The geophysical mapping utilized a suite of high-resolution instrumentation to map the surficial sediment distribution, depth and sub-surface geology: dual-frequency 100/500 KHz sidescan-sonar system, 234-KHz interferometric sonar, and 500 Hz -12 KHz chirp sub-bottom profiler. The survey was conducted aboard the M/V Megan Miller August 9-13, 2007. The study area covers 35 square kilometers from about 0.2 km to 5-km offshore of the south shore of Martha's Vineyard, and ranges in depth from ~ 5 to 20 meters.
Vertical and horizontal position of the SWATHplus system were acquired with Differential Global Positioning System (DGPS) navigation. DGPS data were acquired with the forward, or primary, Coda Octopus F180R Motion Reference Unit (MRU) DGPS antenna and transmitted via a network connection to the SWATHplus data collection software. The F180R MRU is part of the rigid sidemount used to deploy the SWATHplus bathymetric system, and is mounted on the bracket holding the SWATHplus transducers. The F180R MRU consists of 2 L1 antennas used for position and heading. The antennas are mounted on a rigid horizontal pole, 6.76 meters above the F180R MRU, offset in a forward/aft configuration (see 'source contribution' for a full description of the F180R). DGPS accuracy is 1 to 3 meters, depending on the distance from a US Coast Guard coastal repeater station (<http://www.navcen.uscg.gov/>). However, offsets between the winch used to deploy the sound velocity profiler and the DGPS antenna were not recorded. Therefore, water depths provided are intended to give a general depth at the sound velocity profile location (within 1 meter).
Swath bathymetric data were collected using an SEA Ltd SWATHplus 234-kHz Interferometric Bathymetric Sonar (<http://www.sea.co.uk/swathplus.aspx?nav=products> ) during survey operations. The SWATHplus system was mounted as a sidemount and positioned on the starboard side of the R/V Megan Miller. Line spacing varied from 40 meter in ~ 5 to 15 meter water depth to 70 meters in ~15 - 25 meter water depth. Data files were recorded by SEA software SWATHplus 3.05.19 in the SWATHplus SXR format. Data collection parameters are saved into a SWATHplus session file in SEA's SXS format that can be later used for data replay.
Pitch, roll, heading, and heave information were recorded using a Coda Octopus F180R motion reference unit (MRU) (see: <http://www.codaoctopus.com/motion/f180/index.asp>) and transmitted via network connection to the SWATHplus data collection software. The MRU sensor was mounted directly above the SWATHplus transducers, thereby minimizing "lever arm" offsets which can lead to positioning and depth errors in the bathymetric data. The F180R series MRU uses two L1 antennas for position and heading accuracy. These were mounted directly above the MRU with an offset of 6.76 meters. The antennas are mounted on a rigid horizontal pole with a separation of 1 meter and are offset from the MRU in a forward/aft configuration. The forward offset of the primary antenna from the MRU is 0.5 meters, with no port/starboard offset.
Sound Velocity Profiles (SVP): Fourteen sound velocity casts were collected at various intervals during the USGS cruise 07011 using an Applied Microsystems SV Plus v2 instrument (see <http://www.appliedmicrosystems.com/products/productDetails.aspx?id=1> for details). Sound velocity profiles (SVP) were generally collected at the eastern and western ends of the survey area, corresponding to the start and/or end of geophysical line data collection. This procedure was primarily followed to coordinate collection of SVP casts with the beginning or end of trackline. This was acceptable because only small variations in the speed of sound were noted throughout the survey area and negligible refraction artifacts were visible within the swath bathymetric data.
At each pre-defined station (i.e. location), the SV plus v2 was slowly lowered to the seafloor using a small shipboard winch. The SV plus v2 was monitored by a member of the science party and as the instrument made contact with the seafloor, the position (i.e. location) was recorded in the lab using HYPACK navigation software. The position was then manually recorded within the cruise log and SWATHplus acquisition software. Upon recovery, the SV plus was connected to an onboard computer via serial port in order to download data using the SV plus v2 communications software, SmartTalk.
Software: SV plus v2 SmartTalk (no version)
Software: Microsoft Excel 2004; Adobe Photoshop CS3 (10.0.1)
Software: AWK (no version)
Software: ArcGIS 9.2
Software: ArcGIS 9.2
Software: ArcGIS 9.2
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