Effect of Air‐ and Water‐Filled Voids on Neutron Moisture Meter Measurements of Clay Soil

Abstract

After NMM measurements were taken in a tightly installed access tube, either a small or large annulus was installed in the same borehole. Additional NMM measurements were taken with the annulus filled with air, and then water and ρb and θv were measured. The RMSE of the NMM calibration using all 11 installations was 0.02 m3 m−3. However, if two cores were used for calibration, the ratio of NMM‐measured θv to in situ θv was significantly different (p < 0.05) from measured θv half the time (RMSE, 0.012–0.05 m3 m−3). Small air‐filled voids created drier estimates of θv (bias, −0.039 m3 m−3; p < 0.001), wherease small water‐filled voids were not significantly different from the calibration. Air‐ and water‐filled voids from larger annuli were significantly lower and higher (p < 0.001) than core‐measured θv, with biases of −0.068 and 0.080 m3 m−3, respectively. Although this work does not correct NMM‐predicted θv to matrix θv, it does bound NMM error under field conditions in a cracking clay soil.