Building a Fractured Model

Part 8, Part 8: Fractures and Rock Physics

Learning objectives

Combine a background rock with a fracture set into a model
Turn strike, density, and fluid into HTI parameters
See the azimuthal velocity field the model produces
Recognise the background-plus-perturbation structure

Three Geological Choices, One Numeric Model

The rock physics of this part exists to be assembled into a model, and this section does the assembly. Start with a plain isotropic background rock, a $V_p$ p and a $V_s$ s. Add a single fracture set described by three things a geologist can genuinely provide: the strike the fractures run along, the crack density that says how intense they are, and the fluid that fills them. Every earlier step now fires in sequence: crack density and fluid become weaknesses, weaknesses become HTI Thomsen parameters, and the strike orients the whole thing in the horizontal plane.

The result is a complete, numeric anisotropic velocity model: a background plus an oriented HTI perturbation. The card lists exactly what a synthesis engine would receive, and the map shows the azimuthal P velocity field it implies, fast along the fractures and slow across them.

The Perturbation Way of Thinking

Notice the structure: an unperturbed background and a perturbation laid on top. This is how nearly all earth modelling is organised. The background carries the bulk velocity and the layering; the perturbation carries the feature you actually care about, here a fracture set, elsewhere a gas sand or a fault. Building models this way keeps them controllable: turn the strike dial and only the azimuthal orientation moves, raise the density and only the anisotropy strengthens, swap the fluid and only the P response changes while the geometry holds.

You now hold a fractured earth model built entirely from first principles, controllable by three intuitive knobs. The next section, the star of this part, synthesises its seismic signature with the Ruger azimuthal AVO equation and then inverts that signature to recover the fracture strike, closing the loop from geology to data and back.