TEM Prospection for the Definition of the Hydrogeological Setting in a Quarry in Central Italy

In several parts of the World, study, evaluation and safeguard of water resources are very important topics, due to the increasing water request that is responsible for pollution’s problems and shortage of the aquifers. Therefore it is basic to define accurately the potentiality and vulnerability of the aquifers in order to plan their use and to ensure availability over time. In this case study we used the TEM method (transient electromagnetic method) [1, 2, 3] in order to reconstruct the hydrostratigraphy of a regional aquifer that could be affected by a quarry activity (basalts extraction) in Castel Viscardo, Terni, Central Italy. The main aim was to map the impermeable substratum, as a first step to define the local hydrogeological model: the survey area is characterized by the presence of permeable Pleistocenic pyroclastic and lava flows, related to the Vulsinian District activity, overlapping the sedimentary substratum (mainly clays) of Pliocenic age. We have chosen the TEM method, as the electrical response of the hydrogeological units is well resolved, as reported by Floris et al. [4]: lava and tuffs show similar resistivity, but higher than the deeper clays. The conductive response of this latter makes the methodology very suitable, as the detection of conductors is greatly enhanced by EM prospections. The interpretation of resistivity profiles, obtained from TEM soundings, allowed us to understand clearly the electro-stratigraphy contacts, related to the main hydro-lithological layers. TEM methodology is a well known tool for hydrogeology, as it is highly effective, among “the ground-based” methods: it allows to investigate down to great depths (200-300 m and more), by means of small energizing devices, without galvanic contacts with the ground and with high resolution power (mainly in the case of conductive targets), with relatively low costs. It allowed us to investigate, in short time, a relatively wide area (about 10 Km2).