HR: 1340h
AN: S23B-0323    [Abstracts]
TI: Beyond Delay Time Tomography: the Estimation of Rock Physics Properties From P- and S- Velocity Models.
AU: * Vanorio, T
EM: vanorio@geoazur.unice.fr
AF: UMR-G‚osciences Azur, CNRS-UNSA, 250 rue Albert Einstein, Sophia Antipolis, 06560 France
AU: Virieux, J
EM: viri@geoazur.unice.fr
AF: UMR-G‚osciences Azur, CNRS-UNSA, 250 rue Albert Einstein, Sophia Antipolis, 06560 France
AU: Latorre, D
EM: latorre@geoazur.unice.fr
AF: UMR-G‚osciences Azur, CNRS-UNSA, 250 rue Albert Einstein, Sophia Antipolis, 06560 France
AB: The estimation of physical properties of rocks is a task showing manifold applications in studies of the Earth's crust. Joint P- and S- travel time tomography from microearthquake travel times is a basic tool to assess the local velocity structure in seismically active areas and thus, it provides the spatial distribution of elastic properties at a specific time moment. To characterize porosity and fluid content within the shallow crust, both P- and S-wave velocities are required because P-waves are sensitive to changes in pore fluid, whereas being S-waves mainly depending on rock matrix properties they are relatively unaffected by the pore fluid. Specifically, the translation of velocity images into lithology, porosity and pore filling phase images requires quantitative relations relating the rock elastic properties to its bulk properties and pressure conditions: this goal can be achieved by rock physics effective-medium modeling. We present a methodology to estimate the petrophysical properties from passive seismic data by linking, at the tomographic resolution, P- and S- wave velocities to lithology, porosity and fluid phases. To reconstruct rock velocities, models use the Hertz-Mindlin contact theory and the modified Hashin-Shtrikman bounds both for consolidated and unconsolidated rocks as described in Dvorkin et al. 1999. Models require knowledge of both the shear, bulk moduli, and density of the matrix which are computed from those of the individual constituents using the Hill's average formula and those of fluid phases which are determined on pressure and temperature based equations. Before inversion, trends for dry rocks were compared with laboratory measurements at crustal conditions for site-relevant rocks. The effect of the pore fluid is calculated at different reservoir conditions in the low-frequency domain through the Gassmann's poroelastic theory. Rock property images are finally determined by minimizing the difference between the tomographic and the modeled velocities through the use of local and semi-global inversion scheme. The presented technique is applied to the passive dataset collected in the Campi Flegrei hydrothermal system where the seismic velocity structures recovered by delay time tomography now fit the bulk physical properties of three different rock types.
DE: 8180 Tomography
DE: 8424 Hydrothermal systems (8135)
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting