In June of 2007, the U.S. Geological Survey (USGS), in cooperation with the Louisiana Department of Natural Resources (LDNR), conducted a geophysical survey offshore of the Chandeleur Islands, Louisiana. This report serves as an archive of unprocessed digital Chirp sub-bottom profile data, trackline maps, navigation files, GIS information, FACS logs, observer's logbook, and formal FGDC metadata. Gained digital images of the sub-bottom profiles are also provided.
For more information on the seismic surveys see
http://walrus.wr.usgs.gov/infobank/g/g107la/html/g-1-07-la.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.
The data archived here were collected as part of the USGS Barrier Island Comprehensive Monitoring (BICM) project, which is part of a broader study focused on Subsidence and Coastal Change. The purpose of the study was to investigate the shallow geologic framework and monitor the enviromental impacts of Hurricane Katrina (Louisiana landfall was on August 29, 2005) on the Gulf Coast's barrier island chains.
The USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) assigns a unique identifier to each cruise or field activity. For example, 07SCC01 tells us the data were collected in 2007 for the Subsidence and Coastal Change (SCC) study and the data were collected during the first 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 trackline 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 SB-424 and i for SB-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. The Chirp systems used a signal of continuously varying frequency. The towfish is the 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 sub-bottom 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 sub-bottom source employed for Chirp data collection, during this cruise, consisted of an EdgeTech (FSSB) XStar SB-424 and SB-512i towfish running Triton Imaging SB Logger V. 1.6.4.37.73 acquisition software that was towed about 5.83 m behind the GPS antenna. The frequency range was 4 - 16 kHz (SB-424) and from 0.5 - 8 kHz to 0.5 - 6 kHz (SB-512i). Sample frequencies of the data for the 512i and 424 were 21 and 25 kHz, respectively, and record length was approximately 50 ms. A total of 2 lines were collected using the 424 system and 47 were collected with the 512i. For each recorded shot of the Chirp data, 3 channels of trace data are collected. 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. Only channel 0 was used to produce the profiles presented here. However, all channels are included in the SEG-Y data files. The archived trace data are in standard SEG-Y rev 1, 4-byte integer, IBM float format (Norris and Faichney, 2002); an ASCII format is used for the first 3,200 bytes of the card image header instead of the SEG-Y rev 0 (Barry and others, 1975) EBCDIC format. The SEG-Y files may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) Release 41 (Cohen and Stockwell, 2005). 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 version 3.16.0096 PRO software to produce an 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. NOTE: The only supported Web browsers that properly display all features of the interactive profiles are Internet Explorer 8 or greater for Windows and Firefox 3.6 or greater for all platforms. For all 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>. Post-processing of the data also revealed the first shot value recorded to the SEG-Y headers of lines 07i1, 07i4, 07i5, 07i7, 07i9, 07c14, 07c15, 07i16, 07i17, 07i18, 07i21, 07i22, 07i24, 07i27, 07i31, 07i32, 07i35, 07i36, 07i37, 07i40, 07i42, 07i45, 07i47 to be something other than one. A conversion program called Fixping V. 1.0.0 was used to correct this issue; these corrected SEG-Y files are provided in this archive.
