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Seabed Mapping for Marine Protected Area Planning and Monitoring
An Interferometric Swath System and Towed Video Sled Provide Vital Scientific Data for Sustainable Marine Management

By Dr. Peter Davies
Senior Environmental Scientist
Dr. Alan Jordan
Senior Environmental Scientist
and
Edwina Mesley
Environmental Scientist
Marine Conservation Science Unit
New South Wales Department
of Environment, Climate Change
and Water
Sydney, Australia



As part of a range of strategies for improving management of marine environments within Australia’s exclusive economic zone, authorities are actively implementing a system of marine protected areas (MPAs). An important component of the information required to improve marine resource management and planning is an understanding of the extent, distribution and structure of seabed habitats. Mapping classes delineated using acoustic remote sensing and associated ground truthing are commonly based on geophysical features or a combination of biological and physical features. Such classification schemes reflect the limited capacity of acoustic remote sensors to delineate biotic composition, while recognizing the influence of physical factors on the composition of benthic communities.

In Australia, a spatial framework exists for classifying marine areas using bathymetry, sediments, demersal fish assemblages and sponges into bathymetric units, provincial bioregions, biomes, geomorphic units and features, and seabed facies. The framework operates on a variety of scales ranging from national (Interim Marine Coastal Regionalisation of Australia) to regional (bioregional planning) to ecosystem scale (meters to kilometers).

Setup of pole-mounted GeoSwath transducers on RV Glaucus, shown at sea in the Solitary Islands Marine Park.

In New South Wales (NSW) on the east coast of Australia, six marine parks have been created that cover 345,000 hectares, or about 34 percent of NSW state coastal waters. These marine parks contain various types of zones, ranging from “no-take” sanctuary zones to general-use zones where most recreational and commercial activities are allowed.

NSW’s continental shelf is narrow (12 to 60 kilometers wide), with a gently sloping seafloor substrate dotted with outcropping rocky reefs and the occasional island. The ecology of much of the region is strongly influenced by the warm waters of the southward-flowing East Australian Current, which transports larval and juvenile marine organisms from further north. The marine parks are located within six bioregional areas, each of which represent different components of marine biodiversity.

Recent high-resolution swath acoustic mapping of the seabed of NSW continental shelf waters has significantly improved information on the extent, distribution and structure of seabed habitats and associated assemblages. This is used in the effective planning and monitoring of MPAs in NSW. Combining maps of seabed habitats from a number of sources with information on species composition has allowed a greater range of biodiversity surrogates to be used for representing biological diversity in NSW MPAs. This technique has also improved assessment of the effectiveness of MPA locations.

This article describes the habitat mapping program in the Solitary Islands Marine Park, located on the mid-north coast of NSW and one of the largest marine parks in the state, covering an area of 710 square kilometers. Data were collated in the Solitary Islands Marine Park region from previous geological, hydrographic and aerial photography surveys combined with new swath acoustic bathymetry, backscatter and video data, with an emphasis on the seabed habitats and the sessile floral and faunal assemblages.

Methods
Swath acoustic seabed surveys were conducted with a Kongsberg Maritime AS (Kongs-berg, Norway) GeoSwath 125-kilohertz interferometric side scan sonar and associated auxiliary sensors aboard the 8.5-meter catamaran RV Glaucus, operated by the NSW Department of Environment, Climate Change and Water. The system consists of two 30°-mounted 125-kilohertz transducer arrays, a sound-velocity sensor and a Teledyne TSS (Watford, England) motion reference unit (TSS DMS-05), pole-mounted on a V-bracket and deployed over the port-side of the vessel. Horizontal position data are obtained with an OmniSTAR (Houston, Texas) sub-meter differential global positioning system. The swath mapping system collects georeferenced depth and side scan backscatter data to generate high-resolution bathymetric and backscatter spatial information. Swath coverage along a track was determined by a number of parameters, including water depth, power/pulse length, sea conditions and performance of the motion sensor, with the maximum total swath widths often limited to about 250 meters (125 meters per side). Vessel speed was generally around five knots, which, with a ping rate of about six pings per second, resulted in across-track resolution of 1.5 centimeters and along-track resolution of about 30 centimeters.

Soundings were corrected within GeoSwath’s propriety software for heave, pitch, roll and yaw of the vessel, sound velocity and tides. Sound velocity profiles were calculated from conductivity, temperature, depth (CTD) casts using a Sea-Bird Electronics Inc. (Bellevue, Washington) CTD system, and tides were measured using a bottom-mounted Aquatec Group Ltd. (Hartley Wintney, England) pressure sensor adjusted to chart datum using data from two local tidal stations.

Swath surveys were designed to completely cover areas of interest, run with 25 percent overlap of adjacent swaths to improve data quality at the edge of swaths and provide quality assurance information. Raw data files were filtered and gridded using GS+ software from GeoAcoustics Ltd. (Great Yarmouth, England) with side scan sonar transects processed into georeferenced mosaics of bathymetry at five-by-five-meter grid resolution. Backscatter data were also gridded into five-by-five-meter bins after manipulation (including trace normalization) using GeoAcoustics’ GeoTexture software.

