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Preliminary Report 2/25/05: Seismic Site Characterization for the Deep Underground Science and Engineering Laboratory (DUSEL) at Kimballton, Virginia I. Introduction

The objective of this study was to acquire, process, and interpret two seismic

reflection lines on top of Butt Mountain in Giles County, Virginia, as part of an integrated site characterization for the Deep Underground Science and Engineering Laboratory (DUSEL) at Kimballton. Both lines, approximately 3 km in length, are standard multifold seismic reflection data aimed at imaging faults, thrust sheets, and repeated sections of rock in the vicinity of the proposed Kimballton site. Results from this survey support the existing structural model and suggest that the proposed DUSEL facility would lie in competent limestone units. II. Data Acquisition

Both seismic lines (Fig. 3) were collected in June 2004 along strike of the Butt Mountain Synclinorium (Figs. 4, 5, and 6). Seismic explosion charges were used to record 111 and 84 shots at 20 m spacing along Lines 1 and 2, respectively. Shot holes were drilled to 6 m depth and contained 2 kg of seismic explosive. Two 10 kg shots were used near the end of each line and were drilled to a depth of 12 m. Shot holes penetrated a shallow weathering layer of compacted sand into bedrock with the exception of a short segment on the east end of Line 1 where the weathering layer was exceptionally thick and bedrock was not reached.

This survey consisted of 180 stationary geophone stations. 70 geophones were spaced at 10 m on the western end of Lines 1 and 2, and 110 geophones were spaced at 20 m throughout the rest of the lines. Acquisition parameters may be found in Table 1.

III. Data Processing The processing of Lines 1 and 2 (table 2) was done at Virginia Tech and was directed towards producing the strongest reflectors in the upper 2 km to image rock structures of interest to the DUSEL project. Experimentation indicated that 15 m CDP bins provided the best trade-off between reflector continuity and spatial resolution. The stack fold ranged between 40 and 120. Two-dimensional median filters directed between 330 m/s and 2000 m/s were effective at removing the effects of airwaves and ground roll that remained after bandpass filtering. It was noted that shallow reflectors were better imaged at far offsets, and receivers less than 500 m from the source were not stacked in the section. NMO stacking velocities were chosen to maximize the continuity of reflections in the stacked section. Iterative velocity analysis provided nominal signal improvement beyond the first pass of residual statics. The residual statics time gate focused the

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strongest reflectors in the section, which were between 1600 ms and 2100 ms. Migration was attempted, but created an over-smeared image because of the short line compared to the target depths. Interpretation was done on stacked sections only. IV. Regional Geology and Previous Seismic Work The study area lies in the Valley and Ridge Province, which is the result of thin-skinned deformation during the Alleghanian orogeny. Regional structure is dominated by the Saltville, Narrows, and St. Clair thrust faults. The Butt Mountain Synclinorium (Figs. 4, 5, and 6) lies in the Narrows thrust sheet, which is composed of numerous imbricate faults. The stratigraphy of the study area consists of Cambrian through Ordovician carbonates and Ordovician through Silurian sandstones, claystones, and shales. The section is dominated by massive dolostones, which are capped by a thin, mildly metamorphosed clastic package. .

Surface geology, regional borehole data, and balanced cross sections have been integrated into a regional geologic model first published by Woodward and Gray (1985) and modified by Henika (personal communication 2004). This model indicates that detachment zones originate at the base of the Cambrian carbonate package and propagate into imbricate structures in the Ordovician Martinsburg formation. Stacked thrust sheets triplicate sections of rock in the vicinity of the Butt Mountian Synclinorium (William Henika, personal communication 2004).

Previous regional seismic work by Costain and others (1988) near the Bane Dome (Figs. 2 and 7) shows structures such as regional overthrusts and imbricate faults in the Martinsburg formation. Seismic data acquired in the region has been poor due to sub-surface dissolution of carbonates and poor impedance contrast across rock boundaries. V. Interpretation of Seismic Data Seismic interpretation (Figs. 8 and 9) was guided by the balanced cross sections from Henika (Figs. 5 and 6). Conversion of travel time to vertical depth based upon a borehole will be necessary to confirm the identification of formation boundaries.

The most identifiable feature in both Lines 1 and 2 is the robust reflection at 6000

m, which is interpreted to be the boundary between the Rome formation, a mudrock unit, and the crystalline basement. A strong reflector at 5000 m is interpreted as the Nolichucky-Honaker boundary. Mildly dipping events at 3100 m on Line 1 are interpreted to be truncations of the Millboro shale at the base of the St. Clair detachment. Line 2 is more parallel to strike and has a flatter reflection at 3000 m. Reflections between 2000 m and 2400 m on Lines 1 and 2 are interpreted to be internal reflectivity in the Martinsburg formation. These reflections may be caused by imbricate fault slices or by the interbedding of mudstone and limestone. The top of the Martinsburg formation is interpreted as the Narrows fault.

