Monte Carlo-Based Borehole Effect Correction in Uranium Fission Prompt Neutron Logging Using the Epithermal-to-Thermal Neutron Ratio

Borehole effects are a critical source of uncertainty in uranium fission prompt neutron (PFN) logging, particularly in sandstone-type uranium deposits with complex borehole conditions. This study develops a quantitative correction method for borehole diameter effects based on Monte Carlo numerical simulations and the epithermal-to-thermal neutron ratio (E/T ratio). A 1:1 Monte Carlo model of the PFN logging system and a representative sandstone formation model were established to investigate neutron transport behavior under varying borehole diameters and tool positions (centered and eccentered). The simulation results show that both epithermal and thermal neutron count rates decrease with increasing borehole diameter, while the E/T ratio exhibits a nonlinear decreasing trend. This behavior indicates that borehole geometry significantly influences neutron moderation and detection efficiency. Based on the relationship between borehole diameter and the E/T ratio, a correction function for borehole effects was derived using curve fitting methods. The proposed model was validated against Monte Carlo simulation results and experimental measurements, showing relative errors within 10%. The results demonstrate that the proposed method can effectively quantify and correct borehole diameter effects in PFN logging. This provides a physics-based and geometry-dependent correction framework for improving the accuracy of uranium quantification in complex borehole environments.