Archive of Ground Penetrating Radar and Differential Global Positioning System Data Collected in April 2016 from Fire Island, New York

Navigation data acquisition-Position and elevation data for each GPR line were recorded at the time of collection using an Ashtech DGPS receiver and geodetic antenna. DGPS data were recorded concurrently throughout the survey using a similar instrument combination (Ashtech receiver and Thales choke ring antenna) at a base station set up on the National Park Service (NPS) Robins Rest benchmark (REST). The rover unit was mounted 1.302 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 1-second (s) intervals using coordinated universal time (UTC) stamps and saved as a .plt data file.

GPR data acquisition-A total of 90 GPR lines, representing a linear distance of approximately 21 kilometers (km), were acquired during field activity 2016-322-FA from beach as well as developed and undeveloped back-barrier environments. Data were collected from three study sites that were considered representative of the western (Site 1), central (Site 2), and eastern (Site 3) Fire Island geomorphic zones. At sites 1 and 2, back-barrier data were collected along beach access roads and established roads within and connecting local communities. At site 3, 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 RADAN 7 proprietary software in TerraSIRch mode and saved in Data Zero Time (DZT) format. Acquisition settings were adjusted when necessitated by changes to the subsurface geology or surrounding topography/surface conditions and were recorded in the Field Activity Collection System (FACS) logs.

Navigation processing-Base station data were post-processed through the National Geodetic Survey (NGS) On-Line Positioning User Service (OPUS). The time-weighted position calculated from all base-station occupations did not differ significantly from the NPS control coordinates; therefore, the control coordinates were used for post-processing. The base station coordinates were imported into GrafNav, versions 8.5 and 8.7 (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, DGPS positions were computed at 1-s intervals for each rover GPS session and 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. The GPS data were acquired and processed in the World Geodetic System of 1984 (WGS84) (G1150) geodetic datum. Along some GPR profiles that were collected within communities with closely spaced buildings or in coastal forests, the satellite signals were obscured, and post-processed GPS positions could not be computed for every epoch. Gaps greater than about 15 m in length were filled by interpolating elevations from NOAA lidar data collected in 2014. The final navigation files provided in Forde and others (2018) were exported to the North American Datum of 1983 (NAD83) (2011) and North American Vertical Datum of 1988 (NAVD88), derived using the GEIOD12A geodetic model. Trackline files were created from the post-processed GPS data using the Points to Line tool in Esri ArcGIS version 10.3.1.

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 Radan’s 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. 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 DGPS 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 selected profiles to estimate the radar-wave velocities through the sediment. Calculated velocities (N=30) ranged from 0.06 to 0.2 meters per nanosecond (m/ns) and averaged 0.094 m/ns. The processed profile data were re-imported into RADAN 7 and the surface normalization processing algorithm was applied, adjusting the profile to both the measured terrain and the site-specific, averaged radar-wave velocity. Finally, elevation- and velocity-corrected profiles were exported as Joint Photographic Experts Group (JPEG) images.

Added keywords section with USGS persistent identifier as theme keyword.