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Experiences with multibeam sonar in shallow tropical waters
Fisheries Research 35 Ž1998. 143–147
Experiences with multibeam sonar in shallow tropical waters Franc¸ois Gerlotto b
a,)
, Carlos Hernandez
b,1
, Esteban Linares
b,1
a ORSTOM, BP 5045, 34032 Montpellier Cedex 1, France Instituto de Oceanologia, 1ra, no. 18406, Playa, La Habana, Cuba
Abstract The limitations of acoustic surveys using echosounders in shallow waters are discussed in terms of both vertical and horizontal beaming. The advantages of multibeam sonar applied in a vertical plane throughout the water column for shallow water surveying are outlined and illustrated with examples taken from a survey of a shallow water tropical lagoon in Cuba even under conditions of rough weather. q 1998 Elsevier Science B.V. Keywords: Acoustics; Shallow waters; Lagoon; Windy conditions; Acoustic fish stock assessment; Sonar
1. Introduction Acoustic surveys in shallow waters are usually limited by the presence of boundaries such as the bottom and surface reverberation. When using echosounders in vertical mode, the proximity between the transducer and the bottom reduces dramatically the actual sampling volume. Another effect is that fish tend to avoid the presence of the boat. In the case of horizontal echosounding, both the acoustic range and sampling volume can be enlarged but it becomes difficult, if not impossible, to discriminate easily between echoes due to fish and echoes due to the bottom or to the surface. The aim of this paper is to present an alternative methodology, that of a multibeam sonar insonifying a whole vertical crosssection per transmission, for surveying reliably a
) Corresponding author. Tel.: q33 4 67 41 94 00; fax: q33 4 67 41 94 30. 1 Tel.: q53 7 210342.
large shallow water area, such as the Buenavista Bay in north Cuba even under rough weather conditions.
2. Material and methods The small portable multibeam sonar used was a RESON Seabat 6012 which is designed for bottom mapping. It has 60 beams each 1.58 Žat y3 dB. in the multibeam plan and 158 in the perpendicular direction, covering a total sector of 908. The sonar frequency was 455 kHz, with a pulse length of 0.06 ms. The TVG was set at 20 log R. The number of transmissions was 15 prs at ranges of 2, 5 and 10 m, and 13 prs at 25 m range. The data were averaged over four transmissions, which gave 3.75 imagesrs at 10 m range and 2.5 imagesrs at 25 m range. Buenavista Bay, Cuba, is a large shallow tropical lagoon Ž7 = 40 nautical miles. which was sampled acoustically by a series of parallel equidistant transects, using a 10 m boat with an in-broad motor operated at 5 knots Ž; 2.5 mrs.. The sonar head was set at the side of the boat to sample vertically
0165-7836r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 5 - 7 8 3 6 Ž 9 8 . 0 0 0 6 9 - 1
144
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
and perpendicularly to the boat route ŽFig. 1a., as described in the work of Gerlotto et al. Ž1994.. It, therefore, insonified a vertical cross-section of the water column at each transmission ŽFig. 1b.. Fig. 2 shows the sonar image of one cross-section of the vertical water column given by one transmission. The surface and bottom reverberations are clearly visible as well as a school of fish between them. When sailing at 5 knots, the distance between two successive transmissions was 70 cm at 10 m range and 1 m at 25 m range. According to the speed and the beam angle Ž158., the sampling volumes of two
Fig. 2. An example of a single image of one vertical cross-section of the water volume on one side of the boat. The sea surface is observed on the right of the screen, the bottom on the left side and a single school of fish between these.
successive transmissions are overlapping at 2.6 m distance from the transducer. This means that the water body is exhaustively sampled from 2.6 m up to the range limit. During this survey, weather conditions were rough due to strong north winds of more than 25 knots Ž; 47 kmrh. which produced waves of 50 cm height in 2 m depths of water so that the surface noise level was high.
3. Results 3.1. Rough weather and waÕe effects on echosounding
Fig. 1. Deployment of multibeam sonar. Ža. Schematic diagram of a multibeam sonar in horizontal mode being operated along a transect Žfrom the work of Gerlotto et al., 1994.. Žb. The position of the bottom and surface as observed on the sonar screen.
