Research On High Speed Data Acquisition System Of CNET Pulse Neutron Spectrum Logging Tool

Pulsed neutron element logging was an advanced nuclear logging technology that used to acquire the content of elements and minerals by using a pulsed neutron source to generate fast neutrons into the formation,by measuring the gamma rays emitted by the neutron and formation element nucleus,widely used in complex reservoir lithology identification,unconventional reservoir evaluation and so on.CNET was a three-detector multi-function pulse neutron spectrometry logging tool independently researched by China University of Petroleum(East China).The previous acquisition system was the traditional analog nuclear energy spectrum measurement method,which had a long system dead time and low system resolution and other issues.This paper combined digital multi-channel pulse amplitude analysis theory with nuclear logging instrument acquisition system,and proposed a new digital nuclear energy spectrum acquisition system suitable for CNET logging tool to improve the digitization of previous acquisition system and reduce the dead time of instrument data processing,further improving the resolution of the instrument.This paper firstly analyzed the signal characteristics of nuclear detectors,summarized the principle of digital multi-channel pulse amplitude analyzer and the difference with traditional analog measurement systems,the main factors affecting the digital nuclear energy spectrum acquisition system,and discussed and compared several numerical filtering forming methods.Considering factors such as high count rate scene and ballistic loss,the trapezoidal shaping was finally determined as the digital filtering method of the system.Then the theoretical derivation of the trapezoidal forming method and its forming parameters were studied.On this basis,an innovative fast,medium and slow three trapezoidal forming method was proposed as the system’s acquisition scheme.The simulation experiment was carried out using Matlab software,and its feasibility was verified.Secondly,the hardware part of the acquisition system was designed,including FPGA,ADC selection and pre-amplifier design,and peripheral configuration circuit design.Finally,the FPGA internal program was designed,including the implementation of the trapezoidal shaping algorithm and the implementation of the software architecture.The overall architecture of the system included clock module,high-speed ADC control module,and three trapezoidal forming module as the system acquisition scheme,and analyzed and simulated the functional modules of the method.Through the system analog circuit test,energy linear test,and actual field target test,it can be proved that the new digital acquisition system meets the actual measurement needs,and the performance is significantly improved,compared with the traditional nuclear energy spectrum acquisition system.