Hydrologic Significance of Tectonic Fractures Detectable on Airphotos

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Abstract
Photogeologic study has been useful in the location of ground‐water sources in the state of Sao Paulo, Brazil, and could be important to the investigation of water resources in other areas.Until 1963, it was believed that large areas of the state were unsuited for the location of ground‐water wells (as in pre‐Devonian crystalline formations) and others weregenerally considered too hazardous (Paleozoic and Tertiary: sandstone lenses in shales). Since that time, when aerial photographs of the whole state became available, the authorhas verified an earlier hypothesis of his own that there are actually very few areas in the state which do not have suitable locations for water wells, and that photogeology is a most valuable method of prospecting.The water sources are parts of highly developed fault/ fracture systems which are identifiable on aerial photo‐graphs, at least in this warm humid climate with dry season. Over the fractures the rocks are deeply decomposed, the debris are removed in the rainy season, and existence of water in the dry season promotes higher and denser vegeta‐tion along aquiferous fractures, which usually are located in deep straight‐line valleys. Even where competent beds are overlain by more than 100 m of unconsolidated strata, many discontinuous nets of parallel straight‐line segments are detectable probably because all fractures are really faults with significant throw.To be most effective, the method requires stereoscopic study of areas having dimensions on the order of 10 × 10 km. The selection of locations for drilling should be sup‐ported by analyses of the dip of fracture planes (hade) and by other geological‐geophysical methods, especially by subsoil electrical resistivity surveying.

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