Frequency Limit for the Pressure Compliance Correction of Ocean-bottom Seismic Data

The vertical records of ocean-bottom seismometers (OBS) are usually noisy at low frequencies, and one important reason is the pressure compliance resulted from ocean-surface water waves. Water waves of low frequencies and hence long wavelength cause ocean-bottom pressure variations, leading to seafloor deformation. The relationship between the ocean-bottom pressure and vertical seafloor motion, called the compliance pressure transfer function (PTF), can be derived during quiet times without earthquakes. It is utilized to calculate and remove the pressure-induced noise during an earthquake. In shallow waters, water waves of high frequencies also induce compliance noise, which in principle can be removed similarly. However, the compliance PTF increases with frequency, so it is possible to become the same order as the seismic PTF that describes the seafloor deformation generating ocean-bottom pressure. In such a case, because earthquake signals are also present in the pressure records, subtracting the predicted compliance noise may distort the original signals.

In this paper, we analyze the data from 24 OBS instruments with water depth ranging 107-4462 m. We find that for most stations there are four zones in the investigated frequency range (0.01-0.2 Hz), with different correlations between the vertical seafloor acceleration and ocean-bottom pressure. The exact limits for each zone vary station by station, and largely depend the water depth. In frequency zone (I), the vertical seafloor acceleration is mostly composed of pressure compliance noise, and the noise can be removed using the compliance PTF. The compliance PTF is found to be much smaller than the seismic transfer function, so the distortion of earthquake signals is negligible. In zone (II) of higher frequencies, the vertical acceleration and ocean-bottom pressure are largely uncorrelated. In zone (III) and (IV) of even higher frequencies, the compliance noise is negligible, and the ocean-bottom pressure is mostly resulted from the vertical seafloor acceleration. Thus, the approach of removing compliance noise can be applied in frequency zone (I), and in this zone the seismic PTF can be safely ignored.