What is it about?
Ocean waves make noise in the far infrasound range, with a peak power around periods of 5 s. Nobody can hear that, but is the main signal recorded on seismometers, microphones and hydrophones (microphones in the ocean). Our work extends previous theory on how waves make this noise. In particular we consider the effect of water depth on the waves, and the generation of different types of sound waves: some related to seismic surface (Rayleigh waves), others related to seismic waves that travel deep in the Earth, and sound waves in the atmosphere.
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Why is it important?
Seismologists are using this background noise to monitor changes in the solid Earth, such as filling up of a magma chamber before a volcanic eruption or detecting precursor signatures (changes in stresses) before Earthquakes. Understanding better the source of noise can lead to more sensitive methods and in particular a higher temporal resolution. Oceanographers are interested in the ocean wave climate, especially before the era of satellites (i.e. 1993) or buoys. This is important for understanding the properties of the rare extreme storms and separating the trends due to global change from the year-to-year changes associated with El Nino or other climate patterns.
Perspectives
If seismologists manage to understand our oceanographic point of view, they should see that our Fourier analysis of the problem is a natural way to separate the different seismic phases (e.g. Rayleigh and body waves) and lead to a better understanding of microbarom generation. In particular we predicted a strong "site effect" for body waves that has now been verified, and that is very different from the "site effect" for Rayleigh waves. Also, knowing that microbaroms recorded at IMS stations are propagating within 45 degree of the horizontal means that they cannot be associated to Rayleigh or body waves, but rather to acoustic-gravity modes for which the water depth has no impact. We also predicted that this mechanism of noise generation is very weak for waves in shallow water, such as infragravity waves. As a result, the seismic hum (periods around 100 s), cannot be explained by this theory.
Fabrice Ardhuin
Read the Original
This page is a summary of: Noise generation in the solid Earth, oceans and atmosphere, from nonlinear interacting surface gravity waves in finite depth, Journal of Fluid Mechanics, January 2013, Cambridge University Press,
DOI: 10.1017/jfm.2012.548.
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