Observations of Water Column and Bathymetric Effects on the Incident Acoustic Field Associated With Shallow-Water Reverberation Experiments

As a part of the 2013 Targets and Reverberation Experiment (TREX13), measurements of the acoustic field generated by a source used in midfrequency (1.8-3.6 kHz) reverberation experiments are studied at 5 and 6 km range. The TREX13 reverberation sources were placed off the coast of Panama City, FL, USA, in waters similar to 20 m deep, and data discussed here are from a 2-h period in the late afternoon on April 28, 2013. The observed coda of the source signal is partitioned into an initial primary arrival, and a distinct second arrival delayed by roughly 2 s. Characteristics of the two arrivals are studied in terms of the effective number of modes, interference features, and the direction of acoustic intensity, which was directly measured by a vector sensor located at 5 km range. A shift in frequency within the primary arrival is observed over the 2-h measurement period. Frequency shifts are related to a change in range of dislocations, defined as points of complete destructive interference in the acoustic field, that modulate with tidal variation in the sound-speed profile. Precise frequencies are identified with the vector property called circularity, a nondimensional measure of acoustic intensity curl, that is maximal within the vortex-like intensity field within a dislocation. Using the waveguide invariant beta, the frequency shift is used to estimate the tidal change in the thermocline depth. These interference features are absent in the second arrival, which is postulated to be an acoustic path horizontally refracted by the gently sloping bathymetry (similar to 0.4 degrees) forming the coastal environment. A description of the refraction using modal rays is developed, and the transition of the mode from being trapped to leaky is handled as a transition to a virtual mode near the cutoff depth. Models of the primary and refracted arrivals are presented to support the conclusions.