LANDSAT-TM IMAGES IN GEOLOGICAL MAPPING OF SURNAYA GAD AREA, DADELDHURA DISTRICT WEST NEPAL

摘要

Geological mapping in Nepal is generally carried out on the basis of field observations by taking traverses along some routes (trails, and river courses). Traverses have to be limited due to rugged terrain conditions and are often time consuming. Aerial photographs have been utilized for such purpose. However, photographs are often distorted laterally and do not provide an overall view of an area. Satellite Image such as LANDSAT-TM scene provides excellent image characteristics with synoptic view of large areas in different band combination. In this regard, a False Color Composite (FCC) in 4,3,1 of LANDSAT-TM Image and Aerial photographs was used for the present investigation. The study covers an area of ~650sq.km. The image was interpreted using a transparent overlay for delineating possible lithological boundaries and tectonic features of regional significance prior to the field investigation. A number of traverse routes were planned to verify Image features such as tonal difference, drainage pattern and other morphological features of different rock units. The information obtained from the field, Satellite data, Aerial photos and previous maps were transferred into a topographical base map. Based on these data a geological map at a scale of 1:50,000 have been prepared. Image characteristics of granite terrain are found to be less dissected than the areas occupied by phyllites and slates. The contact /boundary between granites and slates established in the field could be extended to inaccessible areas. Similarly, the limestone occurrence recorded along some sections could be mapped on the image through out the area. Most of the lineaments observed on the image were found to be faults. Effects of these faults are observed on displaced ridges, river courses and lithological contacts. The LANDSAT-TM Images have been effective tools in extending lithological contacts and tectonic structures verified during field checking. Rock units are often covered by vegetation and soil developed on them. Drainage patterns of various lithological units are recognizable with experience.