Time- and space-varying interference patterns of broadband acoustic field sampled by drifting buoys during ENVERSE 97 experiments

During the winter of 1997 SACLANTCEN deployed a fixed controlled sound source, a vertical receive array, and drifting hydrophone buoys in a complex coastal environment on the western Sicilian shelf (ENVERSE 97). The acoustic impulse response of the medium was measured in a broad frequency band as a function of range and azimuth from the source, using repeated, large time-bandwidth-product FM transmissions and DGPS positioning. This paper investigates the combined effects of water column and bottom variability upon the space-frequency distribution of the sound-field intensity. The time and space dependence of extracted features such as patterns of field extrema are analyzed and related to the observed environmental conditions. In particular, perturbations of acoustic field invariants are detected and shown to be well correlated with the range-dependent bottom properties and the time-varying ocean sound speed and current fields. Preliminary modeling (C-SNAP) results obtained from concurrent oceanographic and geophysical ground-truth data (enverse 98) are compared to the acoustic measurements to determine their sensitivity to various environmental parameters