Analysis on the frequency dispersion characteristics of seismic wave caused by low frequency sound source in shallow sea

• The time series of seismic wave at seafloor are mostly composed of interface wave and normal mode waves in shallow sea with hard sea bottom.
• The frequency dispersion of normal mode is obviously when frequency is lower than 100 Hz, while interface wave is dispersive only in the infra-sound frequency range.
• The time series of seismic wave is dominated by interface wave when the source frequency is less than the minimal cut-off frequency of normal modes.
• As for shallow sea with soft sea bottom, there is only one real root in the dispersion equation of seismic wave. The root corresponds to the interface wave which propagates along the seafloor at a slow speed.

The elastic wave in seabed caused by low frequency noise radiated from ship is the so-called ship seismic wave which can be used to identify ship target. In order to analyze the wave components and the propagating properties of ship seismic wave, numerical calculation for frequency dispersion curves of seismic wave caused by low frequency sound source in shallow sea has been carried out. According to the numerical example for shallow sea with hard sea bottom, time series of seismic wave at seafloor are mostly composed of normal modes and interface waves. Each normal mode has a well defined low frequency cut-off, on the contrary the interface wave doesn’t have. The frequency dispersion of normal mode is obviously when frequency is lower than 100 Hz, while interface wave is dispersive only in the infra-sound frequency range. The time series of seismic wave is dominated by interface wave when the source frequency is less than the minimal cut-off frequency of normal modes. As for shallow sea with soft sea bottom, there is only one real root in the dispersion equation of seismic wave. The root corresponds to the interface wave which propagates along the seafloor at a slow speed. More normal modes appear when the depth of sea water increases and more source energy transmits through the wave-guide of sea water rather than along the seafloor by the interface wave. Normal modes gradually become the significant carriers of source energy and interface wave weak accordingly.