Kansas Geological Survey, Open-file Report 2005-18
Richard D. Miller, Julian Ivanov, Jamie L. Lambrecht
KGS Open-file Report 2005-18
Final Report to
U.S. Bureau of Reclamation, Denver
June 30, 2005
A reliable measure of seismic prop-erties as a function of depth is important to the U.S. Bureau of Reclamation’s compre-hensive and accurate appraisal of site response and vibration modes in the thin concrete arch East Canyon Dam in Morgan County, Utah. This applied research project measured in-situ compressional and shear velocities using VSP methods, attempted to map the 2-D compressional- (using turning-ray tomography [Ivanov, 2002]) and shear-wave (Miller et al., 1999) velocity field along the toe road and across the crest of the dam, measured compressional- and shear-wave frequency response as a function of amplitude at a point within the dam, at the concrete/rock interface, and in the rock, and mapped events (possibly reflectors) within the upper 350 ft using high-resolution compressional- and shear-wave seismic reflection techniques (Steeples and Miller, 1990). Seismic reflection profiles were designed to provide information concerning the consistency and geometry of layering between boreholes and identify anomalies (within the upper 350 ft and 1000 ft up-stream and downstream) that might influence realizations. These measurements of velocity and maps of bed geometry were acquired to support the more general sitewide characterization underway by the Bureau of Reclamation.
Complexity of the geologic features at this site inhibited confident pre-survey determination as to how well and if any of the routine geophysical techniques for measuring velocity and mapping variability in the near-surface velocity field would provide meaningful data for incorporation with downstream ground motion simulations. A necessary objective of this applied research project was to evaluate the effectiveness of coincident measure-ments of in-situ compressional and shear velocities using checkshot, VSP, MASW (Park et al., 1999), turning-ray tomography (Zhang and Toksoz, 1998), and high-resolution seismic reflection (Steeples and Miller, 1990). All these methods and associated measurements were focused on rocks within 350 ft of the ground surface. Compressional-wave velocity cross-sections from tomography at the toe correlate extremely well with outcrop and reflection data. MASW data from the toe were consistent with the borehole-measured shear-wave velocity and tomography cross-sections processed for the toe line.
Compressional- and shear-wave high-resolution seismic reflection surveys allowed very crude interpretations of near-surface geometries, which were correlated with moderate confi-dence to what was known about the local geology. Seismic reflection is a technique that is hampered by an extremely irregular geologic setting, such as this one. Dipping layers, highly fractured rocks with several major facture orientations, rocky and irregular near-surface material, unsorted and very attenuative conglomerate rocks, and steep out-of-the-plane cliff faces are all physical characteristics of this site that make using shallow, high-resolution seismic reflection extremely challenging. A combination of indirect evidence (localized scatter events, lateral changes in apparent velocity, changes in signal-to-noise, etc.) and geologic data (borehole measurements and samples, outcrop studies, local setting, etc.) incorporated with the interpret-able primary reflected energy arrivals allow an estimation of the gross seismic characteristics and associated geologic features along these CMP profiles.
Any measurement of velocity and interpretation of bed geometry provides important complementary data for the ongoing Bureau of Reclamation site characterization. The more accurate and laterally complete the complementary seismic measurements the more representa-tive the ground motions simulations. Seismic reflection profiles were used to provide informa-tion concerning the consistency, extent, and geometry of layering between boreholes and identify lateral variations (within the upper several hundred feet extending about 300 ft upstream and around 1500 ft downstream) that might influence realizations. Based on several assumptions it appears the high-velocity conglomerate is about 500 ft thick at the dam along the right abutment and thins to about 350 ft thick at the toe where it thickens to about 500 ft about 400-800 ft from the outlet. From 400-500 ft west of the outlet the conglomerate thins rapidly to depths below resolution limits.
Borehole velocity measure-ments were made in two boreholes—one on the dam crest and one downstream. The unique receiver system provided by the Geological Survey of Canada incorporates three independent pods, each with three sensors polarized to record a different component of the wave field. VSP data were acquired and processed in a fashion to facilitate the measurement of compressional and shear velocities throughout the drilled intervals, specifically taking advantage of the fixed separation between pods for improved estimations of interval velocities. Accuracy of the measured velocity was dependent on the consistency of the source wavelet and coupling of the receiver to the borehole walls. Borehole coupling is critical for optimal interfacing with engineering studies of the dam and incorporation with sonic data acquired using standard wireline logging equipment and techniques. With longitudinal and transverse oriented shear-velocity measurements, it is possible to identify zones within the bore-hole possessing shear-wave anisotropy. However, fracture zones in this geologic setting make it difficult to distinguish anisotropy from poor receiver coupling especially if the directionally dependent differences are expected to be subtle.
The two boreholes were drilled with different targets and specific objectives. The down-stream boring (immediately west of the outlet works) targeted near-surface sediments and helped to establish trends/consistency in material properties away from the structure. A deeper boring placed on the dam crest sampled about 53 ft of the dam thrust block and another 350 ft or so of the natural foundation material. The entire portion of the native materials sampled in these boreholes was the conglomerate observed in outcrop. Two pronounced areas with anisotropy and two areas with velocity inversions were identified in the crest well. Fractured rock is prevalent, causing significant scatter of seismic energy. Correlations between the toe and crest boreholes were not made with great confidence, but some inference can be made about the possible depth to the basal contact from the VSP data.
Coincident with but independent of the surface seismic program originally proposed and carried out by the KGS was seismic response analysis of the dam involving an acentric vibrator dwelling at specific frequencies and recording response characteristics along the crest of the dam. Data recorded within the dam, at the dam/rock interface, and in the rock using the three component, three pod borehole receiver as a function of frequency were used to establish empirical characteristics of the dam/rock interface as it relates to the transmission of seismic energy, specifically shear energy.
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Kansas Geological Survey, Geophysics
Placed online Feb. 10, 2006
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