Open-File Report 03-497
The archived trace data are in standard Society of Exploration Geophysicists (SEG) SEG-Y format (Barry and others, 1975) and may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU). Examples of SU processing scripts and in-house (USGS) software for viewing SEG-Y headers (Zihlman, 1992) are also provided. Processed profile images, trackline maps, navigation files, Field Activity Collection System (FACS) logs, and formal metadata may be viewed with a web browser, and scanned handwritten logbooks may be viewed with Adobe Reader.
The boomer energy source consists of capacitors that are charged to a high voltage and then discharged through a transducer in the water. The transducer is towed on a sled at the sea surface and 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), detected by the receiver, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (e.g., 500 ms) and recorded for specific intervals of time (e.g., 100 ms). In this way, a two-dimensional vertical image of the shallow geologic structure beneath the ship track is produced.
The transducer provided 65 joules (J) per shot for 95GFP01, 135 J for 96GFP01, 105 or 208 J for 97GFP01, and 100 J for 98GFP02. The power level used for 94GFP01 is not known. For all cruises, reflected energy was received by an ITI ST-5 streamer and recorded by Triton Elics Delph Seismic acquisition software. The streamer contains 10 hydrophones evenly spaced over a length of 6 m. Only data received by elements 4-8 were summed for 96GFP01, 3-8 for 97GFP01, and 5-8 for 98GFP02. It is unknown which hydrophones were used for 94GFP01 and 95GFP01. 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 10-12 kHz for 94GFP01, 12 kHz for 95GFP01, 16 kHz for 96GFP01, and 12 kHz for 97GFP01 and 98GFP02. Trace length for all tracklines was 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.
Digital data were not recorded for 94GFP01 lines PON531_1 and LINE_9A and 96GFP01 line pon96_8. No data were collected for 95GFP01 line SBP_A1 and 98GFP02 lines lasi_16 and lasi_48a - lasi_54. The original trace files for 94GFP01 lines PON531_2, PON61_1, PON62_1, PON62_3, LINE_7, LINE_10, LINE_14, LINE_17, LINE_18, and LINE_20; 96GFP01 lines pon96_6, pon96_9, and pon96_24; and 97GFP01 lines pon97_1, pon97_12, pon97_16, and pon97_17 were divided into two or more trace files (e.g., LINE_7 became LINE_7a and LINE_7b) because the original total number of traces exceeded the maximum allowed by the processing system.
The unprocessed 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 (Barry and others, 1975). 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 files have a .TRA extension. Additional recording parameters for each trace file can also be found in the .PAR file associated with each .TRA file. However, the .PAR and .PLN files included here are only needed to process or display the data with Triton Elics Delph Seismic software. The SEG-Y data files are too large to fit on one disc, so they have been distributed onto two DVDs with the SEG-Y files for 94GFP01, 95GFP01, and 96GFP01 on Disc 1 and the SEG-Y files for 97GFP01 and 98GFP02 on Disc 2.
Also included on these discs 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.
For 94GFP01 and 95GFP01, DGPS navigation was provided to the acquisition system about every 60 s by a Trimble Nav Trac/Nav Beacon XL DGPS receiver. For 96GFP01 and 97GFP01, GPS navigation was provided to the acquisition system by a Rockwell PLGR GPS receiver. This was done about every 60 s for 96GFP01 and about every 2 s for 97GFP01. For 98GFP02, GPS navigation was provided to the acquisition system about every 2 s by a Trimble Nav Graphics GPS receiver. The accuracy of all receivers used is within 15 m. The data required some editing to remove spurious data points and fix incorrectly recorded dates. 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 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 ASCII text files.
The trackline maps provided in this archive are set in geographic coordinates, NAD83 (unprojected). They were created using ESRI GIS software ArcView 3.2 and 8.1, exported to Adobe Illustrator for further editing, and saved in JPEG format. These JPEG images are viewable with a web browser. Also included on these discs are the ArcView map documents and shapefiles used to create the trackline maps presented here. The map documents, created with ArcView 8.1, are compatible with ArcGIS 8.1 (Windows). The shapefiles, created with ArcView 3.2 and 8.1, may be viewed using other versions of ArcView or public domain software ArcExplorer 2.0 (Windows) and 4.0 (Windows, Mac OS X, Unix, Linux), available from the ESRI website at <http://www.esri.com/software/arcexplorer/index.html>.
FACS logs are available in both HTML and Rich Text Format. Scanned versions of the handwritten logbooks are provided as PDF files.
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.