CALIBRATION, FIELD-TESTING, AND ERROR ANALYSIS OF A GAMMA-RAY PROBE FOR IN SITU MEASUREMENT OF DRY BULK DENSITY

Abstract

This paper describes a new gamma-ray probe for measuring dry bulk density in the field. This equipment can be used with three different tube spacings (15, 20 and 30 cm). Calibration procedures and local error analyses are proposed for two cases: (1) for the case where the access tubes are parallel, calibration equations are given for three tube spacings. The linear correlation coefficient obtained in the laboratory is satisfactory (0.999), and a local error analysis shows that the standard deviation in the measured dry bulk density is small (± 0.02 g/cm3); (2) when the access tubes are not parallel, a new calibration procedure is presented that accounts for and corrects measurement bias due to the deviating probe spacing. The standard deviation associated with the measured dry bulk density is greater (± 0.05 g/cm3), but the measurements themselves are regarded as unbiased. After comparisons of core samplings and gamma-ray probe measurements, a field validation of the gamma-ray measurements is presented. Field validation was carried out on a variety of soils (clay, clay loam, loam, and silty clay loam), using gravimetric water contents that varied from 0.11 0.27 and dry bulk densities ranging from 1.30–1.80 g°Cm−3. Finally, an example of dry bulk density field variability is shown, and the spatial variability is analyzed in regard to the measurement errors.