Archive of Digital Chirp Subbottom Profile Data Collected During USGS Cruise 08CCT01, Mississippi Gulf Islands, July 2008

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Frequently anticipated questions:

What does this data set describe?

Archive of Digital Chirp Subbottom Profile Data Collected During USGS Cruise 08CCT01, Mississippi Gulf Islands, July 2008
In July of 2008, the U.S. Geological Survey conducted geophysical surveys to investigate the geologic controls on island framework from Ship Island to Horn Island, Mississippi, for the Northern Gulf of Mexico (NGOM) Ecosystem Change and Hazard Susceptibility project. Funding was provided through the Geologic Framework and Holocene Coastal Evolution of the Mississippi-Alabama Region Subtask (; this project is also 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, GIS information, FACS logs, observer's logbook, and formal FGDC metadata. Gained digital images of the seismic profiles are also provided.
For more information on the seismic surveys see
These data are also available via GeoMapApp ( and Virtual Ocean ( earth science exploration and visualization applications.
The USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) assigns a unique identifier to each cruise or field activity. For example, 08CCT01 tells us the data were collected in 2008 for research on Coastal Change and Transport (CCT) and the data were collected during the first field activity for that study in that calendar year. Refer to for a detailed description of the method used to assign the cruise or field activity 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, i for 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 system uses a signal of continuously varying frequency. The towfish is a 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 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-512i towfish running Triton Imaging SB Logger (FSSB) v. 1.6.500.9 acquisition software and towed about 16.5 m behind the GPS antenna. The frequency range was 0.5 - 6 kHz. Sample frequency of the data was 21 kHz, and record length was approximately 75 ms. Shot spacing was about 0.386 m. 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, IBM float format (Norris and Faichney, 2002), the first 3,200 bytes of the card image header is in ASCII format instead of EBCDIC format. The SEG-Y files may be downloaded and 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 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 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 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
  1. How might this data set be cited?
    Forde, Arnell S., Dadisman, Shawn V., Flocks, James G., and Worley, Charles R., 2011, Archive of Digital Chirp Subbottom Profile Data Collected During USGS Cruise 08CCT01, Mississippi Gulf Islands, July 2008: U.S. Geological Survey Data Series 620, U.S. Geological Survey, St. Petersburg, FL.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -89.005115
    East_Bounding_Coordinate: -88.630422
    North_Bounding_Coordinate: 30.266262
    South_Bounding_Coordinate: 30.194378
  3. What does it look like? (JPEG)
    Location map of all digital seismic reflection data collected during USGS field activity 08CCT01. This map was created at a scale of 1:728,745. Refer to the Lineage section below for the trackline map creation process and other details. (JPEG)
    Trackline map of digital seismic reflection data collected offshore of West Ship Island (refer to Area 1 on the location map) during USGS field activity 08CCT01. This map was created at a scale of 1:49,700. (JPEG)
    Trackline map of digital seismic reflection data collected offshore of East Ship Island (refer to Area 2 on the location map) during USGS field activity 08CCT01. This map was created at a scale of 1:55,125. (JPEG)
    Trackline map of digital seismic reflection data collected offshore of Horn Island (refer to Area 3 on the location map) during USGS field activity 08CCT01. This map was created at a scale of 1:85,500. (JPEG)
    Zoomed-in trackline map of digital seismic reflection data collected in Dog Keys Pass and offshore of Horn Island (refer to Area 3a on the Area 3 map) during USGS field activity 08CCT01. This map was created at a scale of 1:28,865. (JPEG)
    Zoomed-in trackline map of digital seismic reflection data collected offshore of Horn Island (refer to Area 3b on the Area 3 map) during USGS field activity 08CCT01. This map was created at a scale of 1:60,145.
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 14-Jul-2008
    Ending_Date: 25-Jul-2008
    Data collection interval
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: Archive for Subbottom Profile Data
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      Horizontal X and Y locations (latitude and longitude coordinates) for each shot are provided as ASCII text files, along with the day of year and time (UTC) the shot was recorded.
    2. What coordinate system is used to represent geographic features?
      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.000001. Longitudes are given to the nearest 0.000001. Latitude and longitude values are specified in Decimal degrees. The horizontal datum used is WGS84.
      The ellipsoid used is WGS_1984.
      The semi-major axis of the ellipsoid used is 6378137.000000.
      The flattening of the ellipsoid used is 1/298.257224.
  7. How does the data set describe geographic features?
    Binary data file: Trace data are available as binary files in SEG-Y format. These files have a .sgy extension and range in size from 8 to 457 MB.
    Graphic image file: Printable profiles are available as GIF images. Interactive profiles are available as HTML pages with accompanying JPEG images. The line navigation maps and trackline maps are JPEG images. The trackline maps contain links to the seismic profiles and line navigation maps.
    Navigation file: Navigation files are available as ASCII text files. Provided are raw (unprocessed) shotpoint navigation files, processed shotpoint navigation files, and processed 1,000-shot-interval location files.
    GIS file: The GIS project used to create the trackline maps is provided as a .zip file composed of ESRI map documents, shapefiles, and metadata.
    Barry, K.M., Cavers, D.A., and Kneale, C.W., 1975, Recommended standards for digital tape formats: Geophysics, v. 40, no. 2, p. 344-352. Also available on-line at:
    Cohen, J.K., and Stockwell, J.W., Jr., 2010, CWP/SU: Seismic Unix Release 41: A free package for seismic research and processing, Center for Wave Phenomena, Colorado School of Mines. Available on-line at
    Norris, M.W., and Faichney, A.K, eds., 2002, SEG-Y rev 1 Data Exchange format: Society of Exploration Geophysicists. Also available on-line at
    Zihlman, F.N., 1992, DUMPSEGY V1.0: A program to examine the contents of SEG-Y disk-image seismic data: U.S. Geological Survey Open-File Report 92-590, 28 p. Also available on-line at
    Disc Image Credit: Descloitres, J., MODIS Land Rapid Response Team at the NASA Goddard Space Flight Center, 2001, Mississippi.A2001361.1640.250m.jpg. Available on-line at

