Shallow water infrasonics: Propagation and noise studies with ocean bottom seismometers

Abstract

Sound propagation studies near or below the hydroacoustic cutoff frequency of shallow water ducts have been strongly neglected in the past. The growing interest in bottom-interacting phenomena stimulated the investigation of pertinent propagation mechanisms to identify low-frequency sources or to probe the upper sea floor in-situ. For this purpose three-component ocean bottom seismometers each with a radio link and an external hydrophone have been developed and deployed during several sea trials off the Italian coast near La Spezia. Numerous measurements from small underwater explosions and from the ambient background or passing ships were analyzed with various software packages. The results to be discussed in this paper are confined to the infrasonic regime from 1 to 18 Hz. Shot-data are illustrated by typical seismogram sections, particle hodographs, dispersion contours for amplitude or phase, and attenuation curves, while noise data are presented in the form of amplitude or cross spectra. It can be clearly demonstrated that the infrasonic energy is mainly transmitted via seismic interface waves of the Stoneley- or Scholte-type propagation along the sea floor.