The nature of the inhomogeneous component of the wave‐induced microseismic noise field

Abstract

Spectra of the near‐field microseismic noise induced by nonlinear surface wave interactions are calculated to examine the influence of the inhomogeneous component of the wave‐induced pressure field. The contribution of this component appears to be critically dependent on the thickness of the water column, the properties of the bottom structure, and the position of the observation point. Calculation for a typical six‐layered bottom with an overlying unconsolidated sediment on top of a solid half‐space, in a shallow‐water environment (100‐m water layer), shows that the microseismic spectrum inside the active wave region is dominated by the inhomogeneous component that is some 40 dB higher than the homogeneous component. However, when the bottom is a single solid basement, this component reduces by about 30 dB. Further, when the observation point moves beyond the boundary of the active region, the spectral level drops dramatically and finally approaches the level of the Rayleigh mode of the basement. Applications of these results to the estimation of the ULF noise spectra from microseismic data recorded on shore are discussed. Variation of the spectra with distance from the active region is also examined. [Work supported by ONR.]