GB1408808A - Well logging system with linearity control - Google Patents

Abstract

1408808 Borehole log linearity (gain) control TEXACO DEVELOPMENT CORP 23 Aug 1973 39960/73 Heading G1A A linearity (gain) control for a borehole logging system comprises means for measuring γ-ray counts for two predetermined energy bands of the γ-ray spectrum, means for generating a signal proportional to the ratio of these counts, and means for using this signal to control the linearity of the system. The energy bands selected are centred at 7 and 7.75 MeV corresponding to the top and the flank of an iron emission peak. The ratio of the emissions in these bands maintains a constant value so that deviations in gain of the system can be detected and corrected. As shown, Fig. 1, a borehole log comprises a pulsed neutron source 16 and a detector comprising a scintillation crystal 12 and a photomultiplier 11. A discriminator 13 may be arranged to pass only those pulses above a given height to eliminate the background count. At the wellhead the pulses are amplified and analyzed by means of a multi-channel pulse height analyzer 40 feeding data processing circuits 41 which produce signal representing e.g. the earth porosity which are recorded. Pulses corresponding to two energy bands which are known to produce a constant intensity ratio are selected by means of two single channel pulse height analyzers 23 and 24. The outputs of these analyzers are counted over a suitable time interval and the totals transferred to buffer registers 28, 30 under control of pulses derived from the master oscillator 25, the cycle time of the processing circuitry being slower than the pulsing rate of the neutron source which is also controlled by oscillator 25. The outputs of the buffer registers are applied to D:A converters 31, 32, the ratio of the resulting signals being produced by an analogue divider 33 to provide, via a low-pass filter 34, a control voltage for adjusting the gain of amplifier 22 and, if desired, for controlling the power supply for the downhole circuits. Changes in gain of the system result in an apparent lateral shift of the spectrum thus resulting in a change in the ratio of the signals produced by the selected energy bands which provides a feedback signal to adjust the system gain. A radio-active neutron source may be used instead of the accelerator 16.