Summary

The focus of this study is on shear wave splitting caused by preferential fractures and differential stress. Multicomponentmultisource offshore seismic data events recorded by arrays of three-component (3C) digital seismometers of Malay Basin provides valuable insight into the use of shear-wave splitting (SWS) measurements. The properties of shear wave splitting can be used to infer (1) the directional dependence of polarization (?) in fractures medium, usually parallel to crack orientation; and (2) the measurement of delay time (dt) between the two split S-waves is proponational to the number of cracks per unit volume with splitting uncertainties.

Introduction

Seismic anisotropy (i.e. the dependence of seismic velocities on the direction of propagation) due to fractures causes great interests in modern seismic exploration. When a shear wave propagrates through an anisotropic or fractured media, unlike compressional (P) waves, shear waves are characterized by having different polarization into fast and slow shear waves. Shear waves propagating in anisotropic medium are called inline (SV) and cross-line (SH) (Crampin, 1981; Alford, 1986). The SV-wave is polarized in the plane of propagation, while the SH-wave is polarized orthogonal to the plane of the propagation. However, in layered medium, the SV-wave is polarized in the radial direction, and SH-wave would be in the transverse direction. In isotropic medium, we would expect radial component but not the transverse component (Figure 1a), because it is dominated by SV-wave energy from P-SV conversion. On the other hand in the presence of azimuthal anisotropy, the polarization are determined by the natural axes of the anisotropic medium (Figure 1b) and shear wave energy is subjected to polarization during splitting (Alford, 1986).

Figure 1: Polarization of shear wave propogation. In case of isotropic (a), the polarization is depends on source-receiver configuration. For anisotropic case (b), it is determined by anisotropic fracture orientation (Figure modified from (Bale et al., 2009)

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