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U.S. Geological Survey Open-File Report 2009-1001

Geological Interpretation of the Sea Floor Offshore of Edgartown, Massachusetts
MethodsTitle             PageList of             FiguresConversion             FactorsAbstractIntroductionSettingMethodsBathymetrySidescan             Sonar Seismic             Profiles SedimentsBottom             Photography Data             CatalogSummaryAcknowledgmentsReferences             Cited Contacts
Click on figures for larger           images.
Figure 5. Port-side view of the             National Oceanic and Atmospheric Administration Ship Thomas             Jefferson at sea.Figure 6. The National Oceanic and Atmospheric Administration             Launch 1014 being deployed from the NOAA Ship Thomas             Jefferson. Figure 7. A Klein 5000 sidescan-sonar             system hull-mounted to a National Oceanic and Atmospheric             Administration launch. Figure 8. The RESON Seabat 8101             hull-mounted in the keel cut out of National Oceanic and Atmospheric             Administration Launch 1005. Figure 9. The RESON Seabat 8125             hull-mounted in the keel cut out of National Oceanic and Atmospheric             Administration Launch 1014. Figure 10. Conductivity-temperature-depth             (CTD) profiler.Figure 11. A port-side view of the USGS             research vessel Rafael that was used to collect Chirp and             Boomer seismic-reflection profiles, bottom photography, and sediment             samples in the vicinity of Edgartown Harbor, Massachusetts.Figure 12. The locations of the tracklines             (blue lines) along which the 5 Boomer high-resolution             seismic-reflection profiles were collected during cruise 08034 of             research vessel Rafael. Figure 13. The locations of the tracklines             (green lines) along which the 16 Chirp high-resolution             seismic-reflection profiles were collected during the August 2008             cruise 08034 of the research vessel Rafael.Figure 14. The small Seabed Observation and             Sampling System (SEABOSS), a modified Van Veen grab equipped with             still and video photographic systems, mounted on the aft starboard             side of the research vessel Rafael.Figure 15. Locations of stations at which             bottom samples and photographs were taken during cruise 08012 of             research vessel Rafael to verify acoustic data. Figure 16. Correlation chart showing the             relationships among phi sizes, millimeter diameters, size             classifications (Wentworth, 1922), and American Society for Testing             and Materials and Tyler sieve sizes.Figure 17. Sediment-classification scheme             from Shepard (1954), as modified by Schlee (1973).       
Sidescan Sonar and Bathymetry

Two 29-foot aluminum Jensen launches (1014 and 1005) deployed from the       NOAA Ship Thomas Jefferson acquired sidescan-sonar and       single-beam and multibeam bathymetric data from an area of sea floor of       approximately 37.3 km2 during 2004 (figs. 5, 6). The sidescan-sonar data were acquired with Klein       5000 sidescan-sonar systems hull-mounted to the launches (fig.       7). The systems were set to sweep 100 m to either side of the launch       tracks. The systems emitted port and starboard 50°-wide beams tilted down       20° from horizontal and transmitted at 455 kHz. Daily confidence checks       were made of the sidescan-sonar system by observing the outer ranges of       the sonar images. The data were acquired in XTF (extended Triton data       format), recorded digitally through an ISIS data-acquisition system, and       processed by using CARIS SIPS (Sidescan Image Processing) software for       quality control, and to produce a composite sidescan-sonar image at 2-m       horizontal resolution. As part of this process, the sidescan-sonar data       were demultiplexed and filtered to remove speckle noise and to correct for       slant-range distortions. Image shine through to account for areas of       overlap and auto-contrast adjustment were applied. Contrast enhancement       based on the dynamic range of the data was applied to the mosaic, and it       was then projected into UTM Zone 19 to produce an enhanced image.        Adobe Photoshop CS2 and ArcGIS 9.2 were used to refine the grayscale       stretch and enhance the imagery. 

The single-beam bathymetric data were collected with an Odom       vertical-beam echosounder. The single-beam bathymetry data were       supplemented by multibeam echosounder data that were collected with       hull-mounted RESON SeaBat 240-kHz 8101 and 455-kHz 8125 shallow-water       systems (figs. 8, 9). These systems measure two-way sound traveltime       across a 150-degree swath and 120-degree swath, respectively. The SeaBat       8101 has 101 beams at a 1.5-degree beam spacing. The SeaBat 8125 has 240       beams with a cross-track beam width of 0.5 degrees at nadir. Original       horizontal resolution of the multibeam bathymetric data was 0.5 m;       vertical resolution of the multibeam data is about 0.5 percent of the       water depth. Both bathymetric datasets were acquired in XTF (extended       Triton data format), recorded digitally through an ISIS data acquisition       system, and processed by NOAA, which used CARIS HIPS (Hydrographic Image       Processing System) software to control quality, incorporate sound-velocity       and tidal corrections, and produce a digital terrain model. Because of the       areas of no data between ship tracks, the bathymetric data were regridded       to 25 m at the USGS Woods Hole Coastal and Marine Science Center and       interpolated to eliminate data gaps and to create continuous-coverage       datasets. A more detailed description of the acquisition parameters and       processing steps performed by NOAA and the USGS to create the datasets       presented in this report are included in the metadata files. These files       can be accessed through the Data Catalog section of this report.

