Study on Rayleigh Wave Dispersion and Attenuation in HTI Fractured Porous Media

Summary In near-surface porous media containing aligned fractures and fluids, the propagation of Rayleigh waves is influenced by both fracture geometry and wave-induced fluid flow. These processes introduce frequency-dependent dispersion and azimuthal anisotropy in both phase velocity and attenuation. To investigate these effects, this study applies Chapman-type fracture–pore effective medium theory to describe fluid-saturated fractured media as horizontally transversely isotropic (HTI) media with frequency-dependent complex stiffness. The complex dispersion relation of Rayleigh waves is obtained by combining this model with the Stroh formalism for HTI half-spaces. Numerical results show that Rayleigh-wave phase velocity exhibits clear frequency-dependent dispersion together with azimuthal anisotropy. The fast and slow propagation directions remain consistent with the orientation of the fracture strike. Rayleigh-wave attenuation exhibits a clear relaxation peak within the frequency band associated with wave-induced fluid flow and shows strong azimuthal variation. Parameter analysis further indicates that fracture density primarily controls the strength of azimuthal anisotropy, whereas porosity mainly affects the overall level of dispersion and attenuation. The relaxation time governs the frequency range over which dispersion and attenuation become significant. The combined frequency–azimuth variations of phase velocity and attenuation therefore provide potential constraints for estimating fracture–pore structures in fractured reservoirs using multi-frequency and multi-azimuth Rayleigh-wave observations.