Ambient and Ship-Induced Low-Frequency Noise in Shallow Water

Abstract

This paper describes some features of extremely low-frequency hydroacoustic and seismic noise (1 to 10 Hz) in coastal waters. The data have been collected by a single tri-axial ocean-bottom seismometer equipped with an additional hydrophone. The following aspects are considered.

• a. modification of the background power-spectra by ship noise.

• b. coherence and phase spectra between the seismic components.

• c. direction-finding based on the azimuthal power-distribution.

The hydroacoustic and seismic power-spectra are very similar in general shape. However, for some significant lines the output of the horizontal geophones indicates a higher signal-to-noise ratio. The horizontally polarized energy may be used to determine the direction to the source once the propagation mechanism has been identified by means of all seismic sensors. It has been proved that ship-induced infrasonic noise is propagating in form of seismic interface waves with their characteristic elliptical particle motion.