On Resolution in Geophysical Probing Using Electromagnetic Methods

A model of a surface magnetic loop induction scheme over a two layer earth with fixed parameters, is developed. Using this model, formulas and curves are presented which determine the number of measurements N required to achieve a knowledge of the depth of burial D of a basement layer to within a specified tolerance ¿D. It is shown that the condition N >> D/(¿¿s) where ¿s is the quasi-static skin depth of the upper layer is necessary for the tolerance ¿D to be small. The tolerance is dependent on the mathematical model so that the popular method of using Sommerfield integrals has the consequence that horizontal resolution is much poorer than vertical. The assumptions of large signal-to-noise ratio and highly accurate measurement are necessary for formulas to be valid.