Introduction

The U.S. Geological Survey (USGS) ismapping the sea floor offshore of major metro-politan centers. The New York-New Jersey met-ropolitan area is one of the most populatedcoastal regions in the United States. The NewYork harbor estuary and its offshore area are usedfor waste disposal, transportation, recreation, andcommercial and recreational fishing.

Interpretive maps of the sea-floor geologyprovide a fundamental framework for researchand management in the coastal ocean. Theyshow the composition of the seabed and theshape (topography) of the sea floor; they alsoshow areas of present and past dumping of sedi-ments and pollutants, as well as locations andimpacts of other human activities. They provideinformation on the transport of sediment and helpto define biological habitats. All this informationcan be used to develop predictive models toguide habitat and resource management, moni-toring strategies, and other research studies.

Mapping the Sea Floor

Modern oceanographic surveys use remote-sensing techniques (sidescan sonar (fig. 1), multi-beam echosounding (fig. 2), and high-resolutionseismic-reflection profiling), direct sampling, andvisual observations to characterize the sea floor.The surveys provide a new, highly detailed viewof the sea floor. In contrast to earlier maps thatwere based on widely spaced data, the new digi-tal images are similar in detail to an aerial photo-graph and show the changes in seabed featuresover a wide range of scales. The locations andeffects of human activities, such as waste disposaland bottom trawling, are often clearly observed.Sedimentary features, such as bed forms, provideinformation about the transport and fate of sedi-ments and pollutants and about the importance ofthe underlying geologic structures and the geo-logic history in controlling the present distribu-tion of surface sediments.

Mapping the Area Offshore of NewYork and New Jersey

Sidescan sonar and multibeam systemsemit pulses of sound that reflect off the seafloor. Features of the sea floor are identifiedby the pattern and varying levels of soundreflected (backscattered) from the sea floor.

One of the most striking characteristics of thebackscatter maps of the New York Bight area(fig. 3) is the variability in sediment character-istics over scales of tens to hundreds ofmeters. Sampling, bottom photography, andthe high-resolution seismic observations showthat the bottom sedimentary environments inthis area range from outcropping rock tomuds. The maps show a complex pattern ofsediment properties that is a result of theunderlying geology, modern processes, andanthropogenic activities.

The surveys off the New York-New Jerseyarea are being carried out by the USGS in coop-eration with the U.S. Army Corps of Engineers,Texas A&M University, the State University ofNew York at Stony Brook, the University ofNew Brunswick, the Canadian HydrographicService, Wesleyan University (Connecticut),and Coastal Carolina University.

References Cited

Butman, Bradford, Danforth, W.W., Schwab,W.C., and Bucholtz ten Brink, M.R.,1998, Seafloor topographic andbackscatter maps of the Upper HudsonShelf valley and adjacent shelf, offshoreof New York: U.S. Geological SurveyOpen-File Report 98–616.

Schwab, W.C., Allison, M.A., Corso, W. Lotto,L.L., Butman, B., Bucholtz ten Brink,M., Denny, J., Danforth, W.W., andFoster, D.S., 1997, Initial results of high-resolution sea-floor mapping offshore ofthe New York - New Jersey metropolitanarea using sidescan sonar: NortheasternGeology and Environmental Sciences, v.19, no. 4, p. 243–262.

For more information, please contact:

Bradford ButmanU.S. Geological SurveyWoods Hole Field Center384 Woods Hole RoadWoods Hole, MA 02543–1598

Mapping the Sea-Floor Geology Offshore of theNew York-New Jersey Metropolitan Area

U.S. Department of the InteriorU.S. Geological Survey

USGS Fact Sheet FS-133-98Reprinted November 1999

Tow Vehicle

Boulder

Boulder

Sand

MudGravel

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ing tra

wl sear

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Ship's path

Figure 1. Upper: A sidescan sonar “fish” towedbehind a vessel surveys the sea floor by send-ing sound to either side of the ship’s path.Typical survey swaths (yellow area) are a fewhundred meters wide. Lower: The intensity andpattern of sound reflected from the ocean floorprovide information on the composition of sedi-ments and the topography. Strong reflectionsfrom boulders, gravel, and the walls of trawlgrooves appear as light tones on this datarecord; weak reflections from finer sediments orshadows behind vertical features are dark. Acomposite image, pieced together from multiplesurvey strips, provides an image of the seafloor similar in detail to an aerial photograph.

Figure 2. High-resolution multibeam mappingsystems use sound from arrays of 60 to morethan 150 electronically separated transducers tomeasure water depth as well as sediment char-acteristics of the sea floor. The transducers formbeams a few degrees wide that produce a foot-print of a few square meters on the sea floor inwater depths of 50 meters. Because the systemis fixed to the ship’s hull, the data can be easilygeoreferenced and surveys can be run atspeeds of 15 knots. The multibeam surveys pro-vide a new, highly detailed view of the sea floor.Used with the permission of the University ofNew Brunswick.

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