Estimating moisture contents in the vadose zone using cross‐borehole ground penetrating radar: A study of accuracy and repeatability

Abstract

[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.

Cross‐borehole ground‐penetrating radar for monitoring and imaging solute transport within the vadose zone

“…To minimize problems with drift within the GPR system during data collection, a time zero calibration was conducted every tenth transmitter position. The angle between a line connecting the transmitting and receiving antennas and the horizontal was limited to 45 degrees in order to avoid influences of wave reflections from high angle of antenna offset [Peterson, 2001], low signal-to-noise ratios at high angles, and problems associated with the presence of wires buried within the subsurface for communication with other sensors [Paprocki, 2000;Alumbaugh et al, 2002].…”

“…The entire infiltration experiment was divided into two major phases, a tap water infiltration phase followed by a saltwater infiltration phase. Results of the tap-water infiltration experiment from the first phase are given by Alumbaugh et al [2002], LaBrecque et al [2002], and Chang et al [2004]. In the second phase, sodium chloride solutions were employed to introduce a conductive fluid plume.…”

“…XBGPR velocity tomograms, inverted from the first arrival time of EM waves, have been used to characterize static and dynamic changes in subsurface water content [e.g., Hubbard et al, 1997a;Alumbaugh et al, 2000Alumbaugh et al, , 2002Binley et al, 2001;LaBrecque, et al, 2002], and identify water or contaminant pathways [Hubbard et al, 1997b]. Compared to GPR velocity, GPR attenuation is less frequently used in hydrogeologic investigations, and its application for this purpose is still in its infancy.…”

“…At the STVZ site, Alumbaugh et al [2002] determined a site-specific empirical relationship between TDR-measured dielectric constant and soil moisture content for soil samples to be…”

“…By combining equations (1a) and (2), water content can be estimated from EM wave velocity. This method has been used to tomographically image water content between boreholes using radar velocity information [e.g., Alumbaugh et al, 2000Alumbaugh et al, , 2002LaBrecque et al, 2002].…”