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 ε 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.
This paper examines the attribution of data fields required to generate high resolution soil profiles for support of Computational Test Bed (CTB) used for countermine research. The countermine computational test bed is designed to realistically simulate the geo-environment to support the evaluation of sensors used to locate unexploded ordnance. The goal of the CTB is to derive expected moisture, chemical compounds, and measure heat migration over time, from which we expect to optimize sensor performance. Several tests areas were considered for the collection of soils data to populate the CTB. Collection of bulk soil properties has inherent spatial resolution limits. Novel techniques are therefore required to populate a high resolution model. This paper presents correlations between spatial variability in texture as related to hydraulic permeability and heat transfer properties of the soil. The extracted physical properties are used to exercise models providing a signature of subsurface media and support the simulation of detection by various sensors of buried and surface ordnance.
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.