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Colin Zelt Aron Azaria (M.S. student) Alan Levander Dept. Earth Science Rice University

Utilizing 3-D seismic refraction to investigate shallow groundwater contamination at Hill Air Force Base. Techniques, findings, and comparisons with other seismic data discussed in detail.

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Colin Zelt Aron Azaria (M.S. student) Alan Levander Dept. Earth Science Rice University

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  1. An application of 3-D seismic refraction traveltime tomography to a shallow (<20 m) groundwater contamination site Colin Zelt Aron Azaria (M.S. student) Alan Levander Dept. Earth Science Rice University Zelt et al. 2006; Geophysics, 71, H67-H78

  2. Case study location Hill Air Force Base, Ogden - UT • Chlorinated solvents disposed of in unlined trenches • Dense liquids migrated through unsaturated zone into the shallow aquifer • Paleo-channel eroded in clay layer acted as contaminant trap • Contaminant accumulated in topographic lows of the channel’s bottom (from: Google Earth)

  3. Paleo-channel from well logs Direct measurements • Superfund site since 1987 • Remediation started in early 1990’s • More than 200 boreholes drilled as part of remediation process • Well logs provide point control on the depth to the impermeable clay formation that constitutes the aquiclude • Direct measurements highlight a paleo-channel eroded in clay layer trending north-south • Pools of DNAPL are present at the base of the paleo-channel • The water table lies approximately 9-10 m below surface

  4. Depth to clay 3-D refraction survey geometry • 601 geophones • 596 shots (349 picked) • 2.8m x 2.1m grid spacing • .223-caliber rifle • 187,877 picks

  5. Spectra, picking and reciprocity • 75 Hz center frequency, up to 200 Hz present • 5 ms pick uncertainty • Pick data in 12 azimuth bin

  6. 1-D starting model and final raypaths • Tested three 1-D starting models • “A” is preferred starting model • Every 200th raypath shown

  7. Preferred and alternate models at z=10m

  8. Checkerboard resolution tests

  9. Horizontal slices of preferred 3-D velocity modeland lateral resolution

  10. Vertical (east-west) slices of preferred 3-D velocity model

  11. 75 Hz Fresnel zones

  12. top of clay layer 1000 m/s isovelocity surface 10 m depth horizontal slice Comparison with other seismic data 3D refraction traveltime tomography (Zelt et al., 2006) 2D full waveform inversion (Gao et al., 2006) 3D reflection (Fradelizio et al., 2008)

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