Michele Di Filippo

Volcano–tectonic structures, gravity and helium in geothermal areas of Tuscany and Latium (Vulsini volcanic district), Italy

Abstract Since the early 1980s, geological and structural mapping, gravity, and helium soil–gas studies have been performed in the eastern sector of the Vulsini Volcanic District (Roman Magmatic Province) in an attempt to locate potential geothermal reservoirs. This area is characterised by an anomalous geothermal gradient of >100°C/km, and by widespread hydrothermal mineralization, thermal springs, high gas fluxes, and fossil and current travertine deposits. The results of these surveys indicate the existence of a number of fault systems, with N–S and E–W structures that appear to be superimposed on older NW–SE and NE–SW features. Comparison of the results of the various studies also reveals differences in permeability and potential reservoir structures at depth.

Definition of sinkhole triggers and susceptibility based on hydrogeomorphological analyses

Sinkholes represent a geological risk that is often underrated, mainly due to its very localized nature. In fact, sinkholes occur only under particular circumstances and typically affect relatively small areas. Despite these characteristics, the difficulty in forecasting the precise location and timing of their sudden collapse creates serious problems for civil protection authorities and urban planners. In this framework, identifying the mechanism and thus the triggering factor of sinkholes is strategically pivotal in developing management plans. The present paper addresses the sinkhole-prone area of Il Piano (Elba Island, Central Italy). The integration of hydrogeological surveys, coupled with a thorough study of historical maps and aerial photographs, suggests that the main triggering factor in this area may not be related to water pumping from the karst aquifer, as initially hypothesized. Instead, sinkholes appear to be caused by ravelling and erosive processes occurring entirely in the sedimentary cover when heavy rainfall induces water overpressure within the superficial aquifer.

The importance of the geophysical methods for the determination of the urban subsurface structures

In this work, it is intended to highlight the indispensable significance of the geophysical methods at the level of primary decision-making at urban planning studies and to help resolving a range of problems connected to risk assessment.To this purpose, a research approach that integrated different geophysical methods for local subsoil evaluation using for seismic microzonation studies was carried out in the city of Leonessa. A detailed gravity survey and microtremor observations were made to map and model the thickness of Quaternary deposits (gravel, sand and clay) overlaying the Meso-Cenozoic pelagic basin deposits. The Leonessa basin, the test area of this study, is one of the major intra-mountain tectonic depressions of Central Apennines boarded on the southern side by the great normal fault, here referred to as the Leonessa fault.The study involved a test area of 50 km2 covered by 300 gravity stations and by 30 sites of 3-component single-site observations. The gravity measurements were performed using a LaCoste & Romberg gravimeter mod.D60. Instead, the acquisition of the ambient seismic noise was performed using the REF TEK (Third Generation Broadband Seismic Recorder) model 13 0S-01 equipped with three components 1 Hz sensors. Stations were accurately located with differential GPS (Ashtech Z-Xtreme dual-frequency GPS) that provided centimetric accuracy in elevation. The microtremor data were analyzed by means of the horizontal-to-vertical spectral ratio (H/V). A realistic density of the unconsolidated Quaternary deposits (1.75-2.00 g/cm3) determined on core samples and a density of 2.60 g/cm3 for the basement limestone were used to constrain the 2D gravimetric models. The gravity and microtremor data were compared with a collection of 69 boreholes, 5 well logs, 130 geotechnical field testing and 61 geophysical surveys (MASW and Down Hole) made available by the the municipality of the Leonessa. The 2D models match quite well with all these data.Finally, the data obtained with these two different geophysical techniques allowed us to evaluate the thickness and the fundamental resonant frequency of the Quaternary sedimentary infilling, and to reconstruct the 3D bedrock configuration.We think that, in the process of creating earthquake resistant cities, geological, geophysical and geotechnical investigations have an indispensable significance in the evaluation of suitability for settlement and land use decisions.