Experimental evidence of range dependence of mode cut-on frequency in a scaled wedge-like environment
Résumé
In a previous work, results of laboratory scale measurements of long-range, across-slope propagation of broadband pulses in a shallow-water wedge-shaped oceanic waveguide with a sandy bottom were reported and analyzed [A. Korakas, F. Sturm, J.-P. Sessarego, and D. Ferrand, Proceedings of the 9th ECUA (Paris, France, 29 June-4 July 2008)]. The experiments were conducted at the LMA-CNRS laboratory in Marseille (France). Strong 3D effects like mode shadow zones and multiple mode arrivals were observed, in agreement with theoretical predictions. In the present work, we focus on the frequency dependence of the received signals along the cross-slope direction. The recorded time series are analyzed mode-by-mode. The wave packets associated to each mode are extracted from the time series at several ranges and their spectra are examined by means of appropriate Fourier transforms. It turns out that, as we move out in range across-slope, the low frequency part of the spectral content of the mode wave packets is progressively removed, giving evidence of the frequency dependence of the mode cut-off range. This effect, often referred to in the literature as range dependence of the mode cut-on frequency, is shown here for each propagating mode. In particular, the spectra exhibit interference patterns that are identified as intra-mode interferences.