基于X射线和伽马源的密度测井数值模拟

通过XCOM程序计算和分析不同介质发生光电效应和康普顿散射的衰减系数随射线能量的变化规律，并利用蒙特卡罗模拟方法研究基于X射线和137Cs伽马源的地层散射能谱以及密度测井响应和精度。结果表明，由于所采用的X射线能量比0.662 MeV的伽马射线低，对于同种地层介质，X射线衰减快，密度越大，差异越明显；在源距相同的条件下，较低能量的X射线源地层密度响应灵敏度比137Cs伽马源的高，但受岩性影响更大；在考虑源强的情况下，当X射线源对应源距为33 cm、伽马源对应源距为38 cm时，X射线源的计数统计性与伽马源的计数统计性相当，X射线源的密度响应灵敏度比伽马源的高，且X射线源的密度测量不确定度小，测量精度高，而伽马源的密度测量不确定度大，测量精度低。X射线源可以替代137Cs伽马源进行密度测井，但在仪器设计、能谱处理和密度计算等方面还需要进行深入研究。
The attenuation coefficient of photoelectric absorption and Compton scattering for different energy rays with the medium are calculated and analyzed by XCOM.The Monte Carlo method is used to compare the scattering spectrum, the response of density logging, and the precision of X-ray and Cs-137 sources. The results show that because the X-ray energy is lower than that of the 0.662 MeV gamma rays, the X-ray attenuation is faster in the same formation, and this difference is more obvious withincreased density. At the same source-detector distance, the sensitivity to the density of the lower energy X-ray source is higher than that of the Cs-137 source, but is dependent on lithology.Considering the source intensity,when the source-detector distance is 33 cm for the X-ray source and 38 cm for the Cs-137 source, respectively, the counting statistics is equal between the two sources.The sensitivity to the density of the X-ray source is higher than that of the Cs-137 source, and the uncertainty of density measurements of the X-ray source is lower than that of the Cs-137 source, which indicates that the density logging precision of the X-ray source is higher than that of the Cs-137 source. It is concluded that X-ray source can replace gamma ray source for density logging, but further research in instrument design, energy spectrum processing and density calculation is needed