Under such weather conditions of winds of 25 knots, no usual echosounding could provide reliable results due to the effect of waves which resulted in a boat rolling of 108 on either side of the horizontal. Boat rolls of such dimensions greatly reduce the range achievable with two usual transducers of 118 and 58 beams set horizontally at 0.5 cm depth in water depths of 2.5 m ŽTable 1.. These values of roll angle are those that were encountered during the worst weather conditions of the survey. Moreover, the breaking waves produced strong surface noise. The same level of range reduction may arise with a bottom that is not uniform.
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
145
Table 1 Theoretical range for two usual echo sounder transducers when used horizontally Žtransducer at 50 cm from the surface, total depth 2.5 m.
Maximum range Žno rolling. Minimum range due to surface echo Minimum range due to bottom echo
Large beam Ž118.
Narrow beam Ž58.
8m 1.5 m 7m
18 2.5 m 8.5 m
3.2. Scattered fish using multibeam sonar Individual and small groups of fish could be observed between the surface and bottom boundaries on the multibeam sonar image ŽFig. 3.. These were counted manually and expressed as a number of individuals per ESDU ŽElementary Sampling Distance Unit. or sailing distance. In this survey, ESDU was 5 min sailing at 5 knots or a distance of 770 m. The multibeam sonar samples the water volume exhaustively so that no fish can escape detection at distances greater than 2.6 m from the boat, in contrast to the situation with narrow-beam echosounders. However, sizing fish targets is a problem since the same fish echo may be strong Žwhen the fish main axis is perpendicular to the beam axis. or extremely weak Žwhen the fish main axis is parallel to the beam main axis.. Thus, the only information that can be
Fig. 3. An example of scattered fish observed by multibeam sonar and between the surface and bottom reverberation. The undulating echo on the right of the surface line Ždashed line. is the mirror echo of the bottom. The observed undulation is due to waves on the surface. The weak points observed close to the surface line represent the scattered fish echoes.
obtained by multibeam methodology when using the TVG function set at 20 log R is an estimate of fish abundance. Similar experience is reported by Duncan and Kubecka Ž1993. using a dual-beam echosounder in horizontal mode in rivers where fish orientate to the current and in lakes where fish are more randomly-oriented. However, solutions using various post-processing procedures for obtaining some estimates of fish sizes under these two environments are outlined in the work of Kubecka Ž1996.. 3.3. Schools of fish using multibeam sonar Small pelagic fish, mainly clupeoids, formed rapidly moving schools in these waters which the multibeam sonar is capable of detecting. One school can be observed rather precisely as a series of successive vertical cross-sections separated in time Žat 0.2-s intervals., as shown in Fig. 4. Such a series permits the estimation of the school size from estimates of its maximal ‘length’ Ž L. parallel to the boat pathway, its maximal ‘width’ ŽW . perpendicularly to the boat path and maximal ‘height’ Ž H . in the vertical dimension. The values for W and H can be measured on the echo image for each ping and L can be estimated from the duration time of the fish observation Ž t . and the vessel speed Ž Õ .. Thus, in the school shown in Fig. 4, the maximal value for Hmax is 2 m Žfrom top to bottom. and Wmax is 4 m. Lmax is calculated from Õ s 2.75 mrs and t s 3 s, which gives: Lmax s 2.57 = 3 s 7.7 m. The detection of other behavioural phenomena such as boat avoidance and swimming close to the water surface is possible. From Fig. 4, avoidance of the boat by schools appears to be negligible. For example during the few seconds that the school was recorded, there was no noticeable change in its distance from the boat. In this case, the distance was rather short Ž4 m.. Similar observations have been made with other schools.
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F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
Fig. 4. An example of successive vertical cross-sections through a school of fish during an observation lasting 3 s.
During calm weather, there are problems with reflections from the mirror-like underside of the water surface. Any echoes from fish and the bottom are detected both as true echoes and their reflected mirror-images. These mirror images can be easily
recognised as they are observed above the surface line. When fish schools or scattered individual fish approach close to the surface, their true echo and reflected echo may be connected on the image. It then becomes difficult to locate the surface precisely
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
147
and to distinguish which is the true fish echo ŽFigs. 2 and 3; see also Fig. 4, images 2 and 3.. The sonar data are also affected by the sea movements, but to a lesser extent than with echosounders. With multibeam sonar, the position of the bottom can be observed precisely and its echo cannot be confused with fish echoes. The surface interferences are not so easy to discriminate. Air bubbles form a common surface noise and produce echoes quite similar to fish echoes. When the surface is rough, the part of the fish biomass that lies close to the surface is undetectable acoustically.
sonar provides information on the bottom and surface roughness, gives a good description of fish distribution, and fish school dimensions and fish behaviour. Echosounders provide more quantified indices such as average target strengths and echointegrated biomass. Data from echosounders can be cleaned up with the help of simultaneously-derived sonar information. Given further development of processing and post-processing software for extracting quantitative results from raw data, it is likely that multibeam sonar will become the best tool for shallow water works ŽAnon., 1996..
4. Advantages and disadvantages of multibeam sonar
Acknowledgements
The advantages and defects of sonar are more or less the opposite of those related to echo sounders. It is easy to know precisely the volume of water where no echoes other than fish can be detected. It is possible to obtain indices of fish biomass Žthrough echo counting and school volume. from a large sampling volume and without risking overestimation. The main drawback of multibeam sonar is that analysis of data can only be done by visual analysis of video records, which is not easy. Moreover, it is still not possible to measure the total echo energy for various reasons, namely, absence of calibration and difficulties with averaging data culled from output images. Nevertheless, multibeam sonar is an invaluable tool for direct observation of fish behaviour associated with spatial structure, shape of fish schools and 3-D analysis of movements. The best approach for shallow marine waters is to combine the two acoustic systems. The multibeam
The authors wish to thank Dr. A. Duncan for her scientific suggestions and revision, and her help in the writing of the text.
References Anon., 1996. Three dimension analysis and visualisation of the spatial structure of fish schools using multibeam sonar image processing. ICESrFAST W.G., Woods Hole, 17–19 April, 9 pp. Duncan, A., Kubecka, J., 1993. Hydroacoustic methods of fish survey. National River Authority, R&D Note 196, 136 pp. Gerlotto, F., Freon, P., Soria, M., Cottais, P.H., Ronzier, L., 1994. Exhaustive observation of 3D school structure using multibeam side scan sonar: potential use for school classification, biomass estimation and behaviour studies. ICES CMrB:26, Ref. D, St John’s, Newfoundland, Sept. 1994, 12 pp. Kubecka, J., 1996. Use of horizontal dual-beam sonar for fish surveys in shallow waters. In: I.G. Cowx ŽEd.., Stock Assessments in Inland Fisheries. Fishing News Books, Blackwell, Oxford, pp. 165–178.
Experiences with multibeam sonar in shallow tropical waters Franc¸ois Gerlotto b
a,)
, Carlos Hernandez
b,1
, Esteban Linares
b,1
a ORSTOM, BP 5045, 34032 Montpellier Cedex 1, France Instituto de Oceanologia, 1ra, no. 18406, Playa, La Habana, Cuba
Abstract The limitations of acoustic surveys using echosounders in shallow waters are discussed in terms of both vertical and horizontal beaming. The advantages of multibeam sonar applied in a vertical plane throughout the water column for shallow water surveying are outlined and illustrated with examples taken from a survey of a shallow water tropical lagoon in Cuba even under conditions of rough weather. q 1998 Elsevier Science B.V. Keywords: Acoustics; Shallow waters; Lagoon; Windy conditions; Acoustic fish stock assessment; Sonar
1. Introduction Acoustic surveys in shallow waters are usually limited by the presence of boundaries such as the bottom and surface reverberation. When using echosounders in vertical mode, the proximity between the transducer and the bottom reduces dramatically the actual sampling volume. Another effect is that fish tend to avoid the presence of the boat. In the case of horizontal echosounding, both the acoustic range and sampling volume can be enlarged but it becomes difficult, if not impossible, to discriminate easily between echoes due to fish and echoes due to the bottom or to the surface. The aim of this paper is to present an alternative methodology, that of a multibeam sonar insonifying a whole vertical crosssection per transmission, for surveying reliably a
) Corresponding author. Tel.: q33 4 67 41 94 00; fax: q33 4 67 41 94 30. 1 Tel.: q53 7 210342.
large shallow water area, such as the Buenavista Bay in north Cuba even under rough weather conditions.
