Interpretation of Broadband Ocean-Bottom Seismometer Horizontal Data Seismic Background Noise

The seismic background noise observed on the OBS (ocean-bottom seismometer) recordings is well known to be strong. With the recent appearance of broadband OBS, the background noise in the extended frequency range is even more significant. By taking into account the various degrees of freedom of the seismic sensors, it has been shown that the seismometers are not only translational sensors but also rotational ones with a sensitivity that depends on the installation. The strong similarity of the background noise recorded on the two horizontal traces of the broadband Hippocampe OBS data deployed during the Rosmarin program is interpreted as a rotational motion around the vertical axis. This torsion motion is so important that it dominates the other noise seismic contributions like translations and tilt (rotational motion around a horizontal axis) and represents more than 90% of the background noise signal at the periods between 5 and 50 sec. Use of the difference between the two horizontal traces makes it possible to reduce the background noise made up primarily of this torsion motion and thus to enhance the displacement signal but also strongly modify it. An effort should be made to better control the installation of the sensors on the seafloor. A better solution would be to simultaneously record rotations at the same position as that of the seismometer. Such data will permit the correction of the seismic traces of the OBS data and strongly reduce the background noise.