The role of S-wawe velocities in determining the elastic anisotropy of rocks: number modelling and lab measurement

0444549 - GLÚ 2016 RIV CA eng C - Conference Paper (international conference)
Svitek, Tomáš - Lokajíček, Tomáš - Petružálek, Matěj - Vavryčuk, Václav
The role of S-wawe velocities in determining the elastic anisotropy of rocks: number modelling and lab measurement.
Innovations in Applied and Theoretical Rock Mechanics. Montréal: Canadian Institute of Mining, Metallurgy and Petroleum, 2015. ISBN 978-1-926872-25-4.
[International Congress of Rock Mechanics /13./. Montréal (CA), 10.05.2015-13.05.2015]
R&D Projects: GA MŠMT LH13102; GA ČR(CZ) GAP104/12/0915; GA ČR(CZ) GAP210/12/1491; GA ČR GA13-13967S
Institutional support: RVO:67985831 ; RVO:67985530
Keywords : acoustic properties * body waves * geomechanics * microstructures * seismic anisotropy * wawe propagation
Subject RIV: DC - Siesmology, Volcanology, Earth Structure

The most common type of waves used for probing anisotropy of rocks in laboratory is the direct P wave. Information potential of the measured P-wave velocity, however, is limited. In rocks displaying weak triclinic anisotropy, the P-wave velocity depends just on 15 linear combinations of 21 elastic parameters, called the weak-anisotropy parameters. In strong triclinic anisotropy, the P-wave velocity depends on the whole set of 21 elastic parameters, but inversion for 6 of them is ill-conditioned and these parameters are retrieved with a low accuracy. Therefore, in order to retrieve the complete elastic tensor accurately, velocities of S waves must also be measured and inverted. For this purpose, we developed a lab facility which allows the P- and S-wave ultrasonic sounding of spherical rock samples in 132 directions distributes regularly over the sphere. The velocities are measured using a pair of P-wave sensors with the transmitter and receiver polarized along the radial direction and using two pairs of S-wave sensors with the transmitter and receiver polarized tangentially to the spherical sample in mutually perpendicular directions. We present an inversion method for elastic parameters describing general triclinic anisotropy. We demonstrate on synthetic tests that the inversion becomes more robust and stable if the S-wave velocities are included. This applies even to the case when the velocity of the S waves is measured in a limited number of directions and with a significantly lower accuracy than that of the P wave. Finally, we analyze velocities measured on a rock sample from the Outokumpu deep drill hole, Finland, including the error analysis for several levels of confining pressure.
Permanent Link: http://hdl.handle.net/11104/0247057