Ocean waves and seismic noise
What we know about seismic noiseSince the early days of seismology, the ubiquitous noise with periods 5 to 10 seconds has been clearly associated to ocean wave activity1.The corresponding ground motions have vertical excursions of the order of a few micrometers, hence their usual name microseisms.The detailed relationship between microseisms and ocean waves was only clarified much later, when Michael Longuet-Higgins made the connection between the empirical discovery by Pierre Bernard that seismic noise has a period half of the wave period, and the theoretical works by M. Miche who showed how two wave trains of opposite directions and with the same period can generate noise at half the wave period. The theory by Michael Longuet-Higgins was later extended by Klaus Hasselmann to random waves.
Why study seismic noise?
Because seismic noise is closely related to ocean waves, it can actually be used to measure waves in the middle of the ocean without getting sea-sick: the seismic waves travel to land and any seismometer can record these waves. Unfortunately the interpretation of the seismic noise is not so simple and varies much from one station to another. The IOWAGA team is developping methods to use seismic records for wave measurements.The resulting data may not be very useful for day-to-day wave measurements is many areas because it is not so accurate (the best accuracy we obtained so far is only 20% error for wave heights and 10% error on mean wave periods (Tm0,-1) when data is averaged over 4 hours. In comparison the numerical wave models that we are developping are typically much more accurate (10 to 15% error for the wave height). The nice thing is that seismometers have been around for one century now, and they are probably the only instrument to have recorded accurately the wave climate from 1900 to 1980. Also, they can measure the effect of very very low waves of long periods that ocean buoys hardly see. Finally, because the seismic noise level is linked to the poorly known directional distribution of wave energy, the analysis of seismic noise can also reveal some interesting features of wave directional spectra. So much for oceanographers ...
seismologists want to know where the noise comes from
First of all, having all this background noise without knowing where it comes from is rather annoying. But beyond their natural curiosity, seismologist are also using this noise to probe the structure of the Earth. Imagine an echographic system or sonar in which you just listen for the noise and feel the structure of the Earth from how the noise "sounds" different at different places.
What we do
In collaboration with IPGP, we have developped a model of seismic noise that allows us to compute the sources of seismic noise and understand better the measured signal. There are a few tuning knobs (seismic wave attenuation is the most important one), but it works really well, most of the time. So now we know where most sources of noise are. . Another thing that was done is to derive the theory for seismic body waves: these are waves that travel trough the Earth and only make up a small fraction of the noise field, whereas the more energetic surface Rayleigh waves travel along the Earth surface. This theory predicts that the maximum sources of body waves are not exactly at the same place in the ocean as the surface waves. Early analysis results suggests that the theory is correct.
What next ?
The wave and seismic propagation model will be refined based on a detailed analysis of mismatch between modeled and observed noise.
(1) Bernard, P. (1990), Historical sketch of microseisms from past to future, Phys. Earth Planetary Interiors, 63, 145–150.