Transverse horizontal spatial coherence of deep arrivals at megameter ranges

Predictions of transverse horizontal spatial coherence from path integral theory are tested for ranges from 2000 to 3000 km. A low-frequency (75 Hz) source bottom-mounted at depth 820 m near Kauai transmitted ‘‘m-sequence’’ signals over several years to bottom-mounted U.S. Navy horizontal line arrays in the North Pacific. The arrays acquired only the early, deep arrivals. Coherence lengths estimated from the datasets, on the order of 400 m, show good agreement with lengths calculated from theory. Both measurements and predictions show a dependence on path as well as path length. These coherence lengths correspond via a simple incoherent source model to about 1° in horizontal arrival angle variability. Estimates of scintillation index, log-amplitude variance, and variance of decibel intensity indicate that the fields were partially saturated. There is no significant seasonal variability in these measures. Scintillation index predictions from path integral theory also agree well, although theory identifies these fields as fully saturated. [Work supported by ONR.]