A review of the application of analytical methods to the prediction of transient electromagnetic field responses

Abstract

The effective use of transient electromagnetic fields for mineral exploration depends on a proper understanding of how these fields propagate in and are affected by different geological materials and structures. The analytical methods for investigating these effects form the subject of the review. Some of the basic phenomena which can be explained without need for extensive numerical modelling are discussed. The following examples are among them: The rate of decay of the measured field, which is diagnostic of the general geometry of the conductivity distribution in the ground, can be forecast with simple analytical models. Transient decay curves of a significantly different character can be caused by geological materials whose conductivity or magnetic permeability varies with frequency ? for an earth in which the conductivity is frequency-dependent, changes in sign of the transient may even be observed. A magnetic permeability that varies with frequency is sufficient to explain all the anomalous observations that have been made in surveys carried out over areas where superparamagnetic minerals are present on the Earth's surface. Analytical methods can be used to show that the response of a more-conducting target in a less-conducting host can be represented as the sum of independent host and target responses, plus an interaction term; this result helps to justify the use of anomaly-separation methods to extract target responses. Inversion studies using analytical models show where supplementary data is necessary to define adequately the numerical values of the parameters of the model.