Shot point navigation at even 500 shot intervals for EdgeTech SB-512i chirp seismic-reflection data collected in 2014 by the U.S. Geological Survey offshore of Fire Island, NY (Esri point shapefile, GCS WGS 84)

Scripts within SIOSEIS (version 2013.1.1) 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 (version 2013.1.1) was used to read SEG-Y files, renumber shots starting from one, extract the envelope-detected trace, and write 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. The SIOSEIS script Sio_medianstk applied a two-trace median stack (trim 50 percent of the values furthest from the median amplitude) to improve subsequent water bottom picking.

3. The script Read512i_mstk was used to read renumbered and two-trace median filter stacked SEG-Y files, write a Seismic Unix file, and extract SEG-Y trace header information, including shot number, longitude, 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 18 N coordinates (WGS 84) using Proj (version 4.6.0). 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.

A Python (version 2.7) script was used to apply layback to seismic navigation acquired during the 2014-009-FA survey. Attributes oldEast and oldNorth in the table 2014-009-512i_lbunique.csv', which is included in the zip compressed version of the shapefile 2014-009-FA_512i_500sht.shp', document pre-layback shot/trace positions. Using Easting and Northing coordinates (UTM Zone 18 N, WGS84) produced in the previous step, the script calculated easting and northing differential values between consecutive trace positions. Headings between consecutive traces were calculated using the arctangent function (arctan2(dy,dx)), and reciprocal back bearings were determined using a lookup table. Back bearings were smoothed along track using a moving median function with a window length defined by the user (59 traces during 2014-009-FA). Layback easting and northing offsets were calculated by multiplying the linear distance between the catamaran and the shipboard DGPS receiver (43 m during 2014-009-FA) by the sine and cosine of the smoothed back bearing, respectively. Offset values were then added to the original coordinates to produce layback positions.** The layback routine introduces a stair-step offset in the tie-line seismic shot point and trackline navigation. This is a by-product of the moving median function and appears in the north-south oriented tie-lines. **

An awk script (do_nav_gisprep) was used to process unique, 100 shot, and 500 shot lay-back corrected navigation to a merged csv file that can be utilized by ArcGIS to create a trackline and shotpoint shapefile. Text files containing unique shot point positions for each seismic line with layback-adjusted navigation were concatenated into a comma-delimited text file. Then the layback-corrected position CSV file was imported into ArcMap (version 10.2.2) using 'Add XY data' and saved as points (Geographic, WGS84) in the Esri shapefile format. Additional fields for survey ID, vessel ID, and device ID were added to the attribute table and populated using "Add Field" and "Field Calculator".

Added keywords section with USGS persistent identifier as theme keyword.