Characterizing deep sediments of the Eastern U.S. Continental Shelf from the discrete Doppler spread of moving low-frequency tonal sources

The Doppler shift of tonal sound waves generated by a moving source provides a means to identify source energy radiating from different angles, relative to its motion. In shallow water, the shift imparted on a laterally moving source depends on the elevation angle, and a single generating tone may be received as multiple frequencies, each corresponding to sound propagating at an angle of the discrete normal modes. Using a geoacoustic model consistent with those emerging from studies conducted at the New England Mud Patch (NEMP), predictions of mode angles correlate to the observed mode angles at 57 Hz. For lower frequencies, this study examines harmonic tones from a container ship traversing the eastern U.S. Continental Shelf from west to east over a 150-km range. Observations of multiple non-resonant modes at the lowest harmonics (∼5 Hz) imply an increase in sound speed over an ∼1.5-km-thick sediment layer, terminating in a higher-speed rock basement. These data show a shift in mode angle commencing ∼75 km east of the NEMP, suggesting a significant change in sediment type and thickness.