Azimuthal Velocity
Learning objectives
- See horizontal velocity depend on compass azimuth in HTI
- Read the velocity ellipse: long axis along the fractures
- Relate ellipse flattening to fracture density
- Understand this as the fracture-detection inversion
The Compass Becomes an Instrument
Vertical layering was azimuth-blind, the same from every direction. Aligned vertical fractures are the opposite. A wave running along the fracture strike barely notices the cracks and travels fast; a wave running across them is slowed at every fracture face. So the horizontal P velocity depends on the compass azimuth of travel, and plotting velocity against azimuth in map view traces an ellipse.
The ellipse is not decoration; it is a measuring instrument. Its long axis points along the fracture strike. Its flattening, how much the fast and slow velocities differ, scales with the fracture density. Rotate the fractures and the ellipse rotates with them; open more fractures and it flattens further.
Reading Fractures Without Seeing Them
This is the entire logic of fracture seismology in one picture. A wide-azimuth survey shoots the same subsurface point from many compass directions (the yellow sample points). Each direction returns a velocity, or on real data an amplitude. Fit an ellipse through those measurements and its orientation hands you the fracture strike while its eccentricity hands you the intensity. You have characterised a fracture set kilometres down without a single core.
That is why acquisition geometry and anisotropy are linked: you cannot read azimuthal anisotropy from a narrow-azimuth survey, because you never sampled the ellipse. It is also the bridge into Part 8, where we stop using velocity and switch to amplitude versus azimuth, the Ruger azimuthal AVO equation, which is more sensitive to fractures and is how the effect is exploited in practice. The last section of this part tilts the whole symmetry axis to handle dipping fabrics: TTI.