Velocity-porosity relationships in the upper oceanic crust; theoretical considerations

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doi: 10.1029/92JB01464
Author(s): Berge, Patricia A.; Fryer, Gerard J.; Wilkens, Roy H.
Author Affiliation(s): Primary:
University of Hawaii at Manoa, School of Ocean and Earth Science and Technology, Honolulu, HI, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 97(B11), p.15,239-15,254. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. Univ. Hawaii, Sch. Ocean and Earth Sci. and Technol., Contrib. No. 2650. 82 refs.; illus. incl. 2 tables
Summary: We consider here the application of rock physics theories to investigate relationships between seismic velocities and porosities in the shallow oceanic crust. Classical Hashin-Shtrikman limits ignore void shapes and are too broad to provide useful constraints on velocities and porosities. Making some assumptions about the distribution of void shapes improves the constraints. Theories which ignore crack-crack interactions underestimate the effects of porosities on velocities, thus providing upper bounds on velocities and porosities. "Self-consistent" theories overestimate crackcrack interactions and so provide lower bounds. At the high porosities required to reduce basalt from a P velocity of 7km/s in massive form to the 2.2km/s observed in zero-age oceanic crust, however, the bounds are too far apart to be useful. The theories are strictly valid only for very small porosities. Using an algorithm proposed by Cheng for iteratively building up porosity to create a highly porous medium, analogous to differential computation methods traditionally used to improve upon the self-consistent approach, we have devised two hybrid theories, which we term extended Walsh and extended Kuster-Toksöz. These two theories remain approximately valid at the high porosities of oceanic crustal layer 2A to provide useful upper and lower bounds on velocity for a given porosity and pore aspect ratio distribution. We attempt the inverse problem, determining porosity from a given velocity, using on-bottom refraction data collected on the flank of the East Pacific Rise. For 120ka material with a P velocity of 2.5km/s, if our assumptions regarding the aspect ratio distribution are correct, porosity lies somewhere between 24 and 34%. Resolution on slower, zero-age crust (2.2km/s) is poorer: there we predict a porosity between 26 and 43%. Use of shear-wave information would tighten these bounds. Copyright 1992 by the American Geophysical Union.
Year of Publication: 1992
Research Program: DSDP Deep Sea Drilling Project
ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; 18 Geophysics, Solid-Earth; 19 Geophysics, Seismology; Algorithms; Body waves; Crust; Data processing; East Pacific; East Pacific Rise; Elastic waves; Inverse problem; Mechanical properties; Oceanic crust; P-waves; Pacific Ocean; Physical properties; Porosity; Properties; Rock mechanics; Seismic waves; Seismology; Theoretical studies; Upper crust; Velocity structure
Record ID: 1992065439
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.

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