Three-dimensional tomography and rock properties of the Larderello-Travale geothermal area, Italy

In a geothermal area, a detailed knowledge of the three-dimensional velocity structures aids the management of the field and the further development of the geothermal source. Here, we present a high-resolution study of the three-dimensional S-wave velocity structures from microearthquake travel times for the Larderello-Travale geothermal field, Italy. We have also deduced the Vp/Vs and Vp × Vs parameters for this area to emphasize the deep variations in the physical rock properties due to fluid content and porosity. Furthermore, effective porous medium modelling has been performed for site-relevant lithologies, to improve our interpretation of the results in terms of rock physics signatures. This has allowed us to estimate the variation range of the seismological parameters investigated, as well as their sensitivity for suitable rock under specific physical conditions. Low Vp/Vs anomalies, arising from a lower Vp compared to Vs, dominate the geothermal field of Larderello-Travale. These have been interpreted as due to steam-bearing formations. On the contrary, analysis of Vp × Vs images provides information on the relative changes in rock porosity at depth. Comparison of tomographic section images with previously interpreted seismic lines suggests that the reflective ‘K-horizon’ delineates a transition between zones that have different porosities or crack gatherings. The ‘K-horizon’ also lies on low Vp/Vs anomalies, which suggests a steam saturation zone, despite the reduced porosity at this depth.