Analysis of Crustal seismic data in a geothermal field (Southern Tuscany)

Southern Tuscany is characterised by an extensional regime, crustal thinning and magmatic intrusions. Important geothermal reservoirs and anomalous pore fluids are associated with an intricate geological setting. The area was investigated by applying integrated methodologies to deep crustal seismic reflection data: amplitude versus offset inversion, and theoretical approaches to model overpressure conditions. The seismic transect CROP-18, split into two lines (18-A and 18-B), has imaged the exceptionally high reflectivity of some markers within the metamorphic crystalline basement units and of the medium-lower crust in this region. Joint analysis of the available data enabled us to distinguish between the lithological reflectors and the reflections caused by a predominant fluids, such as overpressure conditions. We detected the presence of vertical conduits, responsible for magmatic intrusions rising from the crustmantle transition to the near-surface layers, as well as overpressured reservoirs. Theoretical models are used to quantify the effect of overpressure in terms of porosity increase, and the consequent density decrease in the area where the main marker characterizing the geothermal fields, the K-horizon, becomes shallow.