Coastal Bathymetry and Backscatter Data Collected in June-August 2023 From the Chandeleur Islands, Louisiana

Acquisition: An ERS consisting of multibeam bathymetry was conducted using two Teledyne Reson SeaBat T50-P multibeam echosounders, in dual-head configuration. The pair of Mills Cross (configuration) transmit and receive arrays, were placed side-by-side within a bracket that oriented them at opposing 30-degree angles (relative to horizontal). The bracket was pole-mounted on the starboard gunnel of the research vessel (R/V) Sallenger, a 26-foot fiberglass vessel. The sonar arrays were oriented athwartships (primary and secondary arrays facing outward and down to port and starboard, respectively) and located approximately 1-m below the waterline when deployed. An Applanix Pos MV Wavemaster II V5 inertially aided navigation system was used for timing, positioning, and attitude. The Inertial Measurement Unit (IMU) was mounted atop the sonar bracket just aft of the pole. Two Trimble GA830 Global Navigation Satellite System (GNSS) antennas were affixed atop the cabin approximately amidships of the vessel, two meters apart, port-starboard. An AML Oceanographic Micro-X sound velocity sensor was used for speed of sound readings at the sonar head. A Sontek CastAway CTD was used for measuring sound velocity profiles. Hydrographic survey acquisition and processing software, HYPACK® and HYPACK HYSWEEP® (version 22.3.1.0) managed data inputs from sensors and produced two files (.RAW and .HSX). Data were collected in WGS84 (G1150) Universal Transverse Mercator (UTM) Zone 16 North (16N) coordinates. Multibeam and backscatter data were collected in three areas: to the east of Hewes Point (referred to as 'MainAOI'), a portion of the single-beam survey coverage (Lyons and others, 2025) north and west of Hewes Point ('SBESCoverage'), and offshore of the southernmost Chandeleur Islands ('SouthernRecon'). Multibeam coverage was extended outside of the main AOI to ensure complete coverage of the survey area during the survey period (west, single-beam survey) and to support geophysical data collection (SouthernRecon).

Processing: Positioning and attitude data were post-processed in Applanix POSPac MMS (POSPac) software version 8.8 Service Pack 1. Raw position files were imported into POSPac and processed with the IN-Fusion PP-RTX processing mode utilizing Trimble CenterPoint RTX technology (Scherzinger and Hutton, 2025), creating smoothed best estimate of trajectory (SBET) files and dynamic error files (SMRMSG) for each day of the survey. Bathymetric data were post-processed in Teledyne CARIS™ Hydrographic processing software HIPS and SIPS version 11.4.28. HYPACK® acquisition files (.HSX) were first imported into CARIS™ and then post-processed navigation files (SBET and SMRMSG) were imported and applied (Caris, 2015). Additionally, Global Positioning System (GPS) tide (GPS tidal corrections utilize GPS height information from real time survey observations to obtain an ellipsoid height) were applied (IHO, 2019). Sound Velocity Profiles (SVPs) were edited in the SVP editor where all observations from the water line to the sonar heads (1 m water depth) were rejected along with any outlying data. After editing, the SVPs were applied to the sounding data. Roll calibration was also performed, as necessary. TPU was calculated from real time values recorded in the HSX and the SMRMSG files (IHO, 2019). Data were filtered to exclude any points with RESON Quality flags of 0, 1, and 2 (these flags are assigned to individual soundings at the time of acquisition and are a metric of pass/fail in regard to both brightness and collinearity: 0 = fail both, 1 = pass brightness only, 2 = pass collinearity only), as well as soundings that had a TPU greater than stated IHO Special Order standards (IHO, 2008). Refraction artifacts were addressed using Refraction Editor. Further editing was performed in Subset Editor. A 1-m bathymetric CUBE surface was computed and a visual and statistical inspection of the data effecting the surface was performed and edited as needed to serve as quality control (Calder and Wells, 2007). Gaps in the continuous surface were caused by limitations in swath width and planned line navigation within shallow waters. The final surface was exported as an x,y,z ASCII file using the Base Surface to ASCII function.

Acoustic Intensity Backscatter data (.7k) were loaded in CARIS Hydrographic processing software HIPS/SIPS version 11.4.28 using the SIPS Backscatter tool. The final CUBE bathymetry surface serves as input for geometric corrections and average temperature and salinity data (derived from the Sontek CastAway CTD measurements taken during acquisition) are utilized by the program to calculate local absorption and transmission loss. These inputs, along with the following settings were entered into the tool, and then processing was initiated [Weighted Average = blending, image type = beam average, weighting = swath, and an Angle-Varying gain = 200]. During processing, the tool automatically creates a pattern file (.bbp) which is used to correct for variations in the beam pattern of the system. Upon completion, a 1-m backscatter mosaic with intensity values ranging from -17.63 to 3.156 dB (decibels) was created for each survey area. After visual inspection, the mosaic was exported as a 32-bit float GeoTIFF (.tiff) using the surface to GeoTIFF function.

Datum Transformation: Final x,y,z data were converted using NOAA's VDatum v.4.2.2 software conversion tool from the WGS84 [G1150] realization to NAD83 [2011] NAVD88, using the GEOID18 model (orthometric height).