GB1384099A - Pulsed neutron logging system with gain compensation - Google Patents

Abstract

1384099 Gain control in well logging systems TEXACO DEVELOPMENT CORP 24 Aug 1972 [20 Sept 1971] 39465/72 Heading G1A In a well logging system comprising a sub-surface probe containing a neutron source 16 and a scintillation crystal 12 with a photo-multiplier 11 for receiving the gamma rays from the areas surrounding the sub-surface probe 2 and wherein the height of the pulses emitted by the photo-multiplier 11 represents the energy of the gamma ray causing it, variations in the gain of the crystal or the photomultiplier are corrected by adjusting the gain of an amplifier 24 in dependence upon difference between the measured location of a peak in the spectrum from its known location. As shown the pulses from the sub-surface probe are amplified and fed into an integrator which holds a gate open for a duration depending on the magnitude of the pulse height and allows a high frequency oscillator 29 to supply a channel address register 30 so that at the end of the duration the register 30 has a binary number representing the pulse height. The circuit shown is such that each binary number (or range of numbers) is associated with a particular energy channel 31 and at any instant each channel contains a number representing the sum of the number of counts (or pulses) that have occurred with its associated energy value. The contents of three channel registers 2, 3, 4 are supplied to a signal comparator circuit 46, and initially one channel register 3 is chosen to coincide with a peak in the spectrum (i.e. the 2À23 MEV hydrogen peak) and the channel register generates a voltage signal on line 25 whose amplitude is proportional to the difference between the contents of channel register 3 and the contents of either channel register 2 or channel register 4 whichever difference is larger. Then if any gain drift occurs the signal comparator 46 will detect a change in the relative proportions and will generate an error signal of appropriate amplitude and sign. The error signal is fed back via line 25 to the control input of amplifier 24 of the pulse height analyser and will cause a change in the gain of the amplifier to compensate for the drift. In a modification a plurality of peaks are monitored providing a plurality of error signals which are fed via a logic circuit which controls the amplifier.