Archive of Chirp Sub-Bottom Profile, Imagery, and Navigational Data Collected in June and August 2023 from the Chandeleur Islands, Louisiana

Chirp acquisition - Sub-bottom data were acquired from one survey platform that utilized two, different EdgeTech portable sub-bottom profiling (SBP) systems. EdgeTech's 3100-P (aka chirp SB-424; frequency range from 4-24 kilohertz (kHz)) and 3400 (aka chirp SB-216; dual frequency 2-16 kHz transducers) SBPs were towed on the port side of the 26-foot research vessel, the R/V Sallenger. In total, 40 lines, equivalent to a total of 135.1 line-kilometers, were surveyed. The seismic source utilized during 2023-325-FA consisted of an EdgeTech 424 or 216 towfish running DISCOVER version 4.09 acquisition software and towed on the port side of the vessel, 3.56 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 4-20 kHz (DOY 217); 3-12 kHz (DOY 219 and 220); or 3-16 kHz (DOY 168), pulse lengths ranged between 5 ms and 20 ms, and recording lengths were 35.5 ms (DOY 219 and 220), 59.18 ms (DOY 168), and 60.47 ms (DOY 217). Based on survey speeds of 3.5-4.5 knots, the shot spacing was roughly 0.450 m. During acquisition, positions from the POS MV IANS 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, IBM 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, 2023-325-FA_logs.zip.

Chirp processing and image creation - The EdgeTech 3100 and 3400 chirp systems recorded the sub-bottom data in JSF and SEG-Y rev. 1 formats. 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) applying automatic gain control, (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 1,000 shots.

Navigation acquisition and processing - A Teledyne Reson T50P dual head MBES system and Applanix POS MV Wavemaster II IANS with two Trimble GA830 GNSS antennas (positioned forward-aft near the aft-starboard of the vessel) were 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 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 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 using GIS software. The final output files are as follows: (1) 2023-325-FA_nav.csv contains unique shotpoint positions (along each line) for the entire survey – roughly every 5 shots, (2) 2023-325-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) 2023-325-FA_sol.csv has the start-of-line locations for each trackline. The concatenated, comma-delimited text files were later imported into ArcMap 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 2023-325-FA_geospatial_metadata files included in the data release.