Seismic Investigations of a Geothermal Field in Southern Tuscany, Italy

Abstract

The Monte Amiata region in the Southern Tuscany, Central Italy, describes a volcanic complex with great significance in terms of the regional fresh water supply, mining and geothermal power generation. Mainly for the latter purpose, the volcanic area of Mt. Amiata has been the subject of extensive geological and geophysical research. The results from these studies have led to successful geothermal production in the Mt. Amiata region since the early 1960s. Today, the most important reservoirs in this area are the Bagnore and the Piancastagnaio fields which are both operated by Enel Green Power. Therefore, in order to get a more detailed understanding of new and promising adjacent areas, five reflection seismic profiles were carried out. We have performed and tested different depth migration methods on those lines. The vital point of depth migration algorithms is the accuracy of the velocity model that is used for the backpropagation of the seismic data. Therefore, we derived a suitable 1D starting model from nearby well logs and VSP measurements. In order to remove the large topography effects along the profiles, we then utilized first-arrival tomography for each seismic line. For the following processing we incorporated these 2D tomographic results into our starting model which compensates for static effects and improves the resolution in the near-surface area. The velocity models were then used in the application of Kirchhoff Prestack Depth Migration (KPSDM) to the seismic data for each profile, respectively. Moreover, we applied a focusing depth migration method, Fresnel Volume Migration (FVM), which limits the migration aperture to the volume that physically contributes to a target reflection/diffraction. This methodology significantly improves the quality of the migrated image and therefore yields a better result than conventional KPSDM or time migration. In comparison with time-domain imaging methods we conclude that depth migration generally provides an increased lateral resolution which is due to its flexibility with respect to lateral velocity heterogeneities in the near surface. This advantage particularly improves the imaging of fine-scale structures and geological faults. In summary, the applied seismic techniques deliver a well-resolved image of the subsurface for a thorough characterization of the geothermal reservoir.