TABLE OF CONTENTS Remote sensing is the collection of data without actual contact with the object being studied. Thus in terms of geology remote sensing includes aeromagnetic and gravity geophysical surveys, which have already been described. This section is concerned only with remote sensing using electromagnetic waves.
The electromagnetic spectrum includes visible light, but ranges from cosmic radiation to radio waves (Fig. 3.89). Remote sensing can be carried out from various elevations, ranging from treetop height to satellites in the upper atmos phere. Thus the two crucial parameters to remote sensing using electromagnetic waves are elevation and the wavelength analyzed. However, these techniques contain many common factors. An energy source is essential; it may be induced (as with microwave radiation used in radar) or natural (as with solar radiation). The sun s electromagnetic waves are filtered through the atmosphere and either absorbed by or reflected from the earth s surface. Reflected waves are modified by surface features of the earth. Some types of wave vary according to thermal variance of the surface, vegetation cover, geology, and so on. These wave variations can be measured photographically or numerically. They are then analyzed, either visually and subjectively or by more sophisticated computer processing The results may then be interpreted (Fig. 3.90).
