Electric and electromagnetic response patterns of fault zones in granites

Groundwater percolation in granitic terrains is highly conditioned by fault zone type structures where the rock has been disrupted to a point as to increase the overall porosity and local permeability. This is particularly the case where a surface sedimentary or weathered layer is nearly absent. For practical purposes that is the situation in granitic terrains in central Portugal where the weathered layer is no deeper than 3 m. In such a situation the variations in geophysical parameters may be considered to be related to variations in the bedrock itself. The geophysical methods considered below include “classic” electrical sounding, multidirectional electrical sounding and VLF/VLF-R profiling.