Aerial methods of geologic survey have proved to be more than just a substitute for field work on the ground. Even good aerial stereo photos cannot replace geologic insight, but they can speed up the work. And, what is perhaps more important, they can provide a view that encompasses all or most of an area affected by a given geologic process.
It is easy to understand the difficulty of the task of piecing together the nature and sequence of geological events from the worm's eye view of a man on the ground when some of the features he is trying to identify may involve a good many tens of miles. In fact, the view provided by air photos in some cases permits the identification of features and interpretation of events that would be missed with ground methods alone. Faults, for example, which are important as indicators of zones of mineralization, and which have an obvious significance for construction projects of all kinds, can be identified by means of aerial photos, because they often are indicated by change in color or tone of a linear character, frequently expressed as alignments of natural features such as trees, streams, and so on. Fracture intersections are especially significant by reason of their association with mineral concentrations.
Aerial photos are also useful in detecting other characteristics that are associated with mineral deposits, such as outcrops of favorable host rock and zones of alteration as indicated by change in tone or color. The view from above is useful in desert and other areas where the ground surface is well exposed. But perhaps it is of even greater use in areas with dense ground cover, for structural units that may be most difficult to perceive by ground investigation alone often leave their imprint on the outlines and tonal differences of vegetation and surface as it appears in an air photo. The scale of the air photos must be small enough to permit the stereoscopic viewing of large enough areas to see evidences of extensive geologic processes. A scale of 1:62,500 (about a mile to the inch) is said to be quite suitable for the study of regional geology, and it can serve other purposes as well. A mosaic permits one to view a large area, but the view is stereoscopic. Furthermore, recognition of the traces of certain processes is hampered by the abrupt, even if slight, changes in tone at the boundaries of the individual photos.
These considerations have led some experts to think that photos taken from satellites will have great importance for the recognition of certain geological features. A print will contain a large area of one negative or photo, all of it with the same sun angle—characteristics that facilitate recognition of features of great length or area. For some features it may be a positive advantage not to be distracted by the detail shown in larger-scale photos. A mechanically reduced low altitude photo will not show the same patterns as a high altitude photo.
All this is not to suggest that studies of regional geology will be done from satellite photos in the future or even that photos take from an airplane are now the exclusive route to geologic understanding of an area. Conventional observation on the ground, perhaps supplemented by observation from a helicopter, is still necessary. The problem is to adjust the proportions between the different types observation to the particularities of each situation.
Adapted from Natural Resource Information for Economic Development, by Orris C. Herfindahl, Published for RFF by The Johns Hot kins Press, 1969.
A version of this article appeared in print in the September 1969 issue of Resources magazine.
