Trackline navigation for 512i seismic-reflection data collected by the U.S. Geological Survey along the Delmarva Peninsula, MD and VA, 2014 (Esri polyline shapefile, GCS WGS 84)

Scripts within SIOSEIS (version 2010.2.25) 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 layback corrected navigation files including trackline shapefiles and shotpoints are summarized below and in the following processing steps. 1. SIOSEIS (version 2010.2.25) was used to read SEG-Y files, renumber shots starting from one, 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. Sio_medianstk was used to remove S-boom cross talk (when multi-channel streamer lines were recorded simultaneously: Julian Day 169-178) with a two-trace median stack and improve water bottom picking. This script was used for all lines even after multi-channel streamer was turned off, due to the improvements made to water bottom picks. 3. Read512i_mstk was used to read median filtered stacked and renumbered 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, and shots with unique navigation coordinates. Geographic coordinates (WGS84) were converted to UTM zone 18 coordinates (WGS84) 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, which are included in the folders within 'seis_images\512i'.

A Python (version 2.7) script was used to apply layback to seismic navigation acquired during the 2014-002-FA survey. Attributes oldEast and oldNorth in the table ‘2014_002_512i_lbunique.csv', which is included in the zip compressed version of the shapefile ‘2014-002-FA_512i_500sht.shp', document pre-layback shot/trace positions. Using Easting and Northing coordinates (UTM Zone 18 N, WGS84) from the trace headers, 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 2014-002-FA ). Layback easting and northing offsets were calculated by multiplying the linear distance between the catamaran and the shipboard DGPS receiver (33 for line 1 or 43 m for all other lines during 2014-002-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.

An awk (do_nav_gisprep) script 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.

The shapefile containing unique shot point positions, created in the previous step, was used as an input for VACExtras points to lines (version 2.1), a VB collection of tools developed by VeeAnn Cross (USGS-WHCMSC), which converts point data to a new polyline shapefile ('2014_002_FA_512i_Tracklines.shp'), based on record order. The 'FileName' field was used to define which points were used to generate each polyline feature. The length of each polyline segment was calculated and included in the attribute field 'Length_km', specifying UTM, Zone 18 N, WGS84, and kilometers with ArcMap version 10.2.2. Additional fields for vessel name, device and survey ID were added in ArcMap

The online links to the data were updated to reflect the new server hosting the data. Additionally, other small edits could be made to the metadata, such as modifying http to https where appropriate. The metadata date (but not the metadata creator) was edited to reflect the date of these changes.

Added keywords section with USGS persistent identifier as theme keyword.