The OBS noise due to deep ocean currents

Abstract

Abstract. Ocean bottom seismometers (OBS) are usually deployed for seismological investigations but these objectives are impaired by noise resulting from ocean environment. We split the OBS recorded seismic noise into three domains, short-period, microseisms and long-period, also known as tilt-noise. We show that the first and third domains are controlled by bottom currents 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 two currents along the full tidal cycle, between neap and spring tides. In the short-period noise band the ocean current generates harmonic tremors that corrupt the dataset records. We show that, in the analyzed cases, the harmonic tremors result from the interaction between the ocean current and mechanical elements of the OBS that are not essential for sea bottom recording and thus have no geological origin. The data from a new Broadband OBS type, designed and built at Instituto Dom Luiz (University of Lisbon)/CEIIA, hiding no essential components from current flow, shows how utmost of the harmonic noise can be eliminated.

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“…Several recent studies attempted to model the harmonic noise generated by the head-buoy and rope strumming in the water columns of the DEPAS OBSs (Stähler et al, 2018;Essing et al, 2021b;Corela et al, 2022); however, this was observed at frequencies between 1 and 10 Hz. For example, Stähler et al (2018) modeled the strumming of the rope and head buoy as it is usually implemented in the Lobsters of the DEPAS pool, including in our deployment.…”

“…Sources of short-period (≤ 0.2 s or ≥ 0.5 Hz) noise can be geological (e.g., local earthquakes, volcanic tremors, etc.) or due to the interaction of sea-bottom currents with the structural components of the OBS (Corela et al, 2022). In addition, noise due to passing ships and marine mammals must be considered.…”