3D Modelling of Seismic Responses from Large Vertical Fractures

Seismic responses from systems of vertical fluid-filled fractures, of mm-scale widths and 100 m-scale lengths (macrofractures), have been simulated in 3D. These fracture responses bear prominent events of scattered compressional (PP) and shear (SV) wave fronts spatially matching the extent of the fracture systems. The response waveforms, relative strengths, and phases differ depending on saturation. In the case of fluid-filled fractures, converted (P-to-S) scattering events recorded on the X component are the strongest while PP scattering fronts recorded on the vertical Z component are much weaker. The responses from dry (gas-filled) fractures are notably stronger than those from the fluid-filled ones, and the scattered PP front dominates in Z-component records. The responses of dry fractures are anisotropic, i.e., differ along and across the fracture plane, but anisotropy is negligible in the case of fluid-filled fractures. Systems of large fractures in reservoirs are detectable in three-component 3D seismic data as 2D amplitude anomalies of P-to-S scattering fronts.