Skip to main content
ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #248577
Title: Acoustic techniques for studying soil-surface seals and crusts

Author
item LEARY, DEL - Jasco Research
item DICARLO, DAVID - University Of Texas
item HICKEY, CRAIG - University Of Mississippi

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2009
Publication Date: 8/18/2009
Citation: Leary, D., Dicarlo, D.A., Hickey, C.J. 2009. Acoustic techniques for studying soil-surface seals and crusts. Ecohydrology. 2:257-262.

Interpretive Summary: Raindrop impact on some types of newly cultivated soils can cause the formation of surface seals and crusts. Surface seals have low hydraulic conductivity which prevents water from penetrating into the ground thereby increasing the total runoff and sediment yield. Increased soil surface strength associated with crusts will prevent the soil from separating from the underlying soil, thus decreasing the total sediment yield. Studying these soil surface features is important to better understand the potential for soil erosion since each attribute can have an effect on total sediment runoff. Acoustic methods were introduced in an attempt to characterize these fragile but highly influential soil surface properties in a non-destructive way. Two different acoustic measurements were used: Firstly, the level of hydraulic sealing was accomplished using an acoustic reflection measurement where essentially the acoustic echo from the soil surface gives a measure of the hydraulic conductivity. Secondly, a measure of the soil surface strength is simultaneously accomplished by measuring the amount of soil surface vibration induced by the incident acoustic wave. This article reports measurements on two soil samples that showed either very high or very low total sediment yield values in flume runoff experiments. Each soil sample was prepared using identical packing methods and then partitioned using a fiber mesh on half the sample surface. This mesh was used to effectively prevent the additional soil compaction due to the rain droplets on half the sample, but allowing an equal amount of water when the soils underwent a rainfall simulation. The acoustic methods show promise for non-destructive monitoring of soil surfaces. The methods developed allow for repeated measurements thereby allowing for unique measures of how soil characteristics can evolve with time.

Technical Abstract: The 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.