Deep and/or shallow electrical resistivity investigation suitable for dual resistivity-induction logging

摘要

A dual resistivity and induction apparatus for the investigation of earth formations traversed by a borehole, comprises an electrode system, supported on a support member, comprising a central electrode and five pairs of electrodes respectively short-circuited and aligned symmetrically about the central electrode at increasing distances therefrom; the central electrode and the electrodes of the fourth and fifth pairs are called current electrodes, the electrodes of the first, second and third pairs are called voltage electrodes; a source of alternating current at a first frequency, f.sub.1, is coupled between the electrodes of the fifth and fourth pairs; a source of alternating current at a second frequency, f.sub.2, is coupled between the electrodes of the fifth pair and an electrode at electrical infinity with respect to the electrode system; a circuit arrangement coupled to the electrodes of the fourth pair for generating current at f.sub.1 from the central electrode for maintaining the potential difference between the first and second pairs of electrodes at substantially zero and for generating current at f. sub.2 from the central electrode for maintaining the potential difference between the first and third pairs of electrodes at substantially zero; first and second impedances, respectively interposed between each of the source of alternating current and the current generating circuit and the fourth pair of current electrodes; and transmitter and receiver coils supported on the support member, for inducing at a third frequency, f.sub. 3, electromagnetic energy; the length, along the axis of the support member, of each of the electrodes of the fifth pair is several orders of magnitude smaller than the relative spacing along the support member of that fifth pair of electrodes to reduce the interference between the electrode system and the coil system.