Provincetown Center for Coastal Studies

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    Seafloor Mapping

     
         
     

    The Seafloor Mapping Program was created when the Nearshore Resource Characterization Mapping Project allowed the Center to purchase a state-of-the-art Interferometric Sonar System that collects coincident bathymetry and acoustic backscatter imagery. This geophysical instrument will allow investigators to capture the seafloor at very high resolution and accuracy in order to conduct research in nearshore, shallow water settings. These studies can provide quantitative data to answer questions regarding sediment transport pathways and bedforms, small, medium and large scale coastal evolution, submerged aquatic vegetation and other relevant science-based and management questions.

    Interferometric sonar yields coincident backscatter imagery (left) and bathymetry (right). These two data sets collectively yield more information when synthesized.

    Interferometric sonar yields coincident backscatter imagery (left) and bathymetry (right). These two data sets collectively yield more information when synthesized.

    This sonar system is one the central pieces of equipment for the program and the department. In fact, investigators within the department are currently working with other scientists at the Center to explore different uses for this system equipment.  Dr. Amy Costa, the director of the Marine and Estuarine Studies Program within the Center’s Department of Ecology, and Owen Nichols, the Project Leader of the Marine Fisheries Research Project also within the Department of Ecology, are developing several studies with Dr. Mark Borrelli to look at the relationship between eelgrass biomass, spatial heterogeneity, and canopy height using the sonar equipment. A proposed study entitled “Interferometric sonar as an eelgrass habitat assessment tool: are sonar-derived indices of spatial heterogeneity, canopy height, and biomass correlated with macrofaunal community composition?” is one such example.

    Acoustic backscatter imagery of ripples and eelgrass

    Acoustic Backscatter Imagery. This is a pre-processed image of the seafloor created using sound (sonar) rather than light. The entire image is 50 m across. The bedforms, or ripples, are spaced approximately 10-15 cm apart. Near the top of this image the seafloor is obscured entirely by eelgrass (Zoestra marina).The black vertical columns in the center are removed during processing and the imagery is ‘stitched’ together to accurately capture the features on the seafloor.

    ONGOING PROJECTS

    1. Nearshore Resource Characterization Mapping Project

    The Provincetown Center for Coastal Studies is conducting a 3-year, state-funded pilot project for the 'nearshore seafloor mapping and resource characterization' in Cape Cod waters. The agreement also 'acknowledges the need to develop effective mapping strategies for nearshore environments'.

    This project, in part, will determine the feasibility of tide-coincident mapping of the interface of the marine and terrestrial nearshore systems at very high resolution. Typically, nearshore mapping occurs in water depths of 10 meters or greater. Closer to shore vessel-based surveying is very labor-intensive, more hazardous and yields data of varying quality. Surveys mapping bathymetry using single-beam sensors on personal water craft capture relatively low-density data and don't include acoustic backscatter data.

    The Center will use a state-of-the-art Interferometric Sonar System to map the nearshore marine system (in 2 meters out to 10 meters of water), which will yield coincident swath acoustic backscatter and bathymetric data. The acoustic backscatter data will ensonify or 'take a picture' of the seafloor using sound. This along with sediment samples and underwater imagery and/or video will provide detailed information with regards to the surficial seafloor geology including grain size, composition, bedforms as well as submerged aquatic vegetation and other biota. The bathymetry will provide another layer of detail for establishing baseline data useful for modelers and ongoing and future studies of the area.

    This next generation interferometric bathymetry system has the ability to generate an average swath to depth ratio of 10:1, while the typical multi-beam bathymetry systems deliver a 3:1 ratio. Therefore, in water depths of 3 meters our new system could generate a 30 meter swath compared to a 9 meter swath with multi-beam systems. This significantly reduces the time needed to map these shallow water areas with commensurate reductions in both the cost and hazards associated with operating a vessel in the nearshore.

    Most past efforts to map the marine/terrestrial interface have used bathymetric data from one survey and terrestrial and shallow water Lidar data from another survey. These surveys were often weeks to months apart. This project will eliminate the uncertainties, or errors, introduced by different: datums, mapping goals, data resolutions/accuracies, or temporally distant surveys. Toward this end, airborne- or terrestrial-based LIDAR (Light Detection And Ranging) will be used to conduct a low-tide survey of the nearshore terrestrial system (beach/dune/bluff ) followed immediately by a high-tide vessel-based survey. This will allow us to overlap spatially and within ~6 hours temporally the marine/terrestrial interface, which will yield a unique dataset for this environment. This area was selected in part for the Areas in Cape Cod Bay to be surveyed have a mean tidal range of ~3 meters.

    Seamless onshore/offshore mapping. Taylor Brown and Ashley Norton use an RTK-GPS unit to collect data points on several intertidal groins.

    Seamless onshore/offshore mapping. Above left and right Taylor Brown and Ashley Norton, respectively are using an RTK-GPS unit to collect data points on several intertidal groins, one of which is shown in the acoustic backscatter imagery (bottom). By using the high accuracy RTK-GPS data points on those groins (2 cm of uncertainty in the vertical), the bathymetry and terrestrial lidar can be tied together with these known data points common to all both datasets, thereby effectively eliminating a major source of uncertainty (different datums) when merging marine and terrestrial datasets.

    2. STATEMAP 2011-2012

    The Center has won a competitive STATEMAP (http://ncgmp.usgs.gov/ncgmpabout/statemap) grant for 2011-2012 to produce a seamless onshore/offshore map out to the 20 m isobath in the Provincetown Quadrangle. More information to follow. 

     
     


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