The Application Of Monte Carlo Simulation To Borehole Gamma-Gamma Density And Spectral Gamma Calibrations

The application of the Monte Carlo statistical processing method, originally developed during the Manhattan project in the 1940s, was originally restricted to government lab work with applications focusing on nuclear weapon design, nuclear reactor design and other radiometric processes. With the development of ever more powerful computers, numerical processing that required a room-full of main frames 25 years ago can now be accomplished on a high end lap-top. While the learning curve on the use of MCNP modeling can be steep, the availability of modern PCs and access to the code now allows others to make use of this powerful tool to assist in the development of better borehole nuclear measurements. It is now possible to find commercial providers who will model downhole tools and predict their response to natural gamma radiation in earth formations, and provide real time spectral concentrations based on MCNP and correlation with known physical models. Stability and repeatability of downhole spectral measurements has been difficult to accomplish and these methods promise much better results. In addition, the application of MCNP to other borehole radiometric measurements, such as gamma-gamma density and litho-density may provide users with much needed confirmation of field calibrations results and the normal drift associated with source half-life, borehole corrections, and overall tool performance. MCNP modeling may also be used to design better source shields and calibration fixtures to allow more accurate and safer field operations. This paper will review some of the current MCNP borehole spectral applications, as well as present examples of MCNP applied to slim-hole gamma-gamma density tools, source shields and calibration fixtures.