A broadband investigation of microseisms and ocean surface gravity waves at the Cascadia array

Abstract:

The Cascadia initiative gathers seismometer and hydrophone data along the American west coast to including northern California, Washington and Oregon. These stations cover a large variety of depths in the same geographical area, allowing for interesting comparisons in similar meteorological conditions. Acoustic noise (velocity and pressure) spectra are studied in the range 0.01 to 10 Hz at various depths. It is known that nonlinear interacting surface waves are the main source of acoustic noise at frequencies around 0.2 Hz with a successful direct model of acoustic spectra from a numerical model of ocean waves (Ardhuin et al. JASA 2013). In that band the main source of error is likely the uncertainties on sediment properties. At frequencies above 0.4 Hz, that direct model of the acoustic noise spectrum is not very accurate probably due to a poor representation of surface wave processes, in particular wave breaking, and their influence on the directional wave spectrum or on the direct generation of noise. Previous study deep stations (~5000 m depth) at H20 or the ACO observatory (Dunnebier et al. 2012) exhibit a strong correlation between acoustic noise level and surface wind speed. Comparison of spectra with meteorological and oceanographic conditions (wind records, wave hindcasts) provide a better way to understand the origin of the noise. The main features are :

- booms and busts (Farrell and Munk 2010).

- better observed correlation of acoustic noise level with wave significant height rather than surface wind speed (see e.g. below)

- effects of station depth.

- saturation of the gravity part (Farrell and Munk 2012).

- other results from the work in progress.