Hard-rock hydrotectonics using geographic information systems in the central highlands of Eritrea: Implications for groundwater exploration

Abstract

Summary In the central highlands of Eritrea, the fractures derived as lineaments from digitally enhanced Landsat TM images are grouped based on their tectonic origins and relative ages into three major sets of geological structures, Set 1, Set 2 and Set 3. Set 1 comprises the oldest NNE–SSW and NW–SE trending tensile lineaments, Set 2 is N–S and WNW–ESE and Set 3 is NE–SW and ENE–WSW. Both Set 2 and Set 3 are conjugate shear fractures. Frequency of groundwater occurrence (producing wells close to fracture lineaments), well yield and distance or locations of well data from the fractures are the main parameters considered in the analysis using geographic information systems (GIS). In the Set 2 and Set 3 shear fractures, well yields at distances of 0-60 m to lineaments are low while well yields at a distance of about 90–120 m away from the major lineaments are high. In the NNE–SSW fractures of Set 1, well yields at distances of 0-60 m from the major fractures are high but much lower than the highest well yields in the shear fractures at about 90-120 m. The relationship between well yield and distance to fracture lineaments in the three sets of fractures indicates that both of the shear fractures are hydrotectonically more significant than the tensile fractures. Of the shear fractures the N–S trending Set 2 lineaments are relatively the most important in terms of groundwater potential. The data summarized in a conceptual hydrotectonic model can be used as a working reference in selecting potential sites for future groundwater exploration in the region. Although drilling sites can be designed within the optimum distances, for example, 90 m distance, along the N–S trending fractures, supplementary detailed geophysical investigations should be carried out prior to drilling. The lineament analysis using GIS thus provides a means for scaling down target areas and design cost-effective and successful groundwater exploration programmes.