Equipment Used: NOAA Ship Okeanos Explorer is equipped with a 26 kilohertz (kHz) Kongsberg EM 304 MKII multibeam sonar. The nominal transmit (TX) alongtrack beamwidth is 0.5°, and the nominal receive (RX) acrosstrack beamwidth is 1.0°. The system generates a 150° beam fan, containing 512 beams with up to 800 soundings per ping cycle when in high-density mode. In waters shallower than approximately 3,300 m the system is able to operate in dual-swath mode, where one nominal ping cycle includes two swaths, resulting in up to 1,600 soundings. Data are recorded using Kongsberg's Seafloor Information System (SIS) software. Collocated to the bathymetric data, bottom backscatter data were collected and stored within the raw files, both as beam-averaged backscatter values, and as full-time series values (snippets) within each beam. During standard data acquisition, the EM 304 multibeam sonar is synchronized with the other active sonars using the Kongsberg Synchronization Unit with the EM 304 multibeam sonar set as the master. Any changes in equipment setup for the year or expedition are detailed in the annual Readiness Report or associated Expedition Report, respectively. For general information about sub-bottom operations, please refer to the NOAA Ocean Exploration Mapping Procedures Manual. Calibrations: At the beginning of each field season, a multibeam geometric calibration (patch test) is conducted to resolve any angular misalignments of the EM 304 multibeam equipment. A patch test is also conducted if any multibeam equipment (e.g., transducers, IMU, antennas) is installed or disturbed. The patch test determines if there are any residual biases or errors in navigation timing, pitch, roll, and heading/yaw (and resolves each bias individually in that order). Whenever possible (and assuming reasonable values), the results of each test are applied in SIS prior to data collection for the following test. Calibration Reports are archived as supplemental documents to the annual Readiness Report throughout the year. A relative backscatter correction was performed in 2021, and the resulting gain values were uploaded to the processing unit. This procedure helps to normalize differences in backscatter values resulting from variable frequencies and pulse durations employed within sectors and among ping modes used during multibeam data acquisition.
Acquisition Corrections: Real-time corrections to the data upon acquisition include the continuous application of surface sound speed obtained with a hull-mounted Reson SV-70 probe, and application of water column sound speed profiles obtained with Sippican Deep Blue Expendable Bathythermographs (XBTs) and/or Seabird CTD 9/11. Sound speed profiles are conducted every four hours, or more frequently as dictated by local oceanographic conditions (typically every two hours when operating in more dynamic areas). Reson sound speed values are constantly compared against secondarily derived sound speed values from the ship’s onboard thermosalinograph flow-through system as a quality assurance measure. Roll, pitch, and heave motion corrections are applied in real-time via a POS MV 320 version 5 or a Seapath-380, using Marine Star DGPS correctors. The motion and positioning unit used will be noted in the processing logs. No tidal corrections are applied to the raw or processed data. Multibeam data quality is monitored in real-time by acquisition watchstanders. Ship speed is adjusted to maintain data quality and sounding density as necessary. Line spacing is planned to ensure one-quarter to one-third swath-width overlap between lines, depending on the environmental conditions and impact on the quality of the outer swath regions. Angles are generally left open (70°/70°) during transits to maximize data collection and are adjusted on both the port and starboard sides to ensure the best data quality and coverage. If outer beams are returning obviously spurious soundings (e.g., due to attenuation or low grazing angle), beam angles are gradually reduced and monitored closely until a high-quality swath was obtained. Processing Steps: The full-resolution multibeam .kmall files are imported into QPS Qimera, and then processed and cleaned of noise and artifacts. Outlier soundings are removed using multiple methods including automatic filtering and/or manual cleaning with the swath and subset editing tools. The default sound speed scheduling method is “Nearest-in-Time; SVP Crossfade 60 sec.” If another method was implemented, it will be noted in the associated log. Data Product Creation Steps: Gridded digital terrain models were created using the weighted moving average algorithm and were exported in multiple formats using QPS Fledermaus software. Some expeditions have several final multibeam grid files in order to keep file sizes manageable and to focus on particular survey areas of interest. The final surfaces are re-projected to the field geographic WGS84 reference frame in QPS Fledermaus software, saved as a .sd file, and then exported to multiple formats (ASCII XYZ text file (.xyz), color image .tif, floating point .tif, and Google Earth .kmz file formats). The .gsf files are used to create daily backscatter mosaics using QPS FMGT. Horizontal Datum: WGS84 Vertical Datum: Data are referenced to the waterline using surveyed vessel offsets and static draft measurements. Software Versions: The version for any software used is noted in the associated Expedition Report. Data Format: Raw data (Level-00) are archived in .kmall format. Processed files (Level-01) are archived as .gsf files. Bathymetry grids (Level-02) are archived as .xyz, color .tif, floating point .tif, .kmz, and .sd. Backscatter mosaics are archived as .sd and .tif formats. Weather, Watchstanding, Processing, and Data Package Logs (.xls) are archived. There is a complete accounting of each individually archived multibeam data file and of each bathymetric surface product in the multibeam data acquisition and processing logs archived with the dataset. Contact: Please do not hesitate to contact NOAA Ocean Exploration (oar.oer.exmappingteam@noaa.gov) with any questions regarding these files. If you are interested in downloading the raw data (Level-00) or cleaned/edited data (Level-01), you can access those data from the NOAA National Centers for Environmental Information for geophysical data https://www.ncei.noaa.gov/products/seafloor-mapping. For questions or assistance, NOAA Ocean Exploration’s Data Management Team can be reached at oer.info.mgmt@noaa.gov.