Why Rocks Are Anisotropic
Learning objectives
- Name the two dominant causes: layering and fractures
- Distinguish VTI (vertical axis) from HTI (horizontal axis)
- Explain why VTI is a depth error and HTI is azimuthal
- See why fractures are found from wide-azimuth data
Two Causes, Two Symmetries
Anisotropy is not exotic; it is the ordinary condition of sedimentary rock. Two causes dominate, and the difference between them is one of symmetry, which is precisely what a modeller must get right.
The first is layering. Shales are built from clay platelets that settle flat, and sedimentary sections stack thin bed on thin bed. Waves travel fast along the layering and slow across it, and the symmetry axis points vertically, across the beds. This is VTI, vertical transverse isotropy. Its defining feature: it looks the same from every compass direction. Rotate your survey azimuth and the anisotropy does not change.
Layering Bends Depth; Fractures Bend Azimuth
The second cause is aligned fractures. A set of parallel, near-vertical cracks makes the rock fast along the fracture strike and slow across it. The symmetry axis now lies horizontal, and the signature becomes azimuthal: this is HTI, horizontal transverse isotropy. The velocity you measure depends on which direction you shoot.
That distinction sets the agenda for everything after. Because VTI is azimuth-independent, it hides as a depth and moveout error: migrate a VTI shale with an isotropic velocity and your reflectors land at the wrong depth and your gathers refuse to flatten. Because HTI is azimuthal, it is both a complication and a gift. Shoot wide azimuths over a fractured reservoir and the direction of fastest velocity points along the fractures while its strength scales with how intense they are. You can read fracture orientation and density straight off the seismic. That azimuthal fracture workflow is the heart of Part 8; the rest of Part 7 gives us the tools, starting with the Thomsen parameters that put numbers on the surface you just saw.