Time evolution of broadband seismic noise during the French pilot experiment OFM/SISMOBS

During the French pilot experiment OFM/SISMOBS (April 22-May 20, 1992) in the Atlantic Ocean, two sets of Guralp CMG-3 broadband seismometers were successfully installed and recovered. The first set, OFF (Observatoire Fond de Puits: Borehole Observatory), was installed inside the DSDP borehole 396B at 296m below the seafloor and the second one, OFM (Observatoire Fond de Mer: Ocean Bottom Observatory), on the sea bottom close to the hole at a depth of 4450 m. A first analysis showed that the broadband seismic noise (5 sps) on the seafloor had the same magnitude as compared with the long period seismic noise (1 sps) recorded at SSB, a continental station of the GEOSCOPE network. The noise recorded in the borehole was disappointedly high. The seismic data obtained during the experiment have been reanalyzed and it is shown that the seismic noise in the borehole is decreasing significantly with time whereas the instrument on the seafloor displays normal variations. A long term experiment (at least one month) is necessary to assess the final noise in the borehole.