MONTE CARLO SIMULATION OF THREE-DETECTOR DENSITY LOGGING
三探测器密度测井的Monte Carlo模拟

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.