[1] During hydrostatic compression conducted within the elastic regime, P and S-wave velocities measured on porous rock samples generally increase with pressure and reach asymptotic values at high pressures. The increase in velocities can be attributed to the gradual closure of compliant cracks, in which case the high-pressure velocities reflect only the influence of the stiff, non-closable pores. A procedure is presented to extract the complete pore aspect ratio distribution from the pressure dependence of dry velocities, assuming that the rock contains a distribution of cracks with different aspect ratios, and one family of stiff pores having an aspect ratio that generally will lie between 0.01 and 1. The model is able to invert successfully many sets of experimental data on dry sandstones taken from the literature. The pore aspect ratio distribution inverted from dry data can then be used to predict saturated velocities as functions of pressure, by introducing fluid into the pores. For ultrasonic velocity measurements that are performed at high frequencies in the laboratory ($ MHz), the predictions of saturated velocities using effective medium theories match well the experimental data for a good number of sandstone data sets. The saturated velocities thus predicted are always more accurate than those predicted from the Gassmann relations, which underpredict the saturated velocities by a large amount. These results are only weakly dependent on the choice of the effective medium theory.Citation: David, E. C., and R. W. Zimmerman (2012), Pore structure model for elastic wave velocities in fluid-saturated sandstones,

[1] 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 fluidsaturated 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. Citation:

Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth's upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves. Structurally bound water (hydroxyl) is believed to affect seismic properties but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth's interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere-asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.

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