Open-File Report 03-088
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 columns. 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 source type. In this way, a two-dimensional image of the geologic structure beneath the ship track is constructed.
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, UTM_401_1.pln, and cent_rmc.pln files are only needed to process or display the data with Triton Elics Delph Seismic software.
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 3000 Hz. The transducer was towed on a sled at the sea surface providing 100 joules per shot. The reflected energy was received by an Innovative Technologies, Inc. (ITI) ST-5 solid core streamer and recorded by PC-based Triton Elics Delph Seismic acquisition software. The streamer contains 10 hydrophones evenly spaced over 6 m. Only data received by elements 3, 4, 5, and 6 were summed. 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 GPS antenna. The sample frequency of the data was 12 kHz, and the total record length was 100 ms. The fire rate was every 0.5 s, which resulted in a shot spacing of about 1 m based on survey speeds of 3.5 - 4 knots.
The chirp data use a signal of continuously varying frequency. The seismic source employed for the chirp data consisted of an EdgeTech X-Star SP424 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. Furthermore, the raising or lowering of the tow fish during a survey to avoid obstacles or follow relief 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 kHz. Tracklines 01c001 - 01c009 were shot every 0.25 s to a record length of 50 ms, and tracklines 01c010 - 01c011b were shot every 0.125 s to a record length of 33 ms. Based on survey speeds of 3.5 - 4 knots, the shot spacing for the fire rate of 0.25 s was about 0.5 m, and the shot spacing for the fire rate of 0.125 s was about 0.25 m. For each recorded shot of the chirp data, 3 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. The original trackline 01c011 was divided into 01c011a and 01c011b because the number of traces exceeded the maximum allowed by the processing system.
GPS navigation was provided to the acquisition system every second by a Trimble-Centurian P-Code receiver. The accuracy of this receiver is to 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 the latitude and longitude coordinates to UTM coordinates for UTM Zone 15. The navigation data have not been corrected to reflect the offset between the source and the GPS antenna. Position fixes for every 500 shots and for the start of line are also provided as an aid for registering of the data after plotting. Because boomer and chirp data are collected at different frequencies, position fixes for each system are provided. 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's 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 your WWW browser.
Included on this disc are the ArcView projects and associated shapefiles used to create the trackline maps presented here. The projects were created with ArcView 3.2 and are compatible with ArcView 3.x (Unix or Windows) and ArcGIS 8.x (Windows). The shapefiles can also be viewed using the public domain software ArcExplorer 2.0 (Windows) and 4.0 (Windows, Unix, Linux), which can 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 FACS logbooks and crew logbooks are also provided.
Also included on this DVD are 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 (boomer data) or every 1,000 shots (chirp data), and produce a filtered (boomer data only) and gained, and a GIF image of each profile. These images can then be displayed using a variety of shareware programs such as ImageMagick (Unix or 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.