Design Of Data Acquisition And Processing System For The Pulsed Neutron Oxygen Activation Tool

Oxygen Activation(O/A)logging is a method which is widely used in oil field to measure water flow.The method can simultaneously measure flow velocity in the annulus or the casing,therefore,it has been applied in injection profile measurement and dropout checking.The traditional measurement of O/A tool only depends on the time spectrum,the measurement results of flow velocity can be affected by the formation changes,and the error is large during dynamic measurement.The designed data acquisition and processing system of the pulsed neutron oxygen activation logging tool collected energy and time spectrum,in addition,corrected the time spectrum through energy spectrum,which improves the accuracy of water flow measurement,thus meeting the demand of dynamic measurement.A multi-channel peak acquisition and counting circuit of pulse signal based on dual MCU has been designed.The main functions of circuit include pre-amplification,baseline recovery,pulse broadening,peak detection and count detection,in which the peak detection circuit adopts the method of CFD(Constant Fraction Discriminator),making the circuit structure small and simple.Based on the assumption that the pulse interval conform to a uniform distribution,the time-sharing technology of interrupt has been adopted,which distributes the multi-channel pulse to the dual MCU evenly,and then transmit the data to logging surface system after the spectrum is formed.The circuit has low power consumption and excellent real-time performance,which can collect energy spectrum and time spectrum under the condition of high count rate.According to the characteristics of collected data and the traditional oxygen activation logging interpretation,a method of obtaining the transit time by combining the energy spectrum and the time spectrum has been proposed.This method aims at the problem that the peak position cannot be accurately identified when external interference is severe,which replaces all count values in the time range with the count values near the oxygen peak in energy spectrum.Thus,the peak position on the time spectrum can be obvious to calculate the transit time when dynamic measurement.The results of Cs137 test and downhole test indicated that the energy spectrum data collected was effective and it can provided more precise downhole flow information,which was conducive to the preliminarily judgement of the aquifer and the calculation of flow velocity in complex situations,thus improving the efficiency of water flow measurement.