Archive of Chirp Subbottom Profile, Imagery, and Geospatial Data Collected in May 2023 from Oahu, Hawaii

Chirp acquisition - Subbottom data were acquired from one survey platform, an Edgetech 424 Portable Subbottom Profiler, which was towed on the port side of the 26-foot research vessel, the R/V Alyce C. 84 lines, equivalent to a total of 110.16 line-kilometers, were surveyed. The seismic source utilized during 2023-310-FA consisted of an EdgeTech 424 towfish running DISCOVER version 4.09 acquisition software and towed on the port side of the vessel, approximately 3 m across-track from the IANS reference point. The data were acquired in J-STAR standard format (JSF) and SEG-Y at 5 hertz (Hz), which equates to 0.20 pings per second. The frequency sweep was 4-20 kilohertz (kHz), pulse lengths ranged between 5 ms and 10 ms, and recording length was approximately 53.2 ms. 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 0.2 s. The binary portion of the seismic data provided in this publication (Forde and others, 2023) 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 subbottom profile images that were processed using SU software. Survey Field Activity Collection System (FACS) logs, geophysical logs, and acquisition geometry figure are also provided as supplemental information in the download file, 2023-310-FA_logs.zip (Forde and others, 2023).

Chirp processing and image creation - The EdgeTech 424 chirp system recorded the subbottom 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 subbottom profiles included in Forde and others (2023). 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 subbottom profile images have tic marks every 200 shots, with labels, larger tic marks, and dashed grid lines every 1000 shots. Not all lines have 1000 shots or more, see the logical consistency report for additional details.

Navigation acquisition and processing - A Norbit Winghead MBES system with an integrated Applanix Pos MV Wavemaster II V5 IANS with two AeroAntenna Technology (AT16575-540Ts-TNCF-000-RG-45_NM-R) GNSS antennas (positioned forward-aft near the aft-starboard of the vessel) and an associated IMU were used for positioning onboard the R/V Alyce C. Real-time positions from the Applanix system were recorded and written to the seismic trace headers via a RS232 cable running from the Norbit topside unit to the topside unit of the chirp system every 0.20 seconds, with unique navigation fixes being recorded every second. During subbottom 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 4 North [N], meters). Navigation files for each subbottom line, saved by line number, were concatenated into comma-delimited text files so they could be represented geospatially using Esri ArcGIS software. The final output files are as follows: (1) 2023-310-FA_nav.csv contains unique shotpoint positions (along each line) for the entire survey – roughly every 5 shots, (2) 2023-310-FA_1000sht.csv includes the 1,000-shot-interval locations that were generated to correlate shot locations on the trackline map with evenly spaced 1,000-shot index markers along the top of the printable profile images, and (3) 2023-310-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) for use in the map document (.mxd) included in this archive (Forde and others, 2023).

Trackline map creation - 2023-310-FA_location.mxd and associated shapefiles (.shp) were created with Esri ArcMap 10.8.2 software. The survey navigation file, 2023-310-FA_nav.csv, was imported into ArcMap and processed using the ArcScript "Points to Line" to create a polyline shapefile of the cruise tracklines called 2023-310-FA_trkln.shp. The 1,000-shot-interval and start-of-line files, 2023-310-FA_1000sht.csv and 2023-310-FA_sol.csv, were imported into ArcMap and saved as point shapefiles. A non-proprietary version of the location map was created by utilizing the ArcToolbox "Map to KML" tool, which converted each layer included in the map document (.mxd) to keyhole markup language (.kml) format prior to them being compressed (zipped) and saved as a .kmz file. The ArcGIS and Google Earth geospatial files provided in Forde and others (2023) are unprojected (geographic coordinates, WGS84). For detailed information about the GIS files provided in this data release, please refer to the individual metadata records (*_metadata.txt and *.shp.xml) accompanying each shapefile and the readme file included in 2023-310-FA_gis.zip (Forde and others, 2023).