Data Processing

Navigation Processing

Shotpoint navigation files, consisting of unique latitude and longitude navigation fixes, were extracted from the trace headers of each chirp line using Seismic Unix (SU) software and are stored (by trackline number) as American Standard Code for Information Interchange (ASCII) text files. In addition to shotpoint navigation data, a 1,000-shot-interval file was generated to correlate shot locations on the trackline map with evenly spaced 1,000-shot index markers along the top of the printable profile images. Shotpoint coordinates were projected from latitude and longitude coordinates (WGS84) to Universal Transverse Mercator (UTM) northings and eastings (WGS84, Zone 16, meters) using PROJ.4 software (http://proj4.org/) and reformatted for use with Environmental Systems Research Institute (Esri) geographic information system (GIS) ArcGIS software. A file containing the location of the start of each line was also created for use on the trackline map. Navigation data were not corrected for offsets between the shotpoint and the GPS antenna (fig. 3).

The survey shotpoint navigation file (2015-331-FA.txt) and 1,000-shot-interval location file (s2015-331-FA.txt) can be accessed from the Data Downloads page.

Seismic Processing

The SEG Y data were processed with SU (Cohen and Stockwell, 2010) software to produce gained graphics interchange format (GIF) images of the subbottom profiles included in this report. Seismic Unix (SU) is a public-domain seismic processing software package written and maintained by the Center for Wave Phenomena at the Colorado School of Mines in Golden, Colorado. The software is compatible with Unix, Linux, and Mac OS X operating systems. Though the SU processing scripts have a variety of functions, in this case they were used to remove navigation, line name, and time information from the SEG Y headers. These scripts also produced a gained GIF image for each profile, which can be displayed using a variety of shareware programs, such as ImageMagick (Unix, Linux, Macintosh) or a web browser. All processing scripts were run on a Mac OS X (version 10.11.6) system.

A representative chirp data processing sequence consisted of: (1) removing navigation data for each shotpoint and converting the .sgy file to SU format, (2) applying automatic gain control, (3) generating a PostScript image of the subbottom profile, and (4) converting the PostScript image to a GIF image.

ArcGIS Map Document

The Esri ArcGIS map document, shapefiles, and seismic profile images supplied in this report are included in 2015-331-FA_arc.zip (48-MB), which also contains a detailed readme file describing the GIS. The trackline location map provided on the Data Downloads page was created in ArcGIS 10.3.1 and exported to Adobe Illustrator for further editing prior to being incorporated in this publication. The shapefiles may also be viewed using other versions of Esri desktop software, including their free software, Explorer for ArcGIS (http://www.esri.com/software/arcgis/explorer/), or other software such as GeoMapApp (http://www.geomapapp.org/), which is compatible with all platforms.

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