The effect of deep ocean currents on ocean- bottom seismometers records

Ocean-bottom seismometers (OBSs) are usually deployed for seismological investigations, but these objectives are impaired by noise resulting from the ocean environment. We split the OBS-recorded seismic noise into three bands: short periods, microseisms and long periods, also known as tilt noise. We show that bottom currents control the first and third bands, but these are not always a function of the tidal forcing. Instead, we suggest that the ocean bottom has a flow regime resulting from two possible contributions: the permanent low-frequency bottom current and the tidal current. The recorded noise displays the balance between these currents along the entire tidal cycle, between neap and spring tides. In the short-period noise band, the ocean current generates harmonic tremors corrupting seismic dataset records. We show that, in the investigated cases, the harmonic tremors result from the interaction between the ocean current and mechanical elements of the OBS that are not essential during the sea bottom recording and thus have no geological origin. The data from a new broadband OBS type, designed and built at Instituto Dom Luiz (IDL – University of Lisbon)/Centre of Engineering and Product Development (CEIIA), hiding non-essential components from the current flow, show how utmost harmonic noise can be eliminated.