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 ms and 1 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 discharged through a transducer in the water. The frequency range of the source is between 300 and 3,000 Hz. The transducer was towed on a sled at the sea surface providing 100-300 joules per shot for 00SCC02 and 135 joules per shot for 00SCC04. Reflected energy was received by an Innovative Transducers, Inc. (ITI) ST5 streamer and recorded by PC-based Triton Elics Delph Seismic acquisition software. The streamer contains 10 hydrophones evenly spaced over 6 m. However, only data received by elements 4-8 were summed, resulting in a higher signal to noise ratio for the data. The streamer was positioned parallel to the boomer sled and laterally separated from it by about 7 m. The sample frequency of the data was 12 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.
During data acquisition, a 6 ms trigger delay of unknown origin was observed in the profiles. This delay resulted in an extra 6 ms being added to the top of each profile. To rectify this problem in the field, a 6 ms recording delay was introduced on some of the tracklines. Lines B00_20 - B00_36 and B00_38a - B00_50 were recorded with this delay. For consistency in this archive, the processed profile images of all other tracklines have been shifted up 6 ms. However, the SEG-Y files have not been altered. Any further processing of 00SCC02 lines B00_01 - B00_19 and B00_37 - B00_38 and all 00SCC04 lines (B00_51 - B00_124) should remove the upper 6 ms of the record.
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 are only needed to process or display the data with Triton Elics Delph Seismic software. No digital data is available for lines B00_05a, B00_14, B00_28a, B00_52 - B00_65, or B00_82. The original trace files for 00SCC02 lines B00_41 and B00_48 were broken up into two or more trace files (e.g., B00_41 became B00_41a and B00_41b) because the original total number of traces exceeds the maximum allowed by the processing system.
To conform to ISO 9660 naming standards, the SEG-Y data files and associated navigation files were all renamed. The "bss" prefix of the original trackline names was shortened to a single "b." For example, trackline "bss00_01a" was renamed "B00_01a."
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. The data required some editing to remove spurious data values and fix incorrectly recorded dates. The edited results were used to generate the trackline navigation 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, 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 map documents and shapefiles used to create the trackline maps. The map documents are compatible with ArcGIS 8.1 (Windows). The shapefiles may also be viewed using ArcView 3.x (Windows, Unix) or public domain software ArcExplorer 2.0 (Windows) and 4.0 (Windows, Mac OS X, 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 original handwritten logbooks are also provided as PDF files.
Also included on this disc are example Seismic Unix 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.
U.S. Department of the Interior, U.S. Geological Survey
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Last modified: 23:48:40 Sat 12 Jan 2013
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