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Seafloor mapping based on multibeam echosounder bathymetry and backscatter data using Object-Based Image Analysis : a case study from the Rewal site, the Southern Baltic

Janowski Ł., Tegowski J., Nowak J.

Oceanological and Hydrobiological Studies

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2018

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Vol. 47, No. 3

248--259

EN

Seafloor mapping is a fast developing multidisciplinary branch of oceanology that combines geophysics, geostatistics, sedimentology and ecology. One of its objectives is to isolate distinct seabed features in a repeatable, fast and objective way, taking into consideration multibeam echosounder (MBES) bathymetry and backscatter data. A large-scale acoustic survey was conducted by the Maritime Institute in Gdańsk in 2010 using Reson 8125 MBES. The dataset covered over 20 km2 of a shallow seabed area (depth of up to 22 m) in the Polish Exclusive Economic Zone within the Southern Baltic. Determination of sediments was possible based on ground-truth grab samples acquired during the MBES survey. Four classes of sediments were recognized as muddy sand, very fine sand, fine sand and clay. The backscatter mosaic created using the Angular Variable Gain (AVG) empirical method was the primary contribution to the image processing method used in this study. The use of the Object-Based Image Analysis (OBIA) and the Classification and Regression Trees (CART) classifier makes it possible to isolate the backscatter image with 87.5% overall and 81.0% Kappa accuracy. The obtained results confirm the possibility of creating reliable maps of the seafloor based on MBES measurements. Once developed, the OBIA workflow can be applied to other spatial and temporal scenes.

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A Novel approach to bottom scattering using a narrow acoustic beam

Zieliński A., Tegowski J., Lubniewski Z., Demkowicz J.

Hydroacoustics

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2004

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Vol. 7

253-258

EN

The acoustic response of the ocean bottom to a probing pulse is a complex and complicated process. This process is influenced by with the form of an acoustic transmitting/receiving beam and by the physical processes involved in sound scattering from the surface and the volume of the ocean bottom. The complexities of these phenomena often obscure an intuitive understanding of the underlying principles of echo formation and its reception. In this paper, we propose a simplistic model for this complex process using filter theory. The bottom is represented as a surface reflector with an acoustic wave front sweeping over it with time-varying velocity. The impulse response of a smooth flat bottom is characteristic of a low pass-filter that will greatly attenuate the impinging high frequency pulse. On the other hand, bottom undulations will modulate the reflected signal such that it can be represented by the impulse response of a band-pass filter. The received echo can be represented as the response of such filter to a high frequency pulse. The characteristics and amplitude of the echo are dependent on frequency spectrum overlap between the transmitted pulse spectrum and the filter frequency response. In the paper, we discuss several cases of interest with the intent to provide a solid intuitive understanding of the echo formation from the system point of view.

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Effects of ship motion on acoustic remote sensing

Tegowski J., Zielinski A.

Hydroacoustics

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2003

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Vol. 6

205--214

EN

Ship motion affects quality of acoustic data collected by various acoustic remote sensing systems used for bottom and fisheries surveying. The angular position of an acoustic beam changes in time from its nominal position vertical to sea surface. This motion affects the acoustic returns from the bottom and other targets by changing their intensity and arrival times. In this paper we illustrate these effects on bottom returns obtained using single beam sonar.