The optimal position of a sidescan sonar towfish fixed to a shellfish vessel for very shallow surveys an experiment in the Dutch Wadden Sea

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1 TNO report 2006-U-RO114/A The optimal position of a sidescan sonar towfish fixed to a shellfish vessel for very shallow surveys an experiment in the Dutch Wadden Sea NITG/Geological Survey Princetonlaan 6 P.O. Box TA Utrecht T F info-beno@tno.nl Date July, 2006 Author(s) Ronnie van Overmeeren Assignor Project number Classification report Title Abstract Report text Appendices Number of pages Number of appendices 10 (incl. appendices) All rights reserved. No part of this report may be reproduced and/or published in any form by print, photoprint, microfilm or any other means without the previous written permission from TNO. All information which is classified according to Dutch regulations shall be treated by the recipient in the same way as classified information of corresponding value in his own country. No part of this information will be disclosed to any third party. In case this report was drafted on instructions, the rights and obligations of contracting parties are subject to either the Standard Conditions for Research Instructions given to TNO, or the relevant agreement concluded between the contracting parties. Submitting the report for inspection to parties who have a direct interest is permitted TNO

2 TNO report 2006-U-RO114/A 2 / 10 Contents 1 Introduction The experiment Conclusion Acknowledgement... 10

TNO report 2006-U-RO114/A 3 / 10 1 Introduction In May 2006, a sidescan sonar survey for seabed habitat mapping and monitoring

The Wadden Sea is shallow and characterised by tidal flats that are cut by tidal channels.

good-quality images. The measurements were carried out with a portable CM2 system (C-Max Ltd.

3 TNO report 2006-U-RO114/A 3 / 10 1 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 1). 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 (in red box) the area where the sidescan sonar survey was conducted. Figure 2 The survey vessel TX-63

TNO report 2006-U-RO114/A 4 / 10 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.

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

4 TNO report 2006-U-RO114/A 4 / 10 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 wheel-house. The towfish was lowered some 0.3 m below the bottom of the ship. However, the sidescan sonar images showed some annoying noise 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

5 TNO report 2006-U-RO114/A 5 / 10 2 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).

TNO report 2006-U-RO114/A 6 / 10 Port Stem

6 TNO report 2006-U-RO114/A 6 / 10 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 m; range 50 m.

TNO report 2006-U-RO114/A 7 / 10 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).

very shallow water. The measurements taken with this geometry produced an image free of the light bands and veil (Figure 6, centre).

7 TNO report 2006-U-RO114/A 7 / 10 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.

8 TNO report 2006-U-RO114/A 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. 8 / 10

TNO report 2006-U-RO114/A 9 / 10 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

This is especially the case when the engines of the boat produce strong turbulence and towing immediately at the rear is no option.

9 TNO report 2006-U-RO114/A 9 / 10 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. 3 Conclusion 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 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.

10 TNO report 2006-U-RO114/A 10 / 10 4 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. Dr. Sytze van Heteren (TNO) played the much appreciated role of critical sound board. The sidescan sonar survey in the Dutch Wadden Sea will contribute to the MESH project (Mapping European Seabed Habitats: and received European Regional Development Funding through the INTERREG III B Community Initiative (

The optimal position of a sidescan sonar towfish fixed to a shellfish vessel for very shallow surveys an experiment in the Dutch Wadden Sea

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)