Seismic Reflection, EdgeTech SB-424 Chirp profile images collected in the vicinity of Buffalo Reef, Michigan, within Lake Superior, during USGS field activity 2018-043-FA, (PNG Images)

PROCESS STEP 1:
SIOSEIS (version 2015.3.1), Shearwater Reveal (version 2019), and Seismic Unix (version 4.2) were used to process SEG-Y data, create navigation files, and plot images. The processing flow and scripts used to produce navigation files including trackline shapefiles are summarized below and in the following processing steps.
1) SIOSEIS was used to read the raw SEG-Y files, extract the envelope-detected trace, renumber shots starting from one, and write out new SEG-Y files.
2) Reveal was use to run the following flows:
A) read_segy.flow - SegyTapeRead read the traces. HeaderMath and UTMLatLong were used to convert the source lat/lon positions from seconds of arc to decimal degrees, project them to UTM Zone 16N WGS 84 meters, and write each to new header words (NRP_LAT, NRP_LON, NRP_X, and NRP_Y). DBWrite wrote the UTM positions for the first channel of each FFID to an internal Reveal database table. Finally, Output wrote the traces to a new file "*.sht-raw.seis" in the internal OpenCPS format.
B) layback_shift.flow - Input read the "*.seis" file and sorted the traces to FFID/CHANNEL. The custom Python module ShotlineLayback (developed by Nathan Miller of USGS-WHCMSC) was used to define the measured horizontal offset between the DGPS antenna and the SB-424 transducer (-8.5 m). The algorithm interpolated a sail line from the source shot positions (NRP_X and NRP_Y), then computed layback positions for the SB-424 shots by translating them back along the sail by the measured offset. ApplyStatic was used to shift the traces down by 2.7 ms to account for the approximate 2 m tow depth of the SB-424 (a sound speed of 1500 m/s was used). Output wrote the shifted traces to a new SEG-Y files in which the trace header words SRC_X, SRC_Y represent the calculated layback coordinates, and REC_X, REC_Y maintain the original DGPS coordinates.
3) The SIOSEIS script read424 was used to read layback and static corrected SEG-Y files, write a Seismic Unix file, and extract SEG-Y trace header information, including shot number, pre-layback and layback longitude and latitude, year, Julian day, and time of day (UTC). Header information from each SEG-Y file was saved to text files after an AWK (no version) filter was used to maintain the first and last shots, shots at multiples of 100, 500, and shots with unique navigation coordinates. Geographic coordinates (WGS 84) were converted to UTM zone 16 N coordinates (WGS 84) using Proj (version 4.9.3). End shots and shots at multiples of 100 may not have unique navigation coordinates. Separate text files containing the first and last shots and even 500 shot intervals were also saved. A 500 shot interval was chosen because it corresponds to the annotation interval provided along the top of the seismic-reflection profile images. Read424 called a Python script 424inlbtoSQL18043.py, written by Wayne Baldwin (USGS-WHCMSC), which imported the CSV files to a Spatialite (version 3.21.0) enabled SQLite (version 3) database, creating two tables containing point geometries for the unique and 500 shot interval navigation. The script also created line geometries from the unique navigation (sorted by LineName and Shot) and wrote them to an additional database table. The tracklines are based on all the shot navigation.
4) The SIOSEIS script siothresh_pltwbt was used to pick the water-bottom time from the layback corrected SEG-Y files, apply a swell filter to smooth water-bottom time picks and shift the trace data by the difference between the smoothed and unsmoothed water bottom time before writing the shifted trace data to a new SEG-Y file.
5) The Seismic Unix script Plot424 read the swell filtered SEG-Y files, saved as Seismic Unix format, and creates 12-inch-high variable density plots of the chirp profiles from the swell filtered SEG-Y files, which are converted to PNG format using ImageMagick (version 6.9.3-4). Images show two-way travel time (seconds) along the y-axis (left margin) and shots along profile (labeled at 500 shot intervals) on the x-axis (along top of profile).

Added keywords section with USGS persistent identifier as theme keyword.

Changed the distribution format name from digital data to PNG to better represent the data. Changed the series issue from a URL to the DOI number.