HR: 1340h
AN: S23B-0326    [Abstracts]
TI: Attenuation Anisotropy and the Relative Frequency Content of Split Shear-Waves
AU: * Carter, A J
EM: a.carter@earth.leeds.ac.uk
AF: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Kendall, J
AF: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: The variation of frequency-dependent seismic wave attenuation with direction (attenuation anisotropy) contains additional information to that contained in velocity anisotropy. In particular it has the potential to distinguish between different mechanisms that can cause velocity anisotropy. For example, aligned fracturing might be expected to cause both velocity and attenuation anisotropy, whilst preferred crystal orientation should lead only to velocity anisotropy. Attenuation anisotropy may also contain useful information about pore-fluid content and properties. We present a methodology for analysis of attenuation anisotropy, and apply it to a microseismic dataset previously analysed for shear-wave splitting by Teanby et al. (2004). The comparison of the relative frequency content of fast (S1) and slow (S2) split shear-waves is a convenient method for examining seismic attenuation anisotropy. Provided that S1 and S2 initially have the same spectral colouring, that no spectral distortion is introduced by the differences between receiver responses of geophone components, and that spectral distortion due to background noise can be ignored or corrected for, we can attribute any differences in their frequency content to attenuation anisotropy. Attenuation anisotropy should be detected by the different (approximately orthogonal) polarisations of S1 and S2 as they pass through the anisotropic medium. In the presence of attenuation anisotropy S1 and S2 should experience different levels of frequency-dependent attenuation. We quantify the differential attenuation of S1 and S2 using a scheme based on the spectral ratio method. We present results from a microseismic dataset acquired in an abandoned oil well at Valhall, a North Sea oil field. The results are surprising in that sometimes the slower arrival, S2, is richer in high frequencies than the faster, S1. This appears to be contrary to results predicted by theoretical crack models for attenuation anisotropy (e.g. Hudson 1981). The mechanism responsible for these observations is not clear. Our differential attenuation measurements correlate with the angles between the initial shear-wave source polarization and the crack normal, the event back azimuths, and the splitting times.
DE: 7294 Instruments and techniques
DE: 5104 Fracture and flow
DE: 5144 Wave attenuation
DE: 7203 Body wave propagation
SC: Seismology [S]
MN: 2004 AGU Fall Meeting