Archive of Digital Chirp Subbottom Profile Data Collected During USGS Cruise 11BIM01 Offshore of the Chandeleur Islands, Louisiana, June 2011

The USGS Saint Petersburg Coastal and Marine Science Center (SPCMSC) - St. Petersburg, FL, assigns a unique identifier to each cruise or field activity. For example, 11BIM01 tells us the data were collected in 2011 during the first 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-ms) profile images of sub-seafloor stratrigraphy. The towfish contains a transducer that transmits and receives acoustic energy and is typically towed 1 - 2 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 time intervals (for example, 0.125 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 11BIM01 consisted of an EdgeTech SB-424 towfish running DISCOVER v. 3.51 acquisition software and towed about 2 m behind the GPS antenna. The data were acquired using a frequency sweep that varied between 4 - 24 kHz; 4 - 20 kHz and 4 - 16 kHz, a 43 kHz sample frequency, and approximately 75 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 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 example SU scripts that allow the user to strip off navigation fixes from the SEG Y headers, along with a fix for every 1,000 shots, and produce a printable, gained GIF image of each profile. The printable profiles provided here are GIF images that were gained using SU software. Refer to the Software page for links to example SU processing scripts and USGS software for viewing the SEG Y files (Zihlman, 1992).