Analysis of frequency‐dependent seismic anisotropy from a multicomponent VSP

Abstract

In this study, we present results from the analysis of a multicomponent VSP for frequency-dependent anisotropy. The recorded four-component shear-wave data were first band-pass filtered into different frequency bands and then rotated to the natural coordinates to separate the fast and slow shear-waves. The polarizations of the fast shear-waves are found to be constant over the whole depth interval, and show no apparent variation with frequency. However, the time delays between split shear-waves decrease systematically as the frequency increases. We apply a timefrequency analysis method based on the wavelet transform with a Morlet wavelet to the data. A strong and consistent variation of shear-wave time delays with frequency is highlighted in the time-delay and frequency spectra. Two mechanisms are likely to explain the observation, i.e. scattering of seismic waves by preferentially aligned open fractures, and fluid flow in porous rocks with micro-cracks and macro-fractures.