Experimental investigation of features of low‐frequency broadband acoustical signal propagation in shallow water

Abstract

In shallow water, the combination of downward refracting sound‐speed profiles and strong sound absorption by the bottom limits the range of (tomographic) transmissions and causes one to use lower‐frequency broadband signals. In this paper, investigations of low‐frequency broadband (25–95 Hz) acoustic signal propagation in a slightly variable shallow waveguide with a negative water column sound‐speed gradient are presented. An experiment was carried out in the Barents Sea along a 14‐km stationary track. In travel time tomography in shallow water, as in deep water, the resolvability and identifiability of the acoustic arrivals is a necessary condition. In this experiment, seven stable arrival peaks are noted. Over 2 hr, these peaks fluctuate in rms travel time by several milliseconds, which is consistent with the measurement accuracy of the system used. Identification of the arrivals using both mode theory and rays with beam displacement, combined with the ambiguity diagram approach of Munk and Wunsch, has shown that some of the arrivals are resolvable modes and some are resolvable rays.