On the imaging of radio-frequency electromagnetic data forcross-borehole mineral exploration

Abstract

SUMMARY Radio-frequency (typically from 0.1 to 20 MHz) electromagnetic methods are powerful tools for locating conductive mineralization in ore exploration and mine development. Yet data interpretation is complicated by the non-linear relationship between the observed electric and magnetic ¢elds and the electrical parameters of the Earth. The principal means of quantifying inversion capabilities is to compute synthetic data sets using accurate numerical models and to perform the inversion under controlled conditions. Our speci¢c interest is in locating 3-D bodies that are highly conductive relative to the host rock. An excellent approximation for this class of targets, at least at radio frequencies, is to assume that the bodies are in¢nitely conductive. The numerical advantage of this assumption is that inhomogeneities can be represented simply as internal boundaries where the total electric and magnetic ¢elds are identically equal to zero. Ensuring numerical stability thus does not require excessive discretization in conductive regions since the maximum grid cell size is determined only by the electrical parameters of the host material. We use a ¢nite-diierence time-domain approach to compute the total electric and magnetic ¢elds everywhere within the background medium and validate the code by comparisons with two analytical solutions. One common means of interpreting radio-frequency electromagnetic data is to assume linearity between the model parameters and physical response and to apply tomographic image reconstruction methods. While relatively simple and inexpensive, the limitations and applicability of tomographic imaging methods to non-linear electromagnetic data acquired in complicated, 3-D mineral exploration environments are not well understood. Our initial study involves applying the simultaneous iterative reconstruction technique to recover images of the electrical properties of a conductive inclusion. Several examples show that the structural geometry of bodies between boreholes can be reliably imaged using both frequency-domain and time-domain data. Phase data seem more amenable to recovering geometry information from tomographic reconstruction methods than amplitude data. However, attenuation data provide better constraints on the electrical properties of the geological media and thus form an essential complement to primarily geometrical information obtained from phase tomography. Non-linear inversion methods will probably be required to incorporate the amplitude data for accurate reconstructions of the subsurface.