Experimental investigation of modal structure of field generated by a pulse-source in a laboratory hydroacoustic waveguide

Abstract

This work contains the results of an experimental investigation of the modal structure of an acoustic field in a model oceanic waveguide. Due to limited propagation range, it is impossible to resolve modes either by arrival times of modal pulses or by Doppler spectra of the received signal. The experiment was done in a laboratory two-layer waveguide where 32 normal waves had propagated at ranges up to 260 cm. As a result of intermodal dispersion, original short pulses radiated at 200 kHz were destroyed, and different parts of the received signal carried information about different modes forming the field in the waveguide. The beginning of the received pulsed signal was formed by the low-number modes, while the area near the back front of the pulse was constructed by high-number modes. Selection of modes was done using Doppler spectra of the received signal by a receiver uniformly moving along the waveguide and information about interference structure was read in the beginning, in the middle, and in the end of each received pulse. Such signal processing allowed to ‘‘impoverish’’ the original modal spectrum and, afterward, to analyze the modal structure of the acoustic field in the waveguide in more detail. [Work supported by RFBR Grant No. 95-02-04565.]