1. 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

2. 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)

3. 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 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

4. 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

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 model and lateral resolution

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

11. 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.

12. 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.