Exploration geophysics is applied to obtain information about the subsurface of the earth that is not available from surface geological observations. Because the electrical resistivity of different earth materials ranges over many orders of magnitude, electromagnetic (EM) methods are used to map the subsurface resistivity structure. Applications can be at any scale—a metal detector searching for coins in beach sands, a detailed resistivity survey to map the influx of conductive seawater into a resistive aquifer, a high powered time-domain electromagnetic sounding survey to map the thickness of resistive volcanics over conductive possibly petroliferous sediments, or a magnetotelluric survey to detect anomalously conductive mantle for a dissertation. However, because most base metal massive sulfide ores are very conductive and provide a strong contrast to their host rocks, mineral exploration surveys have been the prime application of the EM methods and the mineral exploration industry has funded most of the development.
Electromagnetic (EM) methods include an initially confusing variety of techniques, survey methods, applications, and interpretation procedures, which are further complicated by a bewildering array of trade names. Each technique, however, involves the measurement of one or more electric or magnetic field components by an “EM receiver,” from some natural or artificial source of electromagnetic energy—the “EM transmitter.” A useful classification based upon this commonality of EM systems is presented as Figure 1. It is apparent that the measurement is essentially that of obtaining the transfer function of the earth. At most practical current densities, the earth is linear, so the transfer function is simply the output divided by the input. Properties of the earth are interpreted from these transfer functions in ways similar to how properties of an electrical circuit are interpreted from its transfer or system function.
Possibly the most fundamental electromagnetic method is the direct current (dc) laboratory measurement of the output voltage developed from an input current across a sample of rock. From Ohm's law, the ratio of the voltage in volts to the current in amperes is the absolute resistivity in ohm-meters for a rock one cubic meter in volume. The resistivity technique, as categorized in Figure 1, is a generalization of the above lab measurement for arbitrary geometric configurations of the transmitter and receiver electrodes, yet by being dc, it is only a subset of the electromagnetic methods which arise when nonzero frequencies [alternating current (ac)] are utilized by the transmitter.
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.