This Digital Versatile Disc (DVD) document is readable on any computing platform that has standard DVD driver software installed. Documentation on this DVD was produced using Hyper Text Markup Language (HTML) utilized by the World Wide Web (WWW) and allows the user to access the information using a web browser (i.e. Netscape, Internet Explorer). To access the information contained on these discs, open the file 'index.htm' located at the top level of each disc using a web browser. This report also contains WWW links to USGS collaborators and other agencies. These links are only accessible if access to the internet is available while viewing these DVDs.

The archived chirp seismic reflection data are in standard Society of Exploration Geophysicists (SEG) SEG-Y format (Barry et al., 1975) and may be downloaded for processing with public domain software such as Seismic Unix (SU), currently located at http://www.cwp.mines.edu/cwpcodes/index.html. Examples of SU processing scripts are provided in the CHIRP.tar file located in the SU subfolder of the SOFTWARE folder located at the top level of each disc. In-house (USGS) DOS and Microsoft Windows compatible software for viewing SEG-Y headers—DUMPSEGY.EXE (Zihlman, 1992)—is provided in the USGS subfolder of the SOFTWARE folder. Processed profile images, trackline navigation maps, logbooks, and formal metadata may be viewed with a web browser.

ABSTRACT

This archive consists of two-dimensional marine seismic reflection profile data collected in Timbalier Bay and in the Gulf of Mexico offshore East Timbalier Island, Louisiana. These data were acquired in June, July, and August of 2001 aboard the R/V G.K. Gilbert. Included here are data in a variety of formats including binary, American Standard Code for Information Interchange (ASCII), HTML, Portable Document Format (PDF), Rich Text Format (RTF), GIF and Joint Photographic Experts Group (JPEG) images, and shapefiles. Binary data are in Society of Exploration Geophysicists (SEG) SEG-Y format and may be downloaded for further processing or display. Reference maps and GIF images of the profiles may be viewed with a web browser. The GIS information provided is compatible with Environmental Systems Research Institute (ESRI) GIS software.

PURPOSE

Marine seismic reflection data are used to image and map sedimentary and structural features of the seafloor and subsurface. These data are useful in mapping stratigraphy and in assessing other submarine geologic characteristics and features. These data were collected as part of a Louisiana Sand Resources Study done in cooperation with the University of New Orleans, the U.S. Army Corps of Engineers, and the Louisiana Department of Natural Resources. This study is part of the USGS Subsidence and Coastal Change (SCC) project.

SUPPLEMENTAL INFORMATION

The USGS Center for Coastal and Watershed Studies team in St. Petersburg, Florida, assigns a unique identifier to all seismic data collected during each cruise or field activity. The method used to assign the identifier for the cruise or field activity is as follows: YYPPP##, where YY stands for the last 2 digits of the year in which the fieldwork is conducted, PPP is a 3-letter abbreviation for the project the data are collected for, and ## is a 2-digit event tag that represents a discreet leg or time period of fieldwork. Here, for example, 01SCC01 tells us the data were collected in 2001 for the Subsidence and Coastal Change Project and that the data were collected during the first field activity for that project in that calendar year. The naming convention the center uses for each seismic line is as follows: yye###a, where yy is the last 2 digits of the year in which the data were collected, e is a 1-letter abbreviation for the equipment type (i.e. c for chirp and b for boomer), ### is a 3-digit number representing a specific track, and a is a 1-letter abbreviation representing the section of a line if recording was prematurely terminated.

Seismic reflection profiles are acquired by means of an acoustic source (usually generated electronically) and hydrophone or receiver arrays. Both elements are typically towed in the water behind a survey vessel. The sound source emits a short acoustic pulse, or shot, that propagates through the water and sediment column. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor) and detected at the receiver. This process is repeated at intervals ranging between 100 milliseconds (ms) and 1 second (s) depending on the seismic source employed. In this way, a two-dimensional vertical image of the geologic structure beneath the ship track can be interpreted.

The chirp system uses a source signal of continuously varying frequency. The seismic source employed consisted of an EdgeTech X-Star SB-424 tow fish running Triton Elics FSSB software. The tow fish is routinely flown 2 - 5 m above the seafloor. Therefore, water depths shown on the chirp profiles are relative to the depth of the tow fish and not to sea level. Furthermore, the raising or lowering of the tow fish during a survey (to avoid obstacles or follow relief) produces a relative shift in the multiple reflections seen in the profile, which may be confused as a geologic feature. Any elevation change of the tow fish is recorded in the crew logbook. The fish was towed about 10 m behind the Global Positioning System (GPS) antenna, and no correction for this offset has been made. The sample frequency of the data was 25 kilohertz (kHz). All tracklines were recorded to 32 ms. Based on survey speeds of 3.5 - 4 knots and a shot rate of 0.125 s, the shot spacing was about 0.25 m. For each recorded shot, three channels of data were collected. Channel 3 is the "real," or in-phase component of the signal, channel 2 is the "imaginary," or quadrative 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. No SEG-Y data exists for lines 01c003, 01c011, 01c012, and 01c027. The original trace files for tracklines 01c001, 01c002, 01c009, 01c010, 01c015, 01c016, 01c019, 01c029, 01c031, 01c047, 01c048, 01c049, and 01c068 were broken up into 2 or more new trace files (i.e. 01c001 became 01c001a and 01c001b) because the original total number of traces exceeds the maximum allowed by the processing system.

