Acoustic noise generation under plunging breaking waves

The paper presents results of investigations performed in a wave channel in order to determine associations between the dissipation of surface wave energy during breaking and acoustic noise emission. The experiments were carried out in fresh water in the Large Wave Flume (GWK) at the Forschungszentrum K��ste (FZK) in Hanover (Germany). Relationships between the acoustic noise energy and losses of surface wave energy were estimated over the broad acoustic frequency band from 350 to 12 500 Hz, and the characteristic temporal changes of the spectral properties of noise in the breaking process were demonstrated. It was found that the ratio of acoustic noise energy generated during wave breaking to the energy dissipation of single plunging breakers with heights of 1.6-2.8 m were in the 10-9- 10-8 range and found to be in reasonable agreement with the results of some previous experiments performed for smaller scales of breaking wave. The study contributes to the development of a passive acoustic method for the parameterization of sea surface dynamic processes

Acoustic properties of the Pomeranian Bay bottom sediments* Marine acoustics Bottom backscattering Baltic Sea

Abstract The properties of bottom backscattered signals in the Pomeranian Bay have been investigated using a one-frequency single beam echosounder working at a central frequency of 30 kHz. The backscattering strength, time of reverberation and attenuation coefficient in sediments were estimated and mapped for the whole area. The other purpose of the investigations was to verify the acoustic one-frequency multiparametric method in order to determine the sediment-type distribution in the Pomeranian Bay. This method was found as useful tool for sediment identification in the study area

Statistical analysis of acoustic echoes from underwater meadows in the eutrophic Puck Bay (southern Baltic Sea)

In order to monitor the recovery of vegetation from pollution and the success of re-seeding efforts, acoustic echoes from the sea floor, covered and uncovered by underwater vegetation, were collected in Puck Bay (southern Baltic sea) using a 208 kHz Biosonics DT 4200 scientific echo sounder. The echo envelopes were examined and several of their parameters were recommended for further analysis. The possibility of using these parameters to distinguish between a bare sea floor and underwater meadows was tested. The parameters may be helpful in the identification of the species composition of the meadows and in accurate biomass assessment

Geochemical and acoustic evidence for the occurrence of methane in sediments of the Polish sector of the southern Baltic Sea

This paper presents the results of geochemical and acoustic investigations of sediments in the Polish sector of the southern Baltic Sea. Its objective was toindicate areas of gas bubble formation and the occurrence of methane. Over 3000 nautical miles of transects were recorded using a variety of hydroacousticinstruments, and five coring points were selected for further analyses of pore waters(CH4, SO4-2, H2S, NH4+, total alkalinity) and sediments (grain size distribution,Corg, Ntot, LOI and WC). Gas turned out to be present at shallow depths in different forms such as recent and buried pockmarks, and gas-saturated sediments (including gas pockets and seepages). It was found that methane was widespread in the sediments of the study area, both in the surface sediments, e.g. in the vicinity of the Hel Peninsula or in the central Gulf of Gdańsk, and in deeper sediment layers, e.g. in the Gdańsk Deep and the Słupsk Furrow. Chemical analysis showed that as a result of the rapid decomposition of organic matter, sulphates were depleted in the top 20 cm layer of sediments and that methane was produced at relatively shallow depths (in some areas even at depths of 20-30 cm bsf) compared to other regions of the Baltic, reaching concentrations of >6 mmol l-1 in the 30-40 cm layer below the sediment surface. The sulphate-methane transition zone (SMTZ) was 4-37 cm thick and was situated in the uppermost 50 cm of the sediments