Using and Understanding Shaded Relief displays of the Horizontal Derivative

The shaded relief form of display is an important tool which is utilized (and custom-tailored) by EDCON-PRJ to highlight basement fracture zones and to enhance regional and local magnetic lineaments. The following discussion is meant for the geologist or geophysicist who wants a better understanding of shaded relief displays of the horizontal gradient and their significance for exploration and development.

Magnetic anomaly maps, after proper reduction-to-the-pole filtering, have highs centered over magnetic bodies and lows surrounding them. The figure on the following page shows a west to east profile over a magnetic body edge. The magnetic anomaly trace shows a magnetic high on the upthrown side of the magnetic block and a low on the downthrown side. A horizontal derivative filter computes the absolute value of the slope of the curve. The maximum of the horizontal derivative curve appears over the contact.

In map form, the absolute value of the horizontal derivative produces maximum ridges over each of the contacts between magnetic basement blocks and over faults. Contour displays of the horizontal derivative, however, are generally confusing and bland because only the largest values corresponding to the highest amplitude intrusion edges are visible on the contours. The horizontal derivative map requires a better display technique for exhibiting the full dynamic range of the data. The shaded relief display can provide that dynamic range.

For a brief perspective, following is a list of procedures that is involved in producing the dramatic horizontal derivative display:

1. Interpolation of profiles onto a square grid
2. Reduction-to-the-pole
3. Horizontal derivative filtering
4. Absolute value
5. Shaded relief displays with various sun angles

The shaded relief displays highlight basement fracture zones that are difficult to see on the contour displays. Although faults may be visible on filtered magnetic profiles, it is much easier to trace the direction of the fault trends on the shaded relief images than it is on the profiles. The following question-and-answer dialogue helps to explain the production and use of shaded relief images.

What are Shaded Relief Displays?

Shaded relief displays of aeromagnetic data are produced by near photographic imaging of a digitally-stored grid. Synthetic shaded relief images are produced by assigning gray scales to the reflectance as a function of the data gradient. The eyes see the actual sun's reflectance from objects in our visual world. Similarly, the computer can treat the magnetic field as a topographic surface and can compute the reflectance of a synthetic sun's illumination from that surface.

Various sun illumination angles are tested to enhance the desired anomaly trends. The result is an obliquely-illuminated view of the surface, a view that enhances subtle terrain effects and facilitates magnetic basement interpretation. Slopes of the filtered magnetic field "terrain" are more likely illuminated and thus are whiter if they face the false sun's position. Conversely, slopes are less illuminated and thus darker if they tilt away from the sun.

How are Shaded Relief Displays Produced?

After processing and gridding of the aeromagnetic data, the data grids are contoured and edited. The edited grids are then fed into EDCON-PRJ's shaded relief computer program. EDCON-PRJ tests various sun positions (west, northwest, north, northeast, east, southeast, and south) to define the best sun angle of illumination. A third parameter, vertical scale, is adjusted to vary the gray shade contrast to an optimum level. A larger value sharpens contrast while a smaller value enhances subtle gray shade variation to depict subtler features.

After testing and selecting sun angles on an interactive graphics screen, each image is plotted with latitude and longitude tick marks, and sometimes on digital land grids, for location.

How are Shaded Relief Displays Used?

Shaded relief displays are particularly valuable when used in conjunction with other geophysical and geological data and displays. In general, the interpreter is looking for two types of features on the sun shade plots: 1) Linear anomalies highlighted by a perpendicular or near-perpendicular sun position; and 2) shear zones, generally in basement, that are highlighted best by a near-parallel sun angle. It is easier to interpret linear anomalies trending near-perpendicular to the sun angle and shear zones trending near-parallel to the sun angle because a linear anomaly will have its highest reflectance values at angles normal to the sun, and because shear zones are best recognized by terminations and offset in anomalies they intersect. Although they will not be quite as obvious, features at an oblique angle to the false sun angle can also be interpreted. A natural azimuthal bias is minimized by using two or more sun angles.

After careful tracing of linear trends onto mylar overlays, the trends can be compared to lineaments or structures developed from well control. The major features interpreted on the shaded relief displays coincide with and extend features located with other processing and interpretation tools. Additionally, new linear features have been interpreted. When using shaded relief lineaments, it is important to remember the source of magnetic anomalies is generally magnetic basement.

The shaded relief display of aeromagnetic intensity can be used as an interpretation tool to help locate lineaments within a defined area. It can be used to connect structures at the top of magnetic basement, shear zones in basement, also tectonic province boundaries in basement.

An interpreter of shaded relief images needs to keep the sun's position in mind while picking linear and curvilinear trends. If the sun is in the northeast, then fault locations are picked where a lighted slope facing the northeast turns to a dark southwest slope. The theoretical line of the fault or contact is the ridge where the light turns to dark.

The interpretation results in analyzing a shaded relief display can be used to generate additional interpretation tools such as a lineament pick overlay map. Lineament picks are made by different EDCON-PRJ interpreters and compared before the final lineaments are chosen. The lineaments can then be digitized on a digitizing tablet producing x and y coordinates for each end of the lineament. Editing is accomplished by plotting lineaments at the same scale as the interpreted film overlay and by removing any problem lineaments and redigitizing them.

In summary, the shaded relief display can be a valuable graphical tool for helping to discern basement faults, fracturing and lineament trends. When used and interpreted correctly, these displays can lead to a better understanding of regional and localized basement structure, and can be the cornerstone for additional analyses.