USE OF THE HILBERT-HUANG TRANSFORM FOR CHARACTERIZATION OF GASSY SEDIMENTS IN ECKERNFÖRDE BAY

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