Installation and field calibration of neutron-scatter equipment for hydrologic research in heterogeneous and stony soils

Abstract

The commonly used methods of access tube installation and neutron depth probe calibration were impractical in the stony soils of an agricultural watershed in eastern Pennsylvania. Modification of a portable power-driven drilling machine made it possible to drill straight boreholes to a depth of 24 feet through the soil and the underlying shale beds. Backfilling oversize holes around the access tubes with fine soil material gave satisfactory results. Reasonable correspondence was found between calibration curves from heterogeneous, stony sites and more precise data obtained at a site with alluvial, stone-free soil derived from the stony residual soils upstream. The field calibration curves were roughly parallel to the calibration curve supplied by the manufacturer of the probe but were displaced upward by approximately 4000 counts per minute. Use of the factory calibration curve would be adequate for determining relative changes in soil moisture content but would result in serious error if used for calculating the absolute moisture content.