The bathymetry was viewed as a hill-shaded relief (90° to 120° azimuth and 30° to 45° elevation) to provide a quasi-3D image to aid the manual digitization process. Backscatter information, viewed as a grayscale image, was also used in the delineation of reef and nonreef in areas of uncertainty. Areas of rocky reef were digitized by hand at a constant scale of 1:2,000 using a combination of the bathymetry and backscatter information. Overall, rocky reef areas are characterized by regions of relief, grayscale heterogeneity (texture) and higher backscatter intensity (i.e., darker areas), whereas unconsolidated substrates form regions of relatively little relief, varied to homogeneous textural complexity and weaker (lighter) backscatter.
3D representations of the seafloor from swath acoustic data (bathy-metry and acoustic backscatter) in the Sidney Shoals region of the Solitary Island Marine Park.

Ground-truthing of acoustic data was provided by a digital underwater video camera that was deployed along selected transects within swath acoustic survey areas. Information on the distribution of visually dominant benthic biota was also collected from video. Within the Solitary Islands Marine Park, a total of 8.8 linear kilometers of video footage was collected.

Video frames were captured as still images from selected clear sections of video footage, and detailed physical and biological attributes were aggregated into functional physical and habitat classes based on substrate type and dominant species or groups of species. The bathymetric position of classified points was used to describe depth-related trends in habitat distribution and the physical-attribute class was used to assist visual interpretation of swath acoustic and other bathymetric image data.


Results
To date, approximately 275 square kilometers of continental shelf waters within the Solitary Islands Marine Park and adjacent waters have been swath mapped. Collected data are converted to ESRI’s (Redlands, California) ArcGIS format and combined with maps of shallow reefs digitized from aerial photos and underwater video surveys. Geographic information system analysis is then used to create a number of spatial products, such as digital elevation models (with analysis of slope and rugosity), vector-based seabed habitat maps and spatially linked data on macrobenthic floral and faunal assemblages.

The swath mapping has revealed considerably more rocky reef throughout the marine park than was defined previously from the broad-scale bathymetry and single-beam acoustic transects. Extensive subtidal rocky reefs occur adjacent to the mainland headlands and the offshore islands. Other reefs are present as discrete subtidal features. Shallow reef areas are dominated by forests of macroalgae (kelp), filamentous algae and unique subtropical coral communities. As reef depth becomes greater than around 30 meters (and low light limits plant growth), the benthic community is dominated by sponges, sea whips and ascidians. Other habitats include boulder and cobble fields as well as significant sand waves, the gravelly troughs of which support a variety of marine life. The survey revealed that many areas of kelp and sessile invertebrates inhabit areas of patchy, low-lying reef that is partially covered by sand.


Conclusions
The recent swath mapping within the Solitary Islands Marine Park has significantly improved the effectiveness of surrogates in the planning process by defining the extent and distribution of reefs throughout the region and improving knowledge on the spatial structuring of unconsolidated habitats. Overall, benthic communities on rocky reefs throughout the marine park contain subtropical and temperate species, reflecting latitudinal and cross-shelf gradients of water temperatures and ocean currents.

The application of interferometric side scan sonar that collects georeferenced depth and side scan amplitude data provides the capability to use both bathymetry and backscatter data to delineate habitat types. It was found that the GeoSwath system, with its side scan-like approach, provides high-quality backscatter information that, in combination with the bathymetry, is suitable for defining the extent and structure of a range of seabed habitats. The system allows an assessment of surface roughness based on the variation of backscatter amplitude.

Automated methods for seabed classification from swath data will vastly improve the cost-effectiveness of these techniques. The system will soon be upgraded with the addition of an Applanix (Richmond Hill, Canada) POS-MV Wave-Master to improve motion corrections.

In conclusion, habitat mapping is an essential component of the overall research needed to increase the likelihood that MPAs contain a comprehensive and representative selection of biological diversity. Swath-mapping techniques with increased resolution, and often complete coverage, should improve the ability to incorporate marine park planning criteria with detailed spatial information on seabed habitat distribution, particularly when it incorporates spatial heterogeneity. The high-resolution depth data can also be used to examine reef complexity, slope and depth, and the habitat layers can be draped over the digital elevation model to provide a visual 3D representation of the substrate surface. Continual software developments are also providing the capacity for improvements in data integration and visualization and improved data processing flow.


Acknowledgments
The authors thank Tim Ingleton, Joe Neilson, Richard Gardiner and Hamish Malcolm for field support and data processing. Funding for the project was provided in part by the NSW Marine Parks Authority.


References
For a full list of references, e-mail Dr. Peter Davies at Peter.Davies@environment.nsw.gov.au.




Dr. Peter Davies is a senior environmental scientist with the Marine Conservation Unit of the New South Wales Department of Environment, Climate Change and Water. He has more than 15 years of experience in remote sensing of the marine environment for ecosystem-based management applications using both satellite and acoustic methods.

Dr. Alan Jordan is a senior environmental scientist with the New South Wales Department of Environment, Climate Change and Water. He has 23 years of experience in the fields of fisheries assessment, biodiversity surveys and seabed habitat mapping. His current research focuses on the application of swath acoustics to map the extent, distribution and structure of seabed habitats throughout continental shelf waters of New South Wales.

Edwina Mesley is an environmental scientist with the Marine Conservation Unit of the New South Wales Department of Environment, Climate Change and Water. She has seven years of experience in research and spatial analysis in freshwater and marine ecosystems, predominantly for environmental management and conservation planning applications.



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