50-meter grid representing the Holocene sediment thickness (in meters) on the inner-continental shelf offshore of Fire Island, NY (FI_HISO, UTM Zone 18N, WGS 84, Esri Binary Grid)

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.

Seismic Unix (version 4.2) was used to read 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.

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. Unique navigation (containing shot, x, y, and line number) and SEG-Y files were used as input to LandMark SeisWorksTM 2D (R5000) seismic interpretation software.

CHIRP seismic reflection data were interpreted using Landmark SeisWorksTM 2D (R5000) seismic interpretation software. Interpretation consisted of identifying and digitizing erosional unconformities defining the boundaries between Holocene, Pleistocene, and pre-Quaternary seismic units. An isochron representing the two-way travel time between the Holocene transgressive unconformity and seafloor horizon was computed, then sampled at a 20-meter along track interval and exported from SeisWorks as ASCII text. Awk (no version) was used to convert two-way travel times to thickness in meters using a constant seismic velocity of 1500 m/s.

Mass points representing the isopach computed in the previous step were imported into ArcMap (9.3.1) as point features (easting, northing, thickness) using the 'Add XY data' function, then saved as a point shapefile. The ArcMap (9.3.1) Spatial Analyst tool 'Topo to Raster' was used to create an interpolated grid of the isopach with a 50 meter cell size. Inputs for Topo to Raster consisted of the isopach point shapefile (mass points), and a polygon shapefile traced around the input mass-points (hard clip). (Drainage enforcement was not used).

The ArcMap (9.3.1) 'Raster Calculator' was used to set grid cells with thickness values less than 0.5 meters to null using a SETNULL expression.

Edits to the metadata were made to fix any errors that MP v 2.9.30 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). All of the online links to the publication pages and data had to be fixed. The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

Added keywords section with USGS persistent identifier as theme keyword.