USING OCEAN WAVES AS A PASSIVE MASW SOURCE

Ocean waves are considered to be a source of seismic surface-wave energy for subsurface exploration. We test the utility of ocean-source seismic recording, the sensitivity to array orientation with respect to the shoreline direction, and the equivalence of passive- and active-source data acquired in a near-shore environment. The Kohala peninsula of northwestern Hawaiʻi (Big Island), with its exposure to strong trade winds and prominent rock and soil outcrops along sea cliffs, provides an ideal natural laboratory to study ocean noise as a passive seismic source. The results show that passive-source data recorded from ocean waves over a prominent headland in Kohala produce coherent surface-wave dispersion relationships (phase velocity as a function of frequency) that can be modeled for shear-wave velocity in the shallow subsurface, especially when combined with an active source. We also demonstrate the dependence of the array orientation on the quality of the dispersion spectra—an array perpendicular to the shoreline produces a more complete frequency range and higher coherency, relative to an array oriented parallel to the shoreline. The geologic outcrop along the adjacent sea cliff, together with previous geophysical investigations of the site, confirms the utility of the shear-wave velocity-depth modeling constrained by our recordings. Ocean waves can be used for surface-wave modeling of shear-wave velocity in the upper 10-50 m depth with a relatively short array, but can be improved by combining active and passive sources and by orienting the array so as to be parallel to the direction of ocean-wave propagation.