2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): PERETTI, W.R., KNOLL, M.D., BARRASH, W., CLEMENT, W.P., REBOULET, E.C., Center for Geophysical Investigation of the Shallow Subsurface (CGISS), Boise State University, Boise, ID 83725 wrp@cgiss.boisestate.edu
Keywords: GPR, Heterogeneous, Lithostratigraphy, Hydrostratigraphy
A series of surface and borehole ground-penetrating radar (GPR) data sets were acquired across the central wellfield at the Boise Hydrogeophysical Research Site (BHRS), a research wellfield developed for quantitatively characterizing the distribution of permeability in heterogeneous aquifers using hydrologic and geophysical techniques. The surveys successfully image the complex fluvial stratigraphy in the coarse (cobble-and-sand) braided stream deposits at the site. The goal of the GPR surveys is to develop a radar stratigraphy model of the subsurface to (1) define 3-D geometry of stratigraphy (identify and trace bounding surfaces, identify reflection patterns within bounding surfaces) at successive scales of size and resolution, and (2) relate these architectural elements to lithologic units and to hydrologic parameter distributions. Three-dimensional surface GPR reflection data sets were acquired using 25 MHz, 50 Mhz, 100 MHz, and 200 MHz antennas with the depth of penetration ranging from ~22 m to ~10 m for the 25 MHz and 200 MHz antennas, respectively. Using each antenna frequency, the subsurface was divided laterally and vertically into units or radar architectural elements separated by bounding surfaces at strong contrasts in dielectric properties. Each radar zone is described in terms of its radar characteristics, e.g, location, size, shape, orientation, nature of bounding surfaces (impedance contrasts), internal structure (e.g. cross-bedding), and average dielectric properties (derived from borehole level runs and VRPs). The radar stratigraphy model is hierarchical, recognizing smaller scale features at higher frequencies because of greater resolution at higher frequencies; resolution ranges from ~100 cm for the 25 MHz to ~20 cm for the 200 MHz antennas). The radar stratigraphy model is being compared to lithologic and hydrologic parameter distributions (initially, porosity data). Grain size analysis of continuous cores collected in eight wells is used to describe lithology associated with each radar zone. Neutron logs provide porosity estimates for each radar stratigraphic zone. Synthesis of the radar, grain size and porosity information provides a detailed 3-D model of the radar, lithologic, and hydrologic parameter zonation of the aquifer.
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