Scintillations in long-range acoustic transmissions

Abstract

In the Acoustic Thermometry of Ocean Climate (ATOC) program’s Acoustic Engineering Test (AET), broadband 75-Hz center frequency transmissions were recorded on a 700-m-long vertical array, 3252 km distant. Previously reported results from the AET using 12.7-min. averaged data by Colosi et al. [J. Acoust. Soc. Am. 105, 3202–3218 (1999), hereafter referred to as Colosi99] revealed surprisingly weak acoustic scattering for early arriving identifiable wave-fronts; these results have been confirmed using 1.8-min averaged data. It is shown that scintillation index (SI) is a strong function of position along the pulse with smallest values occurring at the peak and larger values occurring at the tails. Intensity PDFs of identifiable wave-fronts are reanalyzed in terms of both peak intensity and integrated pulse energy (IE), but both PDFs are very closely log-normal. Regarding multipathing along the wave-fronts it is found that on average there are 1.7 peaks per wave-front segment per hydrophone. The combined observation of weak scattering and multipathing is a novel result. Colosi99 analyzed the finale in terms of peak scintillations and found a near log-normal intensity PDF. Reprocessing the full field without limiting data to intensity peaks and accounting for mean intensity nonstationarity yields an exponential intensity PDF.