Omer M. Ahmed MSc, 2016 University of Kerala, India. Digital Image Processing and Analysis Techniques for Detection Of Hydrothermal Alteration Zones A Case Study in Siah-Jangal Area, South Eastern Iran By: Zahra Mokhtari & Mohammad Boomeri & Sasan Bagheri Journal: Indian Society of Remote Sensing Keywords Alteration zones . ETM+ . Band ratio . Color composite . Principal Component Analysis . Iran. Geology of Iran Iran comprised of different structural and petrological features, according to Berberian and King (1981) there are nine major lithotectonic units in Iran 1) Zagros Fold Belt 2) Sanandaj–Sirjan Zone 3) Sahand Bazman Belt 4) Central Iran 5) Lut Block 6) Alborz Zone 7) Kopeh Dagh zone 8) Eastern Iran suture zone 9) Makran zone The Siah-Jangal area is part of Makran zone Fig:1 Major lithotectonic structural zones of Iran and location of Siah-Jangal area (modified after Berberian and King 1981) The Siah-Jangal area is located in south eastern Iran within the Makran Quaternary volcanic belt. Which is extends from Iran to Pakistan and made magmatic arc known as Northern Makran magmatic Arc (NMA). In Iranian part of NMA (where in the Siah-Jangal area), there is a wide range of Hydrothermal alteration and mineralization related to magmatic event and great potential for mineral exploration such as Cu and Au. Fig.2 Regional geotectonic setting of the Makran arc-trench system and Neogen Baluchistan volcanic arc (Based on Grando and McClay 2007; Ellouz- Zimmermann etal. 2007) There is a thick mineralized quartz vein in central part of the study Area surrounded by Hydroxyl bearing minerals such as • Phyllic (Quartz serisite and pyrite) • Argillic (alteration of clay minerals) • Propylitic (alteration zones causing by iron and magnesium) The iron oxide minerals that recognized in the area are hematite, goethite and limonite. Fig.3 (a): extending of Hydrothermal alteration zones around the central quartz vein (b):occurrence of iron oxide minerals. Geology of the Area The oldest rocks in this area are mainly siliciclastic deposits and serpentines rocks intercalated with basaltic lava, shales and carbonate rocks. The second unit is Eocene fossiliferous limestone and greywacke. Quaternary volcanic rocks consist of andesitic rocks and dacite they are unconformable with the flysch deposits. The youngest exposures in the area are the Alluvial deposits. There is structural control in the ore deposit types in the area. B D (B): Fossiliferous limestone (C): Sandstone (D): Flysch sediments and lava flow of basalt Other color Quaternary Alluvial plain C Image Processing Techniques The main purpose of the methodology is to extract the mineral alteration zones from the ETM+ data in the area. Why Landsat 7 ETM+ images ? It is consist of eight bands with a spatial resolution 30 m for bands 1 to 7. Band eight is panchromatic band with 15 m spatial resolution. Three processes techniques were done for the image • False Color Composition (FCC) • Band Ratio • Principal component analysis (PCA) The techniques applied ArcGIS 9.3 programs. by using ENVI 4.3 and Color Composite of Images The rule of color composites is to set the most informative band for a particular purpose in the red gun, the next in green gun and the least informative band in blue gun 3 2 1 in R G B Respectively known as true color composite. All other band combinations are termed as False Color Composite (FCC). Two FCCs were done for the area, FCC (bands 7, 4, 1), FCC (bands 3, 7, 5). the best one is 7, 4, 1. Three rock units are identified in the area (Volcanic rocks, Sedimentary rocks and alluvial sediments). Fig :5. FCC (7:4:1); of area Q Quaternary Alluvial plains ES ES Q ES ES Q ES Eocene Sedimentary rocks. ES CV Cretaceous Volcanic rocks white and yellowish ellipses and circles: hydrothermal alteration areas. ES ES CV CV Fig :6. FCC (RGB 3:7:5) of the area there is no obvious variation between the rock units Band Ratio Images Enhancements resulting from the division of brightness values in one band by the corresponding values in another band. To reduce the effects of environment (i.e. topographic slope and shadows). The ratio image provides unique information that are often difficult to detect in a standard image. Two band ratio were done for the image • Band 5 to band 7 was used for hydroxyl absorption and the ratio appears in light tone due to the high content of hydroxyl-bearing minerals. • band 3/1 ratio for iron oxides and the areas with abundant of these minerals indicated as bright pixels. Fig : 9 5/7 band ratio image for the area. The areas with abundant hydroxyl bearing minerals are shown as bright pixels. (Fig. 10) Band 3/1 ratio the areas with abundant of the iron oxide minerals indicated as bright pixels The combinations of 5/7, 5/4 and 3/1 were created as (FCC) image for the detection of hydrous minerals and iron-rich zones in RGB (Fig. 11) The areas with hydroxyl and iron oxide minerals are shown with bright pixels Principal Components Analysis o PCA is linear transformation technique used to emphasize variation and bring out strong patterns in data set. o Creates a new set of images with reduced spectral redundancy and it maybe more interpretable than the original data. o Also used to compress the information content of a number of bands of images in to just two or three transformed principal component images. According to (Crosta Et al. 2003) there are six band (1, 2, 3, 4, 5 and 7) can be used for principal component analysis. Table1 shows the values of the principal components for six bands of ETM+ in the area. • Iron oxides minerals have low reflectance in band 1 and higher reflectance in band 3 based on this, we can predict that iron oxides can be distinguished by dark pixels in the PC3. Fig:12. PC3 image of the iron oxides minerals. The altered areas are shown as bright pixels. hydroxyl bearing zones are shown as bright pixels in gray scale image of PC4. Fig:13 PC4 image of the hydroxyl minerals. The altered areas are shown as bright pixel. • The false color composite Image obtained using combination of hydroxyl image and iron oxide image in the RGB. The bright pixels are hydrothermal altered zones in study area. Fig.14 FCC image (RGB) of hydroxyl and iron oxide image. Summary and Conclusions The major types of alteration found in the SiahJangal area are clay minerals and Quartz vein. The mineral assemblages are iron-bearing minerals and hydroxyl bearing minerals. Image processing techniques are applied to the digital subset ETM+ such as false color composite, band ratio and principal component analysis (PCA). The hydrothermal alterations in the area are Hydroxyl minerals detected by band ratio 5/7 and iron oxide minerals detected by band ratio 3/1. RGB false color composites of 5/7, 5/4, 3/1 indicated the similar altered mineral areas as white color. PC4 shows the distribution of hydroxyl minerals. PC3 images reveals the distribution of iron oxide minerals. There is no obvious different in the hydroxyl and iron oxides images in the Band ratio and PC methods. The spectral transformations on the ETM+ data can provide effective methods in the identification of hydrothermal alteration minerals in arid and semi-arid areas. References: Zahra Mokhtari & Mohammad Boomeri & SasanBagheri. Digital Image Processing and Analysis Techniques for Detection of Hydrothermal Alteration Zones: A Case Study in SiahJangal Area, North of Taftan Volcano, South eastern Iran. Indian Society of Remote Sensing (ISRS), 43(2):363–377 (June 2015). Omer M. Ahmed [email protected]
