Diffractions
Learning objectives
- Explain a diffraction as energy scattered from a point in all directions
- Recognise the diffraction hyperbola and locate its apex over the scatterer
- See the hyperbola flatten as the scatterer deepens
- Connect diffractions to faults, edges, and the role of migration
The Point That Scatters
Part 5 spends the engine built in Part 4 on the events convolution could never make, and the first of them is the diffraction. A flat interface reflects energy back the way it came. But a point scatterer, the tip of a fault, the edge of a channel, a small hard body, has no direction to prefer, so it scatters energy in all directions at once.
On a zero-offset section that scattered energy draws a hyperbola. Its apex sits directly above the scatterer, at the two-way time of the scatterer's depth. A trace off to the side still receives energy, but that energy travelled a longer, slanted path, so it arrives later, and the further off, the later, tracing out the hyperbola's limbs.
Reading and Removing It
Move the scatterer and its apex moves with it. Push it deeper and the hyperbola flattens, because the extra travel to a neighbouring trace becomes a small fraction of the long path down and back. Shallow scatterers give tight, steep hyperbolas; deep ones give broad, subtle ones. This section is a finite-difference exploding-reflector response of a single point, so the hyperbola is computed physics, not a curve drawn by hand.
Diffractions matter twice over. They are signal: their apexes pinpoint faults, pinchouts, and rough interfaces, and a section full of crisp diffractions is a section with sharp structural information in it. And they are the thing migration exists to handle: migration collapses every diffraction hyperbola back to the apex that made it, turning a smear of scattered energy into a point. Every hyperbola you see here is one that convolution left as a straight-truncated reflector in Part 3, the ghost that is now a real, computed event.