Seismic wave propagation in cracked porous media

The movement of interstitial £uids within a cracked solid can have a signi¢cant e¡ect on the properties of seismic waves of long wavelength propagating through the solid. We consider three distinct mechanisms of wave-induced £uid £ow: £ow through con-nections between cracks in an otherwise non-porous material, £uid movement within partially saturated cracks, and di¡usion from the cracks into a porous matrix material. In each case the cracks may be aligned or randomly oriented, leading, respectively, to anisotropic or isotropic wave speeds and attenuation factors. In general, seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid-¢lled cracks if £uid £ow is signi¢cant. In the range of frequencies for which considerable £uid £ow occurs there is high attenuation and dispersion of seismic waves. Fluid £ow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented, resulting in completely di¡erent e¡ects on seismic wave propagation. A numerical analysis shows that all models can have an e¡ect over the exploration seismic frequency range