Seismic azimuthal anisotropy is observed in many areas of the earth, and knowing where it is present is important because it affects the propagation velocity of seismic waves. Not accounting for velocity anisotropy in processing or inversion of seismic data can lead to incorrect images and physical property estimates, and, therefore, incorrect geologic interpretations. While anisotropy has historically been considered a complication, the effect it has on data can be utilized as a source of information, giving an indication of geologic features much smaller than the seismic wavelength.
In this lecture, I will focus on the North West Shelf (NWS) of Australia, an area with significant stress-induced azimuthal anisotropy. I will explain observations of azimuthal anisotropy across the NWS from the regional-to reservoir-scale.
I first give a regional overview of seismic azimuthal anisotropy across the NWS using seismic exploration data. The results show that fast polarization azimuths and maximum horizontal stress direction trends correlate across a geographical area spanning almost 2,000 km, which compares well with published results from earthquake seismology studies. I also discuss why azimuthal anisotropy is detectable in some areas of the NWS and not in others.
I present a rock physics model that reproduces log azimuthal anisotropy observations in unconsolidated sand-shale sequences based on Vshale and depth. This method naturally introduces two new concepts; “critical anisotropy” the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth, where the confining pressure and sediment compaction are minimal and “anisotropic depth limit” the maximum depth where stress-induced azimuthal anisotropy is expected to be observable, where the increasing effects of confining pressure and compaction make the sediments insensitive to differential horizontal stress.
Finally, I demonstrate the importance of accounting for azimuthal anisotropy and acquisition azimuth in 3D and 4D seismic modeling, feasibility, inversion, and interpretation studies. Azimuthal anisotropy does not affect the small angle reflection angles of 3D and 4D AVO, but it can have a significant effect on larger reflection angles. I show that this effect can influence 4D seismic interpretation where there can be an “apparent 4D effect” when reservoir properties do not change, and a “contaminated 4D effect” when reservoir properties do change.
The methods, techniques, and conclusions discussed in this lecture are likely to be useful in other regions where stress-induced azimuthal anisotropy is present.
|Date||Wednesday, 4 March 2020|
|Time||1:00pm - 2:00pm|
|Location||Benson Common Room (Room Gn9, Geology)|
|Contact Name||Mallory Trumbore|