Hydrogeological characterization by electrical resistivity surveys in granitic terrains

摘要

Dikes and veins are the most important hydrogeological structures found in granitic rocks which may act either as groundwater pathways or as impermeable barriers. In both cases there is a contrast between the resistivity of these structures and the resistivity of the host rock. In this paper we present the results of an electrical resistivity study carried out in Geres area, Portugal. Geological setting of Geres area is dominated by a biotite-rich coarse-grained porphyritic granite crossed by quartz veins and basic rock dikes. The Geres - Lovios major fault crosses the area with a general NNE-SSW strike and it has a very important role in the deep flow of the Geres hot springs. Thirty-three vertical electrical soundings were made using the Wenner alpha array. A 3D apparent resistivity model points out the main heterogeneities and the anisotropy of this hydrogeological framework. The lowest values of apparent resistivity belong to the southern sector of the study area, corresponding to the existing spring. In this place, there's another alignment of low resistivity values, probably related to a zone of quartz veins crossing the Geres - Lovios major fault. The resulting node seems to have a decisive role in the ascending groundwater.