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Calibration and Use of Neutron Moisture and Gamma Density Probes in Rocky Soils
Author(s) -
Tokumoto Ieyasu,
Heilman James L.,
McInnes Kevin J.,
Morgan Cristine L.S.,
Kamps Ray H.
Publication year - 2012
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj2012.0167
Subject(s) - water content , soil water , loam , bulk density , soil science , porosity , neutron probe , mineralogy , absorption of water , moisture , neutron , environmental science , materials science , geotechnical engineering , chemistry , geology , composite material , neutron scattering , physics , quantum mechanics , neutron cross section
In situ measurement of water content in rocky soils is difficult because rocks hinder insertion of probes and alter physical properties of the soil used to detect water content. We investigated the impact of rock on neutron probe measurements of volumetric water content (θ), and on γ densitometry measurements of wet bulk density (ρ wet ). Measurements were done in a drum packed with a clay loam having a range of θ and rock density. The annular space between access tube and hole was sealed with expandable polyurethane foam to replicate a field installation in a karst savanna. The relationship between neutron probe count ratios (CR n ) and θ, obtained with different fractions of rock, was nonlinear, and nearly identical to that obtained with soil only, indicating that a single calibration curve could be used, regardless of the amount of rock in the soil profile. The nonlinearity was caused by the foam‐filled void between tube and hole, which affected measurement geometry. Gamma count ratios (CR d ) decreased with increasing wet bulk density, as predicted by theory. However, the foam sealant resulted in higher CR d than predicted by theory because it reduced absorption and increased scattering of γ particles. A utility of using both methods is that dry bulk density can be estimated by subtracting the density of water in the soil volume from ρ wet , allowing locations of the soil profile to be identified where high rock density may restrict water flow and root water uptake.