Focused arrivals in shallow water propagation in the Straits of Florida

Observations of multipath on the western shelf area of the Florida Straits have consistently revealed an intense arrival associated with refracted bottom reflected (RBR) rays/modes. The 'late' or 'focused' arrival is 15 to 20dB higher than other arrivals. Parabolic Equation and Normal Mode propagation models and the shallow water invariant, beta, are used to explain the focusing. The sound speed profile that produces the focusing comes about from a combination of a strong downward refracting layer resulting from the geostrophic equilibrium of the Florida current on top of a well mixed turbulent bottom boundary layer. The resulting c(z) resembles cosh(g(1-z/D)), which produce perfect focusing at every range, that is, all RBR modes have the exact same group velocity.