Acoustical Determination of Physical Properties of Porous Grounds

Hess, Heather Mary (1988). Acoustical Determination of Physical Properties of Porous Grounds. PhD thesis The Open University.



Conventional techniques for determining air porosity, moisture content, air permeability and crusting features are reviewed. Alternative approaches depending upon inversion of data for sound propagation near to and through the soil surface are presented and their results for both soils and snow are discussed. The inversion techniques depend upon models both for sound propagation near grazing incidence and for the acoustic properties of homogeneous and layered porous materials and these models are described.

Particular acoustic methods, based upon the difference in spectra received by two vertically separated microphones above the ground surface and the spectra received by probe microphones below the ground surface, are used on sand, silt, loam, clay and snow.

It is found possible using acoustical techniques to deduce air porosities at and near the surface to within 10% of their conventionally measured values. The acoustic measurements enabled deduction of an effective flow resistivity parameter in which the actual flow resistivity is multiplied by the square of a pore shape factor ratio. The use of this together with acoustically deduced porosity in monitoring soil condition is discussed. Changes in acoustically deduced parameters are shown to occur with moisture content both in soil bin and field experiments. The acoustic techniques prove successful in monitoring surface crusts and sub-surface layering with depth on a finer scale and at smaller depth intervals than those possible with conventional techniques.

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