Determination of soil water content from terrestrial gamma radiation measurements

Abstract

Natural gamma radiation emitted by the soil and measured at the surface with a gamma ray spectrometer is a function of the radioactive activity of the soil and the linear attenuation coefficient. The dependence of the linear attenuation coefficient on soil water content is explored for selected soil water profiles by numerical integration. These soil water profiles were generalized distributions based on gravimetric measurements over a sandy soil at the Central Experimental Farm in Ottawa. A comprehensive analysis of the measurements showed that inhomogeneous water distribution accounted for a 1.8% error in the count rate compared to a 2.6% error associated with random count rate fluctuation and instrument error. The depth of the soil layer contributing to natural gamma radiation at the surface depends also on the water content; 90% of the total radiation is contributed by a dry soil of depth 0.18 m, compared to 0.14 m for a soil with a fractional water content of 0.2. The total expected error in the measurement over the range of soil water encountered (0.03–0.24) is shown to be 0.033 for the 0.10‐m layer and 0.025 for the 0.25‐m layer.

Regional Regolith Parameter Prediction Using the Proxy of Airborne Gamma Ray Spectrometry

“…Moisture is also known to influence gamma emission, directly (Loijens, 1980; Jones and Carroll, 1983; de Groot et al, 2009) or through Rn emission (Grasty, 1997). Few studies have addressed the prediction of regolith chemical or mineral contents.…”

Gamma ray attenuation in the soils of Northern Ireland, with special reference to peat

“…The attenuation of radiometric signal due to water or soil moisture is well-established (Carroll, 1981) and while studies of airborne data sets may note or discard unwanted attenuation features due to water or water-content, extensive studies of the radiometric signal attenuation levels in single baseline (meaning one-off) survey data sets are rare. Repeat (timelapse) airborne radiometric surveys have the capability to provide assessments of snow-water equivalent (SWE) and/or soil moisture by estimation of the variation in attenuation, from a known calibrated baseline, due to varying water content (Caroll, 1987;Loijens, 1980). The ability to assess attenuation levels from one-off survey data is considered here.…”

Emerging methods for noninvasive sensing of soil moisture dynamics from field to catchment scale: a review

“…However, a better assessment of the horizontal footprint using more advanced gamma‐ray transport modeling is required to confirm this. The measurement depth similarly depends strongly on the gamma‐ray energy . At a high energy of 2.6 MeV, the measurement depth above which 90% of the measured gamma rays originate is 24 cm in a homogeneous dry soil with a bulk density of 1.0 g/cm, and 15 cm in a dry soil with a bulk density of 1.6 g/cm.…”