In August of 2013, the U.S. Geological Survey conducted a geophysical survey offshore of Petit Bois Island, Mississippi to investigate the geologic controls on barrier island framework and long-term sediment transport. This report serves as an archive of unprocessed digital chirp subbottom data, trackline maps, navigation files, GIS information, and formal FGDC metadata. Gained digital images of the seismic profiles are provided. The archived trace data are in standard Society of Exploration Geophysicists (SEG) SEG Y revision 0 format (Barry and others, 1975) and may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU). Example SU processing scripts and USGS software for viewing the SEG Y files (Zihlman, 1992) are also provided.
These data are available for viewing using GeoMapApp (<
http://www.geomapapp.org/>) and Virtual Ocean (<
http://www.virtualocean.org/>) multi-platform open source software. In addition, the SEG Y files can also be downloaded from the USGS Coastal and Marine Geoscience Data System (<
http://cmgds.marine.usgs.gov>).
To archive all digital chirp subbottom profile data and associated files collected during cruise 13CCT04.
The USGS Saint Petersburg Coastal and Marine Science Center (SPCMSC) - St. Petersburg, Fla., assigns a unique identifier to each cruise or field activity. For example, 13CCT04 indicates the data were collected in 2013 for the Coastal Change and Transport (CCT) study, during the fourth field activity for that project in that calendar year. Refer to <
http://walrus.wr.usgs.gov/infobank/programs/html/definition/activity.html> for a detailed description of the method used to assign the cruise ID. The naming convention used for each subbottom line is as follows: yye##a, where 'yy' are the last two digits of the year in which the data were collected, 'e' is a 1-letter abbreviation for the equipment type (for example, c for 424 chirp; i for 512i chirp), '##' is a 2-digit number representing a specific track, and 'a' is a letter representing a section of the line if recording was prematurely terminated or rerun for quality or acquisition problems.
All Chirp systems use a signal of continuously varying frequency; the systems used during this survey produce high-resolution, shallow-penetration (typically less than 50-ms) profile images of sub-seafloor stratrigraphy. The towfish contains a transducer that transmits and receives acoustic energy and is typically towed 1–2 meters (m) below the sea surface. As transmitted acoustic energy intersects density boundaries, such as the seafloor or sub-surface sediment layers, some energy is reflected back toward the transducer, received, and recorded by a PC-based seismic acquisition system. This process is repeated at regular intervals (for example, 0.125 seconds (s)) and returned energy is recorded for a specific duration (for example, 50 ms). In this way, a two-dimensional (2-D) vertical image of the shallow geologic structure beneath the towfish is produced.
The seismic source utilized during 13CCT04 consisted of an EdgeTech SB-424 and 512i towfish running Discover v. 3.51 acquisition software and towed about 4 m and 28 m, respectively, behind the Global Positioning System (GPS) antenna. The data were acquired using a frequency sweep of 4–20 kilohertz (kHz), a 43 kHz sample frequency, and approximately 65 ms record length. Based on survey speeds of 3.5 knots, the shot spacing was about 0.450 m.
The binary portion of the unprocessed seismic data is stored in SEG Y rev. 0 (Barry and others, 1975), IBM float format, which is a standard digital format that can be read and manipulated by most seismic processing software packages; the first 3,200 bytes of the file header are in in American Standard Code for Information Interchange (ASCII) format instead of Extended Binary Coded Decimal Interchange Code (EBCDIC) format. The SEG Y formatted trace files have a .sgy extension. The SEG Y files may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). Also provided are example SU scripts that allow the user to remove navigation fixes from the SEG Y headers, along with a fix for every 1,000 shots, and produce a printable, gained Graphics Interface Format (GIF) image of each profile. The printable profiles provided are GIF images that were gained using SU software. Refer to the Software page of this report for links to example SU processing scripts and USGS software for viewing the SEG Y files (Zihlman, 1992).
