ON THE EFFECT OF OVERBURDEN ON ELECTROMAGNETIC ANOMALIES—A REVIEW

The effect of a conducting overburden in EM prospecting intuitively is considered to be one of degeneration of anomalies, in the sense that the detection of a target and the determination of its unknown parameters become more difficult or ambiguous when an overburden is present than when it is absent. Recently, however, Negi (1967) and Negi and Raval (1969) have suggested on the basis of theoretical work that, if a certain combination of the parameters involved occurs, a conducting overburden can make a target more detectable than it would be without any overburden. These theoretical results and the existing experimental evidence are examined in this paper for the possible existence of “negative screening,” as this effect has been called. Due to a number of incorrect assumptions made in the theoretical analyses by the authors who predicted negative screening, their conclusions do not seem to be valid. A fundamental objection in the case of the stratified sphere, for instance, pertains to the assumption that, in presence of the annulus, the contribution of the inner sphere alone to the total external measurable magnetic field can be obtained by simply subtracting the response of the larger uniform isolated sphere from that of the double sphere. Another major objection concerns the notion in the theoretical analyses that detectability is determined by the contribution from the target alone—a quantity which one can never measure—without regard to the simultaneous contribution from the conducting overburden. Defined on the basis of the measurable total response from the system as a whole, detectability falls progressively with overburden conductivity. Although the existing results of model EM experiments are generally indicative of the absence of a phenomenon like negative screening, no clearcut and indisputable conclusion can be arrived at on the basis of model studies. In some recently published experimental results, there is one solitary instance, unnoticed by the experimenters themselves, which could suggest the existence of negative screening. We believe, however, that, due to the many inherent uncertainties in model EM work, conclusions based on theoretical investigations have to be accepted as more reliable until carefully planned model work is carried out with this specific problem in view.