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

Abstract

Computation of soil moisture content from thermalized neutron counts for the T-Farm Interim cover requires a calibration relationship but none exists for 2-in tubes. A number of calibration options are available for the neutron probe, including vendor calibration, 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 soil water content. The calibration method chosen for this study was a computational approach using the Monte Carlo N-Particle Transport Code (MCNP). Model calibration was performed using field measurements in the Hanford calibration models with 6-in access tubes, in air and in the probe shield. The bet-fit model relating known water content to measured 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 the neutron count rate for the neutron shield and the three calibration models for which data were collected in the field. However, predictions for air were about 65% lower than the measured counts . This discrepancy can be attributed to uncertainties in the configuration used for the more » air measurements. MCNP-simulated counts for the physical models were essentially equal to the measured counts with values. Accurate 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 4 virtual models with the entire set covering a moisture range of 0 to 40%. Predicted counts for the calibration models with 2-in access tubes were 40 to 50% higher than in the 6-inch tubes. Predicted counts for water were about 60% higher in the 2-in tube than in the 6-in tube. The discrepancy between the 2-in and 6-in tube can be attributed to the smaller air gap between the probe and the 2-in access tube. The best-fit model relating volumetric water content to count ratio (CR) is of the form e^A x CR^B with A=0.3596 ± 0.0216 and B=0.4629 ± 0.0629 and r^2= 0.9998. It is recommended that the calibration function based on the count ratio, rather than raw counts, be used to avoid the effects of electronic noise in the probe that may arise due to the conditions at the time of measurement. These results suggest that the MCNP code can be used to extend calibrations for the neutron probe to different conditions including access tube size as well as composition without the need to construct additional physical models. « less