IMAGE PROCESSING AND ANALYSIS USING LANDSAT ETM+ IMAGERY FOR LITHOLOGICAL MAPPING AT FAWAKHIR, CENTRAL EASTERN DESERT OF EGYPT Reda Amer, PhD Student Timothy Kusky, Paul C. Reinert, Professor and Chair of Natural Sciences Abduwasit Ghulam, Research Professor Department of Earth and Atmospheric Sciences and Center for Environmental Sciences Saint Louis University St. Louis, MO 63103 USA
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ABSTRACT A data fusion technique is presented for lithological mapping in arid environments. Landsat Enhanced Thematic Mapper (ETM+) visible (VIS), near-infrared (NIR) and infrared (IR) bands have been enhanced using image fusion with a high spatial resolution panchromatic band of the same data set. The Hue, Saturation, and Value (HSV) transform is used to convert RGB coordinates into the color coordinates (HSV). Later, Principal Component Analysis (PCA) is applied on the fused (HSV) image for mapping ophiolitic and granitic rocks at Fawakhir in the Central Eastern Desert of Egypt. A revised lithologic map of the Fawakhir area is proposed based on the interpretation of Landsat ETM image results and field verification work. It is concluded that the proposed methods have great potential for lithological mapping in arid and semi arid regions.
INTRODUCTION Most of the Precambrian outcrops in Egypt are restricted to the area between the Nile and the Red Sea and the adjacent southern Sinai Peninsula. The Fawakhir area is located 93 km west of the Red Sea coast, along the QiftQuseir highway in the Central Eastern Desert of Egypt at Lat. 26˚ 00′ 17″ N, and Long. 33˚ 35′ 42″ E (Fig. 1). The Eastern Desert of Egypt is a part of the Pan-African Arabian-Nubian Shield, occupied by igneous and metamorphic rocks that were formed during the evolution of the Mozambique Ocean and its closure culminating in the East African Orogeny marking the collision between East and West Gondwana and the closure of the Mozambique Ocean 600 Ma ago (Stern, 1994; Kusky et al., 2003). The Central Eastern Desert is almost exclusively built up of island arc magmatic rocks, ophiolitic mélange and associated rocks, together with subordinate molasse-type sediments and late-tectonic volcanics and granitoid intrusives (El Ramly et al., 1993). The Fawakhir area is occupied mainly by ophiolitic mélange represented by serpentinites, metagabbro, and metabasalt, then intruded by granitic rocks, and overlain by Hammamat sediments. The Fawakhir granite pluton hosts El Sid and El Fawakhir Gold Mines, which are two of several gold mines in the Eastern Desert of Egypt that have been extensively worked since Pharaonic and Roman times (Amer et al., 2008). Remote sensing techniques have been used successfully in lithological mapping for the Arabian Nubian shield and for other areas worldwide by several authors (e.g. Abrams et al., 1983 & 1988; Sultan et al., 1986; Sabins, 1997; Abdelsalam and Stern, 1999; Rowan et al., 2003 Gad and Kusky 2006). Sultan et al., (1986) used Landsat TM RGB band ratios (5/7, 5/1, 5/4 * 3/4) for mapping serpentinites in the Eastern Desert of Egypt. Landsat ETM band ratio images (5/3, 5/1, 7/5) in RGB and (7/5, 5/4, 3/1) in RGB Gad and Kusky, (2006) are used for mapping serpentinites in the Barramiya area in the Central Eastern Desert of Egypt. Amer et al., (2008) proposed new ASTER band ratios ((2+4)/3, (5+7)/6, (7+9)/8) in RGB by analysis of the image spectra of the ophiolitic rocks at Fawakhir, Central Eastern Desert of Egypt. They also used the Principal Component Analysis (5, 4, 2) in RGB of ASTER image to present a revised lithologic map of the Fawakhir area.
