In July of 2012, the U.S. Geological Survey conducted a geophysical survey offshore of the Chandeleur Islands, La. to investigate the geologic controls on barrier island framework. This report serves as an archive of unprocessed digital chirp subbottom data, trackline maps, navigation files, Geographic Information System (GIS) information, and formal Federal Geographic Data Committee (FGDC) metadata. Gained digital images of the seismic profiles are also 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). Examples of 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 12BIM03.
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, 12BIM03 tells us the data were collected in 2012 during the third 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), '##' is a 2-digit number representing a specific track, and 'a' is a letter representing the section of a line if recording was prematurely terminated or rerun for quality or acquisition problems.
All Chirp systems use a signal of continuously varying frequency; the system used during this survey produces high-resolution, shallow-penetration (typically less than 50-milliseconds (ms)) profile images of sub-seafloor stratigraphy. The towfish contains a transducer that transmits and receives acoustic energy and is typically towed 1 - 2 meters (m) below the sea's 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 time 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 12BIM03 consisted of an EdgeTech SB-424 towfish running DISCOVER v. 3.51 acquisition software and towed 4 m behind the GPS antenna. The data were acquired using a frequency sweep of 4 - 20 kHz, a 43 kHz sample frequency, and a record length of approximately 30 ms . Based on survey speeds of 3.5 knots, the shot spacing was about 0.450 m.
The binary portion of the unprocessed seismic data are 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 ASCII format instead of 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 examples of SU scripts that allow users to remove navigation fixes from the SEG Y headers, along with the fix provided for every 1,000 shots, and produce a printable, gained GIF image of each profile. The printable profiles provided here are GIF images 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).
