Wind effects in shallow-water acoustic transmission

Abstract

Acoustic transmission in shallow water is degraded in the presence of high winds; losses can exceed 40 dB, after which they cannot easily be measured. Multifrequency studies have used pulse frequencies f from 312 Hz to 4.44 kHz, with path lengths 17, 23, and 137 km, and observations to about 15 m/s wind speed W. The main results show a dB attenuation proportional to f1.5W4, with a coefficient that depends on the season. They can be explained by a quantitative theory involving the scattering loss from the waves on the rough sea surface. But the possibility of a component of the scattering or absorption coming from shallow layers of entrained bubbles is not ruled out. Briefer descriptions cover effects of directional dependence, comparison of rising and falling winds, squalls, breakup of fish shoals, swell, fluctuations, and observed effects on sonar displays.

Concurrent inversion of geo- and bio-acoustic parameters from transmission loss measurements in the Yellow Sea

“…It will be shown in the next section that d at Qiu et al's site is approximately equal to 6.9 m, which implies that t is less than approximately 1 m. The wind speed during this experiment was about 10 knots ͑Qiu, personal communication͒. Attenuation due to surface waves and subsurface bubbles at frequencies below 2 kHz at 10 knots for a water depth of 40 m is less than 0.1 dB/km ͑Weston and Ching, 1989͒. Chemical absorptivity at these frequencies and ranges is also small ͑Thorpe, 1965͒.…”

Propagation and Scattering Effects in Underwater Acoustic Communication Channels

“…7. Other causes of signal attenuation associated with squalls are surface waves, and bubbles due to concurring precipitation [50].…”

“…Frequency-dependent fading statistics, including the mean power, then render narrowband channel models inappropriate and call for more elaborate descriptions [20], [68], [69]. A frequency dependence of the mean received signal power may arise, for instance, due to a frequency dependence of bottom loss [70], [71], surface loss [49], [50], [63], absorption by seawater [7]- [10], and scatterers in the water column [72]. Frequency-dependent fluctuation rates occur when the channel has paths with different Doppler shifts [20], for instance, due to sea-surface interactions and platform motion.…”

Field measurements and modeling of attenuation from near-surface bubbles for frequencies 1–20kHz

“…5) and 17-km (Ref. 6) paths in shallow water (1 -3 kHz in each case) do show an excess attenuation in TL upon the wind speed exceeding ϳ10 m / s. In the latter case it was postulated that near-surface bubbles were responsible for the excess attenuation, with the attenuation going as a function of frequency, f, and wind speed, U, as f 1.5 U 4 . However, converting long-range TL estimates into a more generalized metric, e.g., loss in dB/m, requires assumptions on the precise nature of the paths (eigenrays) responsible for signal delivery.…”