Measurements of mode attenuation coefficients in shallow water

As part of an experimental study of sound transmission in shallow water, the attenuation coefficients of acoustic normal modes in a shallow area of the Gulf of Mexico were determined directly by separating and measuring the individual normal modes. The modes were separated by two methods, a temporal method depending on the differing group velocities of the modes and a spatial filtering method depending on their vertical pressure distributions. Data were taken at source frequencies of 400 and 750 Hz for a variety of sound-speed profiles. With two exceptions, only the first two modes could be identified and measured. The mode attenuation coefficients depend on the prevailing sound-speed profile, but the most reliable experiment yielded attenuation coefficients of 0.61 and 1.58 dB/km for the first and second modes, respectively, at 400 Hz, and 0.30 and 1.06 dB/km for the first and second modes, respectively, at 750 Hz. Assuming that the mode attenuation could be accounted for by bottom attenuation represented by a complex propagation constant, the above results require bottom attenuation coefficients of 126 dB/km at 400 Hz and 378 dB/km at 750 Hz. The latter imply an f1.75 frequency dependence of the bottom attenuation, a result which is supported by the remaining data.