Radar sounding technology developed to explore the subsurface of Mars may soon be used to find water buried deep beneath Earth’s deserts. Image: NASA/JPL-Caltech
A NASA-led team has used radar sounding technology developed to
explore the subsurface of Mars to create high-resolution maps of freshwater
aquifers buried deep beneath an Earth desert, in the first use of airborne
sounding radar for aquifer mapping.
The research may help scientists better locate and map Earth’s
desert aquifers, understand current and past hydrological conditions in Earth’s
deserts, and assess how climate change is impacting them. Deserts cover roughly
20% of Earth’s land surface, including highly populated regions in the Arabian
Peninsula, North Africa, west and central Asia, and the southwestern United States.
An international team led by research scientist Essam Heggy of
NASA’s Jet Propulsion Laboratory, Pasadena, Calif., recently traveled to northern Kuwait to map the depth and extent
of aquifers in arid environments using an airborne sounding radar prototype.
The 40-mHz, low-frequency sounding radar was provided by the California
Institute of Technology in Pasadena; and the
Institut de Physique du Globe de Paris,
team was joined by personnel from the Kuwait Institute for Scientific Research
(KISR), Kuwait City.
For two weeks, the team flew a helicopter equipped with the
radar on 12 low-altitude passes (1,000 ft, or 305 m) over two well-known
freshwater aquifers, probing the desert subsurface down to the water table at depths
ranging from 66 to 213 ft (20 to 65 m). The researchers successfully
demonstrated that the radar could locate subsurface aquifers, probe variations
in the depth of the water table, and identify locations where water flowed into
and out of the aquifers.
“This demonstration is a critical first step that will
hopefully lead to large-scale mapping of aquifers, not only improving our
ability to quantify groundwater processes, but also helping water managers
drill more accurately,” says Muhammad Al-Rashed, director of KISR’s
Division of Water Resources.
The radar is sensitive to changes in electrical characteristics
of subsurface rock, sediments, and water- saturated soils. Water-saturated
zones are highly reflective and mirror the low-frequency radar signal. The
returned radar echoes explored the thick mixture of gravel, sand and silt that
covers most of Kuwait’s
northern desert and lies above its water table.
The team created high-resolution cross sections of the
subsurface, showing variations in the fresh groundwater table in the two
aquifers studied. The radar results were validated with ground measurements
performed by KISR.
“This research will help scientists better understand
Earth’s fossil aquifer systems, the approximate number, occurrence and
distribution of which remain largely unknown,” says Heggy. “Much of
the evidence for climate change in Earth’s deserts lies beneath the surface and
is reflected in its groundwater. By mapping desert aquifers with this
technology, we can detect layers deposited by ancient geological processes and
trace back paleoclimatic conditions that existed thousands of years ago, when
many of today’s deserts were wet.”
Heggy says most recent observations, scientific interest and
data analyses of global warming have concentrated on Earth’s polar regions and
forests, which provide direct measurable evidence of large-scale environmental
changes. Arid and semi-arid environments, which represent a substantial portion
of Earth’s surface, have remained poorly studied. Yet water scarcity and salt
content, changes in rainfall, flash floods, high rates of aquifer exploitation,
and growth of desert regions are all signs that suggest climate change and
human activities are also affecting these arid and semi-arid zones.
The radar sounding prototype shares similar characteristics with
two instruments flying on Mars-orbiting spacecraft: Mars Advanced Radar for
Subsurface and Ionospheric Sounding (MARSIS), on the European Space Agency’s
Mars Express, and Shallow Radar (SHARAD), on NASA’s Mars Reconnaissance
Orbiter. MARSIS, jointly developed by JPL and the Italian Space Agency, probes
the Martian subsurface sediments and polar ice caps to a maximum depth of about
1.9 miles (3 km). SHARAD, also built by the Italian Space Agency, looks for
liquid or frozen water in the first few hundred feet of Mars’ crust and probes
Mars’ polar caps. Both instruments have found evidence of ice in the Martian
subsurface, but have not yet detected liquid water. The Kuwait results
may lead to revised interpretations of data from these two instruments.
The research follows earlier work by JPL scientists to probe the
subsurface of the Sahara desert using
higher-frequency Synthetic Aperture Radar instruments flown onboard three space
shuttle missions in 1981, 1984, and 1994. That work located shallow drainage
networks and large dry basins, suggesting the Sahara
has had extensive surface water activity in its recent geological past.
shallow aquifers and flat surface provided the team with an ideal test
location. Extreme dryness, such as that present in this region of Kuwait, is
necessary to allow the radar’s waves to penetrate deep into the surface and
reflect on water-saturated layers beneath. Kuwait’s flat topography and low
radio noise also reduced clutter and improved the radar signal’s return.
“Results of this study pave the way for potential airborne
mapping of aquifers in hyper-arid regions such as the Sahara and Arabian
Peninsula, and can be applied to design concepts for a possible future
satellite mission to map Earth’s desert aquifers,” says Craig Dobson,
program officer for Geodetic Imaging and Airborne Instrument Technology
Transition programs at NASA Headquarters, Washington. The work is a pathfinder
for the Orbiting Arid Subsurface and Ice Sheet Sounder (OASIS), a NASA
spacecraft mission concept designed to map shallow aquifers in Earth’s most arid
desert regions and measure ice sheet volume, thickness, basal topography and