HR: 16:30h
AN: NS14A-05    [Abstracts]
TI: P-wave and S-wave Tomography for Characterizing an Impoundment Dam
AU: Link, C A
EM: clink@mtech.edu
AF: Montana Tech of the University of Montana, Geophysical Engineering 1300 West Park Street, Butte, MT 59701 United States
AU: * Betterly, S J
EM: sjbetterly@mtech.edu
AF: Montana Tech of the University of Montana, Geophysical Engineering 1300 West Park Street, Butte, MT 59701 United States
AU: Romero, S
EM: stephenromero@fs.fed.us
AF: Geotechnical and Dams Safety Engineer US Forest Service, Region 1 , Missoula, MT 59801 United States
AB: A data set consisting of vertical and shear component seismic data was recorded on the crest of an impoundment dam. The data were recorded for the purpose of characterizing the integrity of the dam before transferal of the property to the public domain. Cone penetrometer results were also available for comparison. Four sets of 24 channels were used to record data from 3-component spiked geophones and vertical component data from gimbaled geophones in a parallel land streamer configuration. A sledge hammer with flat plate and spiked shear source was used for vertical, transverse, and inline source orientations. Left-right and front-back hits were used to generate the shear data. At each source location, five orientations of the source were used: vertical, transverse left-right, inline front-back. Thus the final data set consists of nine-component data. A spread consisted of 24 one meter spaced receiver locations. The source was moved through the stationary spread starting and ending two meters off the spread ends for 28 shots per spread. A total of three spreads were shot with spread move-up of one-half spread length. Data processing was done using a diving wave tomography approach and Rayfract software. The tomography approach uses first arrivals. First arrivals for the shear data were picked from differenced transverse shot gathers to remove compressional wave contamination. The transverse shear gathers were equalized before differencing. The resulting P-wave and S-wave velocity profiles were subsequently used to calculate a Poisson's ratio profile for the dam. The P-wave velocity profile shows zones of high velocity, approximately that of water, correlating with suspected zones of seepage. These zones occur at a depth of nine meters. Near the top of the P-wave profile, two low velocity anomalies and one higher velocity anomaly are apparent. No independent data exist to confirm the nature of these anomalies. The S-wave velocity profile does not exhibit the anomalous zones near the suspected seepage zones seen in the P-wave profile. One of the cone penetrometer locations occurs at the center of the S-wave profile. The S-wave profile exhibits somewhat higher velocities at this location that generally corresponds with higher cone penetrometer S-wave values. A Poisson's ratio profile was calculated from the P-wave and S-wave profiles. Although, the Poisson's ratio profile contains a large number of dubious values, two zones show values that correlate well with identifiable velocity zones in the P-wave and S-wave profiles.
DE: 1857 Reservoirs (surface)
DE: 7223 Seismic hazard assessment and prediction
DE: 7299 General or miscellaneous
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly