Detection of Stones in Marine Habitats using Hydroacoustics

svenja.papenmeier [ at ]


Hard substrates in sublittoral environments are hotspots for marine biodiversity; especially for benthic communities. It is habitat and shelter for flora and fauna, including rare species endangered by threats such as fishing. For fishes, marine mammals and birds hard substrate areas are important breeding and feeding places. In offshore areas those habitats are mainly represented by cobbles and boulders (stones) often located in wide areas of soft sediment. The localization and description of stone occurrences also gains increasing importance for politics. Detailed distribution maps are relevant for resource assessments, coastal management and protection conventions. So far no clear definition and consistent demarcation criterion exists for stony grounds / reefs in the national and international literature. Descriptions like “high concentration of stones” or “reef like structures” are very vague and not sufficient for habitat classification and modelling after e.g. EUNIS or HELCOM HUB standards. Demarcation criterions such as the distance between the objects or a number of objects on a certain area do not exist so far. For both approaches at least the localization of the single objects is necessary. Remote sampling such as trawling, grab sampling or video inspection is inappropriate to locate and sample hard substrates. Generally, a good method to detect objects at the surface of the seafloor is provided by sidescan sonar imaging. Elevated objects block the sonar pulse causing high backscatter at the front and an acoustic shadow (no backscatter) behind the object. The shadow length is strongly related to the location of the object to the sidescan sonar and the object height. Objects close to the sonar fish have a shorter shadow compared to objects of the same size located farther away. This can cause that objects close to the sonar have a shadow smaller than the sonar data resolution and are consequently not detectable. Further limitation in the detection of objects is given by communities living on the hard substrate. The biomass absorbs and scatters most of the acoustic signal so that the typical high backscatter at the front of the object is not present. Areas of benthic communities and / or algae which are related to hard substrates can be detected very well by single beam seafloor classification systems and can supplement sonar data but not area wide. Conventional single beam systems have a relative large footprint in deeper waters (e.g. ~4.5 m at 25 m water depth) which limits the detection of smaller objects. A smaller foot print is given by parametric sediment echo sounders. The nonlinear acoustic propagation enables high resolution (decimeter) data with small footprints (~1.5 m at ~40 m water depth) at low frequencies (e.g. 8 kHz). Identifying objects like stones in sonar data is mostly related to visual-manual, time consuming procedures. To develop an automated identification and demarcation of stone fields sidescan sonar and parametric echo sound data were recorded within the marine protection area “Outer Reef Sylt” (German Bight, North Sea) in May 2013. The investigated area (~ 3.000 km²) is characterized by heterogeneous distributed moraine and marine deposits. The glacial sediments (here: medium sand to gravel size) are partly covered by Holocene marine sediments (here: fine sand size) forming a patchy and thin drape. Sidescan sonar data show stones either in areas of highly heterogeneous distributed substrate (fine sand, coarse sand, gravel) or in wide areas of fine sand. Parametric sediment echo sounder data indicate hyperbolas at the sediment surface (fig. 1). Extensive ground truthing data, especially high quality underwater videos, show that these hyperbolas are caused by objects such as stones (mostly densely populated) lying on the sediment surface. The locations of the hyperbolas fit very well with areas of stone deposits observed with sidescan sonar. An automated procedure to identify and export the hyperbola positions makes the demarcation of stony grounds reproducible, faster and less complex in comparison to the visual-manual identification on the basis of sidescan sonar data.

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Conference (Talk)
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Research Networks
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GEOHAB Marine Geological and Biological Habitat Mapping, 03 May 2015 - 08 May 2015, Salvador-Bahia-Brasil.
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Papenmeier, S. and Hass, C. (2015): Detection of Stones in Marine Habitats using Hydroacoustics , GEOHAB Marine Geological and Biological Habitat Mapping, Salvador-Bahia-Brasil, 3 May 2015 - 8 May 2015 .

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