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Mapping of Ultramafics and Associated Lithological Units Using Satellite Data
Study Area
The Singhbhum and Chhotanagpur regions represent the part of Protero-Epiproterozoic ultramafic complex,(Jain ,R.S and Nautyal). Distinctive iron oxide and clay mineral alterations associated with these rocks are exposed. These have distinctive surface expressions, even though some areas are under extensive forest and soil covers.
The study attempts to identify and map ultramafic and associated lithology.
The Seka and its adjacent areas are geologically a volcanic area, situated in the South East of Ranchi.The Geological processes produced large areas of hydrothermal alterations, characterised by quartz veins and amphibolites.
The study attempts to use Landsat TM data to identify map ultramafic and associated lithology.
Tectonic Setting
The transition from an essentially mantle dominated volcano-tectonic region in the Archaean to a dominant ensiatic sedimentary environment place in parts of the globe in the Proterozoic . Economically important deposits in the Proterozoic fold belt of the Singhbhum Group and Chhotanagpur Gneisses are expected to be confined within the structrully favourable locations.Tamar-Porapahar Shear zone might have acted as the first-order structure , allowing the deposit occurrence.
Geology
The Tamar-Porapahar shear zone is situated within Singhbhum Group of rocks . The area constitutes quartz-mica schist, cherty quartzite and chlorite schist which are intruded by quartz veins and amphibolites. The Geological milieu is associated with sulphide- facies banded iron formations,which is subjected to high grade metamorphism and intense deformations.
Digital Image Analysis
As image processing is highly scene dependent the methodology was experimented in the study area.Landsat TM images were used to map alteration zones and associated lithology in the ultramafics complex .The Landsat TM, IRS 1C images were used to help understand their lithological and structural controls.
Land sat TM Images
We processed Land sat TM data to generate three colour composites for the Seka alteration zones .
Normal colour images using band 7as red , 5 as green and 4 as blues.(Fig1a).
TM image 7,5,4as RGB. (Fig1b ).
TM ratio image using band ratios 5/7as red , 4/5 as green, and 3/1 as blue.( Fig2).
Landsat TM band ratios 5/7and 3/1 are used because clay and iron minerals have reflectance and absorptions features in these bands . Landsat TM band –ratios 4/5 is used because it emphasizes silicate minerals compared to FeO-rich minerals .(Abrams et al 1983).
The 7,5,4 and 7,4,2 Land sat TM/ IRS 1C images provide a synoptic overview of the study area that is useful for mapping regional morphological features such as rivers , bedrock outcrops and soils.Moreover these images are useful in identifying linear structure faults and fractures.
The image (Fig2) is used to discriminate different rock types in the study area.This image is useful in identifying alteration zones.These alteration zones are labelled 1,2,3,4,5 appear red , suggesting the domination of clay minerals.