Monte Carlo simulation of neutron porosity oil well logging tools: Combining the geometry-independent fine-mesh importance map and one-dimensional diffusion model approaches

The generation of first estimate geometry-independent fine-mesh three-dimensional importance maps with simple one-dimensional diffusion models is demonstrated for the Monte Carlo simulation of the neutron porosity ail well lagging tool response benchmark problem. By combining the approach of using simple one-dimensional steady-state diffusion models for calculating neutron adjoint flux with the geometry-independent fine-mesh-based Monte Carlo importance approach previously developed, an automated and efficient variance reduction. method is obtained for this specific problem. A surprising result is that the converged figures of merit after iteration are consistently larger when the initial importance map is based on the one-dimensional diffusion model rather than that obtained from an analog Monte Carlo simulation.