Synoptic View of a Swell Field in the Pacific: from Spaceborne SAR Measurements to Seismic Noise on the Coast
Abstract
Satellite Synthetic Aperture Radar (SAR) operating with a wave mode offers a space and time sampling that provides a global view of ocean swell fields. Following the methodology described by Collard et al. (2009), SAR-derived observations are attributed to identified swell events. Then, estimates of swell peak period, height and direction are calculated along their propagation route in the mid and far field (typically more than 4000 km from the source region). This real-time data set is combined into a synthetic swell field whose main advantage is to compensate for the sparse sampling of the instrument. The ability to represent the energy distribution of the swell field with this method is assessed using buoy data. We also investigate the possible synergetic uses of the SAR-derived swell data and seismic noise. This attempt is motivated by the theory developed by Longuet-Higgins (1950) stating that microseims energy results from the non-linear wave-wave interaction of oppositely travelling waves of the same frequency. The study focuses on a powerful storm event on April 11th 2008 on the South-East of New-Zeeland, with a two-day period of high winds, blowing over 20 m/s as measured by QuickScat and wave heights up to 12 meters, measured along the JASON altimeter track. The concept of virtual buoys described by Collard et al. (2009) is applied at the seismic station locations, around the Eastern Pacific Ocean. The spectral analysis reveals similar linear features, characteristic of the dispersive arrivals of swells from remote storms, with identical arrival times for waves of the same frequency. These structures, as given by the SAR, encompass wave periods from 12 to 16 seconds. Longer waves, forerunners as W. Munk called them, cannot be seen by the SAR since they are not high enough to be discriminated from the background noise. Nevertheless, this set of observation is very well complemented by the ones of the seismic stations which extend from 14 to 22 seconds. Hereby, our approach confirms that the observed micro-seismic activity in the Pacific has a strong contribution from swell, probably due to the reflection to the coast. This result emphasizes the possibility to use the energy of the swell signal measured by the seismic noise to validate the energy dynamic of the synthetic swell field, taking advantage of numerous seismic stations. Additionally, the estimation of transfer functions from the seismic noise to the swell energy would be a unique opportunity to revisit world-wide swell energy of the last 60 years given the historical seismic data available.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS51B1325H
- Keywords:
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- 4275 OCEANOGRAPHY: GENERAL / Remote sensing and electromagnetic processes;
- 7200 SEISMOLOGY