Electrical signatures of a permeable zone in carbonates hosting local geothermal manifestations: insights for the deep fluid flow in the Gargano area (south-eastern Italy)

In northern Apulia (Italy), geothermal evidence can be associated to structural conditions. The area is characterised by a thick calcareous and dolomitic succession, affected by seismogenic faults and karst dissolution, and by the presence of some surficial geothermal pieces of evidence (coastal springs and well waters). In order to contribute to a better understanding of the deep circulation, an audio-magnetotelluric (AMT) survey was carried out. Soundings were acquired at 22 sites along a 27-km long profile crosscutting the Gargano Promontory and the Tavoliere Plain. A 2D inversion was undertaken providing a resistivity model down to 3 km b.s.l.. In the Tavoliere Plain the resistivity mainly increases with depth. Towards the Gargano area, a low-resistivity layer is interpreted as a fracture network which may favour fluid circulation. The supposed geometry of the permeable zone matches well with the structural interpretation proposed by independent studies. Thus, the present AMT study enabled imaging the first order geological structures that channel deep fluids as locally recognised by boreholes and natural manifestations at the surface.