Patent GB-1396456-A (GB)
Title: Pulsed neutron well logging techniques with background radiation removal
IPC: G01V5/10
Filed: 1973-05-25
Abstract: 1396456 Borehole logging apparatus TEXACO DEVELOPMENT CORP 25 May 1973 [19 June 1972] 25277/73 Heading G1A In a borehole logging system for producing inelastic gamma ray measurements which are corrected for background gamma rays caused by other neutron scattering events, the earth formations are irradiated with relatively short duration pulses of fast neutrons, the background gamma radiation is detected in the intervals between such pulses and counts of this background are stored in counter memory means, total gamma radiations due to inelastic scattering of neutrons plus background gamma radiations are detected during each pulse of fast neutrons and counts representive of the total gamma radiations are made, and the total radiation counts are corrected by means of the stored background counts to provide resultant counts. As shown, the neutron source 11, e.g. a denterium-tritium reacter, is pulsed by a pulsing circuit 14, and gamma rays are detected by a crystal 12, e.g. a thallium doped sodium iodide or cesium iodide, and a photo-multiplier 10. A load or iron shield 13 is placed between the source and detector, and a boron sleeve 15 and plate 16 are also provided to shield the detector from thermal gamma rays. The output of the photo-multiplier is applied via a discriminator 18, amplifier 17, driver circuit 19 and cable 8 to a spectrum stabilizer 23 on the surface. The output of this stabilizer is applied to a pulse height analyzer 24 which analyzes the output in dependence upon signals from gates 22, 44. An oscillater 39 controls the opening time of gates 22, 44 and also the pulsing circuit 14. The gate 44 is operated so that the analyzer 24 detects the stabilizer output in the intervals between pulses of neutrons, whereas the gate 22 is operated so that the analyzer 24 detects the output during the neutron pulses, see Fig.2. As particularly described, the system determines both the carbon/oxygen ratio and the silicon/ calcium ratio of the earth formations. As described with reference to, Fig.3 (not shown), the analyzer 24 consists of four differential triggers each defining an energy window respectively representative of carbon, oxygen, silicon and calcium. Considering the determination of the carbon/oxygen ratio, during the opening times of gate 44, the outputs of the two triggers are applied to the respective up terminals of two up/down decade counters. Then, when the gate 22 is opened the outputs of the two triggers are applied to the respective down terminals of the counters until zero is reached, the outputs thereafter being applied to cascaded decade counters in the case of carbon and to one of a plurality of manually selectable decades. When the count in the selected decade reaches its set value, e.g. 10<SP>2</SP> or 10<SP>3</SP>, the value stored in the cascaded decade counters is displayed by means of light emitting diode digital display devices with the decimal point correctly positioned on the display by the manual selector switch so that the C/O ratio is displayed. The Si/Ca ratio is determined in the same manner. The ratios can also be recorded on a chart recorder 30 together with outputs from a porosity computer 27, a line fraction computer 28 and a water saturation computer 29 (not described).