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U.S. Geological Survey Open-File Report 2012-1006

High-Resolution Geophysical Data From the Inner Continental Shelf at Vineyard Sound, Massachusetts


Data Collection and Processing

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Click on figures for larger images
Thumbnail image of figure 1 and link to larger figure. map showing location of the vineyard sound survey area.
Figure 1. Map showing location of the Vineyard Sound survey area (outlined in blue). Abbreviations on figure are explained in the Abbreviations
Thumbnail image of figure 2a and link to larger figure. A photograph of m/v megan t. miller.
Figure 2a.Photograph of a vessel used for the geophysical mapping in this project, M/V Megan T. Miller (2009-0020-FA,and 2010-004-FA). Photograph by Christopher Polloni
Thumbnail image of figure 2b and link to larger figure. Photograph of the m/v scarett isabella.
Figure 2b. Photograph of a vessel used for the geophysical mapping in this project, M/V Scarlett Isabella (2011-004-FA). Photograph by David Foster
Thumbnail image of figure 3 and link to larger figure. map showing shaded-relief bathymetry of the seafloor in vineyard sound massachusetts.
Figure 3. Map showing shaded-relief bathymetry of the seafloor in Vineyard Sound, Massachusetts. Coloring and bathymetric contours represent depth, in meters, relative to Mean Lower Low Water (MLLW) datum. Abbreviations on figure are explained in the Abbreviations.
Thumbnail image of figure 4 and link to larger figure. map showing tracklines with bathymetry data were collected in the vineyard sound survey area.
Figure 4. Map showing tracklines along which bathymetry data were collected in the Vineyard Sound, Massachusetts, survey area. Tracklines are color coded by U.S. Geological Survey (USGS) survey number. Abbreviations on figure are explained in the Abbreviations.
Thumbnail image of figure 5 and link to larger figure. map showing tracklines with acoustic backscatter data were collected in the vineyard sound survey area.
Figure 5. Map showing tracklines along which acoustic-backscatter data were collected in the Vineyard Sound, Massachusetts, survey area. Tracklines are color-coded by U.S. Geological Survey (USGS) survey number. Abbreviations on figure are explained in the Abbreviations.
Thumbnail image of figure 6 and link to larger figure. map showing acoustic backscatter intensity of the seafloor in the vineyard sound survey area
Figure 6. Map showing acoustic-backscatter intensity of the seafloor in the Vineyard Sound, Massachusetts, survey area. Backscatter intensity is an acoustic measure of reflectivity of the seafloor. In general, higher values (light tones) represent rock, boulders, cobbles, gravel, and coarse sand. Lower values (dark tones) generally represent fine sand and muddy sediment. Backscatter intensity scale values are relative digital number (DN) values based an 8-bit data range (0-255). Abbreviations on figure are explained in the Abbreviations.
Thumbnail image of figure 7 and link to larger figure. map showing tracklines with seismic reflection profiles were collected in the vineyard sound survey area.
Figure 7. Map showing tracklines along which seismic-reflection profiles were collected in the Vineyard Sound, Massachusetts, survey area. Tracklines are color-coded by U.S. Geological Survey (USGS) survey number. Abbreviations on figure are explained in the Abbreviations.
The data presented in this report were collected in Vineyard Sound during three geophysical surveys (USGS field activity serial numbers 2009-002-FA, 2010-004-FA, and 2011-004-FA) conducted by the USGS between 2009 and 2011 (fig. 1; table 1). Surveys consisted of 24 hours-per-day operations aboard the M/V Megan T. Miller (2009-002-FA and 2010-004-FA), and the M/V Scarlett Isabella (2011-004-FA) (figs. 2A and 2B).

This section provides basic descriptions of acquisition and processing of the data contained in this report. Detailed descriptions of acquisition parameters, processing steps, and accuracy assessments for each data type are provided within the metadata files for each dataset listed in appendix 1.

Table 1. Survey details for the data collected in the Vineyard Sound study area.

[The field activity survey numbers (for example, 2009-002-FA) are also abbreviated (for example, 09002) in parentheses.]

