Leroy M. Dorman
Anthony E. Schreiner
Marine Phys. Lab. and Geological Res. Div., Scripps Inst. of Oceanogr., Univ. of California, San Diego, La Jolla, CA 92093-0215
Further results are reported of a study of seafloor noise using an array of ocean-bottom seismometers in a sediment pond in deep water off the California coast. Earlier (Schreiner and Dorman, 1990) measurements were presented of coherence between elements of the array that contained interelement spacings in the 8--156 m range and it was shown that they represented the fundamental and first three overtones of seismic surface and interface waves. Here, the ratios of power spectra (horizontal to vertical component) in the 0.500--10 Hz frequency band were used to refine the estimates of the interface wave mode excitation that were made earlier using the interelement coherences. There is a simple frequency dependence in the mode excitation functions. The fundamental mode dominated below 0.65 Hz and is succeeded by the first three overtones in order as the frequency increases. The other transition frequencies are 0.95 and 6.5 Hz. From these excitation functions, the depth dependence of background noise was synthesized, using the vertical wave functions from the known shear velocity structure. For some depths in the 0.8--10 Hz frequency range, the amplitude of vertical component noise is up to 29 dB greater than at the surface, due to the prominence of higher mode interface waves in this band. Varying degrees of noise suppression are obtained outside this band. The noise suppression (or enhancement) is strongly dependent on the relative mode excitation. At frequencies where the fundamental mode dominates there will be noise reduction downhole while at frequencies where the overtones are prominent, there will be noise enhancement at some depths. For horizontal-component noise there is noise suppression with depth at all frequencies. [Work supported by ONR and ONT.]