NEHRP Clearinghouse
- Title
- Density and Velocity Relationships for Digital Sonic and Density Logs from Coastal Washington and Laboratory Measurements of Olympic Peninsula Mafic Rocks and Greywackes.
- File
- PB2006105445.pdf
- Author(s)
- Brocher, T. M.; Christensen, N. I.
- Source
- January 2001, 34 p.
- Identifying Number(s)
- USGS/OFR-01-264
- Abstract
- We present data from 16 sonic and 18 density logs from 22 oil test wells to categorize the sonic velocities and densities of Cenozoic sedimentary basins in coastal Washington. We also present laboratory measurements of compressional-wave and shear-wave velocities for 40 basement rocks outcropping in the Olympic Peninsula and Puget Lowland. These measurements were made on twenty-nine mafic rocks (mainly Crescent Formation volcanics) and eleven greywackes at room temperatures and confining pressures up to 1000 MPa (10 Kbar). These measurements represent the first laboratory measurements of the shear-wave velocities for these important bedrock units. We use these well log and laboratory measurements to examine velocity-density relationships for rock suites found in coastal Washington, the Olympic Peninsula, and Puget Lowland. For two of the suites of rocks investigated here, we note significant departures of these measurements from the density/velocity relationship proposed for sedimentary rocks by Gardner et al. (1974). Finally, we use the average velocity and densities determined for sedimentary units in coastal Washington (the Quinault and Montesano Formations and the Hoh rock assemblage) to obtain estimates of the average seismic reflection coefficients in the sedimentary basins found along the coast of Washington. These estimates suggest that seismic waves might be efficiently trapped in the basins during future large earthquakes in the Pacific Northwest, enhancing the ground motions for the basins relative to nearby bedrock sites.
- Keywords
- Greywackes; Mafic rocks; Seismic waves; Boreholes; Test wells; Compression waves; Shear waves; Sonic velocity; Density; Wave velocity; Velocity; Laboratory tests