Sunshading geophysical data using fractional order horizontal gradients

The interpretation of linear features in geophysical imagery is of considerable importance because they can correspond to dykes, faults, or other geologic discontinuities. One popular tool commonly used for their enhancement is sunshading, which considers the data values as if they were a topographic surface that is illuminated by a light source (or “sun”) which has a specified azimuth and elevation. The reflectance of the light by the surface is calculated and imaged. Features whose strike direction is orthogonal to the sun azimuth reflect the light strongly and are consequently enhanced, while those lying parallel to the sun direction do not reflect the light and become less apparent as a result. The reflectance algorithm uses the first horizontal gradients of the data calculated in two orthogonal directions (Horn, 1982).