Monitoring surface sealing and crusting of soils and glass beads using acoustic techniques

Abstract

When soils dry after a rain they form a crust or seal that changes both the stiffness and hydraulic properties of a thin surface layer which in turn effects the erodability of the soil. This soil surface crusting/sealing is examined by measuring the acoustic to seismic (A/S) transfer function on laboratory samples. An A/S transfer function measurement is a completely noncontact technique using a suspended loud speaker to impinge acoustic energy from the air onto the sample and a laser Doppler vibrometer (LDV) measures the surface particle velocity. Therefore, this technique measures the seismic energy that has been transferred into the soil from the airborne wave. Several soil samples have been investigated. Also, glass beads, being better known samples, are also examined for dry, crusted, and sealed cases. Samples tested show a quantifiable decrease in the seismic energy transferred as the crust forms but is not as sensitive to changes in sealing. An acoustic reflection technique is proposed to measure changes in the hydraulic properties associated with sealing. These measurements are compared to a poroelastic model proposed by Denneman et al.