Title: Author(s): Document owner: The optimal position of a sidescan sonar towfish fixed to a shellfish vessel for very shallow surveys an experiment in the Dutch Wadden Sea Ronnie van Overmeeren (TNO) Ronnie van Overmeeren (ronnie.vanovermeeren@tno.nl) Reviewed by: Sytze van Heteren (TNO). J. White 31 st January, 2007 Workgroup: MESH action: Action 2 Version: 1 Date published: July 2006 File name: SSSposition2006_RvO_v2 Language: English Number of pages: 10 Summary: Reference/citation: Keywords: This work presents some of the results of applying sidescan-sonar methods for mapping and monitoring shellfish in very shallow waters of the Dutch estuaries and tidal flats, and illustrates the influence of the towfish position on the quality of the acoustic images. Sidescan sonar surveys in very shallow water environments often are best executed by fixing the towfish to the survey vessel. This is especially the case when the engines of the boat produce strong turbulence and towing immediately at the rear is no option. The experiment shows that positioning the towfish at the front of the vessel offers the best solution. N/A Sidescan sonar, Shellfish, (Very) Shallow water, Field protocol, Image quality Bookmarks: Related information: Author: Ronnie van Overmeeren Partner: TNO File: GMHM3_Case_Study_Sidescan_Sonar.doc Page 1 of 9 Last saved: 31/01/2007 18:24
Change history Version: Date: Change: 1 July 2006 2 31/01/07 Formatting to MESH layout. Page 2 of 9
Introduction In May 2006, a sidescan sonar survey for seabed habitat mapping and monitoring sublittoral mussel banks was carried out in the Dutch Wadden Sea, off the coast of the island of Texel (Figure Error! Reference source not found.). The Wadden Sea is shallow and characterised by tidal flats that are cut by tidal channels. Such a shallow environment, with some deeper areas, requires an optimal and flexible system set-up to produce full-coverage, good-quality images. The measurements were carried out with a portable CM2 system (C-Max Ltd.) with a HF (325 khz) towfish aboard a shellfish vessel, the TX-63 (Figure 2). Figure 1. Western Wadden Sea with the area where the sidescan sonar survey was conducted. Figure 2. The survey vessel TX-63. Page 3 of 9
This large vessel (12.5 m wide and 45 m long) was designed for cockle fishing on the tidal flats of the Waddenzee and has a very flat bottom, drawing only 0.3 m of water. The vessel is powered by two huge engines on both sides at the rear of the vessel, producing strong turbulence impeding towing the towfish behind the boat, as the mostly very shallow water requires shallow towing immediately behind the boat. For that reason, the towfish was fitted to a steel pole and directly connected to the starboard side of the vessel, not far behind the wheelhouse. The towfish was lowered some 0.3 m below the bottom of the ship. The sidescan sonar images showed some annoying noise however, on the port side of the towfish. This is well illustrated by the sonar track across a mussel bank in Figure 3. The first obvious artefact is the light stripe parallel to the sailing direction. Also, the backscatter in the area between the nadir and the white band is less clear and locally seems to be shaded or veiled. The light stripe and accompanying cloudy zone also form an annoying feature in the mosaics built to map the mussel fields; an example is the mosaic of Figure 4. West port East starboard Figure 3. Sidescan sonar track (TX0605C3) across a mussel bank (central part), recorded by a CM2 towfish fixed to the starboard side of the vessel. Length of section 600 m; range 50 m. Figure 4. Sidescan sonar mosaic across a mussel bank; approximate size 600 x 400 m. Page 4 of 9
The experiment In order to investigate the source of the annoying noise a field experiment was conducted along a track with some easily recognisable features (Figure 5), facilitating mutual comparison of several runs along the same track. The first track, with the towfish fixed at the starboard side of the vessel showed the same type of noise as the example above (Figure 6, right): a light stripe to the port side of the towfish, parallel to the sailing direction and a shaded backscatter in the area between the white band and the centre. As the obvious suspect of the source of the noise was the vessel, as a first check the towfish was moved from starboard to port and with this new configuration the same track was repeated (Figure 5). Indeed, the annoying light band appeared as a mirror image, on the starboard side of the towfish (Figure 6, left). Also in this case, the area between the light band and the nadir appears to be shaded. Figure 5. Navigation tracks of sidescan sonar measurements with the towfish on starboard (blue), on port (green) and on stem (red). Page 5 of 9
Port Stem Starboard Figure 6. Part of sidescan sonar track (OG060503) recorded by a CM2 towfish fixed to the vessel, positioned at port (left), stem (middle) and starboard (right). Length of section ca. 1100 m; range 50 m. Page 6 of 9
Finally, a third experiment was carried out by connecting the towfish with the pole to the stem of the vessel, thus ahead of the body of the ship (Figure 7). The pole was just long enough to lower the towfish some 30 cm below the water surface, 10 cm less than the flat hull of boat to make sure the instrument wouldn't get damaged by hitting the seabed in very shallow water. The measurements taken with this geometry produced an image free of the light bands and veil (Figure 6, centre). Figure 7. The CM2 towfish connected with a pole to the stem of the vessel. The observed effects can be studied in more detail by comparing only short sections of the track (Figure 8). The red arrows denote an identical feature in the ripple marks, which facilitates mutual comparison. The larger width of the white water column area of the image from the stem position illustrates the shallower position of the towfish in this configuration compared to the port and starboard arrays. The larger distance to the bottom results in an image with narrower shadows and a little less contrast than observed in the port and starboard images. All three images reveal clearly the ripple marks on the starboard side of the towfish, but the sonogram obtained to port is disturbed by the light stripe and the ripple marks in the area between this stripe and the centre are more vaguely portrayed. In all three cases, the images to the port side of the towfish are less developed, but the picture obtained with the stem array reveals best the ripple marks on that side. Looking for an explanation for the noise, further proof of the influence of the vessel was provided by the width of the zone of disturbance. Time lines drawn over the tracks learn that the 2-way travel time to the light stripe is 17 ms, both to port and to starboard (Figure 9), which corresponds to a distance of 12.8 m, which is a little more than the width of the vessel (12.5 m). From this, it can be concluded that the source of the noise, both the light stripe and the shading/blurring, is related to the bottom of the ship. An exact explanation of the disturbing process, however, has not yet been found. Page 7 of 9
GMHM3_Case_Study_Sidescan_Sonar.doc Port Stem Starboard Figure 8. Detailed sections of the sidescan sonar tracks of the experiments with the position of the towfish. Arrows highlight identical features for mutual comparison. Page 8 of 9
Port Starboard Figure 9. Detailed sections of the sidescan sonar tracks of the experiment with the position of the towfish with (in red) time scale lines at intervals of 5 ms. Conclusion Sidescan sonar surveys in very shallow water environments are often best executed by fixing the towfish to the survey vessel. This is especially the case when the engines of the boat produce strong turbulence and towing immediately at the rear is no option. The reported experience with the cockle cutter TX-63 showed that ship-provoked noise is produced, even if the towfish is lowered below the flat bottom of the ship. The experiment shows that in such cases, positioning the towfish at the front of the vessel offers the best solution. Acknowledgement The experiments were carried out with the indispensable input and help of Dr. Erik Meesters of IMARES Texel and the crew of the TX-63, Mr. Albert Schagen en Mr. Willem Anton Schagen. Page 9 of 9