4: Electromagnetic Theory for Geophysical Applications
Section 1 Fundamental Electromagnetism
To comprehend the bases and the interpretational techniques of electrical prospecting methods, requires first a knowledge of the tools of electromagnetic theory. The ability to solve a boundary-value problem in electromagnetic theory then becomes the objective. All electromagnetic phenomena are governed by the empirical Maxwell's equations; we must start with them. Maxwell's equations are uncoupled first-order linear differential equations but can be coupled by the empirical constitutive relations which reduce the number of basic vector field functions from five to two. Care must be taken in selecting the form of the constitutive relations pertinent to the earth. In particular, for most earth problems, we assume isotropy, homogeneity, linearity, and temperature-time-pressure independence of the electrical parameters of local regions of the earth. A more complicated earth model is formed by juxtaposition of several such regions.
Figures & Tables
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.