SEISMIC ATTENUATION IN UPPER OCEAN CRUST AT HOLE 504B

Seismic attenuation and its relationship to borehole stratigraphy in t he upper 1.8 km of ocean basement at Hole 504B are determined from ana lysis of vertical seismic profile (VSP) data. VSP data provide unambig uous measurements of seismic amplitude decay along a vertical propagat ion path through the crust, and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spre ading, scattering, and intrinsic loss mechanisms to this decay. About 60% of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts. The remaining amplitude decrease is concentrated in two layers, at 500-650 and 800-900 meters below seafloor (mbsf) (2 25-375 and 525-625 m below basement), across which amplitude rapidly d ecays and the frequency characteristics of the downgoing wave field ar e significantly and permanently modified. Attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by Q of 10 and 8, respectively. It is likely that th e Q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming process es. The shallow interval coincides with a change in alteration mineral ogy deposited by late-stage fluid flow and may separate lower lavas th at were emplaced within the rift lone from upper lavas that were empla ced by off-axis flow through large lava tubes. Intrinsic attenuation i n the deeper horizon is probably due to an increase in porosity and cr acking associated with either intracrustal deformation or subhorizonta l faulting. There is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf (similar to 725 m subbasement).