A series of three-dimensional (3-D) ground-penetrating radar (GPR) data sets were acquired<br>over the central wellfield area at the Boise Hydrogeophysical Research Site (BHRS). The survey<br>region is 30 m x 18 m and encompasses 13 wells. The goal of the surveys is to image the<br>complex fluvial (cobble-and-sand) stratigraphy around the wellfield. These images will be used<br>to construct 3-D models of the sedimentary architecture and to help constrain fine-scale models<br>of hydrologic and geophysical parameters at the site. The data sets were acquired using 25 MHz,<br>50 MHz, 100 MHz and 200 MHz antennas. Depth of penetration ranges from -9.6 m for the 200<br>MHz data to -22 m for the 25 MHz data. Processing significantly improves the reliability and<br>interpretability of the images. The images suggest that the deposit can be subdivided laterally<br>and vertically into several distinct units or radar architectural elements; these elements are<br>typically separated by erosional bounding surfaces. Horizontal bedding, cross-bedding and<br>channel structures are clearly evident in the 100 MHz and 200 MHz data, and a clay layer that<br>underlies the cobble-and-sand aquifer at -20 m depth is successfully imaged in the 25 MHz and<br>50 MHz data. The water table, at a depth of l-2 m, is imaged in the 100 MHz and 200 MHz data.<br>Time slices and vertical cuts through the data volumes are used to identify the shape and<br>orientation of the different architectural elements, and to accurately locate important<br>hydrostratigraphic boundaries. These data are being used to construct a 3-D model of the<br>hydrogeologic zonation of the aquifer. Hydrologic and geophysical parameter values associated<br>with each zone will be determined from additional field measurements (e.g., hydraulic tests in<br>wells, crosshole radar and seismic tomography, transient electromagnetics, and well logs). The<br>3-D GPR surveys provide valuable information about the location, scale and geometry of<br>different stratigraphic units at the BHRS.


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