The world’s deserts are generally remote, inaccessible, and
inhospitable. Hidden among them, however, are hydrocarbon reservoirs,
evaporites, and other mineral deposits, as well as human artifacts
preserved for centuries by the arid climate. In these harsh
environments, the information and perspective required to increase our
understanding of arid-land geology and resources often depends on
remote-sensing methods. Remote sensing is the collection of information
about an object without being in direct physical contact with it.
Remote-sensing instruments in Earth-orbit satellites measure radar,
visible light, and infrared radiation. Radar imaging systems provide
their own source of electromagnetic energy, so they can operate at any
time of day or night. Additionally, clouds and all but the most severe
storms are transparent to radar.
The first Shuttle Imaging Radar System (SIR-A), flown on the U.S. space
shuttle _Columbia_ in 1981, recorded images that show buried fluvial
topography, faults, and intrusive bodies otherwise concealed beneath
sand sheets and dunes of the Western Desert in Egypt and the Sudan. Most
of these features are not visible from the ground. The radar signal
penetrated loose dry sands and returned images of buried river channels
not visible at the surface. These images helped find new archeologic
sites and sources of potable water in the desert. These “radar rivers”
are the remnants of a now vanished major river system that flowed across
Africa some 20 million years before the development of the Nile River
system. Radar imagery also is a powerful tool for exploring for placer
mineral deposits in arid lands.
In 1972, the United States launched the first of a group of unmanned
satellites collectively known as Landsat. Landsat satellites carry
sensors that record “light,” or portions of the electromagnetic
spectrum, as it reflects off the Earth. Landsat acquires digital data
that are converted into an image.
[Illustration: The Landsat simulated true color mosaic (left) shows the
Selima Sand Sheet covering all but rocky areas of the Sahara Desert in
Sudan. On the right, a 50-kilometer-wide strip of Shuttle Imaging Radar,
SIR-A, is placed over the Landsat mosaic to reveal old stream channels
and geologic structures like these. Structures that are otherwise
invisible under the surface sands are potential sources of water, placer
minerals, ancient artifacts, and information on changes of climate in
arid areas (courtesy of USGS Image Processing Facility, Flagstaff).]
The scarcity of vegetation makes spectral remote sensing especially
effective in arid lands. Rocks containing limonite, a hydrous iron
oxide, may be identified readily from Landsat Multispectral Scanner
data. The Landsat Thematic Mapper (TM) has increased our ability to
detect and map the distribution of minerals in volcanic rocks and
related mineral deposits in arid and semiarid lands.
More than a million images of Earth have been acquired by the Landsat
satellites. A Landsat image may be viewed as a single band in
black-and-white, or as a combination represented by three colors, called
a color composite. The most widely used Landsat color image is called a
false-color composite because it reproduces the infrared band (invisible
to the naked eye) as red, the red band as green, and the green band as
blue. Healthy vegetation in a false-color composite is red.
Desert studies still are hampered in many regions by lack of accurate
climate data. Most desert weather stations are in oases surrounded by
trees and buildings and have been subjected to many location and
elevation changes throughout the life of the station. Data from oases do
not reflect conditions from the surrounding desert. A wide variety of
instruments has been used to record measurements over varying lengths of
time and in different formats, making data difficult to interpret and
compare.
To overcome some of these problems in deserts of the American Southwest,
the U.S. Geological Survey (USGS) established its Desert Winds Project
to measure in a standard format several key meteorologic characteristics
of arid lands. Project scientists have successfully established
instrument stations to measure wind-speed, including peak gusts, which
alter the landforms the most. A station recorded a windstorm near
Vicksburg, Arizona, for example, with peak gusts of almost 150
kilometers per hour. Using low-maintenance, automatic, solar-powered
sensors, the stations also measure wind direction, precipitation,
humidity, soil and air temperatures, and barometric pressure at specific
heights above the surface. Data are sampled at 6-minute intervals and
transmitted every 30 minutes to a Geostationary Operational
Environmental Satellite (GOES). From GOES, the data are transmitted to
the USGS laboratory in Flagstaff, Arizona.
The Desert Winds Project’s investigators combine analyses of data with
detailed geologic field studies and repetitive remote-sensing coverage
in order to investigate and understand the long-term changes produced by
wind in deserts of differing geologic and climatic types.
[Illustration: This geometeorologic station of the Desert Winds Project
measures wind speed and direction, soil and air temperature, and
precipitation and humidity in the Great Basin Desert (photograph by
Carol Breed).]
[Illustration: Nitrate workings in a broad valley in the Atacama Desert
of northern Chile, where saline-cemented surficial deposits formed near
a playa lake (photograph by George Ericksen).]