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Seismic Site Characterization for the Deep Science and Engineering Laboratory (DUSEL) at Kimballton, Virginia
Shumaker, Adam Niven
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The National Science Foundation has announced a plan to establish a Deep Underground Science and Engineering Laboratory (DUSEL) for interdisciplinary research in physics, geosciences, biosciences and engineering. The proposed laboratory will extend to a depth of about 2200 meters and will consist of research facilities for long term study. To date, eight sites in North America have been proposed to host DUSEL. One of these sites, known as Kimballton, is located near Butt Mountain in Giles County in southwestern Virginia. Two seismic lines were acquired along the top of Butt Mountain in June of 2004 to support the ongoing integrated site characterization effort by the Kimballton Science Team. Both lines, approximately 3 km in length, are standard multifold seismic reflection data aimed at imaging faults, thrust sheets, and repeated sections of Paleozoic rocks in the vicinity of the proposed Kimballton site. Crooked line geometry, irregular geophone spacing, ground roll, and poor impedance contrasts between juxtapositioned rock units were challenges in processing the data. Non-standard processing techniques included the use of travel time tomography to accurately constrain near surface velocities, the use of 2D median filters to remove ground roll, and stacking only offsets exceeding 500 m. Interpretation of seismic data supports a triplicated stratigraphic section caused by the stacking of the the St. Clair and Narrows thrust sheets. The St. Clair and Narrows faults are interpreted as shear zones within ductile units of the Martinsburg Formation. 3D travel time tomography was used to build a near surface velocity model of Lines 1 and 2 for the purposes of imaging near surface structure and constraining the extent of topographic lineaments, which are interpreted as bedrock joint systems. Interpretation of the velocity models suggests that the broadly folded strata of the Butt Mountain synclinorium dip gently to the east along the hinge surface. The surface extrapolation of the Lookout Rock fault and the intersection of topographic lineaments with the seismic lines are expressed as low velocity zones that extend to depths of 150 m. This may be related to accelerated weathering along jointed rock surfaces. Results of this study have already been incorporated into the NSF proposal submitted by the Kimballton Science Team (http://www.phys.vt.edu/~kimballton/s2p/b2.pdf).
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