Finite‐difference modeling of sonic wavefields with a dipping interface

Abstract

Formation bedding can cause complex wave propagation in a borehole and introduce velocity bias in sonic logs. Because of the lack of symmetry, little is known about sonic wavefields propagating through a dipping bed. In this paper, we investigate effects on the sonic dipole and monopole wavefields across a dipping interface using a 3-D finite‐difference method. For dipole wavefields propagating from a soft to a hard formation across a dipping interface, the transmission is reduced greatly when compared with a horizontal interface. The different transmissions of SV‐ and SH‐waves through the dipping interface result in significant azimuthal amplitude variation and generate large cross‐coupled components. This apparent anisotropy should be taken into account when estimating formation shear anisotropy in a dipping formation. For monopole wavefields, the azimuthal averaging caused by a dipping interface reduces the reflection across an interface. This may affect fracture evaluation using the Stoneley reflection coefficient in a dipping formation.