Navigation was by TSS POS/MV 320 differential global-positioning-system       (GPS)-assisted inertial navigation systems; Hypack MAX was used for       acquisition-line navigation. Sound-velocity corrections were derived by       using frequent SEACAT CTD (conductivity-temperature-depth) profiles (fig. 10). Typically, a CTD cast was conducted every four       to six hours of multibeam acquisition. Tidal-zone corrections were       calculated from data acquired at the Nantucket tidal gauge and a tertiary       tide gauge installed for this project at Edgartown, Massachusetts. 

Seismic Reflection, Sampling, and Photography 

The bathymetric and sidescan-sonar data were verified with Boomer and       Chirp high-resolution seismic-reflection data during August 2008 aboard       the research vessel (RV) Rafael during cruise 08034 and with       sediment samples and bottom photography during September 2008 aboard the       RV Rafael during cruise 08012 (fig. 11). The Boomer seismic data were acquired       digitally along five lines totaling approximately 27.6 km in length with a       GeoAcoustics Boomer source, GeoAcoustics power supply, Benthos AQ4       single-channel streamer, and Chesapeake Systems SonarWiz acquisition       software (fig. 12). The Boomer was fired at a rate of 0.5 seconds       at a power output of 175 joules. The received signal was filtered between       100 and 3000 Hz with a GeoPulse receiver. The Boomer trace data recorded       for 266 ms with a sample rate of 0.533 ms in standard SEG-Y format. The       3.5-kHz Chirp data were acquired digitally along 16 lines totaling       approximately 56.1 km in length with a Knudsen Engineering Limited (KEL)       Chirp 3200 shallow-water system and KEL Sounder Suite software (fig. 13). The Chirp trace data were recorded with a ping       rate of 0.5 seconds for 66 ms with a sample rate of 0.048 ms in standard       SEG-Y format. 

The raw Boomer and Chirp SEG-Y data were read into Promax R2003       processing software. Heave-compensation data from the Teledyne TSS sensor       (mounted on the transducer sidemount pole) were extracted from the SEG-Y       headers and were used to apply a static shift to the trace data and to       mute the water column. An automatic gain control (AGC) with a window       length of 5 ms was applied. The processed data were written to a new SEG-Y       file, and Seismic Unix was used to read the processed SEG-Y files and       extract shot number, horizontal position, year, day, and time of day.       Shot-point navigation data for both Boomer and Chirp datasets were       converted to point and line shapefiles in ArcCatalog.

Surficial-sediment samples (collected 0-2 cm below the sediment-water       interface) and bottom photographs were collected at 37 stations with a       modified Van Veen grab sampler equipped with still- and video-camera       systems (figs. 14, 15). The photographic data were used to appraise       intrastation bottom variability, faunal communities, and sedimentary       structures (indicative of geological and biological processes) and to       observe boulder fields where samples could not be collected. A gallery of       images collected as part of this project is provided in the Bottom Photography section; location       information for the bottom photographs can be accessed through the       Data Catalog section. 

In the laboratory, the sediment samples were disaggregated and       wet-sieved to separate the coarse and fine fractions. The fine fraction       (diameters less than 62 microns) was analyzed by Coulter Counter, the       coarse fraction was analyzed by sieving, and the data were corrected for       salt content. Sediment descriptions are based on the nomenclature proposed       by Wentworth (1922; fig. 16), the inclusive graphics statistical method of       Folk (1974), and the size classifications proposed by Shepard (1954; fig. 17). A detailed discussion of the laboratory       methods is given in Poppe and others (2005). Because biogenic carbonate       shells commonly form in situ, they usually are not considered to be       sedimentologically representative of the depositional environment.       Therefore, gravel-sized bivalve shells and other biogenic carbonate debris       were ignored. The grain-size-analysis data can be accessed through the       Data Catalog section of this report and in the       Sediments section. 

To facilitate interpretations of the distributions of surficial       sediment and sedimentary environments, these data were supplemented by       sediment data from compilations of earlier studies (Poppe and others,       2003) and unpublished datasets from the National Geophysical Data Center.       The interpretations of sea-floor features, surficial-sediment       distributions, and sedimentary environments presented herein are based on       data from the sediment-sampling and bottom-photography stations, on tonal       changes in backscatter on the imagery, and on the bathymetry. For the       purposes of this paper, bedforms are defined by morphology and amplitude.       Sand waves are higher than 1 m; megaripples are 0.2 to 1 m high; ripples       are less than 0.2 m high (Ashley, 1990). The bathymetric grids and imagery       released in this report should not be used for navigation.   

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