Acoustic techniques for studying soil‐surface seals and crusts

AbstractThe impact of raindrops on a soil surface during a rainstorm may cause soil‐surface sealing and upon drying, soil crusting. Soil‐surface sealing is a result of the clogging of interaggregate pores by smaller suspended particles in the water and by structural deformation of the soil fabric, which reduces the infiltration capacity of soils. Soil‐surface crusting refers to the increase in soil strength or mechanical stiffness associated with near surface compaction, densification, or cementation. The formation of soil‐surface seals and crusts may have a profound influence on the erodability of soils, with the consensus being that the reduced hydraulic conductivity due to sealing is a highly significant contributing factor. In this article we discuss two acoustic techniques, one with sensitivity to changes in hydraulic properties (sealing) and the other to changes in mechanical stiffness (crusting). These non‐contact techniques excite the soil using a suspended loudspeaker to impinge acoustic energy from the air (sound) onto the sample. The response of the soil is quantified using a microphone to measure the total pressure above the soil surface and a laser Doppler vibrometer (LDV) to measure the surface solid particle velocity. Changes in soil‐surface hydraulic conductivity are examined by observing the relative change in total pressure at the soil surface. Soil‐surface stiffening is quantified by the ratio of LDV response to the measured total pressure and is referred to as the acoustic‐to‐seismic admittance. Measurements were made on two different soils having different erodability characteristics and subjected to a simulated rainstorm. A fiberglass mesh screen was used to reduce the kinetic energy from the raindrops on one half of the sample. The half of the rained‐on samples for both soils showed a lower acoustic‐to‐seismic admittance than the samples that were only wetted indicating that the raindrop impact produces a stiffer soil surface. The rained‐on side of both soils also has larger acoustic pressures than the wetted side indicating that the impact of raindrops also increases the flow resistivity (decreased hydraulic conductivity) of the surface. Copyright © 2009 John Wiley & Sons, Ltd.