Seismic data were stored in SEG-Y format, which is a standard digital format that can be read and manipulated by most seismic processing software packages. The SEG-Y file format includes a 3,200-byte descriptive header that contains detailed information regarding the data acquisition and processing parameters. All data presented here are stored in SEG-Y, integer, Motorola format. The SEG-Y data files are too large to fit on one DVD, so they have been distributed onto four discs with the SEG-Y data files for lines 01c001a - 01c019b on Disc 1, lines 01c020 - 01c040 on Disc 2, lines 01c041 - 01c060 on Disc 3, and lines 01c061 - 01c081 on Disc 4. The SEG-Y formatted trace data files have a .TRA extension. Additional recording parameters for each seismic data file can also be found in the .PAR file associated with each .TRA file. However, the .PAR and .pln files are only needed to process or display the data with Triton Elics Delph Seismic software. These files are all stored in the SEGY folder located at the top level of these discs.

Differential GPS (DGPS) navigation was provided to the acquisition system every second by a WAAS/Beacon DGPS receiver. The accuracy of this receiver is within 5 m. However, the data required some editing to remove spurious data values. The edited results were used to generate the trackline maps presented here. The navigation data have not been corrected to reflect the 10-m offset between the shotpoint and the GPS antenna. Position fixes for every 1,000 shots and for the starts of lines are also provided as an aid for registering of the data after plotting. All navigation files are stored as flat ASCII text files within the NAV folder located at the top level of each disc. Raw navigation files are stored in the RAWNAV subfolder, edited navigation files in the EDITNAV subfolder, and 1,000-shot interval files in the SHOTNAV subfolder.

The trackline maps provided in this archive are in geographic projection. They were created using ESRI GIS software ArcView 3.2, exported to Adobe Illustrator for further editing, and saved for the web in JPEG format. These JPEG images are viewable with a web browser and can be found in the MAPS folder located at the top level of each disc. Also included on each disc are the ESRI ArcView project and shapefiles used to create the trackline maps presented here. These can be found in the ARC subfolder of the SOFTWARE folder located at the top level of each disc. The project is compatible with ArcView 3.x (Unix or Windows) and ArcGIS 8.x (Windows). The shapefiles can also be viewed using public domain software ArcExplorer 2.0 (Windows) and 4.0 (Windows, Unix, Linux), which can currently be downloaded from the ESRI website at http://www.esri.com/software/arcexplorer/index.html.

Field Activity Collection System (FACS) logs are available in both HTML and Rich Text Format. They can be found in the FACS folder located at the top level of each disc. Scanned versions of the handwritten logbooks are also provided as Adobe Acrobat PDF files which can be found in the LOGBOOKS folder located at the top level of each disc.

Also included on each disc are example Seismic Unix (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 gained GIF image of each profile. These images can then be displayed using a variety of shareware programs such as ImageMagick (Unix or Linux) or a web browser. The example scripts are included in the CHIRP.tar file which can be found in the SU subfolder of the SOFTWARE folder located at the top level of each disc.

DISC CONTENTS

INDEX.htm: a home page for viewing this report using a web browser

DISCLAIM.rtf: the USGS disclaimer for this report in Rich Text Format

METADATA.rtf:  formal FGDC metadata for the seismic reflection data and associated shotpoint navigation maps in Rich Text Format

README.rtf: a Rich Text Format version of this page

FACS Folder: contains the 01SCC01 and 01SCC02 subfolders

• 01SCC01 Subfolder: contains the FACS logs that document the 01SCC01 overview, equipment, and crew in Rich Text Format
• 01SCC02 Subfolder: contains the FACS logs that document the 01SCC02 overview, equipment, and crew in Rich Text Format

HTML Folder: contains all HTML documents used in this report and JPEG images used by the HTML hyperlinks

LOGBOOKS Folder: contains CREW.pdf, a PDF file with scanned images of the handwritten crew logbooks filled out by James G. Flocks, and FACS.pdf, a PDF file with scanned images of the handwritten FACS logbooks filled out by Dana S. Wiese

MAPS Folder: contains the trackline maps as JPEG images

NAV Folder: contains the EDITNAV, RAWNAV, and SHOTNAVsubfolders

PROFILES Folder: contains the 01SCC01 and 01SCC02 subfolders

SEGY Folder: contains unprocessed digital seismic reflection data in SEG-Y format divided among four discs. Also included are the companion .PAR files and .pln file used by Delph Seismic software to process or display the data.

• Disc 1: contains the SEG-Y data files for lines 01c001a - 01c019b
• Disc 2: contains the SEG-Y data files for lines 01c020 - 01c040
• Disc 3: contains the SEG-Y data files for lines 01c041 - 01c060
• Disc 4: contains the SEG-Y data files for lines 01c061 - 01c081

SOFTWARE Folder: contains the ARC, SU, and USGS subfolders