For the past three years, we have been systematically exploring the issues involved in using ground penetrating radar (GPR) for anti-personnel (AP) landmine detection. Our focus has been on testing and understanding the basic issues using existing commercial GPR. We have investigated the following factors affecting landmine detection: mine characteristics, soil physical properties, soil water content, surface roughness, antenna height and signal polarization. Field testing in controlled conditions and numerical techniques have been used to parametrically study response factors. Based on our research, the AP landmine fabrication characteristics are critical in determining the magnitude and response character, spatial processing is essential to see the targets against background variability, optimal spectral bandwidth is 500 to 2000 MHz and the practical issues of deploying sensors in rough field conditions are a major challenge.
The soil water content distribution at two field sites was measured with the air launched surface reflectivity method using a standard GPR system elevated ~1 m above the surface. Time domain reflectometry (TDR) measurements of water content were also acquired at these sites. At one site, water was applied to the surface in two separate experiments and the water content was measured during drainage. At the other site, a water content profile was acquired across two terrain types, a flat grass field and a corn field. Although the GPR surface reflectivity method was able to map the water content distribution at both sites there were substantial differences between these measurements and those acquired with TDR. The main contributors to these differences are likely scattering and changes in the nature of the gradational air/ground interface, related to spatial variability in water content.
Borehole ground penetrating radar (GPR) was used to measure the temporal and spatial variability of soil water content under uniform wetting and drying conditions. Zero Offset Gather (ZOG) surveys were conducted before and during the uniform infiltration and the subsequent drainage experiments using the PuLseEKKO 100 borehole system with 200 MHz antennas in horizontal access tubes. Time domain reflectometry (TDR) data were collected from 10 vertical probes installed at 0. 1 m increments from 0.1 to 1.0 m below the ground surface. The TDR data were used as standard measures of soil water content to compare with the GPR estimated water content. The electromagnetic wave velocity along the survey profile at about 1 .0 m below the ground surface was estimated using ZOG data by picking the arrival time of the first event. Volumetric water content was calculated using a standard empirical relationship between velocity and water content for each ZOG location. Measured higher soil water content zones are potentially preferential flow areas and were observed in consistent locations throughout both the wetting and drying experiments. The radius of influence of the borehole GPR measurements was about 0.5 m determined theoretically and by comparing GPR and TDR data.
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