PRECISE MEASUREMENT OF DEEP ELECTRICAL ANOMALIES

Abstract

A method of electrical prospecting has been developed in which the deleterious effects of superficial inhomogeneities in resistivity are eliminated. An electrode configuration is used consisting of a current electrode at “infinity,” a pair of current electrodes with a constant separation of a few hundred feet which are used alternately, and a pair of potential electrodes with a constant separation of a few hundred feet, collinear with the pair of current electrodes, and at a distance from them which is varied to secure depth resolution. It is demonstrated that superficial influences are eliminated by forming the resistivity increment, the ratio between apparent resistivity determined by use of the closer current electrode of the alternate pair and that determined by use of the farther current electrode of the alternate pair. It is also demonstrated that the area explored is below a point approximately half‐way between the alternate pair of current electrodes. Resistivity increment curves, called Resistologs by the authors, can be correlated from station to station, and after they are correlated the lateral variation in the electrical properties of selected subsurface zones can be studied. As a result of the latter characteristic of the measurements, a possibility exists for the direct location of oil and gas, observational evidence for which is presented. The results of surveys made in Hart County, Kentucky, and in the Sam Fordyce, Seven Sisters, Pettus, Branyon, Dunlap, Oakville, and Rhode Ranch fields of Southwest Texas are displayed.