The effect of a gap between the access tube and the soil during neutron probe measurements

Abstract

The neutron probe is a tool employed for the measurement of water content in a soil mass. The presence of a gap between the soil and the neutron probe access tube, filled with either air or water, inevitably introduces a systematic error in neutron probe readings. In this study, experimental investigations and numerical analyses were carried out to evaluate the effects of this gap on neutron probe calibration. The numerical model was developed based on the multigroup neutron diffusion equations and the finite element method. The experiments were conducted in a heavy clay soil. The results show that an air gap of 2.5–30 mm between the soil and a 50-mm-diameter aluminium tube could lead to an underestimation of soil water content by 5–45%, but significant underestimation was apparent for air gaps <10 mm. It is also found that the neutron count is significantly overestimated if the gap around the access tube is filled with water rather than air, but this effect is most significant for larger gaps. The results of this research clearly indicate that a gap between the neutron probe access tube and the soil profile should be avoided during field installation, and that if a gap between the access tube and soil develops during service, a systematic error will be introduced into measurements.