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SURFACE OF THE EARTH: GLOBAL STUDIES 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL High-Resolution Waveform Tomography at a Ground Water Contamination Site Fuchun Gao, Gian Luigi Fradelizio, Alan Levander, Colin Zelt • Rice University 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. 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. 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. 120
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES Hunting for Ocean Island Moho: The Snipe Bites Back Garrett Leahy & Jeffrey Park • Yale University Vadim Levin • Rutgers University Number of events per station POHA -- 76 RAR -- 166 XMAS -- 67 PPT -- 187 Idealized Data Vertical Radial Transverse RADIAL TRANSVERSE Direct P The structure of oceanic crust conforms in general to a very simple model. However, fine scale variations of this structure in anomalous regions (such as beneath ocean islands) remain poorly resolved. We estimated teleseismic receiver functions (RFs) at three GSN stations (RAR, POHA, XMAS) and one Geoscope station (PPT) on islands in the Pacific region. The RF method is ideal for examining fine structure of the crust because it is sensitive to the interfaces between layers, though less sensitive to average wavespeed. RFs at these ocean island stations display a train of Ps conversions from shallow depths, rather than the isolated Moho conversion typically seen at continental stations. We find that a simple three-layer structure explains the main features of the RFs, with old oceanic crust sandwiched between an extruded volcanic layer and an underplated layer. GSN data indicate that ocean islands share a common structure, a principal feature of which is crustal underplating, with a total crustal thickness of 11-20 km. Pp m s P VERTICAL Pp m s Ps m s Ps Because new receiver function estimators resolve higher frequencies, more detail emerges... V – bottom of the volcano M – original oceanic Moho U – underplated Moho Matched amplitude variation for radial/trans RF on the V interface indicates interface dip or anistropy V oceanic crust reverberation M U G. Leahy and J. Park, Hunting for Oceanic Island Moho, Geophys. J. Int., 160, 1020-1027, 2005. 121
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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