Electromagnetic induction studies

Abstract

This report constitutes an attempt to review the major developments and identify important trends in the broad field of geophysical electromagnetic induction 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 two‐ and 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.

Electromagnetic modeling of 3-D structures by the method of system iteration using integral equations

“…Weidelt (1975) put forward a more general theory for a three-dimensional (3-D) structure in a layered earth. There have been numerous contributions to this topic since then, many of which can be found in the summary works of Hohmann (1983Hohmann ( , 1988, Chave and Booker (1987) and Cerv (1988). More recently, Wannamaker (1991) and Xiong (1991) improved the accuracy of the modeling scheme with refined methods for the numerical integration of the Green's functions.…”

Electromagnetic Induction Studies

“…For long period MT measurements, both on land and on the seafloor, instrument and cultural noise usually are not as important as non‐planar components to the natural source fields and, on occasion, ground motion. These noises often are highly clumped in time, and have motivated development of powerful robust outlier removal methods [ Egbert and Booker , 1986; Chave et al , 1987; Chave and Thompson , 1989]. A comparison of unweighted, single‐pass sorting, and iterative robust methods using long period time series from the EMSLAB project demonstrated the clear superiority of the robust methods, and also the necessity of remote reference fields for optimal noise removal even for periods beyond 10 4 s [ Jones et al , 1989].…”

“…The approximately 400 references contained in the bibliography attest to the diversity of efforts in this field. For background and research efforts prior to 1986, the reviews of Chave and Booker [1987] and Hermance [1983] are recommended.…”

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

“…In the 1980s, there was an initial period of higher usage of SQUIDs in MT field campaigns (e.g., Berktold 1983;Hermance et al 1984). However, it was soon realised that the need for cryogenic cooling with liquid nitrogen or helium and the size of early SQUID sensors made them less practical for field campaigns than induction coil sensors (Chave and Booker 1987). In particular, the need to refill liquid nitrogen or helium regularly makes SQUIDs unattractive and expensive to operate in MT measurements that last from days to weeks or months, especially when located in remote localities.…”