HR: 15:45h
AN: NS43A-07    [Abstracts]
TI: Acoustic Techniques for Studying Soil-surface Seals and Crusts
AU: * Hickey, C J
EM: chickey@olemiss.edu
AF: Nationa Center for Physical Acoustics, Univesity of Mississippi , University, MS 38677, United States
AU: Leary, D
EM: dleary@olemiss.edu
AF: Nationa Center for Physical Acoustics, Univesity of Mississippi , University, MS 38677, United States
AU: DiCarlo, D A
EM: ddicarlo@ars.usda.gov
AF: USDA/ARS National Sedimentation Laboratory, 598 McElroy Dr, Oxford, MS 38655, United States
AB: The impact of raindrops on a soil surface during a rainstorm may cause soil-surface sealing and crusting. Soil- surface sealing is a result of the clogging in interaggregate pores by smaller suspended particles in the water, 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 or densification. The formation of soil-surface seals and crusts have a profound influence on the erodability of soils, with the consensus being that the reduced hydraulic conductivity due to sealing is the more important factor. However, studies note that measured values of seal hydraulic conductivity are few. The reason so few measurements may be because the thickness of the altered surface layer is on the order of millimeters. For example Lee (2006) states that a soil-surface seal consist of two parts: a 0.1mm thick upper skin seal attributed to compaction by the rain drop impact and a deeper 1.5 mm "washed in" zone with decreased porosity due to the accumulation of particles. Bulk density profiles measured using X-radiography show maximum changes in the top 5 mm of the soil. The surface of the ground (soil) has an influence on the propagation of sound outdoors. The porosity, air flow- resistivity, and tortuosity of the ground are the properties, which characterize the influence of the ground on the airborne sound. The air flow-resistivity of a dry soil is equivalent to the hydraulic conductivity of a water-saturated soil. In this presentation 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. The 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 on two different soils having different erodability characteristics and subject to a simulated rainstorm are presented.
DE: 1815 Erosion
DE: 1835 Hydrogeophysics
DE: 1838 Infiltration
DE: 1865 Soils (0486)
DE: 1875 Vadose zone
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly