HYDROMORPHOGEOLOGICAL INVESTIGATIONS IN PARTS OF ATHUR VALLEY, TAMILNADU, INDIA USING REMOTE SENSING DATA

Abstract

One of the primary constraints to economic and social development in the hard rock terrains of the world is the difficulty encountered in developing reliable water supplies for the population. In tropical arid regions surface water may not be avaiable on a regular basis throughout the year. As a result, groundwater, is generally, the only dependable and safe source of water. However, the search for groundwater and its development in hard rock regions pose a number of problems. In this context the recently developed space remote sensing techniques have opened new vistas for targetting potential areas for groundwater within a reasonable span of time. In this study, a systematic investigation of the hydromorphogeological characteristics dedicated for groundwater exploration in the Athur valley (Tamil Nadu,India), has been taken up. The study area covers a total of 2100 sq.km. (Lat. 12° 25' - 12° 49' N and Long. 78° 18' - 78° 45'E). The Athur valley is located in the Precambrian hard rock terrain. It has been observed that some of the wells discharge prolific supply of water whereas others have been dry. Therefore, it was considered that a detailed and systematic study would throw valuable light on hydromorphogeological characteristics of the area and would help in evolving suitable techniques for identifying potential zones and finally delineation of groundwater prospects in this terrain. With the above in view, the following approach has been adopted: * Interpretation of aerial photographs for identifying the detailed landform features, and structural elements. * Interpretation of Landsat MSS and TM images for delineating the regional structures and landforms. * Digital processing of Landsat MSS and IRS-IA data for enhancing various landforms and lineaments. * Interpretation of field groundwater data and analysis ofpumping test data. * Integration of remote sensing data with ground information and preperation of a groundwater potential map. The study has been based on a variety of data viz., aerial photographs, Landsat MSS and TM data, IRS-IA data, topographic maps, field lithologs, water level and water quality data, pumping test data etc. The geological formations in the area consist of Peninsular gneissic suite, charnockites and metasediments. The Peninsular gneisses are mostly hornblende gneiss, biotite gneiss and granitic gneiss. The associated metasediments include amphibolites and magnetite quartzites. The pink and grey granites with a discordant relationship have intruded across the gneissic , , formations as small veins and lenses. Pegmatites and quartz veins ,cUK' make their way along the shear planes. Basic dykes of dolerites criss-cross the area in a reticulate pattern through certain sets of fractures and faults. u The general trend of the rocks on either side of the Athur valley is NE-SW, whereas in the valley, the trend Is ENE - WSW to E - W. The metasediments which overlie the granitic gneisses in the central part of the valley have undergone tight folding and subsequent erosion, exhibiting doubly plunging synforms trending almost E - W. The Landsat MSS and IRS-IA data on digital tapes have been processed to enhance subtle variations in the spectral patterns of ground objects. The nature of Landsat MSS and IRS-IA data, their spectral, radiometric and temporal resolution, data formats etc., have been briefly mentioned. Representative areas in the valley have been selected for digital analysis for feature enhancement. The studies have been carried out using the following digital techniques: linear contrast stretching, band ratioing, brightness index (BI) and principal component (PC) analysis. The enhanced image data have been converted into photograhic products and used for interpretation. It has been found that in the raw Landsat MSS data, the overall separability between different landforms was very poor except for the denudational and structural hills which is attributed to their distinct vegetation cover. Out of the various combinations of the enhanced images used, a few (mentioned below) have been found to be very effective for demarcating clearly the different landform units. Hybrid FCC of FC-3 and BI 4-6, 5-7 FCC of PC-1, 2, 3 FCC of BI 4-7, 4-6, 5-7 in The manifestation of various landform units on the above combinations have been analysed in terms of spectral signatures of ground materials, and the significance of these combinations has been discussed. Further, the following filtered images have been found to be useful in mapping the linear features using the MSS and IRS-IA data: Edge enhanced images of MSS band 7 with Roberts operator FCC of MSS band 7 using three directional filters (Sobel) (NE-SW, NW-SE, E-W) IRS-IA band 4 using various directional filters (Sobel) In the IRS -IA scatterogram plots, all the classes except the pediment were spectrally inseparable. The enhanced IRS-IA data were not found to be so useful in demarcating various landform classes as it pertains to the post-monsoon time (October,1988). A datailed landform map of the area has been prepared with the help of aerial photographs, Landsat images and field observations. The landform units mapped are flood plains, valley fills, piedmont / bajada, buried pediments, denudational hills and structural hills. The most significant contribution from the digitally enhanced data is that it has facilitated the demarcation of boundary between the valley fills and adjoining pediment zones which have great bearing on groundwater exploration. An attempt has been made to deduce the geomorphic evolution of the area. The geomorphic evolution of the study area has been inferred as the result of the combined

effects of the tectonic processes such as block faulting and shearing followed by denudational processes. A regional lineament-tectonic map has been prepared from Landsat MSS and TM images on a scale 1:250,000. Detailed lineament map has been prepared from aerial photo interpretaion on 1:60,000 scale. Besides, digitally enhanced (directionally filtered images) data have been used to extract subdued lineaments especially in the alluvium/soil covered areas. Rose diagrams have been prepared to bring out the major trend of the lineaments. In a sub-image, a more detailed study was carried out for mapping all the possible linear features using directionally filtered images. These have helped in enhancing subtle linear features which are very useful in interpreting the groundwater potential zones along linears vis-a-vis open fractures. Study of regional tectonic features has been attempted to throw light on the regional tectonic setup and its relation with the tectonic elements of the area. The area has been divided into three zones and for each zone structural features have been described in detail. Hydrogeological investigations have been carried out for assessing groundwater potential in the area. The groundwater characteristics of each landform type like pediment, buried pediment, piedmont\bajada, valley fill etc., have been described. The groundwater data have been analysed along with rain fall data. It is found that groundwater levels decline upto 10-15m during May - June. The depth to groundwater level varies considerably and is related to local depth of weathering and fracturing. The analysis of pumping test data has been carried out to determine aquifer characteristics. The valley fills and buried pediment zones are the most promising areas for groundwater development. It has been inferred that the lineaments, faults and fractures have high groundwater potential. Further, their intersections in the favourable landform areas such as valley fills and buried pediments assume greater significance for siting wells. Finally, based on the above study an attempt has been made to identify and map groundwater prospects of the area.