With the development of accurate, flexible, and reliable digital field instruments, electrical methods of geophysics now are limited mainly by a lack of interpretation capability. Forward solutions for one-dimensional (1-D) (layered) models have been available for many years; they consist of Fourier or Bessel integrals, which can be easily evaluated numerically. Inverse solutions for 1-D models also have been used routinely for many years. However, calculating the response of a two-dimensional (2-D) or three-dimensional (3-D) model is much more difficult, except for certain simple geometries such as spheres or cylinders, where conductivity boundaries correspond to constant-coordinate surfaces. Advances in understanding and interpretation for electrical methods now depend on forward and inverse numerical solutions for arbitrary 2-D and 3-D models.
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Over the last two decades there have been significant advances in electromagnetic (EM) methods of exploration, as evidenced by the extensive research carried out at various companies, universities, and government research organizations; by the large number of papers published on the subject; and by the numerous workshops on various EM topics held in conjunction with the SEG Annual Meetings.
Early EM methods were largely designed by the Scandinavians and the Canadians for exploration under glaciated Precambrian shield conditions, where the resistivities of the host rock and overburden are generally high. They did not work well in areas with conductive overburden or host rock. The lack of sophistication in data gathering and processing severely limited their exploration depth. Moreover, early EM systems were relatively heavy, cumbersome, and slow in operation.