Bathymetric data were collected using a SeaBeam 2112 (12 kHz) mulitbeam echo sounding system. The system, originally used for high accuracy (SeaBeam Classic) charting of the U.S. Exclusive Economic Zone (EEZ), is capable of hydrographic charting and seafloor acoustic backscatter imaging in water depths of 50 to 11,000 meters with up to 151 beams (2 degree transmit and receive). Swath coverage varies as a function of depth, from 150 degrees at 1,000 meters, to 120 degrees at 5,000 meters, and 90 degrees at 11,000 meters, with a resolution of two degrees. The swath of coverage on the ocean floor is approximately 75% of the water depth. The system operates at an acoustic frequency of 12 kHz and uses transducer arrays that are flush-mounted on the hull in a T-shaped configuration centered on the ship's keel. The horizontal positions for soundings were acquired with several Differential Global Positioning System (DGPS) receivers and one P-Code receiver: a Trimble Centurian P-code GPS, a Magnavox MX-200 GPS, and a Northstar 941x differential GPS. Horizontal sounding positions were recorded to raw multibeam echo sounder data files via SeaBeam's Sea Survey data acquisition software. A conservative estimate of the positional accuracy of the GPS and P-Code receivers is +/- 3 meters. The data pipeline included transferring the SeaBeam MB41 raw data files from the SGI computer to a processing laptop via file transfer protocol (FTP). The raw files were converted using CARIS 5.3 Hips Sips software. The processing crew maintained the same processing procedures as employed by NOAA hydrographic field units. Once the data were converted, a Digital Terrain Model was generated for visual detection of artifacts and missed depths. The next step entailed reviewing and editing the data with CARIS Swath Edit, followed with CARIS Sub Set mode editing. Both editing processes allowed the hydrographer to eliminate data points that were considered artifacts or out of context with the immediate benthic area. After editing, the weighted mean grid was re-generated with a grid resolution of 150 meters. No tide corrections were applied. Comparison of the present survey depth with depths obtained during cruise RB03-02 along overlapping portions of the lines, revealed that the new data were on average 100 meters shallower but that the difference was probably not constant. Despite the presence of SeaBeam engineers on board, it took 2 days to find the source of the problem. The new software version (1.2.7) that was installed on the ship in Madeira, contained an option called Apparent depth mode (see
http://pubs.usgs.gov/of/2006/1210/data/multibeam/raw_data/raw_data_RB0305/L3_Trip_summary_Aug-Sep_2003.pdf ). This option was turned on as a default. The apparent depth mode option takes the true depth of each beam and calculates the vertical travel time to the sea surface using the sound velocity profile that is in use at that time of acquisition. It then multiplies the travel time by a generic constant water speed of 1500 m/s to calculate an apparent depth, which is the depth that would have been measured by a standard depth finder. The true depth can be back-calculated as follows: (1) Vertical travel time = (apparent depth-ship's draft)/1500 (2) True depth = vertical travel time*true sound velocity profile+ ship's draft, where the ship's draft is 5.6 meters. These procedures were applied to this dataset and the resultant grid produced complies with previous surveys.
