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Errors in Gamma-ray Measurements of Water Content and Bulk Density in Nonuniform Soils¹

ABSTRACT

Recent experimental study of freezing soils has shown gamma-ray measurements of water content and bulk density are in error when the water content and bulk density are not uniform throughout the gamma-ray beam. Many gamma-ray measurements involve nonuniform soils. This study was conducted to determine the magnitude of errors due to nonuniform bulk density and water content. Errors in gamma-ray water content and bulk density were determined for soil-water systems with linear and step-function changes in water content and bulk density and for measurements of water absorption by a nonswelling porous medium. Both single-energy and dual-energy gammaray systems were analyzed.

Large errors in water content and bulk density determined by dualenergy gamma-ray measurements can occur for highly stratified soils. The calculated values may fall outside the range of values within the gamma-ray beam. Relatively small errors occur if the bulk density and water content change linearly in the collimated beam. Both single- and dual-energy systems accurately measure the average water content in the collimated beam if the bulk density of the soil is constant. However, the average water content in the beam may not represent the water content at the middle of the collimated beam and the middle of the preset time period. Errors due to nonuniform soils can be evaluated using the equations and graphs presented.

¹ Contribution from the Dep. of Agron., Oklahoma Agr. Exp. Sta. as a part of project H-1600. Journal Manuscript No. 3450.

² Assistant Professor, Dep. of Agron., Oklahoma State Univ., Stillwater, OK 74074.

Received for publication March 27, 1978. Accepted for publication June 14, 1978.

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