GNGTS 2014 - Atti del 33° Convegno Nazionale

electrodes (leap-frog technique) in order to reduce cumulative errors caused by electrode polarization. The distance between the measuring electrodes was 25 m. The SP values in the measuring net were obtained by adding readings after establishing a SP arbitrary zero value as a point of reference in the area. Moreover, SP measurements were corrected in order to compensate for cumulative errors and the topographic effect. Finally, we obtained a SP map using Surfer 8 for a contour line representation of the distribution of the potential field. Results and conclusions. Fig. 3 shows the results of the integrated geophysical survey carried out at the Sirino Lake. The ERTs acquired with different electrode arrays shows similar results. In particular the Wenner-Schlumberger one (Fig. 3a) is characterized by relatively low electrical resistivity values (< 100 Ωm) down to - 20 m depth and by higher electrical resistivity values (> 700 Ωm) in the lower portion. In the right the ERT shows a low resistivity zone, connected to the lake that crosses the underlying resistive body. The comparison between the electrical resistivity data with the lithostratigraphy obtained by a geognostic drilling investigation and a gamma-ray log (Grassi et al. , 2001b) allows us to reconstruct the complex geological situation beneath the Sirino Lake. In particular, the shallower low electrical resistivity body represents the debris and lacustrine deposits characterized by an inhomogeneous downwards infiltration. The deeper resistive area denotes the presence of a Siliceous Schists body not fractured and therefore with a generally low hydraulic permeability. Moreover the ERT underline the presence of a low resistivity area, on the southern shore of the lake, interpreted as a highly fractured portion of Siliceous Schists involved into mass movements, with a medium-high relative permeability. This area may represent a possible water route escape because is characterized by more permeable soil layers. Results of GPR survey are shown in Figure 3b. In particular, the 40 MHz acquisition shows three main sub-horizontal continuous reflectors probably due to presence of interfaces with different physical properties. An interesting hyperbolic anomaly is located between 90 and 110 meter of the radargram (yellow line). The 400 MHz radargram shows two horizontal reflectors located respectively at an estimated depth of 0.8 and 1.6 meter. At a distance of 27 and 35 meter from the origin of the acquisition there are the two sub-surface anomalies due to the seepage phenomena occurred in the past (red circle). A zone with a similar electromagnetic behaviour is located to 50 m from the origin (green circle). Moreover between 100 and 125 m there is a zone characterized by strong and chaotic reflections, probably due to deposits of filling material. Finally, the obtained SP map (Fig. 3c) shows negative values upstream (down to -350 mV), while there are positive ones (up to 200 mV) in the lower area surrounding the lake. In particular, the SP negative values identify the areas characterized by a downward water flow, while SP positive values represent water accumulation zones. In conclusions the integration of GPR, SP and ERT techniques allowed to identify possible causes of sinkhole problemand characterize the hydro-geological structure of the area.Moreover, joint interpretation of geophysical, geological, hydrogeological and geomorphological data allowed us to estimate the thickness of the lacustrine deposits, describe the main patterns of the subsurface fluid flows in the area, and identify possible water escape routes causing the piping phenomena. Acknowledgements. The authors are grateful for the geological and logistical support the “Micromondo”, the first theme park on the geology, in order to create an impetus to the dissemination of knowledge concerning the Earth Sciences (geologists Patrizia Magnotti and Dario Rizzo, http://www.ilmicromondo.com ) . References Baumgartner F.; 1996: A new method for geoelectrical investigations underwater . Geophysical Prospecting 44 , 71– 98. Binley A. and Kemna A.; 2005: Electrical Methods . In: Hydrogeophysics by Rubin and Hubbard (Eds.), 129-156, Springer. 148 GNGTS 2014 S essione 3.2