2. Material and methods The small portable multibeam sonar used was a RESON Seabat 6012 which is designed for bottom mapping. It has 60 beams each 1.58 Žat y3 dB. in the multibeam plan and 158 in the perpendicular direction, covering a total sector of 908. The sonar frequency was 455 kHz, with a pulse length of 0.06 ms. The TVG was set at 20 log R. The number of transmissions was 15 prs at ranges of 2, 5 and 10 m, and 13 prs at 25 m range. The data were averaged over four transmissions, which gave 3.75 imagesrs at 10 m range and 2.5 imagesrs at 25 m range. Buenavista Bay, Cuba, is a large shallow tropical lagoon Ž7 = 40 nautical miles. which was sampled acoustically by a series of parallel equidistant transects, using a 10 m boat with an in-broad motor operated at 5 knots Ž; 2.5 mrs.. The sonar head was set at the side of the boat to sample vertically
0165-7836r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 5 - 7 8 3 6 Ž 9 8 . 0 0 0 6 9 - 1
144
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
and perpendicularly to the boat route ŽFig. 1a., as described in the work of Gerlotto et al. Ž1994.. It, therefore, insonified a vertical cross-section of the water column at each transmission ŽFig. 1b.. Fig. 2 shows the sonar image of one cross-section of the vertical water column given by one transmission. The surface and bottom reverberations are clearly visible as well as a school of fish between them. When sailing at 5 knots, the distance between two successive transmissions was 70 cm at 10 m range and 1 m at 25 m range. According to the speed and the beam angle Ž158., the sampling volumes of two
Fig. 2. An example of a single image of one vertical cross-section of the water volume on one side of the boat. The sea surface is observed on the right of the screen, the bottom on the left side and a single school of fish between these.
successive transmissions are overlapping at 2.6 m distance from the transducer. This means that the water body is exhaustively sampled from 2.6 m up to the range limit. During this survey, weather conditions were rough due to strong north winds of more than 25 knots Ž; 47 kmrh. which produced waves of 50 cm height in 2 m depths of water so that the surface noise level was high.
3. Results 3.1. Rough weather and waÕe effects on echosounding
Fig. 1. Deployment of multibeam sonar. Ža. Schematic diagram of a multibeam sonar in horizontal mode being operated along a transect Žfrom the work of Gerlotto et al., 1994.. Žb. The position of the bottom and surface as observed on the sonar screen.
Under such weather conditions of winds of 25 knots, no usual echosounding could provide reliable results due to the effect of waves which resulted in a boat rolling of 108 on either side of the horizontal. Boat rolls of such dimensions greatly reduce the range achievable with two usual transducers of 118 and 58 beams set horizontally at 0.5 cm depth in water depths of 2.5 m ŽTable 1.. These values of roll angle are those that were encountered during the worst weather conditions of the survey. Moreover, the breaking waves produced strong surface noise. The same level of range reduction may arise with a bottom that is not uniform.
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
145
Table 1 Theoretical range for two usual echo sounder transducers when used horizontally Žtransducer at 50 cm from the surface, total depth 2.5 m.
Maximum range Žno rolling. Minimum range due to surface echo Minimum range due to bottom echo
Large beam Ž118.
Narrow beam Ž58.
8m 1.5 m 7m
18 2.5 m 8.5 m
3.2. Scattered fish using multibeam sonar Individual and small groups of fish could be observed between the surface and bottom boundaries on the multibeam sonar image ŽFig. 3.. These were counted manually and expressed as a number of individuals per ESDU ŽElementary Sampling Distance Unit. or sailing distance. In this survey, ESDU was 5 min sailing at 5 knots or a distance of 770 m. The multibeam sonar samples the water volume exhaustively so that no fish can escape detection at distances greater than 2.6 m from the boat, in contrast to the situation with narrow-beam echosounders. However, sizing fish targets is a problem since the same fish echo may be strong Žwhen the fish main axis is perpendicular to the beam axis. or extremely weak Žwhen the fish main axis is parallel to the beam main axis.. Thus, the only information that can be
Fig. 3. An example of scattered fish observed by multibeam sonar and between the surface and bottom reverberation. The undulating echo on the right of the surface line Ždashed line. is the mirror echo of the bottom. The observed undulation is due to waves on the surface. The weak points observed close to the surface line represent the scattered fish echoes.
