Joint TEM/AMT Feasibility Studies in Parnaíba Basin, Brazil: Geoelectrostratigraphy and Groundwater Resource Evaluation in Piauí State

The utility of the joint transient electromagnetic (TEM) and audiofrequency magneto-telluric (AMT) methods in stratigraphic and hydrogeological mapping has been evaluated in the southeastern part of the Parnaiba basin. The field experiments employed central-induction and coincident-loop TEM and tensorial AMT depth sounding techniques at several observational stations along a 400 Km traverse. The processed data are in excellent agreement with borehole data and known geology of the area. The results show that all the main stratigraphic formations can be distinguished on the basis of their resistivity characteristics and also allow proposing a zone for the Transbrasiliano lineament. A preliminary groundwater resource evaluation study of the Piaui State transect showed that the prolific aquifers in the basin are easily detectable using the TEM/AMT methods due to the favourable juxtaposition of electrically-contrasting permeable and relatively impermeable geological formations forming confined aquifers of regional extent. It is contended that these regional groundwater reservoir resources may be developed by machine-drilled boreholes to meet the water needs of the region but further detailed geophysical evaluation will be required for a cost-effective borehole siting program. Suggestions are also offered to guide any future detailed exploration for the more localized and shallower reservoir occurrences and resource development possibilities in the basement areas in the southeastern margin of the basin.