Observations of micromultipath formation at very long ranges in shallow water.

Abstract

Long- and very-long-range shallow-water transmission is observed for low-frequency m-sequences transmissions with center frequencies of 50, 100, and 250 Hz. The source is suspended from a ship that is holding position at each of three ranges: 10, 20, and 80 km. The receiver is a bottomed horizontal array. The array is used to resolve out-of-plane arrival angles to a small fraction of a degree by observing phase rolls of the spatial coherence function of the various arriving wavefronts. At the 10 and 20 km ranges, distinct pulse arrivals are observed in one-to-one correspondence to normal mode arrivals predicted with PE propagation models. But at the 80 km range, a large number of distinct arrivals appear with intensities or arrival times inconsistent with deterministic normal mode or ray arrivals. One possibility is that these are micromultipaths from out-of-plane paths after reflections from facets or sloping bottom contours. In fact, each of the arrivals has a unique horizontal arrival angle, and the steeper the out-of-plane angle, the later the arrival—an observation that is verified with broadband PE propagation model computations for both random faceted and gently sloping bottoms.