Most imagery cannot see underground, although there are some very limited exceptions
Above: Landsat image of Surkhandarya province in southern Uzbekistan draped over digital elevation model from NASA's Shuttle Radar Topography Mission Several important archaeological sites are present in this image.
Most imagery, whether from satellites or aircraft, can only be used to image the surface of the ground (or sea). This is because the radiation is absorbed rapidly at most wavelengths through most materials that are found on the Earth's surface. If you wish to get information on subterranean features, then the following may be your only options:
- Dry snow can be penetrated by microwave instruments so that in some cases a layer of snow may be somewhat transparent to a microwave sensor.
- Some types of light can penetrate clear water. Green light, for example, can penetrate shallow coastal waters. Bathymetric surveys of shallow coastal waters exploit this property to measure sub-surface topography.
- It is possible for drones to be used in enclosed or semi-enclosed spaces although this may require specialist operator training (e.g. in cave systems).
- Some types of radar system can penetrate below ground and/or beneath a vegetation layer. Some synthetic aperture radar systems have revealed ancient features in the Sahara desert buried by sand for example. These operational radar systems require specialist interpretation and they do not penetrate significantly into most ground surfaces.
- Specialist airborne radar systems have been used to map the bedrock (and subglacial lakes) underneath Antarctica and Greenland. In general these systems are only available toresearch organisations.
- In some cases, indirect evidence can be used to infer conditions under the surface. For example, small differences in soil moisture or vegetation on the surface as seen in imagery can reveal the subsurface outlines of ancient settlements.