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Ground-Truthing Sea-Floor Data with the New Mini SEABOSS—Mapping the South Essex Ocean Sanctuary
The U.S. Geological Survey (USGS)'s Woods Hole Science Center, in cooperation with the Massachusetts Office of Coastal Zone Management (CZM) and the National Oceanic and Atmospheric Administration (NOAA), is conducting high-resolution geologic mapping of the sea floor to characterize bottom morphology, surficial-sediment distribution, and subsurface geology in the shallow coastal waters of Massachusetts. The long-term goal of this mapping effort is to produce detailed sea-floor geologic maps that will be used in aggregate-resource and benthic-habitat studies by State and other Federal agencies. In May 2004, Dann Blackwood, Brian Andrews, Barry Irwin (all of USGS, Woods Hole), and Seth Ackerman (CZM, Woods Hole) conducted a remote sea-floor ground-truthing survey in the South Essex Ocean Sanctuary, using a new version of the successful SEABOSS (SEABed Observation and Sampling System; see USGS Fact Sheet 142-00). The new, more compact SEABOSS, called “Mini SEABOSS,” is smaller than the original SEABOSS and was specifically designed by Dann Blackwood and Ray Davis (USGS, Woods Hole) for nearshore survey operations. One hundred potential stations were identified within the South Essex Ocean Sanctuary on the basis of high-resolution swath bathymetry and sidescan-sonar data. All of the planned stations were occupied in just three survey days. The survey of a typical station began with a several-minute drift during which bottom video footage and still photographs were collected; most of the surveys concluded with a sediment grab, using the modified van Veen grab sampler. On many surveys, the video drifts were planned across suspected transition zones to pinpoint areas with a distinct change in sea-floor type. The Mini SEABOSS was designed and built to bring the ground-truthing capabilities of the original SEABOSS onto a trailerable research vessel, for coastal, lake, and river research. It offers the researcher two video cameras, a digital still camera, and a bottom grab sampler. It is built around the same 0.1-m2-area van Veen grab sampler as the original SEABOSS but uses a slightly smaller and lighter frame; it weighs about 185 lb. The Mini SEABOSS is deployed from a manually pivoting davit mounted through the gunwale. A fully variable speed electric winch is mounted on the davit. The system can be used to a depth of just more than 40 meters. The grab itself is raised and lowered with a 3/16-inch Aramid fiber winchline with a breaking strength of 5,600 pounds. The davit is secured when the Mini SEABOSS is in the water. The electrical cable is on a spring-wound takeup reel with electrical slip rings that is mounted aft of the sampler. This arrangement protects the 0.38-in. multiconductor cable and keeps the Mini SEABOSS correctly oriented with the boat.
During normal sampling operations, the Mini SEABOSS is not brought into the boat between deployments but is placed outboard and secured to the gunwale, using mounts to hold it in place. With the Mini SEABOSS in this position, the sediment samples can be recovered and placed in containers, minor adjustments can be made, and the grab can be conveniently cleaned out for the next sample without sediment entering the boat. Because of the secure gunwale mounting system, the Mini SEABOSS is quite stable in this position during short transits between stations. During long transits or to service the digital camera, the Mini SEABOSS can be swung inboard and lowered into the cockpit.
The imaging components are all mounted on a fiberglass strut that can be easily removed so that the grab can also be used for basic sampling operations. The digital camera, mounted in a simple machined-plastic housing, generates a 5-MB image. Depending on the card used and image compression, many hundreds of images can be taken before the card is removed and downloaded. The video signals can be recorded in various formats. The 50-W/s flash unit is battery powered and was recycled from the original SEABOSS. The battery-powered lasers are set 15 cm apart for scale measurements. A third laser is positioned at an angle so that when it intersects the other lasers, the Mini SEABOSS is at the proper height off the bottom for a still photograph. Over the course of three days, 597 images were obtained on the South Essex Ocean Sanctuary survey, and only a few minor adjustments and changes were needed. The cable takeup reel performed well and kept personnel from having to handle and coil the conducting cable. The boat could be gently maneuvered when the system was deployed to optimize speed over the bottom and adjust transect direction. The Mini SEABOSS is a tool that can provide useful ground-truth data in shallow regions inexpensively and in a timely manner.
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in this issue:
Cruise to California Seamounts
Scientists Interviewed for Documentary The National Map Comes to Florida
Geologist Joins USGS in St. Petersburg |
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
