APPLICATION OF AN EFFICIENT MATERIALS PERTURBATION TECHNIQUE TO MONTE-CARLO PHOTON TRANSPORT CALCULATIONS IN BOREHOLE LOGGING

This paper describes a simple, accurate and efficient technique for th e calculation of materials perturbation effects in Monte Carlo photon transport calculations. It is particularly suited to the application f or which it was developed, namely the modelling of a dual detector den sity tool as used in borehole logging. However, the method would be ap propriate to any photon transport calculation in the energy range 0.1 to 2 MeV, in which the predominant processes are Compton scattering an d photoelectric absorption. The method enables a single set of particl e histories to provide results for an array of configurations in which material densities or compositions vary. It can calculate the effects of small perturbations very accurately, but is by no means restricted to such cases. For the borehole logging application described here th e method has been found to be efficient for a moderate range of variat ion in the bulk density (of the order of +/-30% from a reference value ) or even larger changes to a limited portion of the system (e.g. a lo w density mudcake of the order of a few tens of mm in thickness). The effective speed enhancement over an equivalent set of individual calcu lations is in the region of an order of magnitude or more. Examples of calculations on a dual detector density tool are given. It is demonst rated that the method predicts, to a high degree of accuracy, the vari ation of detector count rates with formation density, and that good re sults are also obtained for the effects of mudcake layers. An interest ing feature of the results is that relative count rates (the ratios of count rates obtained with different configurations) can usually be de termined more accurately than the absolute values of the count rates.