Archive of Side Scan Sonar and Swath Bathymetry Data Collected During USGS Cruise 13CCT04 Offshore of Petit Bois Island, Gulf Islands National Seashore, Mississippi, August 2014

Swath Bathymetry Acquisition: The interferometric swath bathymetry data were collected aboard the RV Tommy Munro using a SEA SWATHplus-H 468 kHz interferometric sonar system mounted on the starboard side of the vessel. Boat position and motion data were recorded in real-time with 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 associated vessel motion. Real-time corrected positions were acquired with an 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 versions 3.7.17, with positional and calibration offsets pre-defined by a session file(.sxs), allowing for real-time- corrected depths. During the survey, all swath tracklines were recorded in SEA SWATHplus 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 SOS profiles (water surface to seafloor) at intervals throughout the survey.

Side Scan Sonar Acquisition: Side scan sonar was collected simulataneously as the interferometric data using the Klein 3900 dual-frequency side scan sonar system. It was towed off the port side of the RV Tommy Munro approximately 5 meters astern. SSS data were acquired and recorded using Klein SonarPro software (Klein Associates, Inc.) and manage cable out and layback offsets. Horizontal offset values between the side scan sonar towfish and the DGPS antennas were entered into Sonar Pro. The towfish motion was measured dynamically by internal sensors of the instrument, and towfish altitude (height from seafloor) was calculated by SonarPro. Data files were recorded in an Extended Triton Format (XTF).

Swath Bathymetry Processing: Position data recorded by the Coda-Octopus F190R IMU system were corrected in real time via the OmniSTAR HP differential navigation system. 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 ellipsoid height and position, representing the elevation of the seafloor with respect to the geodetic reference frame ITRF05 across the swath range. SWATHplus served as both an acquisition software and initial processing software. Preliminary roll calibration trackline data were collected and processed with Systems Engineering and Assessment Ltd SWATHplus and Grid Processor software version 3.7.17. Instrument offset and calibration values were entered into the session file (.sxs) and the raw data files (.sxr) were processed using the updated system configuration containing roll calibration values, measured equipment offsets, acquisition parameters, navigation and motion from the F190R, SOS at the sonar head, and SVP cast data. Any calibration offsets or acoustic filtering applied in SWATHplus was also written to the processed data file (.sxp). The initial real-time processing datum for the swath and backscatter data was ITRF05, which is the acquisition datum for OmniSTAR HP position and navigation data. All processed data files (.sxp) were imported into CARIS HIPS and SIPS version 8.1 and finalized in version 8.1.7. The NOAA tide zone model and associated verified six minute tide data were applied using the CARIS Load Tide tool. The total propogated uncertainty was calculated. The original sounding data were edited for outliers using the CARIS Swath Editor tool and associated depth filters. Remaining outliers were then deleted manually. A CARIS BASE (Bathymetry with Associated Statistical Error) surface with the associated CUBE (Combined Uncertainty and Bathymetry Estimator) sample surface was created from the edited soundings dataset. A BASE hypothesis is the estimated value of a grid node representing all the soundings within a chosen resolution or grid-cell size (for example, 2 m) weighted by uncertainty and proximity, giving the final value as a "sample" of the data within the specific grid cell. This algorithm allows multiple grid-node hypotheses to be verified or overridden by the user while maximizing processing efficiency. A 2-m resolution CUBE surface was created to perform initial CUBE hypothesis editing followed by a higher 1-m resolution BASE surface cleaning using the CARIS Subset Editor tool. The 2-m resolution BASE surface sample x,y,z data were exported as ASCII in the ellipsoid datum of ITRF05 and MLLW vertical datum.

