HYDROACOUSTICS
ANNUAL JOURNAL |
START | NEW VOL 20 | SEARCH | STATISTICS | PAS - GDANSK DIVISION |
pp. 237-244, vol. 14, 2011 Jarosław Tęgowski University of Gdansk, Institute of Oceanography, Gdynia, Poland Ilia Ostrovsky Israel Oceanographic & Limnological Research, Yigal Allon Kinneret Limnological Laboratory, Migdal, Israel Mateusz Zamaryka University of Gdansk, Institute of Oceanography, Gdynia, Poland Key words: soft muddy sediments; singlebeam echosounder; backscattered signal Abstract: The bottom top layer of the central part of the Eckernförde Bay (Germany) consists of soft muddy sediments containing free methane gas. Locations of gas bubbles trapped in the
sediment and gas seeps visualised with hydroacoustic data have been reported. The main goal
of our study was to examine whether it was possible, using a singlebeam echosounder with
relatively high frequency of the transmitted signal (120 kHz), to detect echo properties that
could be indicative of the occurrence of free gas in the bottom sediments. During three days
of measurements organised by Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel
(Germany), the acoustic data were collected from boards of r/v Polarfuchs and r/v Littorina.
The Hilbert-Huang Transform was applied to detect ‘gassy’ anomalies in backscattered
signals from the bottom. The transformer decomposes signal into finite and small number of
Intrinsic Mode Function (IMF) components with time-dependent amplitudes and frequencies.
Certain IMF components carry information on variability of geoacoustic parameters, which
can be indicative of presence of gas bubbles in the acoustically penetrated sediment as well
as in the water column. Based on the shape of the echo signal envelope and its fading with
range we characterized the signal attenuation in areas where gas was present. The rapid
increase in acoustical wave attenuation in areas of intensive gas ebullition demonstrates good
applicability of the method proposed.
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