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Much of the section is discontinuously reflective, as might be expected in thick sequences of dolostones and limestones where impedance contrasts are poor between juxtapositions of similar rock units. In addition to the better constrained stratigraphic and major fault boundaries, the character of remaining reflectivity consistent with Henika’s model. Moreover, no secondary faults were detected, which suggests the construction of DUSEL at Kimballton should not be hampered by complicated geology. VI. Anticipated Refraction Analysis Travel time tomography will be used to build a near surface velocity model of Lines 1 and 2 from the first arrival times of refracted acoustic waves. Preliminary investigation shows that refracted rays should penetrate to a depth of 300 m to 500 m and will constrain near surface rock structure. A refined velocity model may be used in the reprocessing of Lines 1 and 2 to improve resolution at shallower depths. Travel time tomography results will also be aimed at imaging bedrock joints and fracture systems. Drilling conditions and arrival times in shot gathers indicate that an anomalously thick weathering layer exists in conjunction with a low velocity zone towards the east end of Line 1 (Fig. 10). This low velocity zone is coincident with surface lineaments derived from topographic maps (Fig. 11) (Henika, personal communication 2005). Presence of a low velocity zone might confirm the interpretation of topographic lineaments as bedrock joints and fracture systems, which may affect the construction of DUSEL structures on top of Butt Mountain. VII. Summary Results from seismic Lines 1 and 2 are consistent both with previous seismic work and Henika’s structural model derived from surface outcrops. All data indicate that the geology of the Kimballton area is composed of repeated sequences of Paleozoic dolomite, limestone, and clastic rocks. No faults excepting the thrust sheets were detected on seismic data. Proposed facilities at 1500 m and 2200 m depth would lie in competent carbonate formations. Further analysis of travel time data will help map shallow bedrock joints and lineaments. VIII. Acknowledgements William Henika is gratefully acknowledged for contributing his unpublished cross sections, surface lineament maps, and vast knowledge of regional geology.

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IX. References Domoracki, W.J., Costain, J.K., Coruh, C., 1988. Processing and Interpretation of

Seismic Reflection Data Near the Bane Dome in Bland County, Virginia. Department of Geological Sciences, Virginia Polytechnic Institute and State University.

Gresko, M. 1985. Analysis and Interpretation of Compressional (P-wave) and Shear (SH-wave) Reflection Seismic and Geologic Data over the Bane Dome, Giles County, Virginia. Doctoral Thesis, Department of Geosciences, Virginia Polytechnic Institute and State University.

Woodward, N., Gray, D.R., 1985. Valley and Ridge Thrust Belt: Balanced Structural Sections, Pennsylvania to Alabama. Chapter III, Southwest Virginia, Tennessee and Northwest Georgia Sections. Appalachian Basin Industrial Associates.

X. Figures and Tables Table 1: Acquisition Parameters for Line 1 and 2 Table 2: Processing Sequence Figure 1: Location Map Figure 2: Regional Map with Bane Dome and Butt Mountain Seismic Lines Figure 3: Index Map for Lines 1 and 2. Figure 4: Cross Section Index Map Figure 5: Cross Section H-H’ Figure 6: Cross Section I-I’ Figure 7: Bane Dome Stacked Seismic Section Figure 8: Line 1 with and without Interpretation Figure 9: Line 2 with and without Interpretation Figure 10: Shot Gather with Low Velocity Zone Figure 11: Lineament Map of Butt Mountain

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Table 1 Acquisition Parameters for Line 1 and Line 2 Date Recorded June 2004 Spread Type End-on/Shoot through Number of Channels 180 Geophone Spacing 70 at 10 m / 110 at 20 m Geophone Type 10 Hz single / 4.5 Hz array Shot Spacing 20 m Small Charge Size 2 kg Large Charge Size 10 kg Charge Depth 6 m (small shots) / 12 m (large shots) Recording Instrument 24 bit Geometric StrataView Sample Rate 2 ms Record Length 15 s Record Format SEG 2 Number of Shots Line 1 111 Number of Shots Line 2 84 Table 2 Processing Sequence 1 Read SEG 2 data – 15 s 2 CDP crooked line sort – 15 m 3 Pick first breaks 4 Static shift traces with shot delay – (125ms) 5 Spherical divergence correction 1/r 6 Elevation statics – 4000 m/s 1300 m datum 7 Bandpass filter 15-25-75-120 Hz 8 Velocity filter – median method, remove 2000 m/s and 330 m/s 9 Surface consistent spiking deconvolution 10 Sort by offset – exclude channels < 500 m 11 Sort to CDP domain 12 Velocity analysis – constant velocity stacks 13 NMO correction 14 Max power autostatic – 500 ms gate at 1600 ms

Allow 25 ms shift 15 Bandpass filter 15-25-75-120 Hz 16 Coherency filter 17 Trace display with single trace scaling

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Figure 1. Location of the Kimballton site.

Figure 2: Regional map with location of Bane Dome and Butt Mountain seismic lines.

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Figure 3. Index maps for Lines 1 and 2. Lines not recorded concurrently. Line 1: receivers = yellow, shots = red. Line 2: receivers = blue, shots = red.

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Figure 4: Cross section index map. Cross sections A-A’ through I-I’ from Henika

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< - Seismic Constraint - >

Figure 5: Cross section H-H’ (Henika 2005). < -Seismic Constraint->

Figure 6: Cross section I-I’ (Henika 2005).

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Figure 7. Interpreted stack of Line VTVC2A near the Bane Dome from Domoracki (1988).

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Figure 10. Shot gather with low velocity zone.

Figure 11. Lineament map from Henika (2004).