The character of seafloor ambient noise recorded offshore New Zealand : results from the MOANA ocean bottom seismic experiment

Abstract

We analyze the characteristics of ambient noise recorded on ocean-bottom seismographs using data from the 2009–2010 MOANA (Marine Observations of Anisotropy Near Aotearoa) seismic experiment deployed west and east of South Island, New Zealand. Microseism and infragravity noise peaks are clear on data recorded on the vertical channel of the seismometer and on the pressure sensor. The noise levels in the infragravity band (<0.03 Hz) on the horizontal seismometer channels are too high to show the infragravity peak. There is a small difference (~0.25 Hz versus ~0.2 Hz) in microseism peak frequencies between the two sides of the South Island on all three seismic channels. Our results show clear depth dependence between the peak frequency of infragravity waves and the water depth. We find that the product of water depth and wave number at the peak frequency is a constant, koH = 1.5. This relationship can be used to determine the variation of phase and group velocity of infragravity waves with water depth, and the location of the infragravity peak and corresponding noise notch at any water depth. These estimates of spectral characteristics, particularly low noise bands, are useful for future OBS deployments.