There are two major methods of geophysical surveying used today for gold exploration work, earth resistivity and the refraction seismic method. The basics of earth resistivity are related to an electric current that is artificially induced into the ground and the resistance the ground offers to the flow of current is measured. The basic electrode configuration is almost universally used and consists of electrodes into the ground in a straight line and equidistant apart. A sounding at a fixed point is taken by starting with the electrode close together and expanding them by arbitrary increments until the desired depth is reached. The depth is equal to the electrode spacing.
The ease with which ground passes an electric current is a function of several variables; the more important being moisture, dissolved electrolytes in the moisture and the surface area of soil particles. Some of these variable change with the subsurface environment and the seasons. In this way, the same material may yield vastly different resistivity values, while vesting different minerals can yield similar resistivity values. For this reason, a background in geology, field experience, and good boring control is important and necessary. Many methods of interpretation and data treatment may be employed. the geophysical unit uses the Barnes Layer values method and the Moore Cumulative Curve method of resistivity data reduction. The actual computations and some of the plotting are done by data processing equipment.
Basically, the earth resistivity method yields a relatively large volume subsurface measurement. This is good for subsurface work involving sand and gravel exploration and subsoil material inventories for large volume earthmoving projects. Very useful subsurface information may be obtained from surveys of perimeters of large natural rock formations. Information may be obtained regarding subsurface uniformity and potential infiltration areas may needs especial design and treatment during gold exploration. Since earth resistivity measures average subsoil conditions over a relatively large volume, it must be used where pinpoint information is required for the gold project.
Due to the oxidation of certain types of minerals near surface, electrochemical differences of potential are sometimes generated. Such natural currents when sufficiently pronounced may often be detected and accurately traced on the surface of the ground. If has been found from experience that the practical application of measuring these natural currents is restricted almost entirely to the identification of gold deposits.