Archive of digital chirp subbottom profile data collected during USGS cruise 14BIM05 offshore of Breton Island, Louisiana, August 2014

The USGS Saint Petersburg Coastal and Marine Science Center (SPCMSC) - St. Petersburg, Fla., assigns a unique identifier to each cruise or field activity. The other ID associated with this activity, 14BIM05, indicates the data were collected in 2014 for the Barrier Island Mapping (BIM) study, during the fifth 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: yyi##a, where 'yy' are the last two digits of the year in which the data were collected, 'i' 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 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 system used during this survey produced high-resolution, shallow-penetration (typically less than 50 milliseconds [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 14BIM05 consisted of an EdgeTech 512i towfish running Discover v. 4.10 acquisition software and towed about 20 m, 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, a 40 ms record length on August 14, 2014 and a 65 ms record length for all other days. On the basis of 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 processed and 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).