Archive of Chirp Sub-Bottom Profile, Imagery, and Navigational Data Collected During USGS Field Activity Numbers 2021-326-FA and 2022-326-FA in 2021 and 2022 from Duck, North Carolina

Chirp acquisition - Sub-bottom data were acquired from one survey platform that utilized an EdgeTech 512i portable sub-bottom profiling (SBP) system. The seismic system was towed on the starboard side of the 35-foot research vessel, the USACE LARC, in a catamaran sled so the transducers were 0.91 m below the water’s surface. The vessel and towing configuration allowed seismic data to be collected in < 0.5 m water depth and up the lower beach face (Locker and others, 2017). In total, 60 lines were surveyed. The seismic source utilized during the DUNEX surveys consisted of an EdgeTech 512i towfish running Chesapeake Technology SonarWiz version 5.05 acquisition software and towed on the starboard side and aft of the RTK-GPS antenna (acquisition_gps.tif; fig. 4, Locker and others (2017). The data were acquired in SEG-Y format at 5 hertz (Hz), which equates to one ping per 0.20 seconds. The frequency sweep was 0.7-12 kHz, pulse length 20 ms, and recording length was 46.5 ms. Based on survey speeds of 3.5-4.5 knots, the shot spacing was roughly 0.450 m. During acquisition, positions from the antenna were recorded and written to the seismic trace headers in arcseconds every 0.20 s. The binary portion of the seismic data provided in the data release is stored in SEG-Y rev. 1, IEEE 32-bit float [big-endian byte order] format, which is a standard digital format that can be read and manipulated by most seismic processing software packages; the first 3,200 bytes of the file header are in ASCII format instead of Extended Binary Coded Decimal Interchange Code (EBCDIC) format. The SEG-Y formatted envelope trace files have a .sgy extension and may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). The printable profiles (provided in .gif format) are chirp sub-bottom profile images that were processed using SU software. Survey Field Activity Collection System (FACS) logs and acquisition geometry figure are also provided as supplemental information in the download file, DUNEX_logs.zip.

Chirp processing and image creation - The EdgeTech 512i system recorded the sub-bottom data in SEG-Y rev. 1 format. Enveloped SEG-Y trace data were processed with custom scripts and SU Release 44 (Cohen and Stockwell, 2010) software to produce gained Graphics Interchange Format (GIF) images of the sub-bottom profiles included in the data release. A representative chirp data processing sequence consisted of (1) removing raw [vessel] navigation data for each shot and converting the SEG-Y file to SU format, (2) setting the initial FFID value (trace header bytes 9-12) to one before incrementally increasing it by 1 and then applying automatic gain control to the profile, (3) trimming the trace data TWT to 30 ms (no observable features, aside from multiples, were encountered below that point in the profiles), (4) generating a PostScript image of the traces, and (5) converting the PostScript image to a GIF image. The sub-bottom profile images have tic marks every 200 shots, with labels, larger tic marks, and dashed grid lines every 1000 shots. Shot numbers shown on the sub-bottom profile images correspond to the trace sequence number within the line (trace header bytes 1-4) or trace sequence number within the reel (bytes 5-8) included in the SEG-Y headers for each profile.

Navigation acquisition and processing - A RTK-GPS utilizing a base/rover setup was used for positioning onboard the USACE LARC. Real-time positions from the RTK-GPS were recorded and written to the seismic trace headers from the antenna to the topside unit of the chirp system every 0.20 seconds, with unique navigation fixes being recorded every second. During sub-bottom processing, location data were extracted from the SEG-Y headers of the processed chirp profiles, using bespoke scripts and Seismic Unix (Cohen and Stockwell, 2010) software and output as ASCII text files; PROJ.5.1.0 (https://proj.org/) software was used to project WGS84 latitude and longitude coordinates to Universal Transverse Mercator (UTM) northings and eastings (WGS84, Zone 18 North [N], meters). Navigation files for each sub-bottom line, saved by line number, were concatenated into comma-separated values (.csv) files so they could be represented geospatially in GIS software. The final output files are as follows: (1) 2021-326-FA_nav.csv and 2022-326-FA_nav.csv contain unique shotpoint positions (along each line) for the entire survey – roughly every 5 shots, (2) 2021-326-FA_1000sht.csv and 2022-326-FA_1000sht.csv include the 1,000-shot-interval locations that were generated to correlate shot locations along the tracklines with evenly spaced 1,000-shot index markers along the top of the printable profile images, and (3) 2021-326-FA_sol.csv and 2022-326-FA_sol.csv have the start-of-line locations for each trackline. The concatenated, comma-delimited text files were later imported into ArcGIS Pro and saved as either a point or polyline file (in Esri's shapefile format, .shp) for subsequent use in GIS software packages. For more information about these geospatial files, refer to the DUNEX_geospatial_metadata files included in this data release.