Anisotropy

Geology Has a Grain

Reservoir properties are rarely equally continuous in every direction. A channel sand can persist for kilometres along its axis yet pinch out over tens of metres across it. A single isotropic range cannot capture that, so we use geometric anisotropy: a long major range along the direction of greatest continuity and a short minor range across it.

Azimuth and Ratio

Two numbers set the anisotropy. The azimuth orients the major axis, and it comes from the geology: the channel trend, the progradation direction, the structural strike. The anisotropy ratio is the minor range divided by the major; a small ratio gives long, streaky features, while a ratio of one returns to the isotropic case. The continuity ellipse in the widget draws these two axes and rotates with the azimuth.

Direction Matters

Because continuity is directional, the variogram you measure depends on the direction you walk. Along the major axis it rises slowly and reaches the sill only at a long lag; across it the variogram reaches the sill quickly. Honoring anisotropy is what makes a model reproduce real geological shapes, elongated sand bodies rather than round blobs, so taking the azimuth from the depositional setting is one of the highest-value choices in the whole workflow.