Calibration of a Neutron Hydroprobe for Moisture Measurements in Small-Diameter Steel-Cased Boreholes

Abstract

Acknowledgments I wish to thank Jim Fields of Washington River Protection Solutions and Curt Wittreich of CH2MHill Plateau Remediation Company, LLC for supporting this work as well as the staff at S.M. Stoller Corporation, who provided schematics of probe geometry and field calibration data and were always available to answer questions related to data collection and logging operations. Thanks to Rick McCain in particular for providing schematics on the hydroprobe construction and engaging in helpful discussions that helped to focus the model simulations.v Executive SummaryComputation of soil moisture content from thermalized neutron counts requires a calibration relationship, but none exists for 2-in. tubes installed at the Tank T-106 interim barrier. A number of calibration options are available for the neutron probe, including vendor and field calibration, but none of these methods were deemed appropriate for the configuration of interest. The objective of this work was to develop a calibration relation for converting neutron counts measured in 2-in. access tubes to volumetric soil water content, θ. The calibration method chosen for this study was a computational approach based on Monte Carlo techniques. The Monte Carlo N-Particle Transport Code (MCNP), developed by Los Alamos, was chosen to perform the theoretical analyses of neutron diffusion in air, the probe shield, and in the Hanford calibration models. Model calibration was performed using field measurements in the calibration models with 6-in. access tubes. ) generated mean 16-s counts of 2264 and 2887, respectively. The shield count, N s , typically used as a standard count to compute the count ratio (CR), was 6168. The best-fit model relating water content to neutron counts was an exponential model that was essentially equivalent to that currently being used for 6-in. steel-cased wells. The MCNP simulations successfully predicted N s for the neutron shield and counts N in the three calibration models for which data were collected in the field. However, predicted counts for air, N a , (mean 16-s count= 14) were about 65% lower than the measured counts (mean 16-s count =43). This discrepancy can be attributed to uncertainties in the configuration used for the air measurements. MCNPsimulated counts for the physical models were essentially equal to the measured counts with values of 2370, 1515, and 2840 for models E, F, and G, respectively. Successful prediction of the response in 6-in. casings in the three calibration models was motivation to predict the response in 2-in. access tubes.Simulations were performed for six of the seven calibration models as well as four virtual models with the entire set covering a moisture range of 0 to 0.40 m 3 m -3. Predicted counts for the calibration models with 2-in. access tubes were 40 to 50% higher than in the 6-in. tubes. Differences between the 2-in. and 6-in. tube are attributed to the differences in the measurement geometry. The best-fit model relating θ to CR is of the form . This is more consistent with the ob...

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