Geological Survey of Denmark and Greenland Bulletin 26 ... - Geus

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A 57.44°N 2 km 11.2°E A more recent study undertaken by GEUS in 2005 yielded echo sounder, side-scan sonar and ground truth data from the central part of Læsø Trindel (Fig. 1; Leth & Al-Hamdani 2008) showing that it consists of a 2 × 2 km shallow platform with water depths of 4 to 13 m. In the eastern and south-eastern parts of the survey area, the water depth increases to 45 m (Fig. 2A). The platform consists mainly of till with boulders and cobbles that vary in size and distribution throughout the area. Its central part shows the highest density of cobbles and boulders, and this is the main boulder reef of the region. The surrounding part is characterised by gravelly sand and sand patches with scattered boulders. 0 6 8 10 13 16 19 22 25 28 31 40 50 Water depth (m) B C Methods Field work in 2011 was carried out in two phases: In phase one, bathymetry data were collected with a single-beam echo sounder, and overlapping seabed backscatter images were obtained using side-scan sonar with 100 and 400 kHz frequencies. Information on the sub-bottom layers was obtained with a chirp profiler (1–10 kHz; providing high resolution but low penetration) and a sparker (1 kHz; providing deeper penetration but lower resolution). The area was surveyed with 100 m line spacing to ensure full seabed coverage. Cross lines with 4 km spacing were surveyed for cross-verification (Fig. 1). In phase two, which succeeded preliminary interpretation of the acoustic data, areas of special interest were video filmed with a camera mounted on a remotely operated vehicle, and surface sediment samples were collected with a Van Veen grab sampler. Finally, sediment cores were collected with a 6 m long vibrocorer. The acoustic data, the video films and the grab and core samples were used together with older data for the final interpretation and habitat mapping . Natura 2000 area Substrate type 1a Substrate type 1b Substrate type 2 Substrate type 3 Substrate type 4 ’Bubbling reef’: current project ’Bubbling reef’: 2005 project Fig. 2. The Læsø Trindel Natura 2000 area. A: Bathymetry. B: Side-scan sonar mosaic. The grey scale reflects hard (dark) to soft (light) sediments. C: Interpreted distribution of substrate types and Natura 2000 habitats. Results and discussion The data from 2005 and 2011 cover almost the entire Læsø Trindel Natura 2000 area (Fig. 1) and were used to map th e bathymetry along the sailing lines, to compile a side-scan mosaic and to map the distribution of various substrates and nature types. The depth data from 2011 compare well with the depth data from 2005 which were acquired with a highly accurate multibeam echo sounder (Fig. 2A). The side-scan backscatter mosaic image shows variation in substrate reflectivity due to the different signatures of the different sediments (Fig. 2B). The interpretation of the image was carried out by dividing the image into 50 × 50 m blocks and manually classifying the sediments in each block. 26
Interpretation of the side-scan images together with ground truth samples allowed us to produce a map that shows the distribution of substrate types and the occurrence of Natura 2000 habitat types, using the classification categories suggested by the Danish Nature Agency (Table 1; Fig. 2C). The chirp profiles were also used for the interpretation. The different types of sediments give different seismic signals that enhance the final seabed sediment interpretation significantly. Boulder reefs (substrate type 4) cover c. 38% of the Na tura 2000 area. Between these, substrate type 2 with sand and scattered boulders dominates. The deeper south-eastern parts of the surveyed area are dominated by soft sediment. Sandbanks are not well defined in the Læsø Trindel area, but Substrate Description type Table 1. Substrate classification 1a Soft bottom sediment with silt and mud 1b Sand, including sandbanks 2 Sand, gravel and small stones 10 cm that cover 10 cm with >25% coverage; also with sand, gravel and scattered, small stones 25 m 25 m A B 100 m 7.5 m 100 m 7.5 m C Fig. 3. Examples of ‘bubbling reefs’. A: On side-scan images ‘bubbling reefs’ appear as small areas with lowered reflectivity (arrows). B: Shallow seismic sparker profiles with sites where the water column is disturbed by gas seeping out from the sediments (arrows). C: Snapshots from a video film taken with a camera mounted on a remotely operated vehicle. Gas bubbles are seen in the water column above the ‘bubbling reefs’ (arrows). The reefs have a rich epiflora and epifauna. 27
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Geological Survey of Denmark and Greenland Bulletin 26 ... - Geus

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