Neutron gauge calibration model for water content of geologic media

Abstract

A neutron gauge must be calibrated before it can be used to measure volumetric water content. Calibration is a function of the design of the gauge's probe, the geometry of the access hole, and the composition of the material. It is typically derived from an empirical relationship between counts of neutrons and known water contents. For consolidated rocks, this empirical calibration is difficult. A calibration procedure based on a numerical model of a neutron gauge is developed to overcome the difficulties. The theory behind the model is the Three Group Diffusion theory. The input parameters are the gauge's design, the material's properties plus neutron cross sections, and the geometric proportionality factors for the access hole. The cross sections are determined in a graphite pile. The calibration is sensitive to the material's bulk density, thermal absorption and scattering cross sections. With this procedure a neutron gauge may be calibrated to any geologic material.