Archive of Digital Chirp Sub-bottom Profile Data Collected During USGS Cruises 08CCT02 and 08CCT03, Mississippi Gulf Islands, July and September 2008

The USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) assigns a unique identifier to each cruise or field activity. For example, 08CCT02 and 08CCT03 tells us the data were collected in 2008 for research on Coastal Change and Transport (CCT) and the data were collected during the second and third field activity for that study 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 or field activity ID. The naming convention used for each sub-bottom 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 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. The Chirp system uses a signal of continuously varying frequency. The towfish is a sound source and receiver, which is typically towed 1 - 2 m below the sea surface. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor), detected by a receiver, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (for example, 0.125 s) and recorded for specific intervals of time (for example, 50 ms). In this way, a two-dimensional vertical image of the shallow geologic structure beneath the ship track is produced. The seismic source employed for Chirp data collection consisted of an EdgeTech SB-424 towfish running EdgeTech's DISCOVER version 3.42 acquisition software and towed about 2 m behind the GPS antenna. The frequency range for both cruises was usually 4 - 16 kHz; however, the frequency range for lines 08c535 - 08c548 and 08c578 - 08c583 of 08CCT02 was 4 - 24 kHz. Sample frequency of the data was 43 kHz, and record length was approximately 75 ms. Shot spacing was about 0.386 m. Standard SEG-Y format allows for the collection of 3 channels of trace data for each recorded shot of Chirp data. Channel 2 is the "real," or in-phase component of the signal, channel 1 is the "imaginary," or quadrature component of the signal, and channel 0 is the "envelope," or product of channels 1 and 2. The acquisition software used for these cruises only saves the envelope data; consequenly, only channel 0 was used to produce the profiles presented here. The archived trace data are in standard SEG-Y, IBM float format (Barry and others, 1975), the first 3,200 bytes of the card image header is in EBCDIC format. The SEG-Y files may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). See the How To Download SEG-Y Data page for download instructions. The SEG-Y formatted trace files have a .sgy extension. 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 processed SEG-Y data were exported to Chesapeake Technology, Inc. (CTI) SonarWeb software to produce a geospatially interactive version of the profile that allows the user to obtain a geographic location and depth from the profile for a given cursor position. This information is displayed in the status bar of the browser. NOTE: The only supported Web browsers that properly display all features of the geospatial profiles are Internet Explorer 8 or greater for Windows and Firefox 3.6 or greater for all platforms. For both browsers, the status bar and JavaScript must be enabled. Firefox also requires "Change status bar text" to be checked under Advanced JavaScript Options. If using Internet Explorer 8, you must disable scripts/Active X controls. To do this, right click the information bar (found at the top of the page), left click "Allow Blocked Content...", and then select "Yes". Use of other browsers may result in spurious or no information given in the status window. Firefox may be downloaded at <http://www.mozilla.org/products/firefox>.