In March and April of 2010 the U.S. Geological Survey (USGS), in cooperation with the U.S. Army Corps of Engineers (USACE), conducted geophysical surveys to investigate the geologic controls on island framework from just east of Cat Island, Mississippi, to Dauphin Island, Alabama, as part of a broader USGS study on Coastal Change and Transport (CCT). This report serves as an archive of unprocessed digital Chirp seismic reflection data, trackline maps, navigation files, Geographic Information System (GIS) files, Field Activity Collection System (FACS) logs, and formal Federal Geographic Data Committee (FGDC) metadata. Gained (showing a relative increase in signal amplitude) digital images of the subbottom profiles are also provided.
For more information on the seismic surveys see
http://walrus.wr.usgs.gov/infobank/c/c110gm/html/c-1-10-gm.meta.html,
http://walrus.wr.usgs.gov/infobank/m/m210gm/html/m-2-10-gm.meta.html, and
http://walrus.wr.usgs.gov/infobank/i/i310gm/html/i-3-10-gm.meta.html
These data are also available via GeoMapApp (
http://www.geomapapp.org/) and Virtual Ocean (
http://www.virtualocean.org/) earth science exploration and visualization applications.
To archive all digital Chirp subbottom profile data and associated files collected during cruises 10CCT01, 10CCT02, and 10CCT03.
The USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) assigns a unique identifier to each cruise or field activity. For example, 10CCT01, 10CCT02, and 10CCT03 indicate the data were collected in 2010 to study Coastal Change and Transport (CCT) during the first, second, and third 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 field activity identification (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 or i 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. 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 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 at 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 employed during 10CCT01 consisted of an EdgeTech SB-424 towfish running EdgeTech's DISCOVER v. 3.51 acquisition software and towed about 3 m behind the GPS antenna. The data were acquired using a 4 - 16 kHz frequency sweep, a 43-kHz sample frequency, and approximately 100-ms record length. The source utilized during cruises 10CCT02 and 10CCT03 consisted of an EdgeTech SB-512i towfish also running DISCOVER v. 3.5.0 acquisition software, which was towed about 18 m and 6 m (respectively) behind the GPS antenna. The data were acquired using a 0.5 - 8 kHz frequency sweep, a 43-kHz sample frequency, and approximately 75-ms record length. The trace data are archived in standard SEG-Y rev. 0, IBM float format (Barry and others, 1975), with the first 3,200 bytes of the card image header in ASCII format instead of EBCDIC format. The SEG-Y files may be 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 in this archive 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. 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. PC Instructions: 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. Macintosh Instructions (Firefox Only): From the Menu Bar go to 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.