The Application Of Telluric Currents To Surface Prospecting

The electrical methods for surface-prospecting have been well developed in the last 20 years. The method involves, in the classic form, the sending of an electrical current into the soil, by means of two electrodes A and B, and studying the potential distribution at the surface, by measuring the difference of potential between pairs of points such as M and N. The points M and N are usually placed on a line between, and equidistant from, A and B. By increasing the distance AB, the current sent into the Earth, and likewise the distribution of surface-potentials, become influenced by regions of greater and greater depth below the surface. By varying the form and dimensions of the electrode-arrangements AMNB, called a "quadripole," one may make valuable deductions concerning the nature or thickness of beds, if one knows the electrical properties of these beds. On the other hand, if the form and dimensions of the electrodes AMNB remain fixed, the depth of penetration of currents in the Earth remains, so to speak, almost constant, although it tends to increase where the subsoil is more conductive, and conversely. Therefore, if one moves the constant-dimension quadripole over all the terrain to be Investigated, one achieves approximately a horizontal exploration. This permits a study of tilted beds or faulted horizon-tal beds. When the particular problem becomes deep, of the order of 2000 meters, or more, it is ne-cessary to Increase the distance between the current-sources AB to a length such that the work 'comes very troublesome, especially if the region is wooded. At the same time, the separation the points M and N must be likewise great (for example, one km) in order to obtain differences potential which are not too small; the measurement of potential differences across MN is then difficult because of the presence of parasitic potentials, due to a flow of telluric currents in fourth, and the order of greatness of these currents may exceed the quantities to be measured.