Shear-Wave Splitting

One Wave In, Two Waves Out

AVAz read fractures from P-wave amplitudes but faded when the fractures were wet. Shear-wave splitting is the companion measurement with no such weakness. Send a shear wave into a fractured HTI rock and it cannot remain a single wave. It splits into two: a fast shear wave polarised ALONG the fractures and a slow shear wave polarised ACROSS them. As the two travel, they separate in time.

Two numbers come straight out. The polarisation direction of the fast wave points along the fracture strike. The time delay between the fast and slow arrivals grows with the fracture density, through the tangential weakness (the same $\gamma$ that never depended on the fluid). So one measurement hands you both the orientation and the intensity of the fractures.

The Fluid-Blind Complement

Now change the fluid and watch the splitting refuse to move. Shear motion carries no information about the pore fluid, because a fluid has no shear strength, so the delay is fixed by the fracture geometry alone. That is the exact complement to AVAz. AVAz reads the fluid but weakens when the fractures are wet; splitting ignores the fluid entirely and reports the geometry no matter what. Used together they separate the two unknowns cleanly: splitting nails strike and density regardless of fill, and AVAz then reads the fill once the geometry is fixed.

That completes the fracture toolkit and Part 8. You can now start from a geological fracture description, turn it into rock-physics weaknesses, assemble an anisotropic earth model, synthesise its P and S seismic signatures, and invert those signatures back to strike, density, and fluid. Part 9 steps back from single features to build earth models at scale: deterministic structures, stochastic heterogeneity, and the export formats that carry a model out of this course into a research code.