Relative Air Permeability Research Articles

Acoustic determination of air-filled porosity and relative air permeability of soils

SUMMARY Two acoustic techniques for determining the air-filled porosity and air permeability of soil surfaces, and the near-surface variation of these properties with depth, have been applied to several soils in situ. The techniques depend upon an inversion process using data on sound propagation near to the soil surface and through the soil surface. The measured difference in acoustic spectra received by two vertically separated microphones above the ground surface and by probe microphones beneath the surface were matched theoretically to deduce the porosity of air-filled connected pores and an effective air permeability which depends on pore shape and size distribution. This modified permeability parameter and the acoustically deduced porosity varied realistically with surface sealing and compaction. The acoustically deduced porosities were within 10% of those obtained conventionally. Changes in acoustically deduced parameters occurred with changes in moisture content both in soil bin and field experiments. The acoustic techniques were successful in monitoring surface sealing and near-surface layering on a fine scale.

In Situ Measurements of Soil Physical Properties by Acoustical Techniques

AbstractKnowledge 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.

Gas diffusion, fluid flow and derived pore continuity indices in relation to vehicle traffic and tillage

SUMMARYRelative gas diffusivity, air permeability and hydraulic conductivity were measured in undisturbed soil cores from tillage and traffic experiments. Continuity indices were taken as the quotient of relative diffusivity and air‐filled porosity, and of air permeability and air‐filled porosity (and the square of air‐filled porosity). These were applied to individual measurements or to treatment means. More general continuity indices were derived from the changes in flow or diffusion with porosity, where the variations in porosity were due to both field variability and applied changes of water potential. These indices were the exponent in the relationship between relative diffusivity and air‐filled porosity and the slope of log–log plots of air permeability and air‐filled porosity or hydraulic conductivity and degree of saturation. Some physical significance was attached to the exponents by comparison with models of soil porosity. Positive intercepts of the relative diffusivity or air permeability plots on the air‐filled porosity axes were taken as porosities blocked to gas movement.Continuity indices and flow measurements showed differences between tillage and traffic treatments which did not necessarily reflect differences in bulk density. Intrinsic permeability was better estimated from air permeability than from unsaturated hydraulic conductivity.