Effect of a steel casing on crosshole EM measurement

Abstract

Crosshole electromagnetic (EM) methods have become important tools for detailed evaluation of the underground conductivity distribution. However, when a drill hole is cased with steel, we usually abandon the idea of applying the crosshole EM methods assuming that the casing would absorb most of the energy. In order to examine the effect of the casing on EM signals, we developed a computer program capable of calculating EM fields outside a steel casing due to a current filament source seated inside the casing. We tested this code with some numerical experiments. The frequency range considered here is from dc to IOkHz. Although the energy absorption of the EM signals due to the casing is significant, especially at higher frequencies, the ratio and the phase difference between the EM fields with and without the casing remain constant at any receiver position for a fixed frequency, provided the receiver is some distance away from the casing. This implies that we may be able to overcome the casing effect, and to recover the whole-space EM fields by correcting the observed data with the constant. away from the source borehole. The implication of this finding is very significant because it will allow us to apply cmsshole EM methods to imaging conductivity distributions even if boreholes are cased with steel pipes, provided that at least one receiving borehole that is not cased with steel pipes is available.