GB681681A - Electrical resistivity well logging apparatus - Google Patents

Abstract

681,681. Geophysical exploration. SCHLUMBERGER OVERSEAS SOC. ANON. April 30, 1951 [May 12, 1950], No. 10070/51. Class 37. Apparatus for investigating the strata in a borehole by logging the resistivity of the earth formations traversed, comprises means for measuring the current emanating from an electrode in a lateral direction only, the current along the bore hole being neutralized by auxiliary electrodes. In a bore hole 10 (Fig. 1), containing drilling fluid 11, is a main current electrode Ao maintained at the required potential with respect to earth by a supply source 13, fed through a commutator 19. Located symmetrically above and below Ao are two pairs of potential electrodes M1, M<SP>1</SP>1 and M2, M12, being connected together, preferably through condensers, and two auxiliary current electrodes A1, A2, also interconnected. The potential electrodes are connected through blocking condensers 25, 26 to the input terminals of an amplifier 30, through a commutator 27, operated in synchronism with 19, the output of the amplifier 30 being fed via another synchronous commutator 34 to auxiliary current electrodes A1, A2 and B; the current emanating from A1, A2 being sufficient to reduce the average potential difference between M1, M<SP>1</SP>1 and M2, M<SP>1</SP>2 to zero. The potential difference between a reference electrode N and a point between M1, M<SP>1</SP>1 or M2, M<SP>1</SP>2 is obtained from the junction 39 of two resistors 37, 38 connected across M1, M<SP>1</SP>1 (or M2, M<SP>1</SP>2), and is indicated by a recording galvanometer 43 fed through a synchronous commutator 42, this reading being indicative of the lateral resistance at Ao. In other embodiments : (a) the potential difference between N and any point near the potential electrodes M1 &c. is measured; (b) the main and auxiliary current electrodes, Ao and A1, A2, are interchanged; (c) alternative pairs of potential and/or auxiliary current electrodes are added to be selected by the operation of electromagnetic switches at the surface; (d) the main current electrode is duplicated at A<SP>1</SP>o, A<SP>11</SP>o connected together, and the potential electrodes M1, M2 are replaced by a single electrode M at the centre; (e) additional auxiliary current electrodes A3, A4 are added, and supplied with current through a synchronous commutator from an additional supply source, which may alternatively be connected direct to electrodes Al, A2. An embodiment wherein a certain amount of apparatus is located with the electrode assembly in the bore hole is shown in Fig. 7, the amplifier 74 for producing the balancing current from the auxiliary current electrodes A1, A2 and the main current electrode Ao being energized from an A.C. source 152. Potential measurements are made at electrodes M, M<SP>1</SP> located between M1, M<SP>1</SP>1 and M2, M<SP>1</SP>2 respectively, and recorded at the surface by a galvanometer 88. The static spontaneous potential of current electrode Ao may also be noted by a galvanometer 95. In a further embodiment (Fig. 8), two underwater amplifiers 123, 131 are used to amplify the potential differences between the pairs of potential electrodes M1, M<SP>1</SP>1 and M2, M<SP>1</SP>2 respectively, feeding the auxiliary current electrodes A1, A2 respectively. A capacityfed galvanometer 138 records the average potential difference between each pair of potential electrodes, and a choke-fed galvanometer 142 records the spontaneous potential difference.