Behaviors of Converted Wave in an Azimuthally Isotropic Medium - A Physical Model Study

Abstract

The existence of subsurface fractures provides not only space for the residence of petroleum but also paths of migration. Therefore, subsurface fractures are of great interest to exploration geophysicists. In reflection seismology, a reservoir of vertically aligned fractures is often considered to possess azimuthal anisotropy, or Horizontal Transverse Isotropy (HTI), in terms of seismic anisotropy. The characteristics and information of this specific type of reservoir are widely obtained using seismic attributes, including the azimuthal variation in the P -wave amplitude and velocity, and the fractional difference of split S-waves. Essentially, a converted (C-) wave is initiated by a downward traveling P-wave, which is converted on reflection to upcoming S-waves. Hence, it combines the behaviors of P- and S-waves in theory. Using a forward model study, this study demonstrates the behaviors of a C-waves in a HTI medium, instead of the behaviors of P- or S-waves. Reflections are facilitated on the horizontal symmetry-axis plane of a scaled HTI model along seven different azimuths using end-on shooting arrangement. Using a P-type transducer as a source and an S-type transducer as a receiver, the behaviors of C-waves in a HTI medium are observed. In the acquired profiles, reflections of P-, PS1- (C1 -), and a mixture of PS2- ( C2-) and S1-waves were detected. The phenomenon of C-wave splitting is also observed because of the behavior of an S-wave in a Transversely Isotropic Medium (TIM), and it could be easily identified in the azimuths near the fracture plane. The reflectivity strengths obtained using a Hilbert transform show that the azimuthal variation in the Amplitude Versus Offset (AVO) for both P- and C1-waves are consistent, but the C1-wave amplitude variation depends more significantly on the azimuth than that of the P-wave. Furthermore, the percentage anisotropy of the C-wave computed from acquired data falls right between those of P- and S-waves. By incorporating C-wave splitting and azimuthal AVO variation into traditional signature analyses, our results show that the fracture orientation is more pronounced when the potential reservoir has vertically aligned fractures.