Influence of anisotropic elastic parameters on seismic imaging: numerical and physical modelling studies

Abstract

The wavefront curvature in an anisotropic medium is significantly influenced by the elastic parameters d* (near-vertical P-wave anisotropy), e (P-wave anisotropy) and g (SH-wave anisotropy). The effects of changes in these elastic parameters on the size of the Fresnel-zone have been studied using numerical modelling techniques and measured anisotropy of real sedimentary rocks. Ray paths for a P-wave gather above horizontal reflectors with a tilted axis of symmetry and contrasting velocity characteristics were also studied in order to determine how changes in elastic parameters affect the reflection point dispersal, and hence the smearing effect in stacked seismic data. Anisotropic migration of synthetic and physical modelling data was also carried out to demonstrate the effect of using inaccurate elastic parameters, and hence, wrong velocity functions in migrating seismic data. Numerical modelling results strongly indicate that the anisotropic elastic parameters significantly affect the size of the Fresnel zone, which determines the spatial resolving power for unmigrated seismic data. The magnitude of ray path distortion and reflection point dispersal is found to vary with the value and sign of the elastic parameter d*. Ignoring anisotropy or using incorrect elastic parameters in migration will lead to wrong depth estimation and spatial mispositioning of reflection events. Experimental seismic data obtained using physical modelling demonstrate that anisotropy can significantly affect the spatial resolving power of seismic waves. The extent of these effects depends on the wavefront curvature which is influenced by the elastic parameters and orientation of the symmetry axis. These observations indicate that the imaging of reflection events from the base of thick shale sediments will be affected by anisotropy.