Survey lines along which EdgeTech SB-512i and SB-424 chirp seismic-reflection data were collected by the U.S. Geological Survey offshore of Massachusetts between Duxbury and Hull (DH_SeismicTrackline shapefile, Geographic, WGS84)

'jsftosegy' (no version), a C script developed by Tom O'Brien (USGS - Woods Hole Seafloor Mapping Group) was used to convert raw chirp traces in the JSF format (acquired using EdgeTech J-Star software) to chirp-processed, envelope traces in the SEG-Y rev. 1 standard format.

A script was run that used Seismic Unix (version 4.1) to read the SEG-Y files, write a Seismic Unix file, and extract SEG-Y trace header information, which included shot number, longitude, latitude, year, Julian day, and time of day (UTC). Geographic coordinates (WGS84) were converted to UTM zone 19 coordinates (WGS84) using Proj (version 4.6.0). Before saving the header information to a text file, the data were filtered using AWK (no version) to save only unique shot point locations. For SB-424 data collected during field activity 07001, a separate text file containing even 500 shot intervals was saved. The 500 shot text file maintains the start and end shots, which may not necessarily be unique fixes. Due to the amount of spurious navigation fixes removed from SB-512i data collected during field activity 06012, a 500 shot text file was generated later (see processing step 7) using the linear referencing tools contained in ArcGIS. A 500 shot interval was chosen because it corresponds to the annotation interval provided along the top of the seismic-reflection profile JPEG images, which are included in the '06012' and '07001' folders within 'GIS\hyperlink_images\seisimage'.

An AWK (no version) script was used to apply layback to seismic navigation acquired during portions of the 06012 survey when shipboard RTK-GPS was used in lieu of the non-operational GPS receiver mounted on the towed catamaran vehicle (This applies to files l25f2000 through l84f2000 acquired over Julian days 218 through 222. Attributes OLDEAST and OLDNORTH in the table 'DH_SeismicShot_unique.csv', which is included in the zip compressed version of the shapefile 'DH_SeismicShot_500.shp', document pre-layback shot/trace positions). 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 19, 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 RTK-GPS receiver, 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 fist 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 (including the layback-adjusted SB-512i lines from field activity 06012) were concatenated into a comma-delimited text file ('DH_SeismicShot_unique.csv'), then imported into ArcMap (version 9.2) using 'Add XY data' and saved as points (Geographic, WGS84) in the ESRI shapefile format ('DH_SeismicShot_unique.shp').

The shapefile containing unique shot point positions, created in the previous step, was used as an input for VACExtras (version 1.97), a VB collection of tools developed by VeeAnn Cross (USGS-WHSC), which converts point data to a new polyline shapefile ('DH_SeismicTrackline.shp'), based on record order. The 'FILENAME' field was used to define which points were used to generate each polyline feature. Using XTools Pro (version 4.1.0) for ArcGIS desktop, the length of each polyline segment was calculated and included in the attribute field 'LENGTH', specifying UTM, Zone 19 N, WGS84, and kilometers. In addition, table attributes for the year ('YEAR') and Julian day ('JULDAY') of data collection, device used to collect the data ('DEVICEID'), survey vessel ('VEHICLEID'), and WHSC field activity number ('SURVEYID') were added and populated using the table editor in ArcMap (version 9.2).

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. The source information was incomplete and had to be modified to meet the standard. Only the source time period of content and source contribution was supplied. The format of the time period of content had to be fixed. Other minor edits were probably performed (title, publisher, publication place, etc.). 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.