obtained by multibeam methodology when using the TVG function set at 20 log R is an estimate of fish abundance. Similar experience is reported by Duncan and Kubecka Ž1993. using a dual-beam echosounder in horizontal mode in rivers where fish orientate to the current and in lakes where fish are more randomly-oriented. However, solutions using various post-processing procedures for obtaining some estimates of fish sizes under these two environments are outlined in the work of Kubecka Ž1996.. 3.3. Schools of fish using multibeam sonar Small pelagic fish, mainly clupeoids, formed rapidly moving schools in these waters which the multibeam sonar is capable of detecting. One school can be observed rather precisely as a series of successive vertical cross-sections separated in time Žat 0.2-s intervals., as shown in Fig. 4. Such a series permits the estimation of the school size from estimates of its maximal ‘length’ Ž L. parallel to the boat pathway, its maximal ‘width’ ŽW . perpendicularly to the boat path and maximal ‘height’ Ž H . in the vertical dimension. The values for W and H can be measured on the echo image for each ping and L can be estimated from the duration time of the fish observation Ž t . and the vessel speed Ž Õ .. Thus, in the school shown in Fig. 4, the maximal value for Hmax is 2 m Žfrom top to bottom. and Wmax is 4 m. Lmax is calculated from Õ s 2.75 mrs and t s 3 s, which gives: Lmax s 2.57 = 3 s 7.7 m. The detection of other behavioural phenomena such as boat avoidance and swimming close to the water surface is possible. From Fig. 4, avoidance of the boat by schools appears to be negligible. For example during the few seconds that the school was recorded, there was no noticeable change in its distance from the boat. In this case, the distance was rather short Ž4 m.. Similar observations have been made with other schools.
146
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
Fig. 4. An example of successive vertical cross-sections through a school of fish during an observation lasting 3 s.
During calm weather, there are problems with reflections from the mirror-like underside of the water surface. Any echoes from fish and the bottom are detected both as true echoes and their reflected mirror-images. These mirror images can be easily
recognised as they are observed above the surface line. When fish schools or scattered individual fish approach close to the surface, their true echo and reflected echo may be connected on the image. It then becomes difficult to locate the surface precisely
F. Gerlotto et al.r Fisheries Research 35 (1998) 143–147
147
and to distinguish which is the true fish echo ŽFigs. 2 and 3; see also Fig. 4, images 2 and 3.. The sonar data are also affected by the sea movements, but to a lesser extent than with echosounders. With multibeam sonar, the position of the bottom can be observed precisely and its echo cannot be confused with fish echoes. The surface interferences are not so easy to discriminate. Air bubbles form a common surface noise and produce echoes quite similar to fish echoes. When the surface is rough, the part of the fish biomass that lies close to the surface is undetectable acoustically.
sonar provides information on the bottom and surface roughness, gives a good description of fish distribution, and fish school dimensions and fish behaviour. Echosounders provide more quantified indices such as average target strengths and echointegrated biomass. Data from echosounders can be cleaned up with the help of simultaneously-derived sonar information. Given further development of processing and post-processing software for extracting quantitative results from raw data, it is likely that multibeam sonar will become the best tool for shallow water works ŽAnon., 1996..
4. Advantages and disadvantages of multibeam sonar
Acknowledgements
The advantages and defects of sonar are more or less the opposite of those related to echo sounders. It is easy to know precisely the volume of water where no echoes other than fish can be detected. It is possible to obtain indices of fish biomass Žthrough echo counting and school volume. from a large sampling volume and without risking overestimation. The main drawback of multibeam sonar is that analysis of data can only be done by visual analysis of video records, which is not easy. Moreover, it is still not possible to measure the total echo energy for various reasons, namely, absence of calibration and difficulties with averaging data culled from output images. Nevertheless, multibeam sonar is an invaluable tool for direct observation of fish behaviour associated with spatial structure, shape of fish schools and 3-D analysis of movements. The best approach for shallow marine waters is to combine the two acoustic systems. The multibeam
The authors wish to thank Dr. A. Duncan for her scientific suggestions and revision, and her help in the writing of the text.
References Anon., 1996. Three dimension analysis and visualisation of the spatial structure of fish schools using multibeam sonar image processing. ICESrFAST W.G., Woods Hole, 17–19 April, 9 pp. Duncan, A., Kubecka, J., 1993. Hydroacoustic methods of fish survey. National River Authority, R&D Note 196, 136 pp. Gerlotto, F., Freon, P., Soria, M., Cottais, P.H., Ronzier, L., 1994. Exhaustive observation of 3D school structure using multibeam side scan sonar: potential use for school classification, biomass estimation and behaviour studies. ICES CMrB:26, Ref. D, St John’s, Newfoundland, Sept. 1994, 12 pp. Kubecka, J., 1996. Use of horizontal dual-beam sonar for fish surveys in shallow waters. In: I.G. Cowx ŽEd.., Stock Assessments in Inland Fisheries. Fishing News Books, Blackwell, Oxford, pp. 165–178.