HR: 16:30h
AN: H14C-03    [Abstracts]
TI: Cross-Borehole GPR Resolution Analysis for Vadose Zone Imaging Using Coupled Unsaturated Flow and Electromagnetic Modeling
AU: * Chang, P
EM: pingyuchang@pchome.com.tw
AF: Chia-Nan University, 60, Erh-Jen Rd., Sec 1, Jen-Te, Tainan, 71710 Taiwan
AU: Alumbaugh, D
EM: alumbaug@cae.wisc.edu
AF: University of Wisconsin-Madison, 2258 Engineering Hall, 1415 Engineering Dr., Madison, WI 53706 United States
AB: Although Cross-Borehole (XB) Ground Penetrating Radar (GPR) is used extensively in shallow groundwater investigations, few studies have been undertaken to analyze XBGPR resolution and accuracy in a dynamic flow environment. This is an important step as it allows the user to asses the images for artifacts that may result due to data collection or processing procedures. To analyze XBGPR resolution, two-dimensional unsaturated flow modeling was conducted to simulate a water infiltration experiment that was conducted at the Sandia/Tech Vadose Zone (STVZ) site. Using a petrophysical model determined from controlled experiments and well log measurements, the spatial variation in water content provided by flow modeling was converted to dielectric constant and electrical conductivity. These parameters were used in the Finite Difference Time Domain (FDTD) Electromagnetic (EM) forward modeling to simulate the response of cross borehole radar signals. The synthetic data were processed in the same manner as the real data collected in the STVZ site, and the inverted tomographic images were then compared with both the vadose zone model and the results of the unsaturated flow modeling to analyze the XBGPR resolution. The results from the resolution analysis suggest that the configurations for XBGPR tomography in the STVZ site may underestimate the dielectric constant for clay layers thinner than 1 m. Moreover, attenuation within the clay layer is overestimated near the boreholes but underestimated between boreholes in the inverted images synthesized by FDTD modeling. Compared to the attenuation images, inverted water content images of the FDTD modeling are more representative of the input model. The inverted attenuation also shows that artifacts appear in the time-lapse images during fresh-water infiltration. These artifacts are caused by the use of straight-ray inversion to approximate curved-ray, near-field propagation of the EM waves. Thus, it is necessary to properly incorporate physics into the inversion algorithms in order to correctly invert for the attenuation in a non-steady-state environment.
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 1894 Instruments and techniques
DE: 0600 ELECTROMAGNETICS
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting