GB1354244A - Well logging system - Google Patents

Abstract

1354244 Well logging TEXACO DEVELOPMENT CORP 13 May 1972 22578/72 Heading G4H [Also in Division G1] In a well logging apparatus including a borehole transmitter, Fig. 1 (not shown) which generates a condition signal corresponding to the condition sensed in the borehole and a reference signal, these two signals are received at the surface and the condition signal is compensated for any deterioration thereof in accordance with a change in amplitude of the reference signal to provide a corrected condition signal. The condition signal is produced by a scintillation counter (10, 12) responding to the neutron-induced or natural γ-radiation from the borehole. The reference signal is produced either from an oscillator (14) in the transmitter or by doping the scintillator with an α-emitter. A discriminator may be included in the transmitter to reduce noise, Fig. 7 (not shown). The condition and reference signals are combined, Fig. 3C (not shown) and transmitted to the surface circuitry along an armoured coaxial cable 3. Analogue measurement.-The combined signal is fed to a spectrum stabilizer 59, Fig. 2, where it is compared with a pre-set D.C. level from a source 60. Any variation of the reference signal component from the D.C. level causes an output to a servomotor 55 which alters the position of the wiper of potentiometer 53 so as to maintain the level of the reference signal component. The corrected condition signal is fed to a pulse height analyser 85 and to a plurality, 90-90C, of processing networks. The output from each network corresponds to the number of pulses whose amplitudes are within a predetermined energy range. Recorders 94, 94A each receive two outputs from different networks, and also a signal defining the position of the transmitter in the borehole. Digital measurement.-The combined signal, Fig. 3C (not shown) is fed to an A/D converter 151 controlled by a digital stabilizer 152. The reference pulses in the digital signal provide a reference which is centred in a digital window in the stabilizer. The conversion gain of the converter 151 is adjusted to maintain statistically equal counts in the upper and lower window halves. Any drift in gain in the system will upset the statistical equality in the window halves and the converter gain will be adjusted by the stabilizer to correct for the drift. The corrected output from the converter 151 is applied to switching means 156. This switch is controlled by pulses defining the position of the transmitter in the borehole. The output from the switch 156 is applied, via either one of memory units 160 and switch 188, to a tape recorder 185.