Open-File Report 03-271
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 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 of the chirp data, 3 channels of trace 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 tracklines 01c003, 01c011, 01c012, or 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 Digital Versatile Disc (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.
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.
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. Also included on each disc is the ESRI ArcView project and shapefiles used to create the trackline maps presented here. 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. Scanned versions of the handwritten logbooks are also provided as Adobe Acrobat PDF files.
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.
Although all data published on these DVDs have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and/or the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of this data, software, or related materials.