GB883490A - Apparatus for investigating earth formations - Google Patents

Abstract

883,490. Determining characteristics of earth strata. SCHLUMBERGER WELL SURVEYING CORPORATION. June 9, 1959 [June 23, 1958], No. 19778/59. Class 37. Bore-hole logging equipment comprises a coil or coils for inducing current into the earth and electrodes for contacting the earth, at least one of the electrodes comprising a closed loop and a plurality of spaced conductive surfaces positioned so that the effect on the coil of eddy currents in the electrodes is minimized by an opposing effect of eddy currents in the closed loop. Fig. 1 shows a hollow resinbonded glass-fibre mandrel 32 carrying transmitting coils T1, T2 and receiving coils R1, R2, R3. The coils T1, T2 are connected in series opposition and fed with current at 20 kc/s. from a generator contained in compartment 18a. Coil R1 is connected in opposition to coils R2, R3, their effective output being fed to a phasesensitive detector in compartment 18b. The direct current output is fed to a recorder 28 which plots the resistive or reactive component of the earth impedance against depth. Electrodes A1, A0 feed a current at 400 c/s. into the earth. A common return electrode 21 is provided. Voltages dependent on the earth impedance are picked up by electrodes M1, M2 and fed via a further phase-sensitive detector to recorder 42. The two recorders may be cornbined in a single instrument which may record the ratio of inductive/conductive measurements. The coils shown may be replaced by a single coil. Electrode and coil construction.-Fig. 3 shows coil T2 separated by rubber from electrostatic shields 34, 37 and contained in a recess in the mandrel 32. An outer sleeve 44 of resin-bonded glass-fibre carries the electrodes such as A1 in the form of a number of plates interconnected to a closed loop 47 by conductors 49. The relative position of plates and loops in each set of electrodes is adjusted so that the effect of eddy currents in the loop cancels that in the plates. The effect on the coil system of varying the position of plates and loops is discussed in detail, results being expressed graphically in Figs. 4, 5, 7 (not shown). The X/R ratio of each loop is important, the loops associated with electrodes A0, M1, M2 being made of nickelchromium-iron alloy whilst that of A1 of copper. The axis of one or more loops may be tilted relative to that of the mandrel 32. In Fig. 6 (not shown), the common return electrode 21 is located near the lower end of the mandrel 32. Temperature compensation is achieved by using for one set of electrodes elements in which the R/X ratio at the operating frequency is less than unity and a further set using elements having a ratio greater than unity.