Final Report Summary - AIM-HI (Acoustic imaging of macrophytes and habitat investigation)
Macrophytes comprise all aquatic plants large enough to see with the naked eye, for example seaweed, algae and kelp. They are an important component of all marine habitats as they support biodiversity (including fish), stabilise bottom sediments and maintain coastal water quality and clarity. Some algae are also extensively used in the chemical, pharmaceutical and food industry. But they are highly susceptible to environment variations, impacting on local food webs and ecosystems. In the Arctic, where climate change is the most readily visible, macrophytes are important indicators of habitat health and glacier melting. It is therefore extremely important to map them accurately, rapidly and affordably.
This is best achieved with sonars, analysing acoustic echoes from the seabed and any overlying vegetation. Single-beam echosounders (SBESs) get reliable measurements of heights at single points just below the survey vessel. Sidescan sonars (SSSs) cover larger areas but do not provide heights. Multibeam echosounders (MBESs) cover large areas too, and provide heights and detailed profiles along each of the sometimes hundreds of beams.
- But what is the best way to process these very large datasets?
- MBES are the emerging tool of choice, but how much confidence can we have as the beams look further away, and at very different angles?
- And how much does it vary with the type of MBES used?
These are the objectives tackled by the AIM-HI project.
To address these questions, the fellow has used a large dataset acquired in an Arctic fjord with the host research group, combining MBES and SBES measurements of macrophytes, with some ground-truth. She has also designed and performed her own experiments, in the laboratory and in a large open-water test site, using similar macrophytes freshly collected on United Kingdom shores and measuring them with different MBES types and in different geometries, with ground truth and help from divers. To answer some questions, simple theoretical simulations of acoustic returns were also conducted, helping with the interpretation of some original results.
What is the best way to process the very large datasets generated by MBES surveys?
MBES provide acoustic profiles along up to 240 narrow beams, at angles from vertical to highly oblique. These beams cover more ground than the SBES along the track of the survey vessel, and less ground across the track. Acoustic echoes are also more attenuated, as they come from further away. Correcting for slant ranges is done in conjunction with averaging (over 5 pings) to give comparable results in the centre beam(s) and image enhancement (using mathematical morphology) to correct for any gaps or distortions in the data. Macrophytes are then clearly distinct from seabeds, and from the water column above. Their heights, and seabed depths, are very similar to those measured with SBES, but of course extending much further, enabling calculations of the covers and biovolumes. The fellow further adapted her original processing techniques, based on multidimensional wavelet decomposition of acoustic echoes and fuzzy-logic clustering, to more detailed analyses of the macrophytes cover. These results were invited for presentation at an international conference in 2011 (Kruss et al., 2011) and are currently revised for submission to a peer-reviewed journal (Kruss et al., under revision).
How reliable is MBES data as one moves away from the vertical, imaging macrophytes further away and at increasingly grazing angles?
Depending on the sonar used, up to 30 % of MBES measurements in the centre beams can be affected by noise or lost. This was easily corrected by averaging over several pings, taking advantage of the higher imaging rate of MBES. Analyses of the Arctic dataset further showed that MBES beams at angles more than 55.5 degrees from the vertical were noisy enough to be safely rejected before further analyses. Angles between 50.5 and 55.5 degrees fared little better, with faint seabed traces and no indication of the presence (or not) of macrophytes. At angles between 15.5 and 30.5 degrees, acoustic returns from macrophytes seem to disappear. Theoretical simulations showed this was mostly explained by the physical characteristics of the macrophytes present in the Arctic, thin and with no gas content and therefore very close acoustically to the surrounding seawater. On a positive note, though, angles less than 15.5 degrees from the vertical, and between 30.5 and 50.5 degrees, were extremely helpful in identifying macrophytes, their partial extent and their respective heights above the seabed. The Arctic results were corroborated with measurements in a laboratory tank and in an open-water test site, using freshly collected samples of similar macrophytes and also looking at the role of gas vesicles in the acoustic returns. These results were presented at an international conference in 2012 (Kruss et al., 2012) and are currently written for publication (Kruss et al., in preparation). MBES generally go further than SBES by providing 'snippets' of the acoustic returns along the full water column, enabling better characterisation of the macrophytes cover and seabed substrates.