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Figure 1. Location of the Fawakhir area on a Landsat image of Egypt. This study aims to further explore new methods for lithological mapping in arid and semiarid regions using Landsat ETM+ images. The method involves enhancing the data quality of low spatial resolution (30 m) VIS/NIR and IR Landsat ETM+ bands by fusion with higher resolution (15 m) panchromatic band (8). Principal Component Analysis and histogram equalization are applied on the fused data for mapping the ophiolitic and granitic rocks of the Fawakhir area in the Central Eastern Desert of Egypt. Herein we present a revised lithological map that shows major differences in the distribution and rock units’ boundaries from the published lithologic map of Hassanen (1985) (Fig. 2).
Figure 2. Geological map of Fawakhir area (Hassanen 1985). ASPRS 2009 Annual Conference Baltimore, Maryland March 9-13, 2009
GEOLOGIC SETTING The Fawakhir is covered mainly by ophiolitic and granitic rocks. These rocks are traversed by quartz veins striking nearly NE-SW where El Sid and Fawakhir gold mines are located. The Fawakhir ophiolitie sequence is composed mainly of serpentinites, metagabbros, and metabasalts (El-Sayed et al., 1999). The ophiolite is intruded by post-emplacement younger granites and later basic to acidic dykes that cut all of the Fawakhir ophiolite units. The serpentinite rocks form a large envelope around the Fawakhir granitoid pluton and are regarded as a member of an ophiolitic sequence developed in a supra-subduction zone setting (El-Mezayen, 1983). The basal contact between the serpentinite and the underlying mélange rocks is sharp and marked by a deep thrust fault striking NNW-SSE and a relatively narrow band of dark green schistose amphibolite is located between the ultramafic and the mélange zone (Hassanen, 1985). The interpretation of Landsat ETM and field work showed that the ophiolitic member serpentinite is surrounding the Fawakhir granitoid pluton from the west, north and south but there is no serpentinite in the most of the eastern side. To the west of serpentinite Dokhan volcanics and Hammamat sediments crop out. To the east and south of the Fawakhir granitoid pluton the ophiolitic members’ metagabbros and metabasalts are exposed as elongate bodies crossing the main asphaltic road and thrust over the serpentinites (Fig. 3). The Fawakhir granitoid pluton has intruded the ophiolitic thrust sheets and is composed of two compositionally distinct granitic phases: an earlier grey monzodiorite phase intruded with sharp contact by a larger pink mainly monzogranite phase (Fowler, 2001).
Serp Gr
Gr Serp
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b Quartz Vein Gd Mg
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Figure 3. Field Photographs at Wadi Al Sid showing: (a) Clear granite apophyses (Gr) in serpentinite (Serp); Looking N. (b) Structural contact between serpentinite (Serp) and metabasalts (Mb); Looking W. (c) Intrusive contact between monzogranite (Mg) and granodiorite (Gd); Looking SW. (d): Al Sid gold mine; Looking NE.
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DATA USED Landsat Enhanced Thematic Mapper ETM+ has 9 spectral bands. These include three visible bands (1-3) between 0.4 and 0.7 µm and one near infrared NIR band (4) between 0.76-0.90 µm and two infrared IR bands (5 and 7) between 1.55 and 2.35 µm, and one panchromatic band 8 between 0.52-9.0 μm; in addition to two thermal infrared bands (61 and 62) between 10.40 and 12.5 µm. Landsat ETM+ spectral bands have a spatial resolution of 30 meters for bands 1 to 5 and band 7. The resolution for band 6 (thermal infrared) is 60 meters and resolution for band (8 panchromatic) is 15 meters.
