Using cross borehole ground penetrating radar attenuation tomography for characterizing soil properties in the vadose zone during a two-stage infiltration test

Cross Borehole Ground Penetrating Radar is currently being used to monitor the change in moisture content and to test the feasibility of using attenuation tomography in order to monitor the conductive solute transport in the vadose zone at a test site in Socorro, New Mexico. The test site is a fluvial deposit containing alternating layers of sand and clay. Data collection began in December 1998 using five wells along an 11-meter profile intersecting a three-meter by three-meter infiltrometer. The infiltrometer has been discharging water at a constant rate of 2.7 cm/day since February 1999 in order to ensure the constant flux source. After an initial phase in isolating the specific pre-inversion processing steps necessary to produce the attenuation images, as well as determine the accuracy of this imaging method, our efforts have turned to analyzing the spatial distribution of the substance attenuation properties. The results show that two clay layers with a high attenuation coefficient are identified in the XBGPR images. Comparing the water content determined by velocity tomography, the attenuation increases for about 0.3 Neper/m during the first year of water infiltration, and shows a trend associated to the development of the wetted region. Currently, sodium chloride solution is being discharged through the infiltrometer at the test site, and frequent data collection is conducted to determine changes in attenuation coefficient causing by the saline solution.