How much does the reliability of results vary with the type of MBES used? The Arctic measurements (2007), the laboratory tank measurements (2011-2012) and the open-water measurements (2011) all used the same sonar, and found the same results. The range of angles at which macrophytes can be detected with MBES extends further away (±15 degrees) from the vertical than would be possible with the SBES instruments traditionally used, and encompasses a further range of angles (30 - 50 degrees from the vertical, on either side of the MBES), but there is a 'dark zone' between these angles, explained by the acoustic characteristics of the macrophytes, and data beyond 50-degree angles is not useful at all. Discussions following the presentation of Kruss et al., 2012 showed similar results had been obtained by other teams around the world, although not (yet) published, using a wide variety of other sonars. The fellow used a different MBES, from another manufacturer and working at higher frequency (i.e. susceptible of a higher resolution). Results from controlled tank experiments showed the same effects. The central range seems to extend as frequency increases, but there is still a 'dark zone' over an intermediate range of angles at which acoustic returns from macrophytes disappear. In conclusion, MBES can image further away than SBES, with angular ranges defined by the sonar's frequency.
The fellow's research identified a common processing methodology, applicable to all depths / environments, and adaptable to different sonar types. Our results will hopefully be extended soon by general comparison of values obtained by other teams around the world, modulated by imaging frequencies and types of macrophytes imaged. This 'Marie Curie Fellowship' paved the way for further international collaborations all including the Fellow and host group, and at different stages of development. The socio-economic impacts of this project go beyond pure research, as they can be applied to surveys of marine biomass anywhere. Their take-up has featured in application articles (e.g. McGonigle et al., Estuarine Coastal and Shelf Science, 2011), and in technical reports (e.g. Blight et al., Marine Institute, Ireland, 2011). This fellowship's results will be of interest to national marine institutes, environmental organisations and the macrophyte industry in general. These results have included and been shared with major sonar manufacturers such as Biosonics Inc. (Canada), Imagenex (Canada), Hydroproducts (United Kingdom) and Reson A/S (Denmark, United Kingdom). The general processing methodology has also been considered for monitoring water quality in large reservoirs. The Fellowships results are now being considered for surveying of other marine ecosystems, e.g. at the fellow's returning institution in Poland, and for the monitoring of habitats around marine renewable energy sites, at different national and European levels.
This is best achieved with sonars, analysing acoustic echoes from the seabed and any overlying vegetation. Single-beam echosounders (SBESs) get reliable measurements of heights at single points just below the survey vessel. Sidescan sonars (SSSs) cover larger areas but do not provide heights. Multibeam echosounders (MBESs) cover large areas too, and provide heights and detailed profiles along each of the sometimes hundreds of beams.
- But what is the best way to process these very large datasets?
- MBES are the emerging tool of choice, but how much confidence can we have as the beams look further away, and at very different angles?
- And how much does it vary with the type of MBES used?
These are the objectives tackled by the AIM-HI project.
To address these questions, the fellow has used a large dataset acquired in an Arctic fjord with the host research group, combining MBES and SBES measurements of macrophytes, with some ground-truth. She has also designed and performed her own experiments, in the laboratory and in a large open-water test site, using similar macrophytes freshly collected on United Kingdom shores and measuring them with different MBES types and in different geometries, with ground truth and help from divers. To answer some questions, simple theoretical simulations of acoustic returns were also conducted, helping with the interpretation of some original results.
What is the best way to process the very large datasets generated by MBES surveys?
MBES provide acoustic profiles along up to 240 narrow beams, at angles from vertical to highly oblique. These beams cover more ground than the SBES along the track of the survey vessel, and less ground across the track. Acoustic echoes are also more attenuated, as they come from further away. Correcting for slant ranges is done in conjunction with averaging (over 5 pings) to give comparable results in the centre beam(s) and image enhancement (using mathematical morphology) to correct for any gaps or distortions in the data. Macrophytes are then clearly distinct from seabeds, and from the water column above. Their heights, and seabed depths, are very similar to those measured with SBES, but of course extending much further, enabling calculations of the covers and biovolumes. The fellow further adapted her original processing techniques, based on multidimensional wavelet decomposition of acoustic echoes and fuzzy-logic clustering, to more detailed analyses of the macrophytes cover. These results were invited for presentation at an international conference in 2011 (Kruss et al., 2011) and are currently revised for submission to a peer-reviewed journal (Kruss et al., under revision).
