Correlation of deep seabed structure with waveguide invariant β in ±∞ regime

Abstract

An examination of the received spectrogram levels of merchant ship recordings has identified an important acoustic feature at the very-low frequencies (VLF). The data were recorded on vertical line arrays deployed in the New England Mudpatch during the Seabed Characterization Experiment 2017 in about 75 m of water. The VLF feature occurs when the group velocities of modes 1 and 2 are equal at a frequency f = F. The average value of F from about twenty ship signatures observed in the Mudpatch is about 24.5 Hz. Generally, the observation of this VLF feature is a result of βmn( F) = infinity where βmn is the waveguide invariant for modes n and m. The sign of the waveguide invariant is determined by the sign of the difference between the phase speeds at F. Also, we predict what F would be in two areas south of the Mudpatch, including a second smaller Mudpatch in about 125 m of water and an area near the slope in about 250 m of water where the sediment is reported to be sand over multiple layers of softer sediments. Finally, F is predicted on the Atlantis II seamount with a limestone seabed in about 1650 m of water.