Chirp sub-bottom profiler tracklines collected by the U.S. Geological Survey offshore of Fire Island, NY in 2011 (Geographic, WGS 84, Esri Polyline Shapefile)

Processing Seismic Data: 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 - preparing the data for input into the Landmark SeisWorksTM 2D (R5000) seismic interpretation software. Wayne E. Baldwin performed this and all subsequent process steps.

An AWK (no version) script was used to apply layback to seismic navigation. The script utilized a read-and-do loop to calculate and apply layback offsets to trace positions.
During the initial loop through the script:
1) Easting and northing coordinates (UTM Zone 18, WGS84) for the first five traces of input navigation were read and easting and northing differentials between the consecutive positions were calculated;
2) The signs (+/-) of the differential values were compared to a look-up table to determine the appropriate conversion of the arc tangent (atan2(dy,dx)) angle between consecutive positions to a polar azimuth;
3) The average of the polar azimuths was calculated;
4) The sine and cosine of the average azimuth was calculated and multiplied by the linear distance between the catamaran and the shipboard DGPS receiver (51.5m lines l14f1- l74f1; 43.2m lines l75f1 - ll92f1), providing absolute values for easting and northing offsets, respectively;
5) A look-up table was used to determine the quadrant of the average azimuth and appropriately add or subtract the calculated offsets to the easting and northing coordinates of the first three input traces, producing final layback positions for those traces;
6) Layback and original easting and northing coordinates for the three adjusted traces were printed to a new layback navigation file that also retained additional attributes input records; and
7) Easting and northing coordinates of the fourth and fifth traces, the three azimuths computed between traces two, three, four, and five, and the average azimuth were held as input for calculations conducted in the subsequent loop.
During subsequent loops through the script:
1) Easting and northing coordinates for three additional traces from input navigation were read, and easting and northing differentials were calculated between the consecutive positions, including the last trace position held from the previous loop;
2) Three new polar azimuths were calculated using the differential values, then a new average azimuth was calculated from the three that were held, the new three, and the average held from the previous loop (the previously calculated average was factored into the new average to smooth "kinks" along the layback navigation that can result from significantly different average azimuths calculated from one loop to the next);
3) new layback offset values were computed, and applied to the easting and northing coordinates of the last two traces input during the previous loop, and the first trace input during the present loop;
4) layback and original easting and northing coordinates for the three adjusted traces were appended to the layback navigation file started in the previous loop; and
5) easting and northing coordinates of the second and third traces, the three new azimuths, and the average azimuth from the present loop were held as input for calculations conducted in the subsequent loop.
Near the end of the input navigation file:
1) if less than three traces were present during a new loop, the layback offsets calculated during the previous loop were applied to remaining trace coordinates;
2) layback and original easting and northing coordinates for the remaining adjusted traces were appended to the layback navigation file; and
3) the script reached its end, closed, and saved the layback navigation file.
In this fashion, the script approximated a moving window, in which the average of six trace-to-trace azimuths was used to calculate layback offsets for three central trace positions. Exceptions were at the start of a file, where the first three input trace positions were adjusted using offsets calculated from the average of only four azimuths, and possibly at the end of a file, where remaining traces may have been adjusted using the offsets calculated during the previous loop.

Text files containing unique shot point positions for each seismic line were concatenated into a comma-delimited text file. The text file was imported into ArcMap (version 9.3) using 'Add XY data' and saved as points (Geographic, WGS84) in the Esri shapefile format.

The shapefile containing unique shot point positions was used as an input for VACExtras (version 2.1), a VB collection of tools developed by VeeAnn Cross (USGS-WHCMSC), which converts point data to a new polyline shapefile based on record order. The LineName' field was used to define which points were used to generate each polyline feature, and the fields 'Shot', 'Year', 'JulDay', and 'Shot_end' were chosen to be included as attributes in the output shapefile. Using XTools Pro (version 7.1.0) for ArcGIS desktop, the length of each polyline segment was calculated and included in the attribute field 'Length_km', specifying UTM, Zone 18 N, WGS84, and kilometers. The following fields were manually added to the attribute table: Survey, Acq_system, and Ship.

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.