How reliable is MBES data as one moves away from the vertical, imaging macrophytes further away and at increasingly grazing angles?
Depending on the sonar used, up to 30 % of MBES measurements in the centre beams can be affected by noise or lost. This was easily corrected by averaging over several pings, taking advantage of the higher imaging rate of MBES. Analyses of the Arctic dataset further showed that MBES beams at angles more than 55.5 degrees from the vertical were noisy enough to be safely rejected before further analyses. Angles between 50.5 and 55.5 degrees fared little better, with faint seabed traces and no indication of the presence (or not) of macrophytes. At angles between 15.5 and 30.5 degrees, acoustic returns from macrophytes seem to disappear. Theoretical simulations showed this was mostly explained by the physical characteristics of the macrophytes present in the Arctic, thin and with no gas content and therefore very close acoustically to the surrounding seawater. On a positive note, though, angles less than 15.5 degrees from the vertical, and between 30.5 and 50.5 degrees, were extremely helpful in identifying macrophytes, their partial extent and their respective heights above the seabed. The Arctic results were corroborated with measurements in a laboratory tank and in an open-water test site, using freshly collected samples of similar macrophytes and also looking at the role of gas vesicles in the acoustic returns. These results were presented at an international conference in 2012 (Kruss et al., 2012) and are currently written for publication (Kruss et al., in preparation). MBES generally go further than SBES by providing 'snippets' of the acoustic returns along the full water column, enabling better characterisation of the macrophytes cover and seabed substrates.
How much does the reliability of results vary with the type of MBES used? The Arctic measurements (2007), the laboratory tank measurements (2011-2012) and the open-water measurements (2011) all used the same sonar, and found the same results. The range of angles at which macrophytes can be detected with MBES extends further away (±15 degrees) from the vertical than would be possible with the SBES instruments traditionally used, and encompasses a further range of angles (30 - 50 degrees from the vertical, on either side of the MBES), but there is a 'dark zone' between these angles, explained by the acoustic characteristics of the macrophytes, and data beyond 50-degree angles is not useful at all. Discussions following the presentation of Kruss et al., 2012 showed similar results had been obtained by other teams around the world, although not (yet) published, using a wide variety of other sonars. The fellow used a different MBES, from another manufacturer and working at higher frequency (i.e. susceptible of a higher resolution). Results from controlled tank experiments showed the same effects. The central range seems to extend as frequency increases, but there is still a 'dark zone' over an intermediate range of angles at which acoustic returns from macrophytes disappear. In conclusion, MBES can image further away than SBES, with angular ranges defined by the sonar's frequency.
The fellow's research identified a common processing methodology, applicable to all depths / environments, and adaptable to different sonar types. Our results will hopefully be extended soon by general comparison of values obtained by other teams around the world, modulated by imaging frequencies and types of macrophytes imaged. This 'Marie Curie Fellowship' paved the way for further international collaborations all including the Fellow and host group, and at different stages of development. The socio-economic impacts of this project go beyond pure research, as they can be applied to surveys of marine biomass anywhere. Their take-up has featured in application articles (e.g. McGonigle et al., Estuarine Coastal and Shelf Science, 2011), and in technical reports (e.g. Blight et al., Marine Institute, Ireland, 2011). This fellowship's results will be of interest to national marine institutes, environmental organisations and the macrophyte industry in general. These results have included and been shared with major sonar manufacturers such as Biosonics Inc. (Canada), Imagenex (Canada), Hydroproducts (United Kingdom) and Reson A/S (Denmark, United Kingdom). The general processing methodology has also been considered for monitoring water quality in large reservoirs. The Fellowships results are now being considered for surveying of other marine ecosystems, e.g. at the fellow's returning institution in Poland, and for the monitoring of habitats around marine renewable energy sites, at different national and European levels.