3‐D sonic wavefields with a dipping interface

Abstract

A 3D finite-difference method was used to simulate sonic wavefields in a borehole intersecting a dipping bed boundary. Sonic wavefields due to both volume source and point force are presented to simulate the commonly practice monopole and dipole responses. The analyses are focused on the dipping boundary transmission and reflection coefficients and the apparent shear anisotropy. The authors found that the main effect of the dipping boundary on sonic waveforms is the change in transmission/reflection coefficients for both monopole and dipole sources. For dipole wavefields across a dipping interface from a soft to a hard formation, the transmission is greatly reduced when compared with a horizontal interface. The different transmissions of SV and SH waves across the dipping interface result in large azimuthal amplitude variation. This generates large cross-coupled components. However, there is little azimuthal time variation found in inline components. For monopole wave-fields, a dipping interface appears to be a smoother transition than a horizontal interface.