HR: 09:30h
AN: MR31B-07    [Abstracts]
TI: Elastic Anisotropy of Basalt
AU: * Becker, K
EM: becker@geophysik.fu-berlin.de
AF: Freie Universitaet Berlin, FR Geophysik Malteserstr. 74-100 Haus D , Berlin, 12249 Germany
AU: * Becker, K
EM: becker@geophysik.fu-berlin.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg Haus D, Potsdam, 14473 Germany
AU: Shapiro, S
EM: shapiro@geophysik.fu-berlin.de
AF: Freie Universitaet Berlin, FR Geophysik Malteserstr. 74-100 Haus D , Berlin, 12249 Germany
AU: Stanchits, S
EM: stanch@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg Haus D, Potsdam, 14473 Germany
AU: Dresen, G
EM: dre@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg Haus D, Potsdam, 14473 Germany
AU: Kaselow, A
EM: akaselow@simsoftec.com
AF: Seismic Micro-Technology Alps, Rosegger Str. 15 , Leoben, 8700 Austria
AU: Vinciguerra, S
MR31B-07 AF: Osservatorio Vesuviano, Via Diocleviano 328, Naples, 80124 Italy
AB: Elastic properties of rocks are sensitive to changes of the in-situ stress and damage state. In particular, seismic velocities are strongly affected by stress-induced formation and deformation of cracks or shear-enhanced pore collapse. The effect of stress on seismic velocities as a result of pore space deformation in isotropic rock at isostatic compression may be expressed by the equation: A+K*P-B*exp (-D*P) (1), where P=P_c-P_p is the effective pressure, the pure difference between confining pressure and pore pressure. The parameter A, K, B and D describe material constants determined using experimental data. The physical meaning of the parameters is given by Shapiro (2003, in Geophysics Vol.68(Nr.2)). Parameter D is related to the stress sensitivity of the rock. A similar relation was derived by Shapiro and Kaselow (2005, in Geophysics in press) for weak anisotropic rocks under arbitrary load. They describe the stress dependent anisotropy in terms of Thomson's (1986, in Geophysics, Vol. 51(Nr.10)) anisotropy parameters ε and γ as a function of stress in the case of an initially isotropic rock: ε ∝ E₂-E₃, γ ∝ E₃-E₂ (2) with E_i=exp (D*P_i). The exponential terms E_i are controlled by the effective stress components P_i. To test this relation, we have conducted a series of triaxial compression tests on dry samples of initially isotropic Etnean Basalt in a servo-controlled MTS loading frame equipped with a pressure cell. Confining pressure was 60, 40 and 20 MPa. Samples were 5 cm in diameter and 10 cm in length. Elastic anisotropy was induced by axial compression of the samples through opening and growth of microcracks predominantly oriented parallel to the sample axis. Ultrasonic P- and S- wave velocities were monitored parallel and normal to the sample axis by an array of 20 piezoceramic transducers glued to the surface. Preamplified full waveform signals were stored in two 12 channel transient recorders. According to equation 2 the anisotropy parameters are linear functions of the stress exponents. In order to verify the linear dependence of ε and γ from the stress exponents, these exponents and the anisotropy parameters based on the measured velocities have been computed. Parameter D was found from fitting equation 1 to the experimental data. Our experimental results are in an excellent agreement with a linear relation between the exponential terms and the seismic anisotropy parameters as theoretically predicted by equation 2.
DE: 5102 Acoustic properties
DE: 5114 Permeability and porosity
DE: 5199 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005