Well logging direct memory access system and method

Abstract

A generalized computer-based system and method for acquisition of nuclear well logging data, including downhole generation of energy and time spectral information. A downhole instrument traverses a borehole introducing high energy neutrons into the formation at prescribed depths. Resultant gamma rays are detected and converted into pulses which are amplified and conditioned. A downhole multi-parameter analyzer stores pulse height and arrival time for each detected gamma ray whereby time and energy spectra are formed. Spectra include, alone or in combination, time spectra of the neutron source, inelastic and capture energy spectra. From the source time spectra, peaks are detected and capture and inelastic gates referenced thereto. Also thermal neutron decay between source bursts is derived. From the energy spectra peaks including hydrogen or iron are detected and used for calibration and automatic gain control of the gamma ray detector. High speed buffer means and direct memory access enables access to memory by both the multi-parameter analyzer and CPU so as to avoid inhibiting data acquisition during processing of the acquired spectra by the CPU. Parameters including pulse discrimination level, detector gain, and gate positioning affecting the spectra generated downhole, which may simultaneously include inelastic, capture, and time distribution spectra, are variable in the analyzer by means of CPU control functions.