Abstract
The statistics of intensity fluctuations in dual-band, low-frequency, ocean acoustic transmissions across 500 km in the Philippine Sea 2010 experiment appear to be at odds with the standard micro-multipath theory predicting them. M-sequence encoded acoustic signals at 200 Hz and 300 Hz were transmitted for 54 h from a ship-suspended multiport source to a distributed vertical line array (DVLA) with 149 working hydrophones covering most of the 5500 m water column. Histograms of the received intensity fluctuations are approximately exponential. Intensity fluctuations at the two frequencies are strongly correlated. Scintillation indices and variances of log intensity are high (>1 and >5.57dB, respectively), suggesting the measurements are not yet in a saturated regime but still significantly scattered. However, for all this, the pulses received at the DVLA are virtually the same width as those recorded next to the source; no pulse spreading or arrival splitting is seen. So, the fading observed in the data does not appear to match this causal mechanism. To explore the connection with micro-multipath theory more fully, we compare the intensity statistics from the Philippine Sea 2010 data with those calculated both from simulated random oceans and from Flatté and Rovner's path integral theory. [Work supported by ONR.]
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