[1] Experiments have been conducted to demonstrate the accuracy and precision of moisture content estimates derived from cross-borehole ground penetrating radar (XBGPR) measurements made within the vadose zone. Both numerical simulations and field data demonstrate that although a certain amount of image smearing occurs under ideal conditions the general trends in the spatial variation of the moisture content can be estimated by a simple empirical transformation from images of electromagnetic (EM) wave velocity. The field results are verified by comparing the radar-derived images of volumetric moisture content to neutron log derived values. When an appropriate sitespecific conversion from radar wave velocity to moisture content is applied, a root mean square (RMS) error of 2.0-3.1% volumetric moisture content exists between the two sets. Further comparison of the two different data sets along with analysis of plots of the ray density through each cell indicates that regions of high moisture content are better resolved than regions of low moisture and that most of the discrepancy between radarderived and neutron-derived moisture contents occurs in regions of high moisture content. Better spatial resolution can be provided if dense station spacing is used. However, the amount of extra time required to acquire the extra data may limit the usefulness of the method. Repeatability measurements made with five data sets demonstrate that the precision error of the data acquisition system employed averages about 0.54 ns, which translates to about a 0.5% error in moisture content estimation.
The use of cross-borehole ground penetrating radar (GPR) imaging for determining the two dimensional (2D) in situ moisture content distribution within the vadose zone is being investigated. The ultimate goal is to use the GPR images as input to a 2D hydrologic inversion scheme for recovering the van Genuchten parameters governing unsaturated hydraulic flow.Initial experiments conducted on synthetic data have shown that at least in theory, cross-borehole GPR measurements can provide realistic estimates of the spatial variation in moisture content that are needed for this type of hydrologic inversion scheme. However, the method can not recover exact values of moisture content due to 1) the break down of the empirical expression often employed to convert GPR velocity images to moisture content, and 2) the smearing nature of the imaging algorithm.To test the applicability of this method in a real world environment, crossborehole GPR measurements were made at a hydrologic/ geophysical vadose zone test site in Socono, New Mexico. Results show that the GPR images compare well with the uncalibrated borehole neutron log data. GPR data acquisition will continue once an infiltration test has started, and the results from these measurements will be employed in a 2D hydrologic inverse scheme.
INTRODUCTIONUnsaturated hydraulic conductivity governs unsaturated flow within the vadose zone. The spatial variability of this property is often estimated from measurements of samples collected at a l i i t e d number of boreholes and then extrapolating the results across the region of interest. Often this leads to a detailed knowledge of how the conductivity varies with depth, but little information on how it varies laterally. Also, the sampling process will often disturb the material which can lead to inaccurate measurements. A non destructive technique for estimating the hydrological properties
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