GB939757A - Improvements in well logging apparatus - Google Patents

Abstract

939,757. Geophysical surveying. KOOLAJIPARI TROSZT. Oct. 9, 1959, No. 34358/59. Class 37. Borehole logging apparatus comprises a main electrode A 0 supplied with alternating current I 0 and focusing electrodes A 1 , A<SP>1</SP> 1 supplied with alternating current I 1 . The potential at measuring electrodes M 1 , M 2 is converted by a phase sensitive device to a direct current signal which it employed to regulate the focusing current I 1 so that the potential at electrodes M 1 , M 2 tends to zero. As shown in Fig. 3, the main current I 0 is generated by a variable frequency multivibrator 131, amplified at 132 and fed via a current limiting resistor to electrode A 0 . The output of the multivibrator 131 is also fed via a phase shifter 142 to the auxiliary grid of a heptode 141 and thence via an amplifier 15 to electrodes A 1 , A<SP>1</SP> 1 . The effect of this is to focus the current into a narrow beam through the walls of the borehole. The voltage at electrodes M 1 , M 2 is passed via a transformer 111, an amplifier 112 and a filter 113 (passing only the fundamental) to a summation circuit 121. A reference voltage is derived from the multivibrator 131 via a filter 123 and a phase shifter 124. The summation circuit 121 with a discriminator 122 provides a D.C. voltage, the sign and magnitude of which is dependent on the phase and magnitude of the potential at M 1 , M 2 . The D.C. voltage is indicated by a galvanometer and is fed to the first grid of the heptode 141 to control the current I 1 so as to tend to reduce the potential at M 1 , M 2 to zero. A selective amplifier 161, rectifier 162 and recording galvanometer are provided to monitor the potential at M 1 , M 2 .