Effect of Moisture and Bulk Density Sampling on Neutron Moisture Gauge Calibration

by Richard G. Allen, Utah State Univ, Logan, United States,
G. L. Dickey, Utah State Univ, Logan, United States,
J. L. Wright, Utah State Univ, Logan, United States,

Abstract: Three moisture and bulk density sampling methods were evaluated for use in neutron gauge calibration. Each of the methods was comprised of a single core or portions of a core taken during installation of a neutron access tube. In addition to direct measurement of bulk density, the effect of using 'smoothed', 'probable' and gamma-probe measured bulk density profiles was evaluated. The use of these three alternative bulk density profiles in the computation of volumetric moisture generally had insignificant effect on the resulting neutron gauge calibration equation. The use of a depth-weighted volumetric moisture profile generally improved calibration statistics, but reduced slopes of neutron calibration equations (% moisture per count ratio). Overall, a total core method which used a tractor-mounted, hydraulically operated coring tool provided the most consistent calibrations with lowest standard errors of estimate, although compression of soil along the perimeter of the cored hole increased subsequent neutron count ratios. A 'Madera' down-hole sampler generally provided good calibrations, also. A third, small-volume, down-hole sampler provided valid moisture and bulk density samples; however, the smaller representative volume of the sampler relative to the sampling volume of neutron gauges adversely affected slopes of some calibration equations.

Subject Headings: Calibration | Moisture | Soil water | Statistics | Slopes | Soil compression | Density (material) | Computing in civil engineering

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