Chirp seismic reflection - shotpoints, tracklines, profile images, and SEG-Y traces for EdgeTech SB-512i chirp data collected during USGS field activity 2019-002-FA (point and polyline shapefiles, CSV text, PNG Images, and SEGY data, GCS WGS 84)

PROCESS STEP 1:
SIOSEIS (version 2013.1.1), Seismic Unix (version 4.2), Shearwater Reveal (version 2019), and python were used to process SEG-Y data and extract navigation data. The processing flows and scripts used are summarized below and in the following processing steps.
(1.) The SIOSEIS script sio_renum read the raw SEG-Y files, extracted the envelope-detected trace, renumbered shots starting from one, and wrote out new SEG-Y files. The original shot numbers, which were assigned by SonarWiz sequentially over the duration of an acquisition session despite SEG-Y file changes, are preserved in the raw SEG-Y data.
(2.) If navigation was absent or had spurious coordinates in the SEG-Y trace headers, Hypack raw navigation was merged with trace headers using the Reveal software DBMerge tool based on year seconds derived from the SEG-Y trace headers and the raw Hypack data. This step was done for line13 (l13f1) through line 16 (l16f3) and lines 54 (l54f1) through line 56 (l56f4). For some of these SEG-Y filles (L14F3, L15F3, and L56F3) the Hypack files did not record positions exactly at the beginning or end of the SEG-Y files. These shots that had no position were eliminated.
(3.) Lines 13 (l13f1) through line 56 (l56f4) required a layback shift to be applied with Reveal software. All other files skipped this step. The Reveal flow read_segy called the following modules: SegyTapeRead read the traces. HeaderMath and UTMLatLong converted the navigation reference point lat/lon positions from seconds of arc to decimal degrees, projected them to UTM Zone 19N WGS 84 meters, and wrote each to new header words (NRP_LAT, NRP_LON, NRP_X, and NRP_Y). DBWrite wrote the UTM positions for each shot to an internal Reveal database table '*.shots.raw.db'. Finally, Output wrote the traces to a new file '*.seis' in the internal Reveal format. The custom Python module ShotlineLayback (developed by Nathan Miller of USGS-WHCMSC) defined the horizontal offset between the GPS antenna and the SB-512i transducers (approximately 30.8 m astern). The algorithm interpolated a sail line from the navigation reference point positions (NRP_X and NRP_Y), then computed layback positions for the SB-512i shots by translating them back along the sail line by the total offset, and wrote the calculated layback positions to new header words (SRC_X, SRC_Y, REC_X, REC_Y). A limitation of the ShotlineLayback module is the inability to layback source shot/receiver locations that would have occurred prior to the start of the interpolated sail line (i.e. it does not project the sail line backward). Consequently, lines 13 (l13f1) through line 56 (l56f4) start at shot 16. UTMLatLong converted the layback positions from UTM Zone 19N WGS 84 meters to decimal degrees and wrote each to new header words (NRP_LAT, NRP_LON, SRC_LAT, SRC_LON, REC_LAT, and REC_LON). HeaderMath converted the layback REC_LAT and REC_LON values from decimal degrees to seconds of arc with a scalar of 100, and set header words for navigation coordinate units (geographic seconds of arc with scalar) and time basis code (UTC). Output wrote the processed traces to a new SEG-Y files in which the trace header words SRC_X, SRC_Y, REC_X, and REC_Y represent the calculated layback coordinates.
(4.) The SIOSEIS script sio_medianstk, run on all files, applied a median stack of two-trace gathers (two shots) where a percentage (50 percent) of the trace values furthest from the median value were summed. For each time sample, trace values within the two-trace gather are examined and the median value is determined by sorting the values. This was done to minimize noise introduced when the boomer system was operated simultaneously. The noise reduction simplified automatic sea-floor (water bottom) picking conducted prior to swell filtering in step 6. New SEG-Y files were written with the shot numbers reduced by a factor of two.
(5.) A Seismic Unix script read512i was used to read the median stacked SEG-Y files, extract the cataraft or the layback corrected coordinates, and write a Seismic Unix file. Along with the extracted coordinates, other SEG-Y trace header information, including shot number, year, Julian day, and time of day (UTC) were extracted. 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 500 and shots with unique navigation coordinates. Geographic coordinates (WGS 84) were converted to UTM zone 19 N coordinates (WGS 84) using Proj (version 4.6.0). End shots and shots at multiples of 500 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. Read512i called a Python script 512itoSQL_19002.py, written by Wayne Baldwin, which imported the CSV files to a Spatialite (version 4.3.0) enabled SQLite (version 3.21.0) database, creating three tables containing point geometries for the unique and 500 shotpoint navigation and trackline features.
(6.) The SIOSEIS script siothresh_pltwbt called the process wbt that was used to predict water-bottom times from the envelope SEG-Y traces in each file and write the predicted two-way travel times to a trace header. Files 209F1 and l279f1 skipped picking the water-botom time for the first 4 shots to avoid noisy traces. As a result, the processed files begin at shot 5. A top mute was applied to removewater-column noise and a amplitude threshold set to pick the water-bottom time. To apply a static shift and remove the effect of heave of the cataraft, the SIOSEIS process swell was used to apply a 21-trace smoothing filter to the water bottom picks. Swell computes the average water bottom time of a group of traces, then shifts the middle trace of the group by the difference between the middle trace water bottom time and the average water bottom time. Not all files were processed to remove heave if the sea state was calm enough to eliminate this step. The files that include this step have file names *_s.sgy for SEG-Y files and include the notation "Swell Filter" in the figure title for the PNG image files. These process steps and all subsequent process steps were conducted by the same person - David S. Foster.

PROCESS STEP 2:
The SB-512i 500 shot and trackline features were added (Add Data) into ArcGIS Pro (version 2.4.1) from the SQLite database, then exported (Right click on database feature class > Data > Export Feature) to the new Esri polyline shapefiles '2019-002-FA_SB512i_sht500.shp' and '2019-002-FA_SB512iTracklines.shp', respectively. This step was original done on 20191218 but was redone in May 2022 to correct for missing shapefile attributes that was noted during the data release review. This correction does not affect the subsequent prcoessing step.

PROCESS STEP 3:
The Seismic Unix script Plot512i created variable density postscript plots of the seismic profiles showing 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). The postscript images were converted to 200 dpi portable network graphic (PNG) images using ImageMagick (version 6.9.9-40). The plots are labeled along the bottom of the X axis. Plots with fewer than approximately 1000 traces may have truncated labels.