VELOCITY STRUCTURE IN UPPER OCEAN CRUST AT HOLE 504B FROM VERTICAL SEISMIC PROFILES

Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper o cean crust, providing a baseline for comparison with seismic measureme nts elsewhere. We determine P and S velocities from vertical seismic p rofiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 1 48. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, a nd Poisson's ratio as an indicator of lithology, porosity, and structu re. (1) In the P velocity profile, the change in slope marking the lay er 2/3 boundary coincides with the top of the sheeted dike unit. Seism ic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in the se normal incident VSPs, several second arrivals with velocities indic ative of vertically polarized shear energy are observed. P to S wave c onversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on a ngle of incidence as predicted by plane boundary transmission coeffici ent analysis. (3) Vertical velocities determined from the VSP differ b y < 10% from horizontal velocities obtained from the oblique seismic e xperiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (< 1 cm) p ore structure with no measurable anisotropy. The large-scale, well-ori ented vertical fractures, which are formed tectonically, do not have a detectable effect on compressional wave velocities. (4) High Poisson' s ratio in the upper 300 m of basement coincides with an extrusive lay er composed of pillows and thin flows. Low Poisson's ratio at 850-1150 m below seafloor (mbsf) coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes. Relatively large-aspect ratio cracks are required to produce such low values of Poisson's ratio. The cracks were likely created by hydraulic fracturin g when hot dikes encountered low-temperature seawater.