In August of 2005, the U.S. Geological Survey conducted geophysical surveys offshore of Port Fourchon and Timbalier Bay, Louisiana, and in nearby waterbodies. 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. Filtered and 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/g105la/html/g-1-05-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 Subsidence and Coastal Change (SCC) Study.
The USGS Florida Integrated Science Center (FISC) - St. Petersburg assigns a unique identifier to each cruise or field activity. For example, 05SCC01 tells us the data were collected in 2005 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 is 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 sound source and receiver is a towfish, which is typically flown 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 05SCC01 Chirp data collection consisted of an EdgeTech SB-424 towfish running FSSB v. 2.7 software and towed about 14 m behind the GPS antenna. The frequency range was 4-24 kHz. Sample frequency of the data was 25 kHz, and record length was 53-59 ms. Shot rate was 0.125 s, and shot spacing was about 0.261 m. For each recorded shot of the Chirp data, three channels of trace data are collected. Channel 3 is the "real," or in-phase component of the signal, channel 2 is the "imaginary," or quadrature component of the signal, and channel 1 is the "envelope," or product of channels 2 and 3. Only channel 1 was used to produce the profiles presented here; however, all channels are included in the SEG-Y data files. The unprocessed seismic data are stored in SEG-Y, integer, Motorola 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 .TRA extension. Additional recording parameters for each trace file can also be found in the .PAR file associated with each .TRA file. However, the .PAR and .PLN files included here are only needed to process or display the data with Delph Seismic software. The SEG-Y files do not all fit on one disc and have been distributed onto two DVDs. Disc 1 contains SEG-Y files for lines 05c01_a through 05c13_b, and disc 2 contains SEG-Y files for lines 05c14 through 05c21. 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. 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. Warning! 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. Cautionuse 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.
