Arrangement of the electrodes for an electrical logging system for determining the electrical resistivity of a subsurface formation

摘要

The invention is an electrical logging system for determining the electrical resistivity of a subsurface formation from a cased borehole that may include voltage and current supply means, voltage and current control means, electrode support means, and voltage and current processing means. In particular, the system also may include a first pair of electrodes, current electrodes, and a second pair of electrodes, voltage electrodes. The current electrodes are adapted to be electrically connected at different heights to the casing within the borehole and nearby one of the ends of the borehole for supplying an electrical current to the casing. The space between the current electrodes defines an intermediate casing part through which casing current flows. The casing current flowing through the intermediate casing part causes the casing part below the intermediate casing part to have a different electrical potential than the casing part above the intermediate casing part. This results in a leakage current which flows between the casing part below the first electrodes and the casing parts above the first electrodes by way of the subsurface formation. The pair of voltage electrodes are adapted to be located on the casing part either above or below the current electrode for detecting voltages caused by the leakage current returning to that casing part from the subsurface formation. In this manner, the values obtained by the voltage electrodes are not affected by casing current since only leakage current flows into the portion of casing measured by the voltage electrodes.