Archive of Chirp Sub-Bottom Profile, Imagery, and Navigational Data Collected During USGS Field Activity Number 2024-320-FA in 2024 Offshore of Breton Island, Louisiana

Chirp acquisition - Sub-bottom data were acquired using an Edgetech 3400 Portable Sub-Bottom Profiler (SBP), which was towed by the 26-foot research vessel, the R/V Sallenger. A total of 28 lines were surveyed. The seismic source utilized during FAN 2024-320-FA consisted of an EdgeTech 3400 towfish running DISCOVER version 42.0.1.115 acquisition software and towed on the port side of the vessel, approximately 4 m across-track from the IANS reference point. The data were acquired in J-STAR standard format (JSF) and SEG-Y format at 5 hertz (Hz), which equates to one ping per 0.20 seconds. The frequency sweep was 1-26 kilohertz (kHz) and 3-10 kHz, pulse lengths were 5 ms and 20 ms, and the recording length was 65.1 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 FACS logs and acquisition geometry figure are also provided as supplemental information in the download file, 2024-320-FA_logs.zip.

Chirp processing and image creation - The EdgeTech 3400 dual frequency SBP system recorded the sub-bottom data in JSF and SEG-Y rev. 1 formats. SEG-Y data were saved (with alternating pings supplied by each transducer) as separate files for the 1-26 kHz and 3-10 kHz frequencies. 1-26 kHz data have “A” appended to the end of the file name, for example, 20240809144450A.sgy. The raw JSF data are not provided in the data release but are available upon request. Using the jsf2segy software package (https://github.com/Geosvy/jsf2segy), JSF files were converted to analytic and envelope SEG-Y traces. The envelope SEG-Y files are comprised of the combined frequency data. 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) creating the envelope seismic data file from the analytic data (envelope converted files included in this publication have "_env" appended to the line name), (2) removing raw [vessel] navigation data for each shot and converting the SEG-Y file to SU format, (3) 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, (4) trimming the trace data TWT to 30 ms (no observable features, aside from multiples, were encountered below that point in the profiles), (5) generating a PostScript image of the traces, and (6) converting the PostScript image to a GIF image. The SBP images have tic marks every 200 shots, with labels, larger tic marks, and dashed grid lines every 1,000 shots. Shot numbers shown on the SBP 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 Teledyne T50-P multibeam system with an integrated Applanix Pos MV Wavemaster II V5 IANS with two Trimble GA830 global navigation satellite system antennas (positioned amidships) and an associated IMU was used for positioning onboard the R/V Sallenger. Real-time positions from the Applanix system were recorded and written to the seismic trace headers via a RS232 cable running from the Applanix receiver 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 SU (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 16 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) 2024-320-FA_nav.csv contains unique shotpoint positions (along each line) for the entire survey – roughly every 5 shots, (2) 2024-320-FA_1000sht.csv includes 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) 2024-320-FA_sol.csv has 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 2024-320-FA_geospatial_metadata file included in this data release.