HR: 08:30h
AN: NS11A-01    [Abstracts]
TI: Investigating the Relationship Between Sedimentology and GPR Data at the Borden Aquifer
AU: * Moysey, S
EM: moysey@stanford.edu
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Irving, J
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Knight, R J
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Gaylord, D R
AF: Washington State University, Department of Geology, Pullman, WA 99164 United States
AU: Allen-King, R M
AF: SUNY at Buffalo, Department of Geology, Buffalo, NY 14260 United States
AU: Stelmack, W
AF: SUNY at Buffalo, Department of Geology, Buffalo, NY 14260 United States
AU: Taylor, K M
AF: Washington State University, Department of Geology, Pullman, WA 99164 United States
AB: Ground penetrating radar (GPR) is becoming an increasingly popular subsurface characterization tool. While GPR data can provide important insights on subsurface heterogeneity, it is not always clear how the reflections in a radar image are related to sedimentary structures in the earth. Interpreting GPR reflection data to obtain a model of the subsurface therefore requires insight into how variations in geologic properties are recorded in the radar signal. We designed a field study to investigate this problem in an unsaturated region of the glacio-fluvial sediment that makes up the Borden aquifer (Canadian Forces Base Borden, Ontario). To conduct our experiment, we first collected 200, 450, and 900 MHz GPR data along a 20 m line transecting an undisturbed portion of a sand quarry. Next we employed a backhoe to expose the section of the subsurface that was imaged by the radar to a depth of about 3 m. This allowed us to map the sedimentary section and collect core samples that could later be used for laboratory measurements. The sediment in the area we studied was texturally diverse, ranging in size from clays to cobbles. Distinctive depositional and erosional features, including large displaced sediment blocks several meters long, made the site a good candidate for GPR imaging. However, upon comparing the radar images to our sediment map we found a poor direct correspondence between the two. Using the position of marker beds in the radar image and the sedimentary section, we have been able to identify subsurface velocity variations as one of the causes of complexity in the radar image. The magnitude of the variations (8-14 m/ns) is consistent with the range of velocities expected at the site based on water content measurements made on core samples (3-40% vol./vol.). We note that this large range in velocity reflects a contrast in the water retention properties of different sediments at the site and would not be as significant in studies conducted under saturated conditions. In addition, using finite difference simulations of GPR wave propagation through a simplified model of the site, we have found that data at late times are complicated by wave interference (i.e., the concurrent arrival of reflected energy from different parts of the subsurface at the receiver antenna). This interference makes it difficult to identify sedimentary features in the radar data, like the large sediment blocks, even after constant velocity migration. We have found that the use of velocity analysis and numerical modeling were critical tools that aided in our interpretation of the radar data in this complex geologic environment.
DE: 0669 Scattering and diffraction
DE: 0689 Wave propagation (4275)
DE: 1875 Unsaturated zone
DE: 1894 Instruments and techniques
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly