High-Resolution Waveform Tomography at a Ground Water Contamination Site
High-Resolution Waveform Tomography at a Ground Water Contamination Site
Credit:
Fuchun Gao, Gian Luigi Fradelizio, Alan Levander, Colin Zelt • Rice University/IRIS Consortium
Description
Location of the experiment site in UTM coordinates (left plot), the correlation of the waveform tomography velocity model with two lithologic logs and a depth migrated reflection image (top right), and the reconstructed 3D geometry of the paleo-channel (bottom right). In the left plot, parallel black lines are geophone lines, red dots are source locations in the 3D reflection experiment and the light yellow line is the location of surface spread of the VSP experiment. The color map shows the depths to top of clay based on boreholes. White circles are borehole locations. A velocity model from travel time tomography is used in the depth migration. The origin point (0,0) of the site map (left) is at (570127.22 m, 89219.48 m) in UTM coordinates.
Shallow (< 20 m) High-Resolution seismic compressional velocity models have been constructed at a ground water contamination site at Hill Air Force Base (HAFB) to identify the base of a paleo-channel in a clay aquitard buried beneath alluvium, using data recorded by 630 PASSCAL instruments. The dataset has useful energy between ~10 Hz and ~250-350 Hz. Waveform tomography has been applied to a VSP-2D surface dataset and a 3D surface reflection dataset (Figure, left). The velocity model from the former application (Gao et al., 2005) reveals surprisingly large vertical and lateral velocity heterogeneities, which may compromise some conventional seismic imaging tools (Figure, top right). The vertical velocity gradient is ~80m/s/m in the paleo-channel. Lateral heterogeneities in velocity as large as 200 m/s occurring over ~1.5 m are recovered in the model. The structural details in the model correlate well with two lithologic logs and a post-stack depth-migrated image, using the 2D data recorded at the surface between the two VSP boreholes.
Waveform tomography applied to the 3D surface reflection dataset yields 45 2D velocity models for profiles sorted out from the 3D dataset. The locations of the 45 2D seismic profiles are the 45 geophone lines shown as black lines in (Figure, left). From each of the 45 waveform tomography models, the cross-sectional geometry of the buried paleo-channel can be identified by following the velocity contour of 800m/s. The combination of the 45 cross-sectional images gives the 3D geometry of the paleo-channel bottom (Figure, bottom right). The 3D geometry reconstructs the structural host for the polluted ground water. Since the pollutant DNAPLs are heavier than water, they are believed to pond at the deepest points of the paleo-channel. The reconstructed geometry can be used for placement of extraction wells at the site to aid remediation efforts.
Gao, F. , A. Levander, R. G. Pratt, C. Zelt and G. Fradelizio, Waveform tomography at a ground water contamination site: VSP-surface dataset, Geophysics (in press), 2005.
Photographer / Contributor: Fuchun Gao, Gian Luigi Fradelizio, Alan Levander, Colin Zelt • Rice University
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