The Application of Electrical Transients to Well Logging

The transient response of earth formations may be determined by means of four electrodes mounted colinearly on a sonde. Exponentially decaying current pulses are applied to one pair of electrodes and the resulting voltage transient at the other pair is recorded. It is shown by theoretical analysis and verified by laboratory experiments that this arrangement readily yields the direct resistivity of the formation and provides valuable additional information as well. Introduction Although electrical transients have been employed with moderate success in geophysical prospecting, their application to well logging has received virtually no attention in technical literature. In the conventional direct resistivity logging methods only the zero frequency impedance of the formation under study is measured, and no information is obtained regarding its reactive characteristics. Since the use of certain transients appears to offer some advantages in this respect, experiments were performed on a laboratory scale to investigate this possibility. This investigation was limited to a study of a four-electrode array where both the receiving and transmitting electrode pairs are mounted on a logging sonde, and decaying exponential pulses are applied to the transmitting electrodes.