In Situ Measurements of Soil Physical Properties by Acoustical Techniques

Knowledge of porosity, air permeability, pore structure, and surface layering of soils is desirable in agricultural research. The conventional techniques are invasive, typically requiring sample extraction. When monitoring seasonal changes, the conventional methods disrupt considerable proportions of the test plot surface. The sample area used in conventional measurements may be too small to represent the variations in the test plot. Here, the feasibility of acoustical techniques for monitoring surface air porosity (total porosity minus volumetric water content), air permeability, and pore structure and the variation of these properties to depths of several centimeters below the surface is demonstrated. Test soil plots prepared using three soil materials, masonry sand, a Grenada silt loam (fine-silty, mixed, thermic Glossic Fragiudalf) and a Catalpa silty clay (fine, montmorillonitic, thermic Fluvaquentic Hapludoll), were considered. Variation in water content and compaction of each soil material were considered. Both acoustic reflection and transmission measurements were made in the audio-frequency range. The soils are modeled as air-filled, rigid-framed porous media. The acoustic-reflection measurements involve analysis of propagation data from a small loudspeaker and two vertically separated microphones. The acoustic-transmission measurement requires a specially designed probe microphone. Analysis of the acoustic-reflection data yields qualitative indications of the relative air permeability of the soils. The transmission measurement yields information about the changes in air permeability with depth. Quantitative information on surface porosity, air permeability, tortuosity, and layering is presented by fitting theoretical predictions based on the soil model to the measured sound reflection and transmission data. The acoustically determined air porosity for the soils considered is within 10% of the values determined by gravimetric techniques. (H. Hess and K. Attenborough on leave from the Dep. of Engineering Mechanics, The Open Univ., Walton Hall, Milton Keynes MK7 6AA, England.)