Physical Laws, Electromagnetic Induction, and Rock Properties
Maxwell’s equations provide the starting point to obtain an understanding of how electromagnetic fields can be used to study the structure of the earth and determine its electric or magnetic properties (Maxwell, 1954; Ward and Hohmann, this volume). One equation is used to represent the fact that magnetic fields are caused by electric current flow:
where H is the magnetic field, J is the current density, and D is the electric displacement. This equation represents two kinds of current flow, one (J) in which charge carriers flow through a medium without hindrance, and another, ?D/?t, in which charge separation, and hence an impeding electric field, arises. The first type of current is often called ohmic, or galvanic, while the second type of current is known by the name displacement current.
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.