Archive of Chirp Subbottom Profile, Imagery, and Geospatial Data Collected in 2019 from Rockaway Peninsula, New York

Chirp acquisition - USGS scientists created a purpose-built, towed sled system to mount the 512i chirp subbottom profiler (just below the water's surface) and enable launching and recovery from beach environments. The seismic sled was also configured to help ensure the quality of data collected in exceedingly shallow water depths, such as those observed near the shoreline, over shallow nearshore bars, and within the surf zone. Subbottom data were acquired from one survey platform, the seismic sled, which was towed by the 26-foot research vessel, the R/V Sallenger. 74 lines, equivalent to a total of 210.17 line-kilometers, were surveyed. The seismic source utilized during 2019-333-FA consisted of an EdgeTech SB-512i towfish running DISCOVER version 4.09 acquisition software and towed on the port side of the R/V Sallenger, approximately 14 m behind the global positioning system (GPS) reference point. The data were acquired at 5 hertz (Hz), which equates to a 0.2 s sample interval, using a frequency sweep of 0.7-12 kilohertz (kHz), and recording length was approximately 172.5 ms. Based on survey speeds of 3.5-4.5 knots, the shot spacing was approximately 0.450 m. The binary portion of the seismic data is stored in SEG-Y rev. 0, 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 American Standard Code for Information Interchange (ASCII) format instead of Extended Binary Coded Decimal Interchange Code (EBCDIC) format. The SEG-Y formatted 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 are chirp subbottom profile images that were processed using SU software. Survey Field Activity Collection System (FACS) and geophysical logs are also provided as supplemental information in the download file, 2019-333-FA_logs.zip.

Chirp processing - The SEG-Y data were processed with Seismic Unix (Release 44) software to produce gained Graphics Interchange Format (GIF) images of the subbottom profiles included in this data release. Preliminary processing was conducted by E. Wei in October 2019; however, additional issues were observed in the trace data during the QA/QC process that resulted in the subbottom data being reprocessed by A. Forde in February 2021. A representative chirp data processing sequence consisted of (1) applying a 3000-4000-10,000-12,000 bandpass filter to the analytic data to remove noise from the raw data, prior to creating the envelope seismic data file (each line number had an "e" added as a prefix), (2) removing raw [vessel] navigation data for each shot and converting the SEG-Y file to SU format, (3) inserting DGPS, as arcseconds, into the source coordinate headers (corrected files included in this publication have "_bp_mod" appended to the line name), (4) applying automatic gain control, (5) trimming the trace data TWT to 30 ms (no observable features, aside from multiples, were encountered below that point in the profile), (6) generating a PostScript image of the traces, and (7) converting the PostScript image to a GIF image.

GPS Acquisition - A GPS base station, RIIS, was established near the survey area. RIIS is a USGS installed P/K nail located in the dockage area of the former Riis Landing U.S. Coast Guard Station located just west of the Marine Parkway Bridge on the Rockaway Peninsula. The base station was continually occupied and equipped with a Spectra Precision SP90M GPS receiver recording full-carrier-phase positioning signals (L1/L2) from satellites via Thales Choke-ring antennas, recording at a rate of 0.1 s. Supplementary acquisition and processing information for the navigation data associated with the seismic sled and GPS are included in Rockaway_2019_SBES_metadata.txt (Stalk and others, 2020).

Navigation acquisition and processing - An Ashtech ProFlex 500 unit with a global navigation satellite system antenna (positioned amidships) was used for positioning onboard the R/V Sallenger. Real-time positions from the ProFlex were recorded and written to the seismic trace headers via a RS232 cable running from the ProFlex receiver to the topside unit of the chirp system every second. During subbottom processing, vessel coordinates previously saved to the seismic headers were replaced with post-processed GPS. Once updated, location data were extracted from the SEG-Y headers of the processed chirp profiles, using Seismic Unix software and output as ASCII text files; PROJ.5.1.0 (https://proj4.org/) software was used to project WGS84 latitude and longitude coordinates to Universal Transverse Mercator (UTM) northings and eastings (WGS84, Zone 18, 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) 2019-333-FA_slednav.csv contains unique shotpoint positions (along each line) for the entire survey, (2) 2019-333-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) 2019-333-FA_sol.csv has the start-of-line locations for each trackline. The concatenated, comma-delimited text files were later imported into ArcGIS 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.

Trackline map creation - 2019-333-FA_location.mxd and associated shapefiles (.shp) were created with Esri ArcGIS 10.6 software. The survey navigation file, 2019-333-FA_slednav.csv, was imported into ArcMap 10.6.0.8321 and processed using the ArcScript "Points to Lines" to create a polyline shapefile of the cruise tracklines called 2019-333-FA_trkln. The 1,000-shot-interval and start-of-line files, 2019-333-FA_1000sht.csv and 2019-333-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 .mxd document to keyhole markup language (.kml) format prior to them being compressed and saved as a .kmz file. The ArcGIS and Google Earth geospatial files provided in this publication are unprojected (geographic coordinates, WGS84). For detailed information about the ArcGIS 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 2019-333-FA_gis.zip.