Patent GB-1144203-A (GB)
Title: Dual channel well logging system
IPC: E21B47/12
Filed: 1966-04-19
Abstract: 1,144,203. Photo-electric bore hole logging. PAN GEO ATLAS CORP. April 19, 1966 [April 19, 1965], No.17154/66. Heading G1A. [Also in Division H4] In a well logging system in which two subsurface detectors supply information to a surface unit, pulses from the two detectors are each converted into a composite pulse form, each pulse comprising two portions of opposite polarity and of substantially equal area. These pulses are transmitted along a single cable to the surface unit where they are converted back to unidirectional pulses having no overshoots of the opposite polarity. The unidirectional pulses from the two detectors are arranged to have opposite polarity so that they can be separated and fed to two meters. The system may be employed in other acoustic or radiation well logging. In a radiation well logging system, the detectors 12, 14, Fig.1 e.g. scintillation counters or gas-filled counters, produce sharp pulses, of which selected ones are amplified by preamplifiers 16, 18 and stretched by monostable vibrator circuits 20, 22. The output from circuit 20 fires transistor 84, causing a Zener diode 86 to conduct heavily and a pulse to be induced in cable 10 via transformer 24. When the transistor is turned off at the end of a pulse, the energy stored in the transformer is dissipated producing a large overshoot portion of opposite polarity to and substantially equal area as that of the initial output pulse from the transformer. Similar pulses are produced by circuit 22, having the reverse polarity to those produced by the circuit 20. A device may be provided in the subsurface unit for disabling either circuit 20 or 22 while the other is generating an output pulse, so that only one pulse is transmitted along the cable 10 at a time. At the surface, the pulses are applied to a differential amplifer 50, and clipped by diodes 121, 122 between two clipping levels (219), (221), Fig.2A (not shown), which can be varied by varying the setting of a potentiometer 120 to remove noise. Since it is necessary for the correct functioning of the integrator circuit 54 that the areas of the two portions of each pulse should be exactly equal, a potentiometer 120 is adjusted until the difference in areas of the pulse portions is a minimum for the pulses from both the detectors. To equalize the areas completely, a potentiometer 126 is provided which functions to add area to the initial pulse portion and to remove area from the overshoot portion regardless of polarity. These pulses are then applied to an amplifier 123 where they are integrated to provide unidirectional pulses, Fig. 2E (not shown), having a polarity which depends on which detector they originate from. These pulses are then fed to an amplifier 142, whose output stage is differential so as to provide two outputs 56, 58 for two rate meters 60, 62, one of which accepts pulses originating in detector 12 and the other those originating in detector 14. If it is required to use a single conductor cable for the logging operation, switches 44, 46 are provided which connect the top and bottom ends of the conductor 42 to ground, so that it constitutes the grounded outer sheath of a single conductor cable. In addition, the winding 40 of the transformer 24 is removed from the circuit so that only the output appearing across the winding 36 is transmitted to the earth's surface.