• Geologic setting
  
• Survey Methods

Survey Methods

Multibeam bathymetry and backscatter data were acquired using the SeaBeam 2112 multibeam echo sounder. The 12KHz system uses up to 150 beams covering approximately 150 degrees swath width at depths less than 1,000 meters, and 120 degrees swath width for depths up to 5,000 meters. The swath width decreases to approximately 90 degrees for depths greater than 5,000 meters.

The operation of the SeaBeam system is dependent upon the depth of the water, i.e. the deeper the water, the longer the pulse width, the slower the ping rate, and the narrower the swath width. The multibeam swath width in the deeper regions was between 10,000 meters and 12,000 meters ( 5.3 nautical miles to 6.5 nautical miles). This multibeam swath coverage allowed the survey line spacing to be established at 6 nautical mile intervals with an east - west azimuth. The pulse width ranged between 7 ms and 20 ms, with ping intervals varying between 8 seconds to 20 seconds.The SeaBeam system recorded the raw data files to a Silicon Graphics Incorporated (SGI) Origin 2 computer.The data files were automatically opened and closed with a 6 Mb limit.This enabled adequate and efficient data management procedures.The total file size for the processed 2003 SeaBeam bathymetric data was approximately 557 Mb.

Sound Velocity

Sound velocity corrections were required by the SeaBeam acquisition system to account for water column temperature and salinity variations.The cast data were derived from three separate sources.A deep water Conductivity Temperature and Density (CTD) cast was obtained prior to the start of SeaBeam data collection.A Seabird Electronic Model 9/11 Conductivity Temperature and Density probe was used for obtaining CTD cast data.The maximum depth obtained with this instrument was 3,000 meters.The Levitus Tables were used to supplement the cast data with velocity values for water depths between 3,000 meters and 9,000 meters. Levitus Tables represent the annual average water velocity structure for a one degree by one degree area centered at -66.5 degrees longitude and 19.5 degrees  latitude. This water velocity profile is in the form of discrete (depth, velocity) points where the depth is in meters and the velocity in meters per second. For additional information on how the profile was used see the Excel spreadsheet SVP_2003 in the data\svp directory.Additional speed of sound data were recorded using the Sippican Expendable Bathythermograph (XBT).XBT c ast data were limited to range of 760 meters to 1365 meters depth.A Seabird Electronic Model SBE-21 Thermosalinograph (TSG) was constantly monitored for surface sound velocity values.Monitoring the TSG values enabled the scientific crew to determine when additional sound velocity data were required.   Sound velocityvalues were entered in the acquisition system for real time application with beam forming and beam steering capabilities.Variations in sound speed due to temperature and salinity differences within the water column affect the ray path and two way travel time of each formed beam from the SeaBeam transducer to the sea floor. The correct sound velocity variations are thus required to account for any "bending" of the ray path, since this will affect the final depth measurement of each formed beam.No evidence of multibeam swath cupping or frowning was evident during data acquisition and processing.

Data Processing

Data processing was performed by NOAA hydrographers.The data pipeline included transferring the SeaBeam MB41 raw data files from the SGI computer to a processing laptop via file transfer protocol (ftp).Once the transfer was complete, the raw file was converted using CARIS 5.3 Hips Sips software.The processing crew (NOAA hydrographers) maintained the same processing procedures as employed byNOAA hydrographic field units.  Once the data were converted, a Digital Terrain Model (DTM) or dirty DTM (has not been cleaned for noisy data) was generated for visual detection of artifacts and missed depths.The next step entailedreviewing and editing the data with CARIS Swath Edit, followed with CARIS Sub Set mode editing.Both editing processes allowed the hydrographer to eliminate data artifacts or outliers.

After data editing, the weighted mean grid (or DTM) was re-generated with a grid resolution of 150 and 200 meters.A colorized by depth and grey toned surface model was exported as a TIF image and submitted to USGS team members. The 150 meter weighted grid was also exported as an ASCII XYZ data set that was converted to a gridded UTM (prt03final.gutm) file which can be read by GeoZui3D. GeoZui3D is a highly interactive 3D Visualization system developed in the Data Visualization Research Lab at the Center for Coastal and Ocean Mapping at the University of New Hampshire. It is designed to support research by providing support for innovative data visualization. It is also intended to be a practical and useful tool for fusing multiple sources of georeferenced Data. GeoZui3D, CARIS 3D Viewer, and IVS Fledermaus were used for visual data interpretation by USGS geologists.The exported ASCII XYZ data set was also converted into a MapInfo Vertical Mapper Grid that was used to generate a 150 meter contour table.

Final data products submitted to USGS team members included the 150 meter resolution, weighted grid image files, both colorized by depth with sun illumination and gray tone surface model; 500 meter interval contour file; backscatter mosaics with 25 and 75 meter resolution; GeoZui3D ASCII gutm grid file with 150 meter grid interval.The image files were used in a project poster that described the results and interpretations of the geologic/bathymetric survey and are included in the ArcView GIS project file.