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USGS Scientists Conduct Comprehensive Seafloor Mapping off the Delmarva Peninsula
U.S. Geological Survey (USGS) coastal and marine scientists continually work to provide information for improving the resilience of coastal habitat and infrastructure to future storms and sea-level rise. In the aftermath of Hurricane Sandy, which impacted the Northeast U.S. coast in the fall of 2012, new research efforts have been directed at mapping the seafloor, imaging materials on and beneath the seafloor, and collecting sediment samples to characterize the geologic and surficial sedimentary framework of the inner continental shelf along the Delmarva (Delaware, Maryland, and Virginia) Peninsula (see “Hurricane Sandy Response—Linking the Delmarva Peninsula’s Geologic Framework to Coastal Vulnerability” and “Linking Coastal Processes and Vulnerability—Assateague Island Regional Study”).
As part of this project, a group of USGS scientists and technologists who dubbed themselves “Team Delmarva” conducted a 40-day research cruise, from June 15 to July 25, 2014, aboard the motor vessel (M/V) Scarlett Isabella. The team collected sediment samples and geophysical data on seafloor depths and surface materials, plus images of sediment layers beneath the seafloor, that will provide new insights into the past, present, and potential future of this coastal system. Their findings will inform a variety of management applications, such as assessments of sand resources, habitats, and coastal vulnerability.
Led by geologists Laura Brothers and Rob Thieler of the USGS Woods Hole Coastal and Marine Science Center (Woods Hole, Massachusetts), the team acquired just over 4 terabytes of geologic and seafloor mapping data, along a linear distance of more than 4,500 kilometers (2,800 miles) and over an area of more than 3,200 square kilometers (1,200 square miles). The team used the latest technology to collect swath bathymetry (seafloor depths), seismic-reflection profiles (cross-sectional views of materials beneath the seafloor), acoustic backscatter (data that provide information about seafloor materials and roughness), water-column properties, and samples of seafloor sediment.
Such an effort required an extensive array of gear, including two seagoing lab vans, three winches, two instruments mounted on the side of the vessel, four instruments towed by the vessel, two underwater cameras, a sediment grab sampler, several navigation systems, and a suite of backups. Fortunately, the team had an excellent platform, the M/V Scarlett Isabella, that comfortably accommodated all the gear and personnel and was able to navigate the shallow water (minimum of approximately 8 meters [26 feet] water depth) of the inner shelf.
The new data provide a more detailed look than ever before at the geologic and geophysical properties of the area’s seafloor and underlying sediments. Swath bathymetry data were collected using a 234-kHz Submetrix sidescan sonar. A moving vessel profiler (designed to gather data at multiple depths while being pulled behind a moving ship) measured the velocity of sound traveling through the water column—information that is used to improve the accuracy of bathymetric data. Acoustic backscatter data, which give an indication of seafloor type (such as gravel, sand, or mud), were collected using the side-mounted Submetrix sonar and a towed Klein sidescan sonar. In addition to aiding in geologic interpretations of sediment type and texture, these data can be used in mapping and understanding benthic habitats. An Edgetech 512i high-resolution subbottom profiler imaged stratigraphy from the seafloor to approximately 20 to 40 meters (65 to 130 feet) beneath the seafloor, enabling geoscientists to map the sedimentary deposits and determine the volume of mobile sediment that may influence how the Delmarva coastline changes in response to storms and climate change. Another seismic-reflection system employing a multichannel hydrophone (underwater microphone) array and a new sound source called the “S-Boom” imaged even deeper stratigraphy, as much as approximately 100 meters (330 feet) below the seafloor. This boomer seismic system provided data that will help define the deeper geologic framework and sedimentary history of the Delmarva Peninsula.
The research cruise collected an impressive amount of data that will keep the team busy for months to come. Products expected to come out of this research, in addition to scientific publications, are regional bathymetric and acoustic-backscatter maps and other geospatial data products. These products lay the foundation for defining the geologic framework, which is critical to understanding coastal-change processes and vulnerability. This work is part of a long-standing, multipronged effort by the USGS Coastal and Marine Geology Program to conduct research that allows coastal stakeholders—including communities, landowners, and U.S. Department of the Interior agencies such as the National Park Service and U.S. Fish and Wildlife Service—to make scientifically informed coastal-management decisions.Read a profile of co-chief scientist Laura Brothers in “This Woman ROCKS,” this issue. Learn more about USGS Delmarva research at “Hurricane Sandy Response—Linking the Delmarva Peninsula’s Geologic Framework to Coastal Vulnerability.”
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Seafloor Mapping off the Delmarva Peninsula
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