RESUMEN
An anomalous dispersion, e.g., when low frequencies arrive earlier whereas the high frequencies arrive later, was observed in the signal arrivals recorded by a single deep-sea bottom-mounted vector sensor. Numerical simulations and modal analyses, based on a three-layer range-independent model, are applied to interpret the anomalous dispersion. Results indicate that the arrival with anomalous dispersion corresponds to trapped modes in the low sound speed sediment and can be observed when both the source and receiver are deployed near the seafloor. Furthermore, the cutoff frequencies, dispersion characteristics, and energy distributions of trapped modes are also performed in this paper.
RESUMEN
Very-low-frequency (VLF) sound has significant potential for underwater detection and estimation of geoacoustic models of the ocean bottom structure. In marine settings, one type of VLF sound is the interface wave. These waves, trapped near the fluid-solid interface, are called Scholte waves, and this is the subject of this study. A field experiment was carried out in the South China Sea with the objective of exciting Scholte waves and investigating the propagation. The data were acquired by an ocean bottom seismometer, deployed on the seafloor. A large volume airgun array near the sea surface provided the sound source. The fundamental and three higher-order mode Scholte waves were excited. The Scholte waves are investigated by seismograms and a phase velocity inversion. The observed frequencies are in the range of 1.0-2.9 Hz. The energy attenuation is proportional to 1/r at the peak frequency 1.4 Hz. The shear wave speed structure, down to 600 m beneath the seafloor, is revealed from the dispersion curves by a least-squares inversion algorithm. The inversion result shows that the shear wave speed is below 300 m/s in the uppermost layer, which explains well the weak excitation of Scholte waves in this experiment.
