Acoustic Determination of Near-surface Soil Properties

The state of the ground can change dramatically in response to changing meteorological influences and physical disturbances of the ground (e.g. tilling) that are important to many civilian and military activities. Permeability is the fundamental parameter of a porous media that controls whether a surface is an acoustically hard one, through which fluids may not easily penetrate, or conversely a more transparent surface, across which gas and water may readily move. Permeability is the property that controls pressuredriven processes including rain infiltration in soils, surface-atmosphere gas exchange, and acoustic response of the ground. In this paper we describe results of preliminary field tests of the acoustic response of a sand surface under several conditions of moisture and disturbance. We compare these measurements to results of our theory by which a point acoustic source can be used to remotely determine the soil permeability on scales from centimeters to tens of meters across the ground surface. In addition to acoustic means of determining the permeability, we also make a number of direct, co-registered measurements of permeability for comparison.