METHOD OF DETECTING GEOLOGICALLY ANOMALOUS BODIES LATERAL TO A BOREHOLE BY COMPARING ELECTRICAL RESISTIVITY MEASUREMENTS MADE USING SHORT-RANGE MEASURING MEANS AND A LONG-SPACED ELECTRODE ARRAY

摘要

An improved method of detecting geologically anomalous bodies lateral to a borehole and determining the distance thereto utilizes a long-spaced electrode array with a current source and a current sink electrode located within the borehole. A short-spaced electrode array or other short-range electrical resistivity measuring means such as an induction log is used to obtain resistivity values for the formation surrounding the borehole. A long-spaced electrode array comprising two potential measuring electrodes, M and N, and two current electrodes, A and B, positioned within the borehole, is utilized to make resistivity measurements of the formation surrounding the borehole, said measurements reflecting the presence of electrically resistive geologically anomalous bodies located at lateral distances from the borehole up to a distance on the order of twice the spacing between the source or sink electrode and the closest potential measuring electrode. The electrodes of the long-spaced array are positioned so that B lies within the borehole and above N, N is positioned above M, M is positioned above A, and the BN distance is approximately equal to the AM distance. The short-range resistivity values are then compared with the resistivity measurements from the long-spaced electrode array, significant differences therebetween indicating the presence of an electrically resistive anomaly lateral to the borehole and the magnitude of said differences indicating the distance from the borehole to said anomaly.