GB2019004A - Well Logging Methods and Apparatus - Google Patents

Abstract

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 A0 and five pairs of electrodes M1, M'1; M2 M'2; M3 M'3; A1 A'1; A2 A'2 respectively short-circuited and aligned symmetrically about the central electrode A0 at increasing distances therefrom. A source of alternating current at a first frequency, f1, is coupled between electrodes A1 A'1 and A2 A'2 a source of alternating current at a second frequency, f2, is coupled between electrodes A2 A'2 and electrode 8 at electrical infinity with respect to the electrode system. A circuit arrangement 15 coupled to electrodes A1 A'1 for generating current at f1 from electrode A0 for maintaining the potential difference between electrode pairs M1 M'1 and M2 M'2 at substantially zero and for generating current at f2 from electrode A0 for maintaining the potential difference between electrode pairs M1 M'1 and M3 M'3 at substantially zero. First and second impedances are respectively interposed between each of the sources of alternating current f1 f2 and the current generating circuit and the electrodes A1 A'1. The length, along the axis of the support member, of each of the electrodes A2 A'2 is several orders of magnitude smaller than the relative spacing along the support member of that pair of electrodes to reduce the interference between the electrode system and the coil system R1 R2 R3 and T1 T2 T3 of the conventional induction log operating at a third frequency. <IMAGE>