The aim of geophysical surveys is to obtain information on subsurface geology. While execution of surveys using specific techniques may differ in detail, it will almost invariably consist of three steps: surveying, data processing, and data interpretation. A successful survey will yield more information on the geological target—its existence, location, shape, size, etc. New information is obtained by interpreting geophysical data. The success of a survey depends to a large extent on decisions made before the survey initiation. An exploration geophysicist working for a mining company is often asked the following question: Can we use geophysics in prospecting for this particular commodity? If yes, what techniques should we use and how do we specify survey parameter. Decisions that are usually based on experience often cannot be justified scientifically. The proper approach would be to carry out test surveys to investigate the physical properties of the target and other bodies that might interfere with its response. In recent years, exploration geophysics has progressed beyond target finding to mapping subsurface geology.
Analyzing the sequence of geophysical survey steps as shown in Figure 1, the main flow (surveying, processing, interpretation) and the associated areas of research can be identified. To make an intelligent decision on the use of a technique, the geophysicist should have at least a rudimentary knowledge of the physical properties of the target and the surrounding media the response of which might interfere with target identification. Most physical property studies have been done in the laboratory on samples collected in the field. While this approach may be satisfactory for some geophysical methods (gravity, magnetics), it is not for others. Electrical properties of earth materials vary substantially (by several orders of magnitude) depending on whether they are measured in situ or in a laboratory. It is virtually impossible to simulate real conditions in the laboratory. An attempt can be made to recompose the original water content, but microinhomogeneities typical of many geological environments (e.g., rock fractures and their frequency and variation with depth) cannot be duplicated.
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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.