RELATIVE CONTRIBUTION TO SIGNAL BY GROUND ELEMENTS IN TWO‐COIL INDUCTION LOGGING SYSTEM*

Publisher Website
Google Scholar
Abstract
A two‐coil induction logging system in an infinite homogeneous medium has been theoretically analysed for the relative contributions of various ground elements to the total signal at the receiver. Unlike an earlier approximate treatment by Doll (1949), the present analysis takes skin effect into account (while neglecting displacement currents). Contour diagrams depicting relative contributions from every ground point and diagrams for vertical and radial/lateral investigation characteristics are presented. The results differ significantly from Doll's in the following points: The relative contribution is frequency and conductivity dependent; It does not show symmetry with respect to the center of coil system; It alternates between positive and negative values with distance; Real and imaginary contributions for a given conductivity and frequency are different from each other. The possibilities of a horizontal‐horizontal sonde is briefly examined and found to be inferior to the usual sonde because of its higher sensitivity to the mud column and the invaded zone.