Archive of Digital Chirp Subbottom Profile Data Collected During USGS Cruises 09CCT03 and 09CCT04, Mississippi and Alabama Gulf Islands, June and July 2009

The USGS SPCMS assigns a unique identifier to each cruise or field activity. For example, 09CCT03 and 09CCT04 tell us the data were collected in 2009 for research on Coastal Change and Transport (CCT) and the data were collected during the third and fourth field activities 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 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 and 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 and SB-424 Chirp systems used during these surveys produce high resolution, shallow penetration (less than 50-ms) profile images beneath the seafloor. 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 during 09CCT03 consisted of an EdgeTech SB-424 towfish running EdgeTech's DISCOVER v. 3.50 acquisition software and towed about 2 m behind the GPS antenna. The frequency range for the entire cruise was 4 - 20 kHz. Sample frequency of the data was 43 kHz, and record length was 100 ms. The source utilized during 09CCT04 consisted of an EdgeTech SB-512i towfish also running DISCOVER v. 3.50 acquisition software; however, it was towed about 9 m behind the GPS antenna. The frequency range varied between 1 - 10 kHz and 0.7 - 12 kHz. Sample frequency of the data was 43 kHz, and record length was 75 ms. Based on survey speeds of 3.5 knots, the shot spacing for both cruises 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 are in ASCII, instead of EBCDIC format. Some data were collected in the EdgeTech's proprietary JSF format and later converted to SEG-Y; all remaining lines were collected in both formats. The SEG-Y files may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). 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 version 3.16.0096 PRO 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 cursor position; this information is displayed in the status bar of the browser. Please note that clicking on the profile image switches it to "Expanded View" (a compressed image of the entire line), and cursor tracking is not available in this mode. 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.com/en-US/firefox/>. Macintosh Instructions (Firefox Only): From the Menu Bar go to Firefox > Preferences > Content (tab) > Checkmark "Enable JavaScript"> Click Advanced... > Checkmark "Change Status Bar Text". Verify that the status bar is enabled by selecting View from the Menu Bar and looking for a checkmark next to "Status Bar"; if the check is missing, highlight and select the text to activate the bar.