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U.S. GEOLOGICAL SURVEY
Data Series 284

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Soil-Moisture Profile Instrumentation

Measurements of soil-water content were col­lected at depth from neutron-probe access tubes using a CPN 503 Hydroprobe manufactured by Campbell Pacific Nuclear International, Inc. The probe uses a 50 mCi Americium-241:Beryllium neutron source, or in System International units, a 1.85 GBq source. This source emits fast neutrons that are not detected by the neutron detectors in the probe. The fast neutrons propagate through the soil, collide with hydrogen atoms in soil water, become thermalized or slowed, and are then reflected back as slow neutrons. The reflected slow neutrons are detected in the tube of the probe and the surface electronic sensor counts each slow neutron event. Counts are accumulated for a specified time interval and recorded. Soil-water content is proportional to the number of slow neutron reflec­tions counted. Because the neutron source and its detector tube can vary in radiation-flux emissions and detections with time, count ratios are used to normalize the field counts for a given set of measurements. Count ratios are calculated by dividing the field counts by standard counts obtained while the neutron probe is within its shield above the ground.

Access tubes at both the vegetated, native soil profile (VS1, VS2, and VS3; fig. 2) and the devegetated, native soil profile (NV1 and NV2; fig. 3) are large-steel tubes with a 140-mm outside diameter and a 6.4-mm wall thickness. These access tubes were installed at the vegetated, native soil site (July 1984) and the devegetated, native soil site (September 1988) using a pneumatically driven downhole-hammer system (Tyler, 1988). Access tubes at the two simulated waste trenches (ET1, ET2, WT1, and WT2; fig. 3) are small-steel tubes with a 51-mm outside diameter and 3.0-mm wall thickness. These access tubes were installed during trench construction (September 1987) with tubes placed in holes that were hand-augured below the trench floor prior to backfilling (Andraski, 1996). Three access tubes (VS1, VS2, and VS3; fig. 2) were used at the vegetated, native soil profile with the neutron probe reaching a maximum recording depth of 13.75, 29.75, and 13.75 m, respectively. Two access tubes were used at each of the other sites (fig. 3) with a maximum neutron probe depth of about 5.25 m.

Neutron counts were accumulated for 30-second time interval at each depth selected within the individual access tubes. At the vegetated, native soil profile (within the instrument shaft area; fig. 2), single readings were obtained at each depth, and readings were obtained at intervals of 0.25 m to a depth of 10.75 m and then 0.5 m below that depth. At the devegetated, native soil profile and the two trenches (within the simulated trench area; fig. 3), two readings were obtained at each depth, and readings typically were obtained at intervals of 0.25 m.

Using count ratios, calibration equations were developed with coefficients of determination greater than 0.96 (Andraski, 1997, p. 1904). The standard error of estimate for small-diameter tubes ranged from 0.017 m³/m³ for measurements at the 0.15 m depth to 0.012 m³/m³ for measurements at depths greater than 0.15 m. For large-diameter tubes, the stan­dard error of estimate was less than 0.009 m³/m³ at all depths.

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