Multi-method Geophysical approach for groundwater prospecting in granitic rocks – NW Portugal case study

Abstract

Groundwater prospecting is of great importance for human activity, because a great amount of water supply for domestic, agricultural and industrial uses comes from the exploitation of this resource (Chandra et al. 2010). Groundwater exploration in granitic geological environments is a task yielded with particular singularities, especially when compared with other bedrocks, mainly associated to its reduced permeability (Pereira 1999; Lima 2000). Aquifer formations in granitic rocks is usually associated to geologic structures (e.g. faults, fractures, alteration mantles (regolith), quartz and pegmatite veins and dykes, basic rock dykes, among others) (Carvalho 1984). The continuous identification and detection of these structural features in depth is of particular difficulty when only surface data are available (Mapping, Remote Sensing, etc.). Consequently, within this context, geophysical prospecting and exploration becomes a crucial and effective targeting tool for hydrogeological-productive structures. With the main objective of determining the most suitable site for a ground water well construction, in a granitic area in Northern Portugal, a multi-method approach of two distinct geophysical surveys was pursued: a combined resistivity and ground magnetic survey. Previously, detailed geological mapping and remote sensing studies were carried out, in order to attain more control over the geophysical data. The ground magnetic survey consisted in total magnetic field measurements, according to a predefined 10x10 meters grid of 108 points, with the use of a proton magnetometer, model G-856 from Geometrics. The magnetic anomaly map was obtained after data treatment with MagMap200, Surfer 11 and Voxler softwares. The resistivity survey comprised a 180 meter profile with 10 meters electrode spacing, collared after magnetic dada interpretation, using a Syscal R1 Plus resistivity meter, with data being treated through RES2DINV software (Loke 2001). The used resistivity meter allows the selection of several electrode arrays, with the Wenner-Schlumberger one being used in this study (Milsom 2000). After the analysis of the ground magnetic survey data, it was possible to infer three main structural features, with approximately N-S direction, and three less conspicuous features of W-E and NW-SE directions. These features were interpreted as fault-like structures, indicative of hosting productive aquifers. The spatial overlap amongst the electric and magnetic anomalies was also flagged. The geometric connection between the 2D resistivity pseudo-section and the ground magnetic survey (plant) allows a more comprehensive geophysical anomaly projection in depth, lowering “apparent dip effects”. Taking into account the combination of different weighting factors, such as regional and local geology, wells productivity of nearby groundwater boreholes and the geometry of the geophysical signature of resistivity and magnetic lows, it was deduced that the most favourable underground target for the presence of groundwater was given by the anomaly overlap pointed west of the map. Thusly the surface location for a vertical drill hole was planned to intersect the target in depth. The 120 meter depth drill hole confirmed the presence of a productive aquifer in the predicted depth interval. The pumping test allowed confirming a productivity of 4000L/h for the borehole. According to Pereira &Almeida (1994), this productivity is above average regarding fractured rock hosted aquifers in Northern Portugal. The results of this work allow concluding that the multi-method approach in geophysical prospecting can be a crucial and useful tool in hydrogeological groundwater exploration in granitic areas. The given possibility for data integration in a three dimensional model allows more reliability of the geophysical model and its association with local geological model and field observations. This allows an efficiency and precision enhancement of underground structure targeting that could not be achieved with a uni-method approach, given the discrete nature of the geophysical signatures of groundwater hosting structures in granitic terrains.