DOWNWARD CONTINUATION AND ITS APPLICATION TO ELECTROMAGNETIC DATA INTERPRETATION

The existing methods of downward continuation were developed primarily for the interpretation of gravity and magnetic data, and, therefore, assume the validity of Laplace’s equation. The data collected in electromagnetic surveys, strictly speaking, obey Maxwell’s wave equation. However, in practice, two approximations are frequently made. First, the effects of displacement currents are neglected in view of the fact that the frequencies used are low. Second, the contribution of the country rock to the measured field is assumed to be nil, as its electrical conductivity is usually much smaller than that of the target body. Experience seems to indicate that these two approximations are permissible in general. It will be seen that, under these two approximations, Maxwell’s wave equation degenerates to that of Laplace, and the existing techniques of continuation become applicable to electromagnetic field data as well. This hypothesis has been tested by using continuation to interpret 20 electromagnetic profiles, involving a variety of theoretical, model, and field data. It has been found that the depths to the tops of the anomaly‐causing bodies can be determined with acceptable accuracy by this method.