technical reports are generally not cited. Nevertheless, the number of references exceeds 600, attesting to the health of the discipline. Additional information on EM induction research may be found in the proceedings of the most recent semiannual Workshops on EM Induction held in Victoria, Canada, in 1982, Ile-Ife, Nigeria, in 1984, and Neuch•.tel, Switzerland, in 1986.