Determination of soil water content from terrestrial gamma radiation measurements

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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.