Elastic wave velocities within oceanic Layer 2 from sonic full waveform logs in Deep Sea Drilling Project holes 395A, 418A, and 504B

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doi: 10.1029/JB095iB06p09189
Author(s): Moos, Daniel; Pezard, Philippe A.; Lovell, Michael A.
Author Affiliation(s): Primary:
Stanford Univ., Dep. Geophys., Stanford, CA, United States
Lamont-Doherty Geol. Obs., United States
Univ. Nottingham, United Kingdom
Volume Title: Special section on Logging and downhole measurements in Deep Sea Drilling Project/Ocean Drilling Program deep crustal holes
Source: Special section on Logging and downhole measurements in Deep Sea Drilling Project/Ocean Drilling Program deep crustal holes. Journal of Geophysical Research, 95(B6), p.9189-9207. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 38 refs.; illus. incl. 2 tables, sketch map
Summary: Multichannel full waveform acoustic logs were recorded in Deep Sea Drilling Project (DSDP) hole 418A during Ocean Drilling Program (ODP) leg 102, DSDP hole 395A during ODP leg 109, and DSDP hole 504B during ODP leg 111, to provide nearly continuous measurements of elastic wave velocities as a function of depth within oceanic layer 2 for different spreading rates and crustal ages. The velocities depend primarily on the morphology of the basalt. Massive units have V p above 5 km/s, V s above 2.8 km/s, and V p /V s below 1.9. Their velocities increase with depth as expected from crack closure due to confining stress and are lower where fractures are present. V p and V s within pillows are quite variable but in general are lower than in massive units, and V p /V s is higher. Velocities within breccias are similar to or lower than those of pillows. Within a given morphology the elastic properties do not depend on spreading rate. Within pillows, velocity variations depend primarily on the degree of alteration infilling and the mineralogy of the infilling material, and secondarily on conditions at emplacement. Coherent shear arrivals are sometimes not found within shallow pillows, and in extreme cases the compressional arrival is also incoherent, because of scattering resulting from the similarity between the sonic wavelength and the pillow size. This "sampling bias" in the sonic log is the most likely cause of somewhat higher average sonic than seismic velocities measured within extrusive basalts at shallow depths. Other causes include the fact that laterally finite massive units may be oversampled by borehole measurements and that voids between pillows and fractures, whose size is within an order of magnitude of the sonic wavelength, have less effect on sonic than on seismic velocities. The crustal section can be separated only roughly into seismic layers on the basis of the sonic velocities. Changes in velocity with depth within the extrusive section at a single site giving rise to previously defined seismic layers 2A and 2B are due to a change with depth in the properties of the pillows, associated with differences in the amount and type of infilling materials, rather than to a change with depth in the relative proportion of pillows and massive units. Because alteration occurs early and its history is different at different depths, depth-dependent variations in pillow properties are (1) established at young age and (2) persist even in old crust. Within the intrusive section of DSDP hole 504B, velocities are similar to or higher than those measured in saturated minicores at atmospheric pressure, and seismic and sonic velocities are approximately equal, suggesting that little large-scale porosity is present. Copyright 1990 by the American Geophysical Union.
Year of Publication: 1990
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; Acoustical logging; Atlantic Ocean; Basaltic layer; Basalts; Crust; DSDP Site 395; DSDP Site 418; DSDP Site 504; Deep Sea Drilling Project; Elastic waves; IPOD; Igneous rocks; Interpretation; Leg 102; Leg 109; Leg 111; Leg 137; Leg 140; Leg 148; Leg 45; Leg 52; Leg 53; Leg 69; Leg 70; Leg 78B; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Tectonophysics; Upper crust; Velocity; Velocity structure; Volcanic rocks; Well-logging
Record ID: 1990053607
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.

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