Patent GB-899979-A (GB)
Title: Apparatus for investigating the electrical resistivity of earth formations
IPC: G01V3/24
Filed: 1960-06-07
Abstract: 899,979. Geophysical surveying. SCHLUMBERGER WELL SURVEYING CORPORATION. June 7, 1960 [June 16, 1959 (2)], No. 20001/60. Class 37. In an earth strata conductivity measuring arrangement, the pad 17, Fig. 1, carried on pivoted arms 19, 20 from an I-beam support 15, is of an electrically-conductive material such as iron and is arranged to emit a constant surveying current Is, Fig. 5, from a rectangular electrode A0, Fig. 2 (not shown), let into the front face of the pad and insulated therefrom, and focusing currents If, from the surrounding pad face including the end wall 24 and the reverse side 25. The currents are supplied from a 500 C./sec. signal generator 51 and leakage of the focusing current through the mud cake is avoided by the edge currents. A narrow monitoring electrode M is provided in an area between the survey and focusing electrodes for automatic adjustment of the focusing current and in consequence improved focusing, the difference in potential between electrodes M and A0 being maintained substantially at zero by energizing electrodes A1 and B1 from the output of a high-gain feedback amplifier 53 which is energized by the said potential difference. The pad is insulated from arms 19, 20 by insulator bushes 32, 33 and the formation conductivity is indicated by the difference in potential between monitor electrode M, and a remote reference electrode N mounted near the top of an insulated length of the support cable immediately adjacent the sonde, the signal being amplified in amplifier 54, and detected in a phase-sensitive detector 55 to which the signal generator output is also supplied. The D.C. output is fed to indicator 59 over line 58, and a phantom on the power supply. An electrode B1 at the bottom of the sonde attracts the focusing current and limits the penetration of the survey current to approximately 10 inches and gives good results for the area flushed by drilling fluid. Pad 18 on the opposite side of the bore comprises a rubber pad 18 carrying on its face an array of relatively small electrodes A, M1, M2, and having an electrode B embedded in its back. Electrode A is energized by a 200-cycle signal from generator 62 which is also connected to electrode B, and indicators 68, 72 respond to the potentials between M1 and M2, and M2 and remote electrode N, respectively, to give a measure of the mud cake resistivity and the formation resistivity to a shallow depth. A dipmeter apparatus, Fig. 6, includes three electrode pads 116a-116c of conductive material such as iron, mounted on an elongated support member 115, and each comprising an insulated coil 133a, Fig. 7, in an annular recess 118a, Fig. 8, the surface area within the recess, although comprising an integral part of the pad, constituting the survey electrode. The pads are carried by arms 140a-140c, 142a-142c attached to the support member 115, the latter being attached at their lower ends to a slidable collar 143. The support and arms are insulated except for portion 144 which is energized to emit additional focusing currents. An inclinometer is contained in the support member, and a centring guide comprising spring arms 150, 152, connected to collar members 153, 154, one of which is slidable on the support is provided. The first 100 feet of cable above the support is insulated and an electrode B is mounted thereon. The cable is raised or lowered by a winch 157 and a measuring wheel 168 provides a drive for the indicator which may be of the photographic recording type. Power is supplied via unit 164 and slip-rings 159 to a 20kc. signal generator in the support, and voltages proportional to the survey currents induced in the coils 133a are conveyed to the recorder through amplifiers and phase-sensitive circuits as in the previous embodiments. The dip can be estimated by a comparison of the recordings. A pad including a survey current electrode insulated within a recess in the face of the pad is described with reference to Figs. 11 and 12 (not shown).