三探测器密度测井的Monte Carlo模拟

为了改善传统岩性密度测井仪的缺陷，本文基于传统的双 源距探测器密度测井仪，提出在负源距范围内增加一个反散射探测器，构成新的三探测器密 度测井仪器. 利用Monte Carlo方法通用程序MCNP(3B)，从光子与地层相互作用的机理出发 ，计算得到了反散射探测器、长源距探测器和短源距探测器的光子通量的能谱分布、光子通 量与源距的关系、光子通量与地层密度的关系、源距与探测深度的关系以及计数能窗等. 从 结果看，三探测器密度测井仪的长、短源距探测器对地层的响应关系与双源距密度测井仪的 长、短源距探测器一致，而反散射探测器对地层具有明确的响应关系，其响应关系与长、短 源距探测器近似相反，且其计数率很高. 因此，在负源距范围内增加第三个探测器是可行的 ，这将有利于提高密度测井的测量精度和垂向分辨率. 同时表明了Monte Carlo方法在核测 井仪器早期研制中的有效性，对仪器设计具有指导作用.

MONTE CARLO SIMULATION OF THREE-DETECTOR DENSITY LOGGING

To amend the defects of traditional litho density logging tool, we propose a scheme of three detector density logging tool. It adds a backscatter detector in negative spacing range on the basis of traditional dual detector density tool. With Monte Carlo general program MCNP(3B), it starts from the mechanism of the interaction between photon and formation. Then it calculates various responses of backscatter detector, long spacing detector and short spacing detector, gains photon flux spectrum distribution, the change rules of photon flux with spacing, the relations between photon flux and formation density, the relations between spacing and detecting depth, and count window. Results show that the responses to formation of long and short spacing detectors of the three detector density tool are the same as that of traditional dual detector density tool. The backscatter detector has a high count rate and a clear response to formation, which is almost reverse to that of long and short spacing detectors. Therefore, it is feasible to put the third detector in negative spacing range, which will improve measuring precision and vertical resolution. At the same time, it indicates that the Monte Carlo method is effective in the early research of nuclear logging tool, and can direct tool design.