IAMG 2001--Cancún Technical Program--Session A

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Qualitative and Quantitave Analysis of Fractures in Precambrian Formations and Their Potential for Underground Water Storage by Tumkur R. Gopinath, Universidade Federal da Paraìba, Brazil, and Cloves R.S. Da Costa, Universidade Federal da Paraìba, Brazil The understanding of the occurrence and exploration of underground water in crystalline rocks is difficult due to the complex hydrological properties of these igneous and metamorphic rocks. The fractures in these rocks will provide necessary porosity and permeability to become aquifer. But it is very difficult to detect these hydrodynamic properties in the subsurface and the presence of the underground water. The objective of the research is to elucidate the nature of the fractures and the conditions that govern the hydrodynamics of the crystalline rocks and try to discover the systematic in the occurrence of fractures and areas favorable for underground water storage. The study area of 100 km 2 is located near Campina Grande-PB, Brazil. During fieldwork the following data were collected: lithological descriptions, strike and dip of the fractures, distance between fractures and fracture types. The field data was used to prepare fracture maps of each sub area, statistical analysis of the fractures, classification of the fractures based on the distance between the planes The water wells existing in the region were plotted on the map to observe the correlation between fractures and water occurrence. In the study area the frequently occurring fractures varies from N30W to N45E with dip varying from 300 to 900. The fractures were classified based on the frequency of occurrences and spacing, in to locally frequent fractures, regionally frequent and regional fractures. The correlation between the fractures and the transcurrent faults of the region was made. In the shear zone, fractures tend to be locally very frequent, zone of micro-fractures and fragmented rocks. Drainage and valley formation in the study area are directly related to the fractures where wide valleys with indications of water are always parallel to tensional fractures or in the intersections between fracture planes. This study showed that the tensional (open) fractures dipping vertically, serve as channels for the water flow and the flow depends on the available contact area. Water storage occurs in areas where there are big intersection of the closely spaced fractures generally near shear zones with high concentration of micro-fractures and rock fragmentation. Other condition necessary for water trapping from vertical flows is the presence of horizontal joints or lithological contacts such as pegmatite intrusion with in schistose rocks. It is these horizontal fractures, which receive water from almost vertical fractures and conduce water discharge towards the wells. Paper in PDF format Gopinath and Da Costa, Acrobat PDF, 351k.