GB1511278A - Measurement of subsurface formation lithology - Google Patents

Abstract

1511278 Neutron well-logging TEXACO DEVELOPMENT CORP 12 Sept 1975 [12 Dec 1974(<SP>3</SP>)] 37532/75 Heading G1A [Also in Division G4] In neutron well-logging, the measured 7-ray spectrum of the earth formation surrounding the borehole is compared with a composite weighted mixture of standard 7-ray spectra of substances postulated to exist in the formation, to derive the relative concentrations of elements in the formation; and these values are further combined with a function of the macroscopic thermal-neutron capture cross-section of the formation to derive its primary parameters. A pulsed fast neutron source 9, 8 (e.g. a D-T accelerator) irradiates a well-casing 4 and surrounding earth formations 3. γ-rays resulting from thermal-neutron capture, detected by a scintillatorphoto-multiplier 12, 11, provide energy-responsive pulses to gates 21a, 21b, 21c, which gate the pulses during three periods following each neutron pulse, the last period containing substantially only background radiation. Pulses thus sorted are applied to a #-computer 23 for calculating the macroscopic thermal-neutron capture cross-section for the formation, and to a pulse-height analyzer 25 for separating the γ-ray energy levels. The measured spectrum formed by the analyzer 25, and standard spectra, stored in a source 30, of known substances which might exist in the formation, are fed to a comparison computer 42 which, by least-squares estimation, finds the relative concentrations of those substances which best fit the measured spectrum. This information, combined in a parameter computer 46 with information from the #-computer 23, yields the quantities of the substances present, for application to a recorder 50.