Survey

Vessel

Begin Date

End Date

Backscatter

Seismics

Bathymetry

2009-002-FA
(09002)

Megan T. Miller

28-May-09

18-Jun-09

Klein 3000
132 kHz

EdgeTech 512i
0.5-12 kHz

SWATHplus
234 kHz

2010-004-FA
(10004)

Megan T. Miller

15-May-10

4-Jun-10

Klein 3000
132 kHz

EdgeTech 512i
0.5-12 kHz

SWATHplus
234 kHz

2011-004-FA
(11004)

Scarlett Isabella

7-May-11

17-May-11

Klein 3000
132 kHz

EdgeTech 512i
0.5-12 kHz

SWATHplus
234 kHz

Bathymetry

Approximately 340 km2 of bathymetric data were acquired using a Systems Engineering & Assessment, Ltd. (SEA) SWATHplus interferometric sonar operating at a frequency of 234 kilohertz (kHz) (fig. 3; table 1). During surveys 2009-002-FA and 2010-004-FA, the sonar transducers were mounted on a rigid pole from the starboard side of the M/V Megan T. Miller (fig. 2A), about 2.4 meters (m) below the water line. During survey 2011-004-FA, the sonar transducers were mounted on a rigid pole from the port side of the M/V Scarlett Isabella (fig. 2B) about 2.17 m below the water line. A motion reference unit (Coda Octopus F180) was mounted directly above the sonar transducers and continuously measured vertical displacement (heave) and attitude (pitch and roll) of the vessel during data acquisition. Sound-velocity profiles were collected approximately every 2 hours using a hand-casted Applied MicroSystems SV Plus sound velocimeter (2009-002-FA) or an ODIM Brooke Ocean MVP30 moving vessel profiler (2010-004-FA and 2011-004-FA). Data were collected with a sonar transmit power of 6 to 8 (on a relative scale ranging from 1 to 15 representing 0 to 100 percent power levels), transmit length of 12 to 43 cycles, and a receive length of 3,072 or 4,096 samples, depending on the survey.

Navigation was recorded with a Global Positioning System (GPS) antenna mounted on top of the pole, directly above the SWATHplus transducers. Horizontal and vertical offsets between navigation and attitude antennas and the SWATHplus transducers were applied during acquisition in the configuration files for the SWATHplus and Coda Octopus F180 software. Data were collected along approximately 3,920 kilometers (km) of tracklines spaced 75 to 100 m apart to obtain overlapping swaths of data and complete coverage of the seafloor (fig. 4). The average speed of the survey vessel was 5 knots (nautical miles per hour). The swath width collected by the SWATHplus system was adjusted based on the trackline spacing for the specific survey.

Differential Global Positioning System (DGPS) navigation was used to determine the horizontal position (x- and y-coordinates) of the GPS antenna mounted above the SWATHplus transducers with centimeter accuracy. The vertical height of the antenna was determined by real-time kinematic (RTK) GPS-corrected coordinates that were transmitted to the survey vessel from a base station established at the USGS Marine Operations Facility (MOF) in Falmouth, Massachusetts (fig. 1). Vertical water-level heights were referenced to Mean Lower Low Water (MLLW) datum using the value published for the tidal benchmark (National Ocean Service identification number 8447930) located in Woods Hole, Massachusetts. GPS calibration measurements were made at the vertical benchmarks and referenced to the MOF base station. SWATHplus acquisition software and the CARIS Hydrographic Information Processing Software (HIPS) were used to process the raw bathymetric soundings. Navigation data were inspected and edited to eliminate erroneous fixes. Soundings were adjusted using corrections from the motion reference unit (MRU), RTK-GPS water-level heights, and sound-velocity profile data. Spurious soundings were eliminated, and the final processed soundings were gridded at a resolution of 5-m per pixel (fig. 3). The bathymetric tracklines, 5-m depth grid, and 5-m hill-shaded grid are available in appendix 1.