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Arnell S. Forde
    • Shawn V. Dadisman
    • James G. Flocks
    • Charles R. Worley
  2. Who also contributed to the data set?
    Funding for this study was provided by the USGS CMGP. We thank R/V G.K. Gilbert captain Richard Young of the USGS - St. Petersburg, FL, for his assistance in data collection. This document was improved by the reviews of Xan Fredericks (Jacobs Technology Inc. - St. Petersburg, FL) and Kim Haag (USGS Florida Water Science Center - Tampa, FL).
  3. To whom should users address questions about the data?
    James Flocks
    600 4th Street South
    St. Petersburg, FL

    (727) 803-8747 ext. 3012 (voice)

Why was the data set created?

To archive all digital Chirp subbottom profile data and associated files collected during cruise 08CCT01.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: 2009 (process 1 of 5)
    Chirp processing: The SEG-Y data were processed with Seismic Unix to produce gained GIF images of the seismic profiles included in this report. A representative Chirp data processing sequence consisted of (1) stripping off navigation data and channel 0 for each shot and converting to SU format, (2) applying automatic gain control, (3) displaying PostScript, and (4) converting of PostScript image to a GIF image. All the processed SEG-Y data were then exported to CTI SonarWeb software to produce an interactive, geospatial version of each profile that allows the user to obtain a geographic location and depth for a cursor position on the profile. Refer to the Supplemental Information section above for details on how to use the interactive profiles.
    Date: 2009 (process 2 of 5)
    Navigation processing: Shotpoint navigation files and 1,000-shot-interval location files were extracted from the trace headers of each line using Seismic Unix software. These files were then processed with PROJ.4 software ( to generate UTM coordinates from latitude and longitude coordinates and reformatted for use with Environmental Systems Research Institute (ESRI) GIS software. NOTE: During data collection of lines 08i71 - 08i79, the navigation system failed; consequently, only one latitude and longitude fix (-88.936298 30.204590, the 1st shot of 08i71) was recorded. This single navigation fix was subsequently assigned to all the preceding lines in both the raw and processed shotpoint navigation files.
    Date: 2009 (process 3 of 5)
    Trackline map creation: The trackline maps and shapefiles provided in this archive were created using the Points to Lines ArcScript (D. Rathert, along with ESRI ArcGIS 9.3 software and then exported to Adobe Illustrator for further editing. The maps were later converted into JPEG format by Adobe GoLive v. 8.0.1. The maps are unprojected (geographic coordinates, WGS84). The cities and interstates layers used on the trackline maps were created by ESRI. The USGS is the originator of all other layers used. A map of trackline navigation for each seismic profile was also produced with CTI SonarWeb software.
    Date: 2010 (process 4 of 5)
    Data Series preparation: In addition to the process steps described above, the following steps were taken to produce this Data Series: the handwritten logs were scanned and saved as PDF files, digital FACS logs were created using the handwritten logs and personal accounts of the crew members and saved as PDF files, and an HTML-based format was used to present the various parts of this archive.
    Date: 24-Jan-2017 (process 5 of 5)
    Keywords section of metadata optimized for discovery in USGS Coastal and Marine Geology Data Catalog. Person who carried out this activity:
    U.S. Geological Survey
    Attn: Alan O. Allwardt
    Contractor -- Information Specialist
    2885 Mission Street
    Santa Cruz, CA

    831-460-7551 (voice)
    831-427-4748 (FAX)
  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
    The validity or accuracy of marine seismic reflection profiles is highly qualitative and depends on equipment and operating condition variables. Visual inspection of the images rendered from the data did not show any major anomalies.
  2. How accurate are the geographic locations?
    As the seismic reflection data were acquired, the position of the vessel was continuously determined by a Coda Octopus F190 receiver, which is accurate to within 20 cm. Positions were recorded and written to the trace headers in latitude and longitude coordinates about every 1 s. The ASCII navigation files and trackline maps have not been corrected to reflect the approximately 16.5-m offset between the Chirp and GPS antenna. Refer to figure 1 included with this archive (Data Series 620) for a diagram of the acquisition geometry, including the GPS offset measurements. Line navigation maps and interactive profiles generated with CTI SonarWeb software have been corrected for the GPS offset.
  3. How accurate are the heights or depths?
    These data are not to be used for bathymetry. Depths shown on the Chirp profiles are relative to the depth of the towfish, not to sea level. Furthermore, the raising or lowering of the towfish 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 towfish is recorded in the logbooks and is evident on the seismic profiles by steep, abrupt shifts in the seafloor return.
  4. Where are the gaps in the data? What is missing?
    These data are collected along tracklines (2-D) and are therefore inherently incomplete. Geologic details between lines must be inferred. An unknown software malfunction resulted in no SEG-Y data being recorded to the headers for lines 08i124 and 08i139 - 08i144.
  5. How consistent are the relationships among the observations, including topology?
    This dataset is from one field activity with consistent instrument calibrations.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: None. These data are held in the public domain.
The USGS requests to be acknowledged as originators of the data in future products or derivative research.
  1. Who distributes the data set? (Distributor 1 of 1)
    Arnell Forde
    U.S. Geological Survey
    600 4th Street South
    St. Petersburg, FL

    (727) 803-8747, ext. 3111 (voice)
    Contact_Instructions: All of this report except the SEG-Y files is available on-line.
  2. What's the catalog number I need to order this data set? U.S. Geological Survey Data Series 620
  3. What legal disclaimers am I supposed to read?
    This DVD publication was prepared by an agency of the United States Government. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty expressed or implied is made regarding the display or utility of the data on any other system, nor shall the act of distribution imply any such warranty. The U.S. Geological Survey shall not be held liable for improper or incorrect use of the data described and (or) contained herein. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.
  4. How can I download or order the data?
    • Availability in digital form:
      Data format: The SEG-Y standard format (Barry and others, 1975) consists of the following: a 3,600-byte reel identification header, with the first 3,200 bytes consisting of an ASCII header block followed by a 400-byte binary header block, both of which include information specific to line and reel number; a trace data block that follows the reel identification header, with the first 240 bytes of each trace block consisting of the binary trace identification header; and seismic data samples that follow the trace identification header. in format SEG-Y Size: 17100
      Media you can order: DVD (format ISO 9660)
      Note: UNIX, Linux, DOS, Macintosh
      Data format: The GIS project used to create the trackline maps is composed of map documents, shapefiles, and metadata. Map documents were created with PC- compatible ESRI ArcGIS 9.3 software. The shapefiles provided may also be viewed using other versions of ArcView, ArcGIS, or public domain software ArcExplorer ( GeoMapApp ( software is compatible with all platforms. in format map document, shapefile, metadata Size: 127
      Network links:
      Data format: SEG-Y data download
      Network links:
    • Cost to order the data: Prices vary.

    • Special instructions:
      Publications are available from USGS Information Services, Box 25286, Federal Center, Denver, CO 80225-0046 (telephone: 1-888-ASK-USGS, e-mail:
  5. What hardware or software do I need in order to use the data set?
    Use of SEG-Y data requires specialized seismic processing software, such as public domain software Seismic Unix (

Who wrote the metadata?

Last modified: 19-Mar-2019
Metadata author:
Arnell Forde
U.S. Geological Survey
600 4th Street South
St. Petersburg, FL

(727) 803-8747, ext. 3111 (voice)
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

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