The immediate objective of a geophysical survey is to obtain some information about the interior spatial distribution of one or more of the earth's physical properties from a limited set of measurements of a related physical field made on the earth's surface (or another accessible place). In the case of an electromagnetic (EM) induction survey, the most relevant physical property is the electrical conductivity, and it is sensed by means of a time-varying magnetic and/or electric field. The procedure of converting field measurements to a physical property distribution is termed modeling or interpretation, and the formal corresponding mathematical process is termed inversion.
Geophysical inversion is difficult in the best of circumstances because of numerous intrinsic ambiguities. For EM methods in particular there is an additional problem. The basic laws that relate the EM field to the physical property distribution are well known (Maxwell's equations) and a quantitative and calculable relationship between the physical measurements and the property structure can be established for certain idealized cases. However, we still lack practicable modeling capabilities that enable quantitative prediction of the EM field configuration produced by an arbitrary physical property distribution of even moderate complexity.
Geologic scenarios are extremely varied, and few actual cases can be described accurately in terms of simple geometric forms like plane horizontal layers. Thus, only rarely can we feasibly turn geophysical observations directly into a reliable picture of earth structure simply by application of an automatic process. Generally, a human interpreter is still needed to guide the interpretation process, and this human needs to have a good qualitative understanding of how physical earth structure can interact with EM fields. In addition the interpreter should be able to mentally extrapolate beyond calculable cases and to select more important features of the data from less important ones. Our objective in this tutorial paper is to assist readers in developing such an ability by discussing the various physical processes which arise in some simple situations.
Figures & Tables
Electromagnetic Methods in Applied Geophysics, Volume I, Theory presented the mathematical and physical foundations common to all EM methods. The purpose of Volume I was to help facilitate the understanding of the theory involved and to provide a limited amount of interpretational aids. Volume II, Applications is devoted to a method-by-method treatment of the principal EM techniques in common use.