Digital Image Processing of Landsat7 data and GIS Application for Geological Investigation in Jebel Erba Area, _ Red Sea Hills, NE Sudan _

Abstract

Digital image ‘processing and Geographic Information System techniques, together with a limited reconnaissance ground truthing, have been used to investigate the geology of the area around J. Erba and J. Oda of the Sudanese‘ Red Sea Hills (RSH). The methodology fulfilled and the procedures followed during this study enabled the establishment of geodatabase named GeoRSl-I and the production of two geological maps of scale l:250,000. The area of study occupies the central part of RSH of Sudan that belongs to the Nubian Sector of the Arabian-Nubian Shield (ANS). It forms part of the Gebeit terrain that is known to be created among the other four terrains of NE Sudan (Tokar, Heya, Gabgaba and Geri) during the Pan-African event (950-50OMa) due to subduction-related processes and collisional tectonics and sutured together byidismembered-ophiolite belts and major shear zones. The geology of the area comprises high-grade gneisses, volcanosedimentary sequences metamorphosed in the greenschist facies, all intruded by syn and syn-to-late orogenic calc-alkaline intrusives and post-orogenic alkaline bimodal gabbro-granite complexes. The coastal plain is geologically characterized by Cenozoic siliciclastic and shallow marine rift- related sedimentary sequences. Pliocene-Pleistocene is represented by the thick older gravel unit and the emergent linear reef terraces. I - - i Several digital image processing techniques have been implemented to enhance the Landsat7 ETM+ digital satellite data. A Digital mosaic that covers almost all the Sudanese Red Sea Hills has been created using nine Landsat7 scenes. A 3x3 high-pass filtered color composite image with a central coefficient of 16 proves to create the most optimized images that appear relatively brighter and more saturated. Four color composite triplets of band ratio images; B3/B5, B3/B1, B5/B7; B5/B7, B5/B4, B3/Bl; B5/B7, B5/B1, (B3/B4)(B5/B4) and B5/B7, B3/Bl, B4/B3, have been composed to assist geological mapping. Directed band ratioing of the ratio color composite (B5/B7, B5/B4, B3/Bl) resulted in a well enhanced image that is characterized by satisfying saturated colors,‘ strong retention of ‘topographic expression and well pronounced lithological discrimination. Unstandardized and standardized principal component analyses have been done using the six reflected bands as well as a four selected bands. Some of their resulted images show relatively clear Landcover differentiation. Detailed lineaments investigation has been done using high-pass filtered, directional-filtered images as well aslthne DEM. The lineaments ofithe study area show a clear bimodal pattern where a dominant N-S (360° i 10°) and subdominant E-W (90° i 10°) trend. The N-S lineaments are attributed to the shear fracture system that resulted from Heya-Gebeit terrain collisional episode and the NW-SE compressional forces that prevailed during the Neoproterozoic time. The E-W trending lineaments have been interpreted to result from the locally deflected, landward extension and propagation of the Red Sea rift-related transform faults. Dykes and dyke swarms present in the study area display a prominent unimodalpattern with a prominent E-W direction. These dykes were probably emplaced during a compressive condition with a minimum principal compressive stress that trends 350°-360°. This direction corresponds to the main direction of the N-S trending Pan‘-African sinistral master faults related to Onib-Sol Hamid and Baraka major shear zones. Shuttle Radar Topography Mission (SRTM) data of 90m resolution that covers the study area have been Well utilized. Contour maps, shaded relief images as well as 3D scenes have been deduced from the SRTM data. The 3D scenes enhanced the visualization of the different geospatial data stored in the constructed geodatabase. A GIS-based geodatabase (GeoRSH) has been designed for the study area. It is structured to include all the available spatial data that have been stored in four group layers namely; the Infrastructure, Topography, Geology and Processed Satellite data group layers. GIS cartography has been implemented" to assist the final design of the two geological maps of scale l:250,000. The final layout of the two maps has been prepared in a GIS framework.