1-Meter Sample Resolution Interferometric Swath Backscatter Data Collected in 2012 from the Chandeleur Islands, Louisiana (U.S. Geological Survey Field Activity Number 12BIM03)

Swath Bathymetry Acquisition: The interferometric swath bathymetry data were collected aboard the R/V SurveyCat using a SEA SWATHplus-m 234 kHz interferometric sonar system mounted on a sled attached to a rail system, fastened between the catamaran halls allowing the instrument to align directly below the GPS antennae to minimize geometry errors. Boat position and motion data were recorded in real-time using a CodaOctopus F190R wetpod inertial measurement unit (IMU) mounted underwater between the transducer heads to minimize lever arm geometry errors between the observed depths and vessel motion. Real-time corrected positions were acquired via the OmniSTAR HP (High-Precision differential global navigation satellite system) satellite constellation subscription. OmniSTAR HP position correction data and motion data from the IMU were integrated with interferometric soundings in the SWATHplus software package, with positional and calibration offsets pre-defined by a session file (.sxs), allowing for real-time- corrected depths. Prior to deployment, all equipment offsets were surveyed in dry dock with the use of a laser total station. During the survey all swath data were recorded in SEA raw data format (.sxr). A Valeport Mini Sound Velocity Sensor (SVS) was attached to the transducer mount and collected continuous speed of sound (SOS) measurements at the depth of the transducers. These values were directly read and incorporated into the SWATHplus acquisition software giving real-time speed of sound at the transducer while underway. In addition, a separate sound velocity profiler (Valeport miniSVP) was used to collect speed of sound profiles (water surface to seafloor) at intervals throughout the survey.

Interferometric Swath Bathymetry Acquisition and Processing: Position data recorded by the Coda-Octopus F190R IMU system were corrected in real time via the OmniSTAR HP differential navigation service. The IMU applied real-time motion corrections for heave, roll, and pitch to the vertical component of each position fix. The corrected positions were integrated with the observed bathymetric values to calculate a final position and ellipsoid height representing the elevation of the seafloor with respect to the geodetic reference frame ITRF2005 (ITRF05) across the swath range. The Systems Engineering and Assessment Ltd program SWATHplus serves as both an acquisition software and initial processing software. Preliminary roll calibration data were collected and processed using SWATHplus and Grid Processor software versions 3.7.17. Instrument offset and calibration values were input into the session file (.sxs). The raw data files (.sxr) were then collected and processed with the updated system configuration file, which contained roll calibration values, measured equipment offsets, acquisition parameters, navigation parameters, speed of sound (SOS) at sonar head, and the SVP cast data. Any acoustic filtering applied in SWATHplus was also written to the processed data file (.sxp). The initial datum for the swath and backscatter data was ITRF05, which is the real-time acquisition datum of OmniSTAR HP position and navigation data.

Acoustic backscatter analysis: The same raw data files (.sxr) were imported into SonarWiz5® version 5.06, and bottom tracking was applied to define the first returns from the seafloor through the water column directly below the swath transducers. Once the seafloor was defined, sonar beam angle correction was applied to remove the water column from the data and minimize target shadows in the data. A comprehensive backscatter mosaic was created from the individual swath lines and was adjusted for contrast and brightness. The final mosaic was exported in GeoTIFF format and imported into the ArcGIS project where the histogram was adjusted to enhance the display of seafloor surface.

Added keywords section with USGS persistent identifier as theme keyword.