Hydrogeophysical Characterisation of an Aquifer Area in Spa, Belgium

Abstract

The region of Spa has been well-known for the quality of its waters for a long time. This region is still a vast resource for groundwater nowadays. Prospection for water resource has been guided by local geological knowledge and the use of drillings to identify suitable spots for water production. Geophysical methods were implemented in order as a time-saving and cost-efficient method I order to guide the location of future drillings. This work focuses on the implementation, processing and interpretation of the electrical resistivity tomography (ERT) geophysical method. This method measures the underground response to an electrical current passing through. This response, known as electrical resistivity, is essentially linked to the abundance of clay material in the underground, the interconnectivity of the solid grains within the rocks, and, most remarkably, to the amount of free water. This study takes place in the cambro-ordovician context of the Stavelot-Venn Massif. The field of investigation is divided into two distinct geological regions: on the north-east lies the first district, called Area 1, characterized by quartzite-rich environment; on the south-west lies the second area, called Area 2, where clay-rich geological materials such as clay and phyllite are abundant. Both regions have been affected by two orogenetic episodes and carry the scars of such events. These take the form of faults and folds, as well as various degree of fracturing within rock layers. Features related to faults and intense fracturing are key to the water production prospect. In this region, faults act as lifts for deep waters, and fractured layers may form suitable water reservoirs. ERT is particularly appropriate in this context for it shows strong contrasts of between massive rock layers and free waters. Faults and highly fractured area bearing water appear clearly on profiles. Some ambiguities may still appear due to occasional artifacts and loss of resolution, particularly at depth. The method was implemented in four distinct regions. The first region, PleinFays, revealed the presence of two faults in a transition area between the clay-rich Area 1 and a quartz-rich area in the south-east. The second region, Américains, revealed the presence of a fault deep enough to be explored for water production. The third region, Belle Hé, revealed a deep area that could correspond to a water reservoir, but suffers from ill-resolution. The fourth area, Fontaines, revealed the presence of a fault that could be exploited for water production as well. Eventually, ERT proved to be an efficient tool to identify geological structures related to water production. Further research could be carried out in order to map the horizontal and vertical expansion of identified faults.