Very low frequency marine seismic noise

Abstract

Some ocean bottom seismic records from Western Australia show very low frequency noise amplitudes that are larger than peak-to-peak amplitudes of seismic reflections on hydrophone records. The noise period is longer than a 5000 ms record length. As a result, it is hard to suppress this noise using traditional seismic processing filters without generating large truncation edge effects. Spectra from 100 trace windows were computed using a Burg multiple segment algorithm. Low frequency peaks near 0.12 Hz dominate the spectra from either hydrophone or vertical accelerometer sensors with or without air gun array shots. The low frequency peaks correspond with wavelength peaks near 500 m. Therefore, the apparent speed of propagation of this low frequency noise is about 60 m/s. This is close to the speed of gravity water waves for the OBC sensor depths. Since this marine seismic noise is at a very low frequency and not generated by the seismic source, a best practice would be additional, analogue, low cut, filtering in the field to suppress its effects on seismic records and processing. Alternatively, digital recursive filters can be designed to be applied with initial condition constraints derived for each seismic trace. The very low frequency noise can then be suppressed in the computer without truncation edge effects.