Fig. 3

Overview of different types of noise recorded in the target frequency band. For visual and illustrative reasons, the dataset is first filtered below 100 Hz. Panel (a) illustrates the impact of the surface activity associated with the operation of the geothermal field where the acquisitions are conducted. From left to right, the dataset is shown in the time-depth domain, then in the frequency-wavenumber (f-k) domain and, finally, back into the time-depth domain once filtered in the f-k domain. The f-k filtering consists in keeping the hatched areas (middle figure) and suppressing the energy related to known noise sources. The dashed line highlights the signature of a down-going wave propagating from the surface along the well at 1500 m/s. Panel (b) shows, in the time-depth domain, a strong local signal related to workover in one of the wells near to TH3