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High Vp/Vs ratio: Saturated cracks or anisotropy effects?

Wang, X-Q; Schubnel, Alexandre; Fortin, Jerome; David, Emmanuel; Gueguen, Yves; Ge, H-K

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We measured Vp/Vs ratios of thermally cracked Westerly granite, thermally cracked Carrara marble and 4% porosity Fontainebleau sandstone, for an effective mean pressure ranging from 2 to 95 MPa. Samples were fluid-saturated alternatively with argon gas and water (5 MPa constant pore pressure). The experimental results show that at ultrasonic frequencies, Vp/Vs ratio of water saturated specimen never exceeded 2.15, even at effective mean pressure as low as 2 MPa, or for a lithology for which the...[Show more]

dc.contributor.authorWang, X-Q
dc.contributor.authorSchubnel, Alexandre
dc.contributor.authorFortin, Jerome
dc.contributor.authorDavid, Emmanuel
dc.contributor.authorGueguen, Yves
dc.contributor.authorGe, H-K
dc.date.accessioned2015-12-10T23:16:16Z
dc.identifier.issn0094-8276
dc.identifier.urihttp://hdl.handle.net/1885/64984
dc.description.abstractWe measured Vp/Vs ratios of thermally cracked Westerly granite, thermally cracked Carrara marble and 4% porosity Fontainebleau sandstone, for an effective mean pressure ranging from 2 to 95 MPa. Samples were fluid-saturated alternatively with argon gas and water (5 MPa constant pore pressure). The experimental results show that at ultrasonic frequencies, Vp/Vs ratio of water saturated specimen never exceeded 2.15, even at effective mean pressure as low as 2 MPa, or for a lithology for which the Poisson's ratio of minerals is as high as 0.3 (calcite). In order to check these results against theoretical models: we examine first a randomly oriented cracked medium (with dispersion but without anisotropy); and second a medium with horizontally aligned cracks (with anisotropy but without dispersion). The numerical results show that experimental data agree well with the first model: at high frequency, Vp/Vs ratios range from 1.6 to 1.8 in the dry case and from 1.6 to 2.2 in the saturated case. The second model predicts both Vp/Sv and Vp/Sh to vary from 1.2 to 3.5, depending on the raypath angle relative to the crack fabric. In addition, perpendicular to the crack fabric, a high Vp/Vs ratio is predicted in the absence of shear wave splitting. From these results, we argue the possibility that high Vp/Vs ratio (>2.2) as recently imaged by seismic tomography in subduction zones, may come from zones presenting important crack anisotropy. The cumulative effects of crack anisotropy and high pore fluid pressure are required to get Vp/Vs ratios above 2.2.
dc.publisherAmerican Geophysical Union
dc.sourceGeophysical Research Letters
dc.titleHigh Vp/Vs ratio: Saturated cracks or anisotropy effects?
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume39
dc.date.issued2012
local.identifier.absfor040499 - Geophysics not elsewhere classified
local.identifier.ariespublicationU3488905xPUB1032
local.type.statusPublished Version
local.contributor.affiliationWang, X-Q, Key Laboratory of Seismic Observation and Geophysical Imaging
local.contributor.affiliationSchubnel, Alexandre, Ecole Normale Supérieure
local.contributor.affiliationFortin, Jerome, École Normale Supérieure
local.contributor.affiliationDavid, Emmanuel, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationGueguen, Yves, Ecole Normale Superieure
local.contributor.affiliationGe, H-K, China University of Petroleum
local.description.embargo2037-12-31
local.bibliographicCitation.issue11
local.identifier.doi10.1029/2012GL051742
local.identifier.absseo970104 - Expanding Knowledge in the Earth Sciences
dc.date.updated2015-12-10T09:51:19Z
local.identifier.scopusID2-s2.0-84862178465
CollectionsANU Research Publications

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