HR: 11:50h
AN: S42A-06    [Abstracts]
TI: Debunking Urban Legends about Seismic Velocities in Urban Basins Using Oil Industry Sonic and Density Logs
AU: * Brocher, T M
EM: brocher@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States
AB: I use compressional-wave (sonic) and density logs from 130 oil industry boreholes to debunk several simplistic myths about velocity-depth functions commonly used in site response estimates and computer modeling of seismic waves in basins. The logs are being used to help construct a new 3-D velocity model for the greater San Francisco Bay Area, and to test tomography and gravity models of the region. The logs sample strata to an average depth of 2 km, some as deep as 5.6 km, in several urban sedimentary basins, mainly in northern California. The logs sample primarily Pliocene to Upper Cretaceous clastic sedimentary rocks including shales, sandstones, and conglomerates: higher-velocity chemical precipitates such as carbonates, dolomites, anhydrites, and halites are notably missing. Myth #1: Velocity increases monotonically with depth. Many logs show abrupt, pronounced, and thick low velocity zones often associated with an abrupt downward reduction of the grain size of the sedimentary strata. These low velocity zones often record regional uplift in which coarse grained and higher velocity strata are deposited above finer grained strata. These regional low velocity zones, with thicknesses of several hundred m, may represent an efficient wave guide for seismic wave propagation. Myth #2: There are only small regional variations in the seismic velocities of the basin strata. In the San Francisco Bay Area, logs from wells extending from the Great Valley to offshore basins show a pronounced westward increase in sonic velocity associated with relatively high velocity Miocene strata. Myth #3: The impedance contrast at the bottom of basins is depth independent. Most wells penetrating basement at depths less than 1.5 km around basin edges exhibit a large (30%) impedance contrast. In the Bay Area, wells penetrating basement at depths greater than 3 km in the middle of basins show only a modest impedance contrast. Thus, the logs predict pronounced basin edge effects and more modest conversions in the geographic center of the basins. Myth #4: Strata of the same age have similar velocities. Miocene strata west of the Calaveras fault have much higher seismic velocities than those east of the fault. Eocene strata in the Great Valley have lower seismic velocities than those in the San Francisco peninsula. Myth #5: Velocities systematically increase with the age of the strata. In the Bay Area, coarser-grained Miocene strata west of the Calaveras fault have higher velocities than finer-grained Upper Cretaceous strata in the Great Valley. Myth #6: Vertical velocity gradients always decrease with depth. Numerous logs from offshore basins on the continental shelf and in the Salinas Valley exhibit a velocity gradient that increases with depth within Miocene age strata. Myth #7: Velocities in the basins are low. In the Bay Area, logs from several basins show sonic velocities up to 4.7 km/s at depths between 2 to 3 km, even for Miocene strata, that are comparable to velocities in the underlying Franciscan complex basement. Myth #8: Sonic velocity-density relations are regionally uniform. Although well logs demonstrate that on average the sonic velocity and density obeys Gardner's rule, strata in Bay Area basins have 4-5% lower densities than predicted by this rule.
DE: 7223 Seismic hazard assessment and prediction
DE: 5102 Acoustic properties
DE: 7212 Earthquake ground motions and engineering
DE: 0915 Downhole methods
SC: Seismology [S]
MN: 2004 AGU Fall Meeting