Numerical and Physical Modelling of P-Wave AVO Response for Fractured Media

In this study we investigate variations of P-wave reflection coefficients with offset (AVO) and with azimuth (AVA) for a transversely isotropic medium with a horizontal axis of symmetry (HTI) in contact with water, and also with another HTI medium. Aligned, equidistant, vertical fractures, inbedded in an isotropic background, were used to represent an HTI medium. To grasp the effect of fractures their contribution was computed separately and then added to the isotropic background. The total response of a medium was then obtained by adding the two.

An approximate equation is used to compute the partitioning of seismic energy at layer interfaces. The solution compares well with the results obtained in physical modelling experiments. To analyse AVO and AVA effects we used 3-D multi-azimuth transmission and reflection surveys. The results show that AVA effects are significant even at moderate angles whilst they are dominant at large incident angles. Routine 2-D AVO analysis, as commonly performed, is clearly unsuitable for HTI media and should be preceded by AVA analysis.