Interferometric Backscatter Processing: The interferometric backscatter (IFB) was processed using SXPTools (build 175). SXPTools is a research project of the US Geological Survey that uses command line programs to enhance the quality of the backscatter from SEA SWATHplus processed data files using an empirical gain normalization scheme (Finlayson, USGS, unpub. data, 2009). The processed (.sxp) files were gridded using MBSystem (Version 5.3) and exported in ESRI ASCII Grid format. The grid was imported into ESRI's ArcGIS Version 10.2, converted to a raster file with the ASCII to Raster Tool. The final mosaic was exported in GeoTIFF format referenced to ITRF05 UTM Zone 16N.

Side Scan Sonar Processing: The side scan sonar acoustic bakscatter data from the Klein 3900 was imported into SonarWiz5 version 5.06, and bottom tracking was applied creating a slant range corrected record to define the seafloor 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. A backscatter mosaic was created from the individual SSS lines and adjusted for contrast and brightness. The final mosaic was exported in GeoTIFF format and imported into Esri's ArcGIS version 10.2 project, where the histogram was adjusted to enhance the display of seafloor surface material. Missing tracklines from the mosaic grid are a direct result of equipment issues with the Klein 3900 during the cruise. Data was either corrupt, noisy, or not collected in these areas

Datum transformation: Using the transformation software VDatum version 3.2, the interferometric swath bathymetry data were transformed horizontally from ITRF05 to the World Geodetic System of 1984 version G1150 (WGS84_G1150) and second transformed into the North American Datum of 1983 (NAD83) reference frame. The vertical datum was not transformed and remained in MLLW. The single text file (13CCT04_v2_IFB_04_2m_ITRF05_MLLW.txt) had to be broken down into six segments for VDatum conversion.

Gridding Bathymetric data: Using Esri ArcGIS, the swath soundings were imported into ESRI’s ArcMap version 10.1 and gridded using the Geostatistical Analyst Tool's "radial basis function". This allowed the user to adjust interpolation parameters with regard to real data spatial resolution and orientation when predicting values. This method provided for a less biased representation of the dataset as a whole. A cross-validation report can be generated when using the radial basis function that helps the user understand how well the model will predict data values at locations without data points. In general the cross-validation takes one point and predicts its position using the weighted surrounding data values. This predicted value is compared against the actual value of that data point and general statistics are computed. The validation report for the swath bathymetry 50-m grid is listed below. The radial basis function surface was exproted to a raster file. A bounding polygon representing the extent of survey tracklines was created and converted into a raster mask using the ArcGIS "polygon to raster" conversion tool. The DEM was then clipped to the raster mask using the ArcGIS Spatial Analyst "extract by raster mask" tool. It is common, in large-area, shallow water surveys, to find wide data gaps between tracklines relative to the swath width surveyed. To minimize this concern in the final DEM, the ArcGIS Spatial Analyst "neighborhood" low-pass raster- data filter was applied. The raster image was then exported as a GeoTIFF.

Error: A comparison of the DEM versus the sounding (x,y,z point data) was plotted to evaluate how well the DEM represented the original sounding data quantitatively and spatially. This was done using the error results from the cross validation report that is produced from the radial basis function process. The validation reports the measured value which is the x,y,z point data, the predicted value from the surface, and error value which is the differece between the two values. From this data the root mean square error (RMS) was calculated. The difference between the DEM and the original sounding values were plotted, color coded, and placed against the backdrop of the DEM. This provided a visual aspect of the spatial representation of error.

DEM conversion from Raster to Point: The 50-m DEM was converted to a point shapefile using Esri's ArcMap "Raster to Point" conversion tool.

DEM point shapefile conversion to text file: The point shapefile was then exported to a text file.

Trackline Map Creation: The tracklines were exported from HYPACK in Keyhole Markup Language (KML) format and imported into ESRI ArcGIS version 10.1. In ESRI ArcGIS, the trackline data were projected from the GPS acquisition datum (World Geodetic System of 1984, WGS84 G1150) to the North American Datum of 1983 (NAD83), Universal Transverse Mercator (UTM) Zone 16 North and saved to ESRI shapefile format.

Added keywords section with USGS persistent identifier as theme keyword.