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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Detection by side-looking radar of geological structures
under thin cover sands in arid areas
International Institute for Aerospace Survey and Earth Sciences (ITC), Enschede, Netherlands
ABSTRACT: Images of SIR-A and SIR-B shuttle radar have been studied for penetration capability of the micro-
waves in arid areas. In the Red Sea hills (Sudan), the radar information was compared with Landsat Images and
Large Format Camera (LFC) photos. Particularly on the latter, much detail could be obtained through stereo
analysis. A radar strip over the central Iranian desert was compared with hand-held shuttle photography. In
both Sudan and Iran it appeared that the radar images revealed some information of the bedrock in the sandy
areas otherwise not visible on the images obtained in the visible or near infrared part of the spectrum,
indicating a certain penetration capability of microwaves through thin cover sands. The differences between
"dike lineament" interpretation on radar image and LFC photos are not so much influenced by the penetration
capability of microwaves, but much more by the influence radar look direction has on the enhancement or de
preciation of lineaments.
The proportion between the contribution of surface
scattering and that of volume scattering towards the
radar return signal in a side-looking radar survey
is still much subject to study and discussion. It
varies with radar variables (wavelength and polari
zation) and flight configuration (incidence angle),
but is also dependent on the target variables such
as surface roughness and dielectric properties of
the target material.
The practical question is can we learn how much of
the backscattered received energy comes from surface
scattering and what part is contributed by volume
scattering, and, if volume scattering is present,
how deep will the energy penetrate to give a still
detectable return signal of a subsurface interface
between two different materials.
Translated towards the field of application in
geology, the question is will microwave remote sen
sing give some information on geological structures
or rock formations which are lightly covered, for
example by windblown sands or superficial soil
cover. This question may be of great importance to
many geologists working in desert-like environments
where a thin sheet of aeolien sand may cover vast
tracks of rock formations and obscure the tectonic
Studies with microwave remote sensing in areas
under hyperarid conditions have revealed that buried
channels may be detected (Breed et al., 1983).
McCauley et al. (1982) demonstrated that from the
Shuttle Imaging Radar (SIR-A) images, buried valleys
and geological structures could be detected under
light sand cover of the Salima sand sheet in Egypt.
The actual penetration depth was found to be in the
order of 1 m for the sand sheet and about 2 m for
the dry sand dune areas.
On the basis of samples taken in the field and
their dielectric parameters measured in the labora
tory, a theoretical depth penetration of 6 m was
The discrepancies between the actual measured and
the modeled depth penetration is principally a re
sult of the difficulty defining parameters effecting
volume scattering of the inhomogeneous medium
between the surface and subsurface interface. The
roughness and orientation of the subsurface inter
face will affect further the angular backscattering
pattern at depth.
Rebillard and Ballais (1983) reported penetration
of microwave energy through a thin sheet of drift
sand 10-15 cm thick in the Algerian Bas-Sahara.
To study empirically if sand-covered geological
structures could be detected on side-looking radar
images, a comparative study was made of SIR images
with other remote sensing data such as Landsat,
Large Format Camera (LFC) and space photos of arid
areas in Iran (Dash-e-Lut) and Sudan/Egypt (Red Sea
hills and eastern desert).
A shuttle imaging radar strip (SIR-A) over central
and eastern Iran was compared with Landsat images
and shuttle hand-held space photography over the
same zone. Particular attention was paid to possi
ble differences in the desert-like sandy areas,
where geological structures may be covered by thin
aeolian sand sheets or colluvial bahada material.
The area under consideration is classified as
arid. The meteorological data of the Nehbandan
station (32°20'N, 57°35'E), situated about 150 km
south of the radar strip, give a mean annual preci
pitation of 100 mm/year. The dry period is from
June to November when precipitation is negligible.
December and especially January are the wetter
months, when local heavy showers give an irregular
rainfall distribution. The SIR-A images were ob
tained towards the end of the dry period on 14
November 1981, making penetration in the dry areas
more likely.
The Shuttle Imaging Radar had a frequency of 1.28
GHz (L band 23.5 cm), HH polarised. The depression
angle of the radar was 43 degrees, ± 3°.
By carefully scrutinizing data take 35-36, an E-W
strip which runs over the Dash-e-Kavir and Dash-e-
Lut, it was noticed that in an area south of the
village of Esfideh (33°36'N, 59°40'E) in the pro
vince of Horasan (east Iran), two strike ridges
appear over much longer distances on the radar
image than on the hand-held space photo (fig. 1A
and 1B between a and b).
According to the geological map of Iran (National
Iranian Oil Company, 1959) the age of the outcrop
ping rocks is Eocene to Oligocene. In the Dash-e-
Lut area, rocks of this age are principally of a
terrestric nature. The two strike ridges appear as
well-defined horizons with a white color on the