A technique, now obsolete, for determining the presence of a thin permeable [[bed]] enclosed in a thick highly resistive [[formation]]. The technique also permits the recording of an approximation to the [[static_spontaneous_potential|static SP]]. The method involves the measurement of two [[curve|curves]] by a [[sonde]] with a special electrode arrangement. An [[SP]] measuring electrode is placed between two [[potential]]-monitoring electrodes and two current electrodes in symmetrical array. One curve is recorded with the SP electrode while the potential-monitoring electrodes are maintained (by current electrodes) at the static SP of [[shale|shales]], and the other curve is recorded while the potentialmonitoring electrodes are maintained at the static SP of permeable beds. A comparison of the curves will reveal SP anomalies at the permeable beds, and means to derive static SP. See [[SP]] and [[SSP]].