Archeological Remote Sensing
Now more than ever, archeological research is interdisciplinary: botany,
forestry, soil science, hydrology - all of which contribute to a more complete
understanding of the earth, climate shifts, and how people adapt to large
As a species, we've been literally blind to the universe around us. If
the known electromagnetic spectrum
were scaled up to stretch around the Earth's circumference, the
human eye would see a portion equal to the diameter of a pencil. Our ability
to build detectors that see for us where we can't see, and computers that
bring the invisible information back to our eyesight, will ultimately contribute
to our survival on Earth and in space.
The spectrum of sunlight reflected by the Earth's surface contains information
about the composition of the surface, and it may reveal traces of past human
activities, such as agriculture. Since sand, cultivated soil, vegetation,
and all kinds of rocks each have distinctive temperatures and emit heat
at different rates, sensors can "see" things beyond ordinary vision
or cameras. Differences in soil texture are revealed by fractional temperature
variations. So it is possible to identify loose soil that had been prehistoric
agricultural fields, or was covering buried remains. The Maya causeway was
detected through emissions of infrared radiation at a different wavelength
from surrounding vegetation. More advanced versions of such multi-spectral
scanners (Visible & IR) can detect irrigation ditches filled with sediment
because they hold more moisture and thus have a temperature different from
other soil. The ground above a buried stone wall, for instance, may be a
touch hotter than the surrounding terrain because the stone absorbs more
heat. Radar can penetrate darkness, cloud cover, thick jungle canopies,
and even the ground.
Remote sensing can be a discovery technique, since the computer can be
programmed to look for distinctive "signatures" of energy emitted
by a known site or feature in areas where surveys have not been conducted.
Such "signatures" serve as recognition features or fingerprints.
Such characteristics as elevation, distance from water, distance between
sites or cities, corridors, and transportation routes can help to predict
the location of potential archeological sites.
Computational techniques used to analyze data.
1. sun-angle correction
2. density slicing
3. band ratioing
4. edge enhancement
5. synthetic color assignment
7. multichannel analysis
Remote Sensing Instruments
Many features which are difficult or impossible to see standing on the ground
become very clear when seen from the air. But, black and white photography
only records about twenty-two perceptible shades of gray in the visible
spectrum. Also, optical sources have certain liabilities, they must operate
in daylight, during clear weather, on days with minimal atmospheric haze.
Color Infrared Film (CIR):
Detects longer wavelengths somewhat beyond the red end of the light spectrum.
CIR film was initially employed during World War II to differentiate objects
that had been artificially camouflaged. Infrared photography has the same
problems that conventional photography has, you need light and clear skies.
Even so, CIR is sensitive to very slight differences in vegetation. Because
buried archeological features can affect how plants grow above them, such
features become visible in color infrared photography.
Thermal Infrared Multispectral Scanner (TIMS):
A six channel scanner that measures the thermal radiation given off by the
ground, with accuracy to 0.1 degree centigrade. The pixel (picture element)
is the square area being sensed, and the size of the pixel is directly proportional
to sensor height. For example, pixels from Landsat satellites are about
100 feet (30 m) on a side, and thus have limited archeological applications.
However, pixels in TIMS data measure only a few feet on a side and as such
can be used for archeological research. TIMS data were used to detect ancient
Anasazi roads in Chaco Canyon, NM.
Airborne Oceanographic Lidar (ADI):
A laser device that makes "profiles" of the earth's surface. The
laser beam pulses to the ground 400 times per second, striking the surface
every three and a half inches, and bounces back to its source. In most cases,
the beam bounces off the top of the vegetation cover and off the ground
surface; the difference between the two give information on forest height,
or even the height of grass in pastures. As the lidar passes over an eroded
footpath that still affects the topography, the pathway's indentation is
recorded by the laser beam. The lidar data can be processed to reveal tree
height as well as elevation, slope, aspect, and slope length of ground features.
Lidar can also be used to penetrate water to measure the morphology of coastal
water, detect oil forms, fluorescent dye traces, water clarity, and organic
pigments including chlorophyll. In this case, part of the pulse is reflected
off the water surface, while the rest travels to the water bottom and is
reflected. The time elapsed between the received impulses allows for a determination
of water depth and subsurface topography.
Synthetic Aperture Radar (SAR):
SAR beams energy waves to the ground and records the energy reflected. Radar
is sensitive to linear and geometric features on the ground, particularly
when different radar wavelengths and different combinations of the horizontal
and vertical data are employed. Different wavelengths are sensitive to vegetation
or to ground surface phenomena. In dry, porous soils, radar can penetrate
the surface. In 1982, radar from the space shuttle penetrated the sand of
the Sudanese desert and revealed ancient watercourses. Using airborne radar
in Costa Rica, prehistoric footpaths have been found.
Beaming radar pulses into the ground and measuring the echo is a good way
of finding buried artifacts in arid regions (water absorbs microwaves).
Man-made objects tend to reflect the microwaves, giving one a "picture"
of what is underground without disturbing the site.