A COMBINED WEIGHT WINDOW AND BIASING APPROACH BASED ON A SUBSPACE IMPORTANCE MAP IN THE MONTE-CARLO SIMULATION OF GAMMA-GAMMA LITHODENSITY LOGGING TOOL RESPONSES WITH THE REVISED MCLDL CODE

A combined weight window and biasing approach has been developed to op timize the specific purpose McLDL code for simulating the spectral res ponse of gamma-gamma lithodensity logging tools, The method is based o n the use of photon importance maps which are strongly dependent upon the geometric configuration of the logging tool, A subspace in the for mation is taken as an importance region and a one-group importance gen erator is used in conjunction with the current McLDL code, The splitti ng of a photon only occurs within the important subspace, and directio n biasing and the exponential transform are applied to drive photons i nto that region or the detector. The approach has been tested on a com putational benchmark problem and the Atlas Wireline Services' commerci al gamma-gamma lithodensity logging cool. Compared to the results from the previous McLDL code, this new approach has eliminated the unaccep tably large weight fluctuations and substantially improved the code's performance in computational accuracy and efficiency by decreasing the relative standard deviation in total detection yield by an order of m agnitude and increasing the figure of merit by more than 70 times, Als o, the revised McLDL code is more efficient than the general purpose M onte Carlo code MCNP in the simulation of gamma-gamma lithodensity log ging tool responses.