METHODOLOGY Image Fusion Technique Image fusion techniques deal with integration of complementary and redundant information from multiple images to create a composite image that contains a better description of the scene (Wen and Chen, 2004). The fusion of two data sets can be done in order to obtain one single data set with the qualities of both (Saraf, 1999). Image fusion is a method to enhance the quality and spatial resolution of an image by combining the spectral information of low spatial resolution imagery with high spatial resolution imagery. The resultant image has high spectral resolution and the same quality as a high spatial resolution image. In this study we applied image fusion techniques to the same data type. The low spatial resolution (30 m) Landsat ETM VIS/NIR and IR spectral bands are combined with the high resolution (15 m) Landsat ETM+ panchromatic (band 8). The low spatial resolution (30 m) Landsat ETM VIS/NIR and IR image is resized into (15m) spatial resolution. Spectral bands (7, 4, 2) are selected to be RGB coordinates. The Hue, Saturation, and Value (HSV) transform is used to convert RGB coordinates into the color coordinates (HSV). Hue ranges from 0-360, where 0 and 360 = blue, 120 = green, and 240 = red. Saturation ranges from 0 to 208 with higher numbers representing more pure colors. Value ranges from approximately 0 to 512 with higher numbers representing brighter colors (Kruse and Raines, 1994). HSV transform replace the value band with the high-resolution image, automatically resample the hue and saturation bands to the high-resolution pixel size using a nearest neighbor, bilinear, or cubic convolution technique, and finally transform the image back to RGB color space (Welch and Ahlers, 1987). The output HSV image has a wide range of color with pixel size of 15 m (Fig. 4).
Principal Components Analysis (PCA) Principal components analysis (PCA) allows redundant data to be compacted into fewer bands so the dimensionality of the data is reduced (Fig. 5). An important advantage of PCA is that most of the information within all the bands (represented by the variance) can be compressed into a much smaller number of bands with little loss of information (Gibson and Power 2000). The bands of PCA data are noncorrelated and independent, and are often more interpretable than the source data (Jensen, 1996). Because multispectral data bands are often highly correlated, PCA transformation is used to produce uncorrelated output bands. The first PCA band contains the largest percentage of data variance and the second PCA band contains the second largest data variance, and so on; the last PCA bands appear noisy because they contain very little variance, much of which is due to noise in the original spectral data. PCA bands produce more colorful color composite images than spectral color composite images because the data are uncorrelated (Richards, 1999). PCA is applied to the fused image the resulting image has high spatial resolution and a wide range of color that make the different lithological units easily discriminated (Fig. 6).
RESULTS AND CONCLUSION The results of this study demonstrate that the image fusion using Hue, Saturation, and Value (HSV) transform is very helpful in the image interpretation. Principal Component Analysis (PCA) of the fused image demonstrates better effectiveness in mapping different rock units in the Fawakhir area (Fig. 6). Serpentinites are identified by blue colors, metagabbros have green colors, metabasalts have pale blue colors, grey granite has brown colors and pink granite has a pale green color. PCA image shows better discrimination between different rock units. Field data are subsequently used together with the results of visual interpretation of the PCA image to prepare a detailed geological
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map for the Fawakhier area (Fig. 7). Comparison between the results of PCA of Landsat ETM fused image and the published geologic maps showed differences of distribution and boundaries of some lithological units. This paper demonstrates new and effective techniques for using Landsat ETM+ images for lithological mapping in arid regions like the Fawakhir area in the Central Eastern Desert of Egypt. The image fusion techniques comprises enhancement of the image quality and spatial resolution using Hue, Saturation, and Value (HSV) transform and applying the Principal Component Analysis PCA on the fused image. The resultant image is used to distinguish between ophiolitic rocks which include (serpentinite, metagabbro, and metabasalt) and granitic rocks (grey and pink granites). It is recommended to use the HSV transform for image fusion and applying PCA on the fused image for better discrimination between different rock units. Comparison between the results derived from the proposed new methods and field work clearly show that the new methods were successful in lithological mapping of the Fawakhir area. Therefore, we suggest that these techniques may be used as time- and cost-effective approaches for lithological mapping in the Arabian–Nubian shield and other arid areas.
Figure 4. Landsat ETM+ Hue, Saturation, and Value (HSV) fused image.
ASPRS 2009 Annual Conference Baltimore, Maryland March 9-13, 2009
A
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Figure 5. 2-D scatter plot of: (A) Landsat ETM+ Hue, Saturation, and Value (HSV) fused image. (B) Principal Component Analysis (PCA) reduced the data dimensionality of HSV image.
Figure 6. Principal Component Analysis (PCA) of Landsat ETM+ fused image. ASPRS 2009 Annual Conference Baltimore, Maryland March 9-13, 2009
Figure 7. Lithological map of Fawakhir area, Central Eastern Desert of Egypt.
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