Aron Azaria (M.S. student) 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

Hill Air Force Base, Ogden - UT 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 One of this superfund sites is located in Utah, just north of Salt Lake city. Chlorinated solvents used mostly to clean jets engines were disposed off in open trenches. The contaminant migrated through the sediments column and accumulated in paleo-channel eroded in a clay layer. HAFB sits on a terrace ~ 100 m above a rural area, were they started to detect contaminant in the shallow aquifer groundwater. (from: Google Earth)

Direct measurements Paleo-channel from well logs 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 The site was listed in the superfund priority list in 1987. Remediation started in the early 90s. Following the traditional approach, a number of direct measurements were performed, highlighting the depth of the clay layer, considered to be the contaminant’s trap. Direct measurements mostly along channel’s bottom Wells clustered in specific areas considered topographic lows Area not sampled, both in the channel and especially outside

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

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

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

Preferred and alternate models at z=10m

Checkerboard resolution tests

Horizontal slices of preferred 3-D velocity model and lateral resolution

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

75 Hz Fresnel zones Map of the buried channel inferred from 3-D post-stack depth migrated reflection data. (b) Depth to the 1000 m/s isovelocity surface from 2-D full waveform inversion of 45 east-west 2-D reflection lines extracted from the 3-D dataset (Gao et al., 2006). (c) Horizontal slice of the velocity model perturbations at 10 m depth from 3-D refraction traveltime tomography (Zelt et al., 2006). The 8 and 11 m depth-to-clay contours from the well data are labeled. White edges are unsampled regions.

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) 1000 m/s isovelocity surface top of clay layer 10 m depth horizontal slice Map of the buried channel inferred from 3-D post-stack depth migrated reflection data. (b) Depth to the 1000 m/s isovelocity surface from 2-D full waveform inversion of 45 east-west 2-D reflection lines extracted from the 3-D dataset (Gao et al., 2006). (c) Horizontal slice of the velocity model perturbations at 10 m depth from 3-D refraction traveltime tomography (Zelt et al., 2006). The 8 and 11 m depth-to-clay contours from the well data are labeled. White edges are unsampled regions.