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 is constructed.
The boomer energy source consists of capacitors that are charged to a high voltage and then discharged through a transducer in the water. The frequency range of the source is between 300 and 3,000 hertz (Hz). The transducer was towed on a sled at the sea surface providing 105 joules per shot. The reflected energy was received by an Innovative Technologies, Inc. (ITI) ST5 streamer and recorded by PC-based Triton Elics Delph Seismic acquisition software. The streamer contains 10 hydrophones, all of which were used. The phones are evenly spaced every 2 feet (0.6 m). The streamer was positioned parallel to the boomer sled and laterally separated from it by about 7 m. The sled was towed about 20 m behind the Global Positioning System (GPS) antenna, and no correction for this offset has been made. The sample frequency of the data was 12 kilohertz (kHz). All tracklines were recorded to 100 ms. Based on survey speeds of 3.5-4 knots and a shot rate of every 0.25 s, the shot spacing was about 0.5 m.
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 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 included are only needed to process or display the data with Triton Elics Delph Seismic software. No SEG-Y data exists for tracklines SC696_12, SC696_13, SC696_14, SB629_19, and SB629_20.
GPS navigation was provided to the acquisition system approximately every 2 seconds by a Rockwell PLGR GPS receiver. The accuracy of this receiver is within about 15 m. 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 20 m offset between the shotpoint and the GPS antenna. Position fixes for every 500 shots and for the start 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 unprojected, set in geographic coordinates, World Geodetic System 1984 (WGS84). They were created using ESRI GIS software ArcView 3.2 and 8.1, 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 this disc are the ArcView map documents and shapefiles used to create the trackline maps presented here. The map documents are compatible with ArcGIS 8.x (Windows). The shapefiles can also be viewed using ArcView 3.x or public domain software ArcExplorer 2.0 (Windows) and 4.0 (Windows, Mac OSX, 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 provided as PDF files.
Also included on this 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 500 shots, and produce a filtered and 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 this DVD 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.
U.S. Department of the Interior, U.S. Geological Survey
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Last modified: 23:45:47 Sat 12 Jan 2013
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