碳氧比能谱测井中能谱及探测器响应的数值模拟

利用蒙特卡罗中子粒子(MCNP)联合输运程序,对在井几何条件下地层中碳、氧、硅、钙等元素发生非弹性散射产生的伽马射线的测量谱及不同探测器的响应进行了模拟。结果表明,同种尺寸的BGO探测器比NaI(Tl)探测器的探测效率大;在谱处理过程中BGO探测器选择光电峰和第一逃逸峰的和作为特征能窗,而NaI(Tl)探测器以光电峰和第一、第二逃逸峰的和作为特征能窗。探测效率以及伽马光子计数与源距关系的模拟结果显示,长源距探测器晶体几何体积应为短源距探测器晶体体积的8倍左右时,二者才具有相当的测量精度。此模拟结果与实验谱具有较好的一致性,这为碳氧比能谱测井的谱处理奠定了基础。

Numerical simulations of energy spectrum and detector response in carbon/oxygen spectral log

The inelastic scattering Gamma-ray spectroscopy and different detectors response of C, O, Si and Ca were simulated under the condition of the log geometry by Monte Carlo n-particle transport code. The responses of BGO and NaI(Tl) detectors were performed, which show that BGO detector works more efficiently than the same size NaI(Tl) detector. The sum of the photopeak and the first escape peak was selected as a characteristic energy window using BGO detector in the spectrum processing, while the sum of the photopeak, the first and the second escape peaks was selected using NaI(Tl) detector. The crystal geometry size of the long space detector is about eight times as large as that of the short space detector by simulation of detection efficiency and the relation of the Gamma-ray counts with the different spaces in order to get equivalent statistic precision. The simulation result is agreement with the experimental spectroscopy, which lays a solid foundation for spectroscopy process of carbon/oxygen spectral log.