Polarization of SKS, ScS and S waves retrieving the parameters of anisotropy from the has been analyzed on digital 3-components seis-observed splitting of shear waves. Let us conmograms recorded at 9 stations of the recently sider a linearly polarized S-pulse entering a installed GEOSCOPE broadband network. Splitting known homogeneous anisotropic zone, defined by was observed for about 15 records, and is inter-its 6 x 6 symmetric matrix of elastic constants. preted as being due to seismic anisotropy beneath Crampin (1984) showed how the eikonal equation the stations. From every record, it is possible reduces to an eigenvalue problem. For a given to retrieve the fast direction at the station, polarization azimuth and a given angle of and the time difference between the fast and slow incidence, the fast and slow S-velocities and polarization directions. Some 30 observations of corresponding eigenvectors are easily computed. quasi-linear S-pulses also help constrain the The incident S-pulse projects on the two perpendirection of fast velocity. S•nthetic seismograms dicular eigenvectors, which correspond to the were built to test the validity of the method fast and slow polarizations. The two components used to retrieve the parameters of anisotropy. travel in the medium with different velocities, Typical time-differences are about 1 sec, until they get out of the anisotropic region. If corresponding, for example, to a 80 km-thick zone we then observe the S-pulse in the transversal of perfectly oriented pyrolite. The fast direc-plane, we no longer have a linearly polarized tions obtained at 3 continental stations in pulse.

An original method is presented that allows us to measure the local shear-wave birefringence properties over any depth interval. It requires the acquisition of two shear-wave vertical seismic profiles (VSPs), each with different initial polarizations of the shear wave. The method is based on the estimation of a two by two matrix (called the propagator matrix) that represents a linear operator between two states of polarization. No information is required about layering above the zone of interest (in particular, about the weathering zone). If these two states of polarization correspond to the direct downgoing shear wave at two different depths z 1 and z2, the operator represents the transmission properties between the two depths. Under the previous hypothesis, this operator is independent of the source polarization and can be accurately estimated by a least-squares method in the frequency domain. Physically, this operator is a multicomponent deconvolution, whose column vectors represent the state of polarizations at a depth Z2 for two linear and INTRODUCTION

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