Application of drill-hole electromagnetic (EM) techniques used in exploring for bodies of conductive mineralization, can detect and define the conductive target itself or other geologic features (for example, fault zones) which may lead indirectly to discovery of a mineralized deposit. Drill-hole applications can be broadly defined to include any configuration where either the source or the receiver (or both) is submersed in the lower half-space. While this conceptual definition includes underground work in mines with conventional surface equipment, often man-made installations (metallic pipes, tracks, etc.) preclude such work. Accordingly, a drill hole is required to allow entry of all or part of the prospecting system into the ground. Such drill holes may be collared either on surface or in underground workings.
Drill-hole applications could be considered the basis for the generalized problem of EM prospecting in that no geometric specifications can be predefined. Surface applications are more restricted because, as is known beforehand, both the transmitter and receiver are confined to the surface (or slightly above it in airborne applications) and therefore can be assumed to lie in the same, roughly horizontal, plane. In principle, similar restrictions in geometry cannot be assumed for borehole applications. Therefore instrumentation, operational procedures, and interpretation facilities must be designed to accommodate many different geometries. However, any particular system is, at least in part, confined even more tightly than on surface. Therefore, from an operational point of view, borehole methods could be considered specialized. Specialized equipment is needed because of the hostile environment of the drill hole, the relatively low occurrence of application compared to surface methods, and because of the geometric restrictions of the drill hole (for example, tilt of the field cannot be measured by geometric nulling).