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:

×H=J+∂D∂t,1

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.