In June of 2006, the U.S. Geological Survey conducted a geophysical survey offshore of Isles Dernieres, Louisiana. This report serves as an archive of unprocessed digital Chirp seismic reflection data, trackline maps, navigation files, GIS information, Field Activity Collection System (FACS) logs, observer's logbook, and formal FGDC metadata. Gained digital images of the seismic profiles are also provided.
The archived trace data are in standard Society of Exploration Geophysicists (SEG) SEG-Y format (Barry and others, 1975) and may be downloaded and processed with commercial or public domain software such as Seismic UNIX (SU). Example SU processing scripts and USGS software for viewing the SEG-Y files (Zihlman, 1992) are also provided.
For more information on the seismic surveys see
http://walrus.wr.usgs.gov/infobank/g/g106la/html/g-1-06-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 Coastal Monitoring project, which is part of a broader study focused on Subsidence and Coastal Change (SCC).
The USGS Florida Integrated Science Center (FISC) - St. Petersburg assigns a unique identifier to each cruise or field activity. For example, 06SCC01 tells us the data were collected in 2006 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 ID. The naming convention used for each seismic 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, b for boomer and 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 2-5 m above the seafloor. 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 Triton Imaging SB Logger (FSSB) V.1.6.421 acquisition software and towed about 10-m behind the GPS antenna. The frequency range was 4-20 kHz. Sample frequency of the data was 25 kHz, and record length was 30 ms. Shot spacing was about 0.668 m. During data collection, a vertical striped pattern was observed in the profiles. This anomaly was caused by an acquisition software malfunction that resulted in zero values being recorded for traces at miscellaneous intervals. For each recorded shot of the Chirp data, three 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 printable profiles provided here are GIF images that were gained using Seismic UNIX software. The processed SEG-Y data were exported to Chesapeake Technology, Inc. (CTI) SonarWeb software to produce interactive versions of the profiles that allow 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 that the only supported Web browsers that properly display all features of the interactive profiles are Internet Explorer 6 for Windows and Firefox 1.5 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 Settings. 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 to be something other than one. The year and date were also omitted from the SEG-Y headers. A conversion program called FixPing V.1.0.0 was run to correct these three errors.The unprocessed seismic data are stored in SEG-Y, 4-byte integer format, which is a standard digital format that can be read and manipulated by most seismic processing software packages (Barry and others, 1975). The SEG-Y formatted trace files have a .SEG extension. Also provided are example Seismic UNIX 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 gained GIF image of each profile.
