National Shipbuilding Research Program

: 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 pressure-driven 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.

Acoustic Determination of Near-Surface Soil Properties

Spatial variability of surface soil moisture in a sloping grassland field

Soil moisture affects soil thermal and dielectric properties and may cause false alarms in detecting manmade objects when dielectric or thermal discontinuities exist in the soil. The spatial variability of soil moisture changes with time and it is important to understand this behavior because it is relevant for detection of small targets, and for modeling background moisture and temperature. Surface moisture of the top 6 cm of soil was sampled on regular grids with an impedance probe at a 0.1-m interval during wetting and drying events, both four days in duration. Maximum variances for data collected in August 2004 increased with decreasing mean moisture, as soil dried following a soaking rainfall. Maximum variances in June 2005 decreased over several days of intermittent rain as the soil rewetted following a prolonged drought. Spatially dependent ranges of approximately 0.5-m lag distance and exponential model fits were consistent among all the data sets, despite changes in moisture, moisture trend, and sample variance. The procession of spatial variation is described by variograms that transition from high to low maximum variances (sills) for wetting events, and from low to high maximum variances for drying events. A linear relationship between the maximum variance and mean of square root of e was consistent for both years, except when the soil was incompletely wetted after a drought. The highest spatial variance in moisture that produced the most variable background for small target detection occurred as a consequence of the incomplete or uneven wetting following a drought.

Spatial and temporal variation of 10-cm background soil moisture

: 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 pressure-driven processes including rain infiltration in soils, surface-atmosphere gas exchange, and acoustic response of the ground. Traditionally it has been assumed that atmospheric acoustic waves do not significantly penetrate the ground. In this paper we describe a new result showing that for some common ground surface materials, acoustic wave propagation in the atmosphere can induce pressure propagation into the ground to sufficient depths to permit the non-intrusive detection of soil permeability across the ground surface.

Non-Intrusive Detection of Soil Properties for Pressure-Driven Processes

Soil moisture is highly variable in space and time and affects the performance of electromagnetic sensors through its effects on thermal and dielectric properties. This research focused on characterizing soil moisture variability at spatial scales relevant to the sensing of small targets. Surface moistures of the top 6 cm of soil were collected on regular grids with an impedance probe. Measurements were made at 0.1-m resolution over 3- × 4-m and 3- × 5-m grids at a short grass site on silt loam. Tall grass and bare soil sites on gravelly silt loam were sampled at 1.0-m resolution over 20- × 30-m and 10- × 30-m grids. Exponential models fit to sample variograms of the 0.1-m resolution data show that soil moistures were spatially dependent over a distance of 0.5 m. Maximum variances (variogram sill), for data collected over a four-day span following a rainfall event, increased linearly with decreased mean moisture level as the soil dried. The revealed structures can be exploited to simulate soil moisture variation temporally and spatially. The impedance probe"s ability to reproduce variation in volumetric water content observed with conventional oven drying methods was demonstrated prior to the field experiment. Separate tests demonstrated that the probes can be used interchangeably. The impact of sparse surface grass on the moisture variation measured with the probe was also demonstrated to be small under the conditions tested.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-resolution surface soil moisture variability at a midwest site

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High-resolution surface soil moisture variability at a midwest site

Spatial and temporal variation of 10-cm background soil moisture