FGDC FORMAL SEISMIC METADATA
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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 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 100 joules (J) per shot for lines 01b001-01b046 and 01b082-01b093 and 200 J per shot for lines 01b047-01b080. The reflected energy was received by a streamer and recorded by PC-based Triton Elics Delph Seismic acquisition software. In an attempt to resolve unknown acquisition problems, streamers were switched throughout the first cruise between Innovative Technologies, Inc. (ITI) ST5 and SN Technologies NexGen streamers. Only the NexGen streamer was used for the second cruise. Both streamers contain 10 hydrophones. These are evenly spaced every 2 feet (0.6 m) on the ITI streamer and every 2 m on the NexGen streamer. Only data received by elements 4-7 were summed for lines 01b001-01b028, by elements 5 and 6 for lines 01b029-01b089, and by elements 7 and 8 for lines 01b091-01b093. The streamer was positioned parallel to the boomer sled and laterally separated from it by about 7 m for lines 01b001-01b080, which were collected on the R/V G.K. Gilbert, and by about 4 m for lines 01b082-01b093, which were collected on a 21-foot Proline. The sled was towed about 20 m behind the Global Positioning System (GPS) antenna on the R/V G.K. Gilbert and about 12 m behind on the Proline. 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 0.5 s, the shot spacing was about 1 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 01b002 and 01b081.
For lines 01b001-01b080, Differential GPS (DGPS) navigation was provided by a CSI DGPS Max WAAS/Beacon DGPS receiver, whose accuracy is within 5 m. The DGPS string was fed to a navigation computer running Hypack software, which converted the data to Zone 15 Universal Transverse Mercator (UTM) coordinates. The navigation was then fed to the Delph Seismic acquisition system approximately every second and recorded in the SEG-Y headers. For lines 01b082-01b093, GPS navigation was provided by a Trimble Centurian P-Code receiver, whose accuracy is within about 100 m. The GPS string was fed directly to the acquisition system approximately every second in latitude and longitude and recorded in the SEG-Y headers. Both UTMs and latitude and longitude are provided for all lines. 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 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, North American Datum 1983 (NAD83). 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, 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.
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