Observations of Infragravity Waves at the Monterey Ocean Bottom Broadband Station (MOBB)

David Dolenc and Barbara Romanowicz

Introduction

Ocean bottom broadband seismic observations show increased noise level when compared to land recordings. Infragravity waves are an important source of background noise at periods longer than 20 seconds. Long-period background noise can partially be removed by post-processing (Crawford and Webb, 2000; Stutzmann et al., 2001). At the same time, observations of infragravity waves can help us better understand their generation from short-period waves. We investigated correlations between the infragravity waves recorded at MOBB and the short-period ocean wave data recorded at nearby ocean buoys. MOBB was installed 40 km offshore in the Monterey Bay at a water depth of 1000 m in April 2002 in collaboration between Berkeley Seismo Lab and Monterey Bay Aquarium Research Institute (MBARI) (McGill et al., 2002; Uhrhammer et al., 2002). It comprises a three-component broadband seismometer with a temperature sensor, a water current meter measuring current speed and direction, and a differential pressure gauge (DPG). The station is continuously recording data which are retrieved, on average, every three months.

Results

We calculated power spectral density (PSD) of the MOBB vertical component for 1-hour long periods for all the data recorded until February 2004. The results were compared to the spectral wave density (SWD) of the ocean waves as recorded on the eight NOAA buoys located from offshore Southern California to Alaska. The results clearly showed that the width of the infragravity wave band recorded on MOBB was best correlated with the energy of the ocean waves recorded at the local NOAA buoy 46042, located just 23 km W of MOBB. To further explore observed correlation between the MOBB PSD and the buoy SWD, we focused on a 7-day period and included 6 more buoys that were closest to MOBB, and had data available. We calculated correlation coefficient between 1-hour long MOBB PSD and buoy SWD values. Results for 5 of the buoys are presented in Figure 18.1. The highest correlation can be observed for the closest buoy (46042). The result also shows that the 7-17 sec ocean waves are best correlated with seismic waves with 30-200 sec period. A weaker correlation can be observed with the other two buoys located over the continental shelf to the north (46012 and 46014). Correlation with the buoy located to the south (46011) and the one further offshore (46059) is much smaller. We plan to further investigate these observations as well as include pressure and tides data to better understand where and how the energy is transferred from the ocean to the seismic waves. Once the reliable DPG data from the MOBB are available we will also use them to remove the long-period noise from the MOBB vertical channel to improve the seismic data quality.

Figure: Correlations between power spectral density for the MOBB vertical channel and spectral wave densities measured on the closest buoys. Results for a 7-day period are shown. A color version of this figure is on page [*].
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References

Crawford, W.C., and S.C. Webb, Identifying and removing tilt noise from low-frequency (\(<\)0.1 Hz) seafloor vertical seismic data, Bull. Seism. Soc. Am., 90, 952-963, 2000.

McGill, P., D. Neuhauser, D. Stakes, B. Romanowicz, T. Ramirez, and R. Uhrhammer, Deployment of a long-term broadband seafloor observatory in Monterey Bay, EOS Trans. Amer. Geophys. Un., 83, F1008, 2002.

Stutzmann, E., J.-P. Montagner, A. Sebai, W.C. Crawford, J.-L. Thirot, P. Tarits, D. Stakes, B. Romanowicz, J.-F. Karczewski, J.-C. Koenig, J. Savary, D. Neuhauser, and S. Etchemendy, MOISE: A prototype multiparameter ocean-bottom station, Bull. Seism. Soc. Am., 91, 885-892, 2001.

Uhrhammer, R., B. Romanowicz, D. Neuhauser, D. Stakes, P. McGill, and T. Ramirez, Instrument testing and first results from the MOBB Observatory, EOS Trans. Amer. Geophys. Un., 83, F1008, 2002.

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