A Combined Hlem-Tem Approach To Well-Siting In Deeply-Weathered Granitic Terrains In Piaui State, Brazil

The location of prolific fracture-zone aquifers under a thick (> 50m) weathered mantle is a major hydrogeological<br>problem. This paper proposes the use of combined HLEM and TEM profiling techniques for improved mapping of<br>concealed fracture-zones in basement areas affected by deep lateritic weathering. In 1997, the prefecture of São<br>Raimundo Nonato in southern Piaui State of Brazil initiated a major groundwater development scheme in rural basement<br>areas. The first phase of the project involved the use of geophysical methods to accurately locate drilling sites in several<br>villages in the region. The weathered mantle is more than 50m thick in the dominantly granitic terrain and we jointly<br>deployed the HLEM and TEM methods at all the sites and obtained concordant results. Several examples of the results<br>of this borehole siting program are presented in which 100 percent success was achieved using the combined TEMHLEM<br>approach.