The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
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Figure 3. RADARSAT-1 scene, September 1998 
Since geometric distortions are normally introduced to satellite 
data during acquisition due to satellite drift and Earth’s rotation, 
highly accurate geometric correction is crucial before any 
image processing. Therefore, the RADARSAT-1 and the 
Landsat TM images were geometrically corrected and 
registered. Well distributed control points were interactively 
selected on both images. The coordinates of these points were 
compared to determine a polynomial equation for adjustment 
between them. The images were thus rectified according to the 
Universal Transverse Mercator (UTM) projection. Finally, they 
resampled at (15 m) resolution using cubic convolution 
technique. Once the geometric correction was completed, the 
images have been fused using the IHS transformations as 
follows; the Landsat TM image is transformed into the IHS 
perceptual color space. Since intensity is related mainly to the 
brightness of the spectral responses, RADARSAT-1 image 
histogram has been stretched to match the variance and average 
of this computed intensity. Then, it was directly substituted and 
the inverse IHS-to-RGB transformation is performed as shown 
in the schematic diagram (Figure 4). 
Figure 4. Schematic diagram of the fusion technique 
The resulted fused image has high spatial enhanced details and 
spectral parameters, because the component of intensity has 
high correlation with spatial variation and the components hue 
and saturation correspond with the spectral characteristics of the 
image, as depicted in the false color composite (Figure 5). 
Figure 5. The resulted fused image of the study area 
4. RESULTS AND DISCUSSION 
The previous procedures led to an excellent spectral 
discrimination and interpretation of the study area. However, a 
problem of slight color distortion (variation in hue) appears 
after the fusion process, since the panchromatic image and the 
intensity image are different. Such a problem has been reported 
by many authors (Pellemans et al., 1993) and (Wang et al., 
2005). Somewhat better performance can be expected when the 
available data-sets have strong correlation (Kalpoma and Kudoh, 
2007). The study area is underlain by metamorphic rocks which 
are represented by: i) mafic-ultramafic granulite and ii) 
orthogneisses (felsic granulite), paragneisses (quartzite, quartz- 
feldspar gneiss) and BIF. These rocks are intruded by younger 
granitic rocks with sharp intrusive contacts. These rocks are 
uncomfortably overlain by the Gilf Formation (Paleozoic), Abu 
Ras Formation (Mesozoic) and Quaternary sediments (Khattab, 
et al., 2002). 
The most important finding of this study is the appearance of 
features beneath the sand surface on the fused Landsat TM and 
RADARSAT-1 images. These features are not observable at all 
on the Landsat TM image of similar spatial resolution (Figure 
6). Therefore, new geological and structural information were 
achieved with regard to the drainage pattern, lithological and 
structural features. The output fused image brought up the 
buried drainage pattern (W) in a most revealing manner. 
Excavations in this area indicated that the fine-grained sand 
dunes in this locality have a thickness of only (0.5-1 m) and the 
aridity of the sand and soils permitted radar subsurface 
penetration and capturing of the feature in the returned signal. 
One major NE trending drainage lines are revealed by 
RADARSAT-1 data to lead into the eastern side of the study 
area. The fused image also revealed some subsurface 
precambrian structures such as foliations, folds (D1 and D2) 
and faults (FI and F2) that control the distribution of the 
gneissic rocks and associated BIF in the study area. These are