Coastal Multibeam Bathymetry and Backscatter Data Collected in June 2024 From Wallops and Assawoman Islands, Virginia

Acquisition: An Ellipsoidally Referenced Survey (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 2023 Q3) managed data inputs from sensors and produced two files (.RAW and .HSX). Data were collected in WGS84 (G2139) Universal Transverse Mercator (UTM) Zone 18 North (18N) coordinates.

Processing: Positioning and attitude data were post-processed in Applanix POSPac MMS (POSPac) software version 9.1 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 Qimera® Hydrographic processing software version 2.6.2. HYPACK® acquisition files (.HSX) were first imported into Qimera® and then post-processed navigation files (SBET and SMRMSG) were imported and applied. Additionally, Global Positioning System (GPS) tide (GPS tidal corrections utilize GPS height information from real time survey observations to obtain an ellipsoid height) was computed and sound velocity profiles were applied (IHO, 2019). Roll calibration was also performed, as necessary, and values were applied to the data (QPS, 2021). CUBE Uncertainty “layer reflects the standard deviation, scaled to 95% confidence level, of the accepted soundings that contributed to the selected hypothesis.” (QPS, 2021). 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 Total Propagated Uncertainty (TPU) greater than stated IHO Special Order standards (IHO, 2008). Refraction artifacts were addressed automatically using the Technical University of Delft (TU Delft) sound-speed inversion tool (Beaudoin and others, 2018) and manually using the Refraction Editor filter. Further manual editing of data points that deviated significantly from the surrounding surface was performed in Slice Editor (QPS, 2021) and a 1-m bathymetric CUBE surface was computed (Calder and Wells, 2007). Small gaps in the continuous surface were caused by limitations in swath width and planned line navigation within shallow waters. Large data gaps in the southern portion of the survey were the result of a 65-degree beam angle filter applied to this section to remove erroneous data. The final surface was exported as an x,y,z ASCII file using the Dynamic CUBE Surface to ASCII function.

Backscatter Mosaic Creation: Acoustic Intensity Backscatter data were extracted from HSX files in Qimera® Hydrographic processing software version 2.6.2. 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. GSF files were exported from Qimera® and imported into FMGT for backscatter mosaic creation. These inputs, along with the following settings were entered into FMGT, 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 (QPS, 2021). Upon completion, a 1-m backscatter mosaic with intensity values ranging from 9.6875 to -65.625 dB (decibels) was created, and the mosaic was exported as a 32-bit floating point GeoTIFF (.tiff) using the 'Export Floating Point GeoTIFF' function.

Datum Transformation: Final x,y,z data were converted using NOAA's VDatum v.4.7 software conversion tool from the WGS84 [G2139] realization, in meters, to NAD83 [2011] Virginia South State Plane Coordinate System NAVD88, in US survey feet, using the GEOID18 model (orthometric height).