Ground Penetrating Radar and Global Positioning System Data Collected from Fire Island, New York, March-April 2021

Navigation data acquisition - Position and elevation data for each GPR line were recorded at the time of collection using a Spectra Precision SP90M GPS receiver and geodetic antenna. GPS data were recorded concurrently throughout the survey using a Trimble R-10 receiver and antenna at a base station set up on local benchmarks at Bellport Beach (designated U374) and Robins Rest (designated REST) for the HHB and POW surveys, respectively. The rover unit was mounted 1.320 m above the GPR antenna and was connected to the GPR's digital control unit. Raw GPS positions were output to the GPR control unit in real time as a National Marine Electronics Association (NMEA) GGA string at 0.5-second (s) intervals using coordinated universal time (UTC) stamps and saved as a .PLT data file.

GPR data acquisition - A total of 25 GPR lines (lines 001-013 for HHB; 014-019, 021-026 for POW), representing a linear distance of approximately 12.5 km, were acquired during USGS FAN 2021-312-FA within developed and undeveloped back-barrier environments at the HHB and POW study sites. At POW, data were collected along beach accesses and established roads within and east of the local community. At HHB, which is located entirely within the Otis Pike Wilderness Area, data were collected along washover fans and along hiking trails through back-barrier scrub/shrub vegetation. The GPR data were collected with a Geophysical Survey Systems, Inc. (GSSI) TerraSIRch SIR System-3000, which acquires single-channel GPR data and includes a Digital Control Unit (model DC-3000) and 200-Megahertz (MHz) antenna. A 16-inch survey wheel (GSSI model 620) was attached to the back of the antenna and calibrated prior to surveying to ensure an accurate revolution-of-wheel to distance ratio. Distance mode was used to record the raw GPR data and the raw GGA position string was recorded in real time via an Acumen SDR data logger. GPR traces were collected at a maximum rate of 64 scans per second with a vertical resolution of 1,024 samples per scan and an Infinite Impulse Response (IIR) filter was used (Lowpass = 600 MHz, Highpass = 50 MHz) to increase the signal-to-noise ratio of the recorded GPR data. Data were acquired using the SIR 3000 in TerraSIRch mode and saved in GSSI's proprietary Data Zero Time (DZT) format.

Navigation processing - Base station data were post-processed through the National Geodetic Survey (NGS) Online Positioning User Service (OPUS, https://geodesy.noaa.gov/OPUS/). The monument for NGS mark U374 at Bellport Beach was damaged at the time of the survey; therefore, the time-weighted coordinates calculated from all base-station occupations at U374 were used for post-processing. The time-weighted position calculated from all base-station occupations at REST did not differ significantly from the National Park Service control coordinates; therefore, the control coordinates were used for post-processing. The base station coordinates were imported into GrafNav, version 8.9 (NovAtel Waypoint Product Group), and the data from the rover GPS were post-processed to the concurrent base-station session data. The final, differentially corrected, GPS positions were computed at 0.5-s intervals for each rover GPS session. Along lines 015, 021, 022, and 025 (POW), the post-processed navigation data was exported in American Standard Code for Information Interchange (ASCII) text format as a NMEA GGA string, which replaced the uncorrected real-time rover positions recorded during acquisition (Forde and others, 2018). Along most GPR lines; however, there were significant dropouts (possibly attributed to atmospheric conditions and (or) the height of the back-barrier vegetation relative to the height of the rover antenna) that resulted in large gaps in the post-processed navigation data. Along these lines, elevations at the raw rover x,y coordinates were interpolated from lidar data collected in January 2020 by the USACE (USACE JALBTCX, 2020) using Esri ArcMap version 10.8.1. Comparison of post-processed and lidar elevation profiles along each GPR line showed good agreement except in beach and dune environments where the Fire Island coastline had eroded landward between 2020 and 2021. Therefore, for lines 001, 002, 005-011 (HHB), 014, 016-019, 023, 024, and 026 (POW), the elevation values in the raw navigation string were replaced with post-processed (beach and dune environments) or lidar (everywhere else) elevations; the NMEA checksum string was recalculated for each navigation string in Matlab version R2021A. Processed navigation data are provided in the North American Datum of 1983 (NAD83) (2011) coordinate system and processed orthometric elevations are reported relative to the North American Vertical Datum of 1988 (NAVD88), derived using the GEOID12B geodetic model. The final navigation files provided in Forde and others (2022) were saved as ASCII text format (.gga) data files.

GPR processing - Reflexw Version 7.2.2 (Sandmeier Scientific Software) geophysical near-surface processing and interpretation software was used to process the GPR data. GPR data were acquired in DZT format and later imported into Reflexw, where they were converted into a DAT file. For archival purposes, a non-proprietary version of the raw data was created by exporting the DAT file from Reflexw and saving it in ASCII 4-column format (.asc). The data were processed in a consistent order: (1) static correction was applied; (2) the mean value was subtracted (dewowed); (3) header gain applied during acquisition was removed; (4) manual Automatic Gain Control (AGC) gain was applied; and (5) post-processed GPS data were imported into the trace headers. Data were visually inspected after each step listed above and before elevation-corrected profiles were exported; all profiles were analyzed for errors or data gaps in the navigation and trace data to ensure data quality was maintained throughout. Hyperbola analyses were performed on all profiles to estimate the radar-wave velocities through the sediment. Calculated velocities at HHB (N=13) ranged from 0.06 to 0.20 meters per nanosecond (m/ns) and averaged 0.11 m/ns. Calculated velocities at POW (N=12) ranged from 0.08 to 0.35 meters per nanosecond (m/ns) and averaged 0.19 m/ns. The processed profile data were re-imported into GSSI RADAN version 7.5.18 software and the surface normalization processing algorithm was applied, adjusting the profile to both the measured terrain and a pre-determined site-specific radar-wave velocity. To facilitate comparison with data collected in 2016, radar-wave velocities of 0.08 m/ns (HHB) and 0.10 m/ns (POW) were applied based on the results of hyperbola analyses reported by Forde and others (2018). The elevation- and velocity-corrected profiles are provided as DZT-formatted files and were also exported as Joint Photographic Experts Group (JPEG) images. Processed GPR profile trackline files were exported from RADAN 7 in Keyhole Markup Language (KML) format, projected to the Universal Transverse Mercator (UTM) 18 North (18N) coordinate system, and appended with profile-specific data acquisition and processing parameters in Esri ArcMap version 10.8.1.