Archive of Digital Chirp Subbottom Profile Data Collected During USGS Cruise 10BIM04 Offshore Cat Island, Mississippi, September 2010

The USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) - St. Petersburg, Fla., assigns a unique identifier to each cruise or field activity. For example, 10BIM04 tells us the data were collected in 2010 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, i for 512i 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 EdgeTech SB-512i Chirp system used during this survey produces high-resolution, shallow-penetration (typically less than 50-ms) profile images of sub-seafloor stratigraphy. The towfish contains a transducer that transmits and receives acoustic energy; it was housed within a float system (built at the SPCMSC), which allows the towfish to be towed at a constant depth of 1.07 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 10BIM04 consisted of an EdgeTech SB-512i towfish running DISCOVER v. 3.51 acquisition software and towed about 10 m behind the Global Positioning System (GPS) antenna. The data were acquired using a frequency sweep that varied between 1 - 10 kHz and 0.5 - 8 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 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 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).
The SEG Y files are available on the DVD version of this report or on the Web, downloadable via the USGS Coastal and Marine Geoscience Data System (http://cmgds.marine.usgs.gov). The data are also available for viewing using GeoMapApp (http://www.geomapapp.org) and Virtual Ocean (http://www.virtualocean.org) multi-platform open source software.