Neutron Probe Calibration in Unsaturated Tuff
- A. N. Elder and
- Todd C. Rasmussen
The measurement of water content in unsaturated media using neutron moisture probes requires a calibration relationship between neutron counts and water contents. Techniques for obtaining the relationship in unconsolidated geologic media may be unsuitable for consolidated media due to the difficulty of extracting undisturbed samples for water content analysis. The calibration relationship, θ = α0 + (α1 + α2ρb)C = α3ρb, with C = neutron counts, and ρb = dry bulk density, provided a good predictor of volumetric water content, θ, for consolidated media at the Apache Leap Tuff Site. Four techniques were employed to obtain data necessary to construct this relationship. Two methods employed a neutron transport code to predict neutron counts from scattering and absorption neutron cross sections. The first method calculated cross sections from elemental compositions, while the second used cross sections obtained from rock samples placed in a graphite pile containing a neutron source. The third method used in situ neutron counts measured in two access holes constructed in unsaturated rock with known water contents estimated from rock fragments excavated surrounding the access holes using a pneumatic hammer. The fourth technique used neutron counts from crushed rock packed into containers and maintained at prescribed water contents. Neutron cross sections obtained from elemental analyses were smaller than graphite-pile values, although the differences were insignificant at the 95% confidence level and the difference in rock water content predictions was small. The neutron transport model provided neutron count estimates consistent with in situ counts and counts from saturated crushed rock in containers.
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