Acoustic Backscatter

Approximately 340 km2 of acoustic backscatter data were acquired during the three surveys using a Klein 3000 dual-frequency sidescan sonar (132/445 kHz) that was towed approximately 20 m astern and 5 to 10 m above the seafloor. Navigation was recorded with a GPS antenna mounted on top of the acquisition van (surveys 2009-002-FA and 2011-004-FA) or on the pole directly above the SWATHplus (bathymetry system) transducers (survey 2010-004-FA). Horizontal offsets between the GPS antenna and the towed sidescan-sonar system, including the linear layback associated with the amount of cable out were measured and accounted for in the Klein SonarPro acquisition software. During most of the survey, sidescan-sonar data were acquired with a swath width of 200-m (100-m to either side of vessel). Approximately 3,570 km of data were collected along survey tracklines spaced 75 to 100 m apart to obtain overlapping swaths of data and complete coverage of the seafloor (fig. 5). For all three surveys, the transmit pulse was set to 50 microseconds (low frequency) and 25 microseconds (high frequency) with a 12-decibel (dB) fixed gain for both frequencies.

The low frequency (132-kHz) sonar data were processed using Xsonar/ShowImage software (Danforth, 1997) to correct for slant-range and beam-angle distortions. Each survey line was mapped into geographic space at 1-m pixel resolution, then imported into PCI Geomatics Geomatica software and combined into a single mosaic. The mosaic was exported as an 8-bit georeferenced tagged image file format (TIFF) image (fig. 6; appendix 1).

Seismic–Reflection Profiling

Approximately 3,800 km of chirp seismic-reflection data were collected in the Vineyard Sound survey area using an EdgeTech Geo-Star FSSB subbottom profiling system and an EdgeTech SB-0512i towfish (frequency modulation swept frequency 0.5-12 kHz), which was mounted on a catamaran and towed between 30 and 50 m astern of the survey vessels (fig 7). SonarWiz seismic-acquisition software was used to control the Geo-Star topside unit, digitally log trace data in the SEG-Y rev. 1 format (IEEE floating point), and record GPS navigation coordinates to the SEG-Y trace headers (in arcsecond of latitude and longitude, multiplied by a scalar of 100). During 2009-002-FA, data were acquired using a 0.12-second (s) shot rate, a 20-millisecond (ms) pulse length, and a 0.7- to 12-kHz frequency sweep, with trace lengths of approximately 66 m (1,250 samples per trace and 0.053-ms sample interval). During 2010-004-FA, files l113f1 through l115f1 were acquired using a 0.25-s shot rate, a 5-ms pulse length, and a 0.5- to 8-kHz frequency sweep, with recorded trace lengths of approximately 200 ms (4,340 samples per trace and 0.046-ms sample interval). The remaining files from 2010-004-FA (Julian days 141–155) were acquired using a 0.25-s shot rate, a 50-ms pulse length, and a 0.5- to 4.5-kHz frequency sweep, with recorded trace lengths of approximately 199 ms (4,328 samples per trace and 0.046-ms sample interval). During 2011-004-FA, data were acquired using a 0.25-s shot rate, a 5-ms pulse length, and a 0.5- to 8-kHz frequency sweep, with recorded trace lengths of approximately 200 ms (4,340 samples per trace and 0.046-ms sample interval). Traces were converted from two-way travel time to depth in meters, assuming a constant speed of sound in seawater of 1,500 meters per second (m/s).

Seismic-reflection data were processed using SIOSEIS (2011) and Seismic Unix (Stockwell and Cohen, 2008). All navigation data were extracted from trace headers, edited, and saved as ASCII text files. Water column portions of the traces were muted (2009-002-FA and 2010-004-FA data), and the effects of sea-surface heave were minimized (2010-004-FA data). A gain was applied to trace amplitudes using a time varying function (2009-002-FA and 2010-004-FA data) and normalized using automatic gain control. Profiles of the final processed trace data are included in this report as 8-bit, grayscale, variable-density plots in portable network graphic (png) format. The final shot-point and trackline navigation are available in appendix 1. Additional details about the acquisition and processing of seismic-reflection data can be found in the metadata for the trackline and shot-point spatial datasets and in the metadata for the png profile images in appendix 1.


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