Variability, Coherence and Predictability of Shallow Water Acoustic Propagation in the Straits of Florida

Abstract

Results of two shallow water propagation experiments are analyzed and compared with model predictions using observed environmental parameters as model inputs. The site of the experiments is off the coast of South Florida near Ft. Lauderdale nearby the future location of the planned Acoustic Observatory. Unique to the Florida Straits Propagation Experiments (FSPE) is an autonomous source that transmits broad band pulse-like signals at each of six center frequencies from 100 to 3200 Hz in octave steps. The transmissions last for 28 days and are received with a 32 element vertical array that is connected to shore by fiber-optic cable. Pulse arrivals along water born paths are identified by comparison with PE and normal mode model predictions. Three mode/ray groups of arrivals are identified: 1) RBR arrivals, which refract in the water column and interact with the bottom below the critical angle. These modes have low loss and nearly identical group velocities so that they coalesce to form a very intense focuse d arrival, 2) SRBR arrivals, that are spread in time and have increasing bottom angle with mode number and 3) numerous and mysterious late arrivals that couple with deep layers of the bottom and rapidly attenuate with higher frequency. The ocean environment, near the edge of the Florida Current, is highly variable with a saturated GM internal wave field and relatively large sub-inertial fluctuations from eddies and stream meanders. Sound speed fluctuations are generally 1 to 2 orders of magnitude larger than observed in the deep ocean. The bottom is composed of unconsolidated carbonate granules that have the density of sand and attenuation of fine sediment. Fluctuation statistics and coherence are computed and modeled in a parameter space of range, depth and frequency. The acoustic propagation, like the environment, is highly variable and complicated and many new interesting dependencies are revealed.