Applicability of Remote Sensing Techniques to Groundwater Exploration in Semi-Arid Hard Rock Terrain - A Systematic Approach
Groundwater exploration in hard rock terrain involves complex decisions, attributable to a lack of reliable data and the heterogeneous nature of the geological environment. Targets for drilling of boreholes are commonly selected on the basis of knowledge, or perceptions, of groundwater occurrence in a specific area. The decision process includes a data-handling phase during which adequate information has to be collected, evaluated, and appropriate methods have to be applied for processing.
Satellite remote sensing data, Landsat TM and SPOT Panchromatic imagery, with different spatial, spectral, and temporal characteristics have been evaluated for their potential use in groundwater exploration in semi-arid and hard rock terrain. An area in the Northern Province of South Africa was selected for the research project. A template has been developed to facilitate the characterisation of lineaments interpreted from remote sensing data with regard to their inferred hydrogeological significance. The lineaments have been subsequently investigated in the field using geophysical techniques followed by exploratory drilling to assess the groundwater potential at selected targets.
Desk studies aimed at comparing lineaments with existing borehole records and other groundwater indicative data, such as drainage systems and vegetation have not convincingly proved lineaments as favourable targets for groundwater exploration. However, the field investigations verify interpreted features, and drilling results indicate that success rates, in terms of fewer dry boreholes and higher yields, can be considerably improved by adopting a remote sensing approach to borehole siting.
A methodology has been formulated where the aspects of incorporating remote sensing data and other groundwater indicative data in a Geographical Information System (GIS) environment are emphasised to delineate target areas for effective borehole siting. The applicability of various sources of information is discussed and it is demonstrated how data can be processed to increase the understanding of groundwater occurrence in hard rock terrain. Ancillary data incorporated in the GIS include; topography, geology, watersheds, drainage, vegetation, borehole records, and community information. Various methods of integrating multiple data sources are discussed. Differential GPS techniques have been used to increase the accuracy of the borehole database, rectify images, and assess the spatial accuracy of the GIS data. A basic cost-benefit analysis demonstrate that the accrued costs for the implementation of a remote sensing approach can be motivated even with minor improvements in borehole success rates and for relatively small-scale groundwater exploration projects.