Patent GB-728564-A (GB)
Title: Improvements in or relating to well logging apparatus
IPC: G01V3/24
Filed: 1953-06-08
Abstract: 728,564. Electric well logging apparatus: electric analogue computers. SCHLUMBERGER WELL SURVEYING CORPORATION. June 8, 1953 [June 6, 1952], No. 15777/53. Class 37. Apparatus for determining the resistivity of the earth formations traversed by a bore hole containing conductive drilling fluid wherein a plurality of symmetrically arranged electrodes are located in the borehole, two of the electrodes being fed with A.C. at different frequencies or alternative pulses of D.C. and the potential differences between other electrodes produced by each current-emitting electrode being determined, is characterized in that the measured potential differences are combined in a computer in such a way that the result is indicative of the lateral resistance of the earth formation at the electrode assembly. As shown (Fig. 1) with the electrode assembly of Specification 681,681, current electrode Ao is fed with A.C. at frequency f 1  from a source 15; interconnected current electrodes A1, A2 are fed with A.C. at a frequency f 2  from a source 21; the potential between interconnected potential electrodes M 1 , M 2  and a reference point 31 is filtered at 26, 27 to give a voltage e 1  at frequency f 1  across resistor 28 and e 2  at frequency f 2  across resistor 29; the potential between interconnected potential electrodes M 1 <SP>1</SP>, M 2 <SP>1</SP> and electrodes M 1 , M 2  is filtered at 33, 34 to give a voltage Ú 1  at frequency f 1 , across resistor 35 and e 2 <SP>1</SP> at frequency f 2  across resistor 36. These voltages are separately fed into a computer 37 described below, where the value of is determined and shown on a meter 38 as indicative of the lateral resistance at the electrode assembly. The electrode assembly may alternatively be that shown in Specifications 691,721, 696,535, 701,498, the latter incorporating additional symmetrical electrodes to enable the earth resistivity to different depths of penetration to be obtained. Measurement of the spontaneous potential at electrodes M 1 , M, may be simultaneously made. The computer (Fig. 2) is shown with an electrode array of the type described in Specification 696,535. The potential difference between electrodes M 1 , M 1 <SP>1</SP> (e 1 <SP>1</SP> + e 2 <SP>1</SP>) is fed via transformer 53 to a variable gain amplifier 44 whose output G (e 1 <SP>1</SP> + e 2 <SP>1</SP>) is taken to a primary winding 45 of a further transformer 47, having another primary winding 46 fed with the voltage (e 1  + e 2 ) between electrode M 1  and ground. Thus the secondary winding 50 of transformer 47 has a voltage (e 1  + Ge 1 <SP>1</SP>) at frequency f 1  and (e 2  - Ge 2 <SP>1</SP>) at frequency f 2 , which is fed through a filter 49 which rejects the voltage component at frequency f 1 , the remaining voltage being amplified at 51 and rectified at 52 to give a D.C. voltage proportional to (e 2  - Ge 2 <SP>1</SP>) which is fed to the amplifier 44 to vary its gain G sufficient to cancel the f 2  frequency voltage in the output circuit. Hence (e 2  - Ge 2 <SP>1</SP>) is made equal to zero, i.e. G = e 2 /e 2 <SP>1</SP>, and the output of amplifier 48 is now (e 1  + Ge 1 <SP>1</SP>) or as required, and is rectified at 56 and indicated at 69. In an alternative form of computer (Fig. 5) a servomotor 134 is fed with a signal from an amplifier 135 whose D.C. input is the voltage e 1 <SP>1</SP> across resistor 35 after rectification at 125, and a proportion b of the rectified voltage e 2 1 determined by the slider 131 of a potentiometer 129, the input to amplifier 135 thus being (e 1 <SP>1</SP>- 6e 2 <SP>1</SP>), and the output signal being in a sense to cause the motor 134 to drive the slider 131 to make this signal zero. Thus when the motor 134 is at rest the valueof b is e 1 <SP>1</SP>/e 2 <SP>1</SP>. The rectified voltage e 2  appears across a potentiometer 136 whose slider 137 is also driven by the servomotor 134, the tapped-off voltage being e 2  e 1 <SP>1</SP>/e 1 <SP>2</SP>, and is added in resistance network 139 to the rectified voltage e 1  to give the required (e 1  + e 2 .e 1 <SP>1</SP>/e 2 <SP>1</SP>)which is indicated on meter 38. Specification 683,561 also is referred to.