Very low frequency seismo-acoustic noise below the sea floor (0.2-10 Hz)

Abstract

In this thesis, the sources and propagation of sub-seafloor microseismic noise between 0.2 and 10 Hz are investigated. There are two fundamental interface wave modes for noise propagation in the crust: the Stoneley and pseudo-Rayleigh wave. In soft bottoms only the Stoneley interface wave can exist. Longer wavelength noise propagates as Rayleigh and Stoneley modes of the ocean+crust waveguide. During the Low Frequency Acoustic Seismic Experiment (LFASE) a borehole array (SEABASS) was deployed in the Blake Bahama Basin in DSDP Hole 534B. The 0.3Hz microseism peak is nearly invariant with depth at a level of 65 and 75 dB rel 1 (nmls2)2) 1Hz for the vertical and horizontal components respectively. At 100 m depth, the mean microseismic noise levels above 0.7Hz are 10dB and 15-20dB quieter for the vertical and horizontal components respectively. Higher modes in the spectra show narrow bandwidth depth variability in the noise field. Particle motion analysis further verifies that noise propagates through the array as Rayleigh/Stoneley waves. Numerical modeling for hard and soft bottom environments shows that heterogeneities on the order of a seismic wavelength radiate energy into the water column and convert acoustic waves in the water into small wavelength Stoneley waves observed at the borehole.