Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

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doi: 10.1029/98JB00766
Author(s): Swift, Stephen A.; Lizarralde, D.; Stephen, Ralph A.; Hoskins, Hartley
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 103(B7), p.15,361-15,376. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 87 refs.; illus.
Summary: Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 148. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, and Poisson's ratio as an indicator of lithology, porosity, and structure. (1) In the P velocity profile, the change in slope marking the layer 2/3 boundary coincides with the top of the sheeted dike unit. Seismic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in these normal incident VSPs, several second arrivals with velocities indicative of vertically polarized shear energy are observed. P to S wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis. (3) Vertical velocities determined from the VSP differ by <10% from horizontal velocities obtained from the oblique seismic experiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (<1 cm) pore structure with no measurable anisotropy. The large-scale, well-oriented 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 layer 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 fracturing when hot dikes encountered low-temperature seawater. Copyright 1998 by the American Geophysical Union.
Year of Publication: 1998
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Body waves; Crust; DSDP Site 504; Deep Sea Drilling Project; Depth; Elastic constants; Elastic waves; Equatorial Pacific; Fractures; Geophysical profiles; Geophysical surveys; Hydraulic fracturing; IPOD; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; P-waves; Pacific Ocean; Poisson's ratio; Porosity; S-waves; Seismic profiles; Seismic waves; Surveys; Upper crust; Velocity structure; Vertical seismic profiles
Coordinates: N011335 N011338 W0834348 W0834357
Record ID: 1998056380
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute.