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In order to conform to International Organization for Standardization (ISO) 9660 naming standards, the SEG-Y data files and associated navigation files were all renamed. Each trackline's original "bss" prefix was shortened to a single "b." For example, trackline "bss00c_01a" was renamed "b00c_01a."
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 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 is constructed.
The chirp system uses a source signal of continuously varying frequency. The seismic source employed consisted of an EdgeTech X-Star SB424 tow fish running Triton Elics FSSB software. The tow fish was 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. Note that the raising or lowering of the tow fish during a survey (to avoid obstacles or follow relief) also 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 GPS antenna, and no correction for this offset has been made. The sample frequency of the data was 25 kilohertz (kHz), with a shot rate of every 0.25 s and a record length that varied between 20 and 32 ms. Refer to the GIF profile images for the exact record length of each trackline. Based on survey speeds of 3.5 - 4 knots, the shot spacing was about 0.5 m. For each recorded shot, three 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 lines b00c_02, b00c_52, b00c_68 - b00c_72, and b00c_121 - b00c_124a. The original tracklines b00c_38, b00c_41, b00c_48, and b00c_66 were divided into b00c_38a and b; b00c_41a, b, and c; b00c_48a and b; and b00c_66a and b 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 DVDs with the SEG-Y data files for lines b00c_01a - b00c_23 on Disc 1, lines b00c_24 - b00c_49 on Disc 2, lines b00c_50 - b00c_84 on Disc 3, and lines b00c_85 - b00c_120 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.
Global Positioning System (GPS) navigation was provided to the acquisition system every second by a Trimble-Centurian P-Code receiver. The accuracy of this receiver is within 100 m. However, the data required some editing to remove spurious data values. The edited results were used to generate the trackline maps presented here. In addition, we have converted latitude and longitude to Zone 15 Universal Transverse Mercator (UTM) coordinates. The navigation data have not been corrected to reflect the 10 m offset between the source 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. Included on each disc are the ESRI ArcView project and shapefiles used to create the trackline maps presented here. The project is compatible with ArcView 3.x (Unix, 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 (RTF). Scanned GIF images of the original handwritten logbooks are also provided.
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, 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.
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