NEUTRON MOISTURE METER CALIBRATION IN LARGE DIAMETER BOREHOLES

In studies of soil moisture flux in the arid, alluvial basins of Nevada, drilling technologies have been developed to easily and economically install 15-cm diam. steel casing to depths of up to 100 m. Laboratory and field calibration of a low source strength (3. 7 multiplied by 10**8 Bq or 10 mCi) neutron moisture meter was carried out to determine the impact of such large casing on tool performance. Both laboratory and field tests showed that a quadratic calibration equation relating returned thermal neutron counts (N) to the volumetric water content ( theta ) of the form N equals b//0 plus b//1 theta plus b//2 theta **2 was acceptable to determine soil profile water content. The density of thermal neutrons returned to the detector was considerably reduced when compared to reported fluxes in traditional small diameter aluminum access pipe. This reduction is attributed to the increased air gap between the tool and the soil, and neutron capture by the steel casing.