bedrock
igeV
lenetrated
; from bed-
.ogical data
'ts of Aer-
>f Presinian
■anite, res-
, the rock
l is gneiss,
less than
> much as
,e causes
> the rock.
in from Ba-
100th appea-
i the MSS
:k constitutes
; because of
.c constant,
the long
a lot of
and blom
if the space
r sis to
>AR, tney
■ied Deneath
,h alluviate,
im JPL buried
ia of Mevada
1 SIR-B flight
isor has a
lenetration
mditions:
of the sand;
' covering
.ekuwula and
:evere arid
.fall is be-
,d even lower
., the aridity
2 in Badain
iobao and the
> times of
that it is
■ areas.
ie above dis-
,as penetrated
and sand in
activity to
WO KINDS OF
characteris-
d SIR-B in-
ism of dunes
mmon ground
tation. In
ectric con-
■y and meto-
alue is 6.67
There is no
e roughness
se of ripple-
is means that
trie constant
nes and other
t on radar
gnificant'
orosity of
incident
dered an im-
ent returns,
the geome-
sition rela-
It is well known that the typical slipface
angle of crescentic dune is approximately to
a constant (32’) while the one of gentle face
is far less than this value. Most of gentle
face angles are less than 10° . It has been
proved that the strongest return will be met
when radar wave illuminates on a uniform
medium perpendicularly. The side-looking
radar can meet this condition when it works
on zero incident angle (£), or when the sum
of the operating depression angle (£) and
the slope angle (o() is 90 degree . It. is
also known that the depression angle of ra
dar is the complementary angle of its view
ing angle, so when SIR-A. depression angle is
43° , then, ot plus /3 i s equal to 73°.
Two possible explanations to this 13° de
gree difference compared with therotical va
lue of 90 degree are given. Firstly, the
overlapped ripple-mark and secondary dunes
on slipface of the dunes with slope angle of
32 degree make it more of actual scatterring
surface angle than normalized 32 degree, and
therefore is more approximate 90 degree ;
secondly, the 90 degree is the value at which
the strongest scatterring will occur. In fact,
the echo energy changes in a range of inci
dent angles with 90 degree, as its center.
It is easy to explain the mechanism from
three deserts with the above theory. The
reason of why there is the strongest return
slipface vertical to radar's illuminatung
direction is that the sum of and (S is the
greatest in the situation; the reason for
the recieving of weaker energy by radar when
it illuminates slipface in slanting angle is
that the sea tterring surface is reduced by a
lower value; when radar wave illuminates
at the gentle surface, the latter becomes
a mirror reflector because of a too low
value and therefore no echo takes' place.
The penetration theory of radar is a very
complicated problem and many researchers
have been engaged in its study for quite a
long time. We take the radar scatterring
modle by Elachi et al. to explain the pene
tration of radar wave to the surface of Ala-
shan Plateau.
The arid and uniformed sand layer like that
of Tiekuwula and Aertengaobao areas is a kind
of low loss layer which leads to the inci
dence of microwave pulse towards lower layer
of sand and the refrection between the sur
face of air and sand layer. As the result of
refraction, the incident angle becomes lower,
which is the incident angle when wave reaches
the plane between sand and bedrock. Just as
mentioned above that the lower the incident
angle, the stronger the return is, the geo
logical body covered by dry sand can, on the
contrary, produce an enhanced imaging effect
on radar images. The study indicates that
the penetration of radar wave is deeper pro
vided that there are higher incident angle
and HH polarization, which is the condition
SIR-A can meet.
7 SUMMARY AND CONCLUSIONS
SIR-A and SIR-B images of the Alashan Plateau
have been analyzed and compared to Landsat
images and other data. Based on observations
and interpretion of the imagery, the fol
lowing conclutions can be made.
The bright return from non-vegetation sand
dunes on two radar images is caused by dunes
slipfaces oriented to the radar beam. The
slipfaces oriented perpendicular to the radar
beam have a maximum echo. The return strength
decreases with a reduction of the angle be-
tween the radar beam and the long axes of
dunes' slipface. Sand dune in which the slip'
faces are away from radar have no return.
Secondary sand dunes that have the slipfaces
oriented towards the radar beam also produce
a high radar return that is clearly shown on
radar images. Radar illumination direction
is a critical factor to the delineation of
non-vegetated sand dunes. Radar return from
vegetation around interdunal lakes is also
easily detectable and can be used to iden
tify these arid land featurea.
SIR-A and SIR-B radar penetration capabi
lity is demonstrated with the detection of
Precambrian metamorphic rocks beneath a thin
cover of sand on the alashan Plateau. The
same features are not visible on Landsat im-
gery. Therefore it is deemed possible to
find other subsurface geological features
using SIRt-A and SIR-B radar images in the
arid areas of China.
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