Velocity model estimation by means of Full Waveform Inversion of transmitted waves: An example from a seismic profile in the geothermal areas of Southern Tuscany, Italy

We propose an FWI strategy that makes use of transmitted waves as input data and utilizes both global and local optimization methods to estimate the P-wave velocity model of the subsurface. We envisage that our approach may be applicable to difficult seismic land data, like those from geothermal areas characterised by complex geological structures. As a test case, we considered the CROP/18A seismic reflection profile that crosses the geothermal field of Larderello (southern Tuscany, Italy). The aim is to estimate the P-wave velocity model down to a few kilometres depth below the surface that could be used as complementary information to the standard seismic reflection image which, in this case, does not show interpretable reflections in a range of depths accessible to industrial drillings. One innovative aspect of the inversion we propose with respect to conventional FWI approaches is its independence of a starting model that, ideally, should reproduce the true long wavelength velocity structure of the subsurface and that may be rather difficult to obtain in case of low quality data and complex geology. We lessen the dependence on knowledge of a suitable starting model by performing a sequence of two inversions. First, we employ a genetic-algorithm (GA) based inversion, a global optimisation method that does not require any specific starting model, resulting in a long wavelength, low-resolution velocity model. This model then becomes the starting model for a second FWI, driven by a local optimisation algorithm, aimed at bringing in the fine details of the subsurface velocity structure. The reliability of the final model is checked by comparing observed and predicted waves for many common shot gathers along the seismic line and through the matching between the velocities measured by check shots in two nearby wells and the FWI velocities in the same locations. Many details of the velocity field, likely related to metamorphic and igneous formations, become apparent and may complement the interpretation of the standard reflection image. From these results, it appears that the use of transmitted waves and of the FWI approach discussed here may effectively improve the information for geophysical interpretation of challenging seismic land data, such as those that characterise many areas of geothermal exploration