|U.S. Geological Survey Data Series 237|
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). Example SU processing scripts and USGS software for viewing the SEG-Y files (Zihlman, 1992) are also provided.
The boomer plate is an acoustic energy source that consists of capacitors charged to a high voltage and discharged through a transducer in the water. The transducer is towed on a sled at the sea surface and when discharged 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 a receiver, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (for example, 0.25 s) and recorded for specific intervals of time (for example, 100 ms). In this way, a two-dimensional vertical image of the shallow geologic structure beneath the ship track is produced.
A C-Products C-Boom power supply was used for lines 04b01-04b25, and an Applied Acoustic Engineering CSP 300 power supply was used for lines 04b26a-04b37. For all lines, an Applied Acoustics AA200 plate was used and the power supplies provided 100 J per shot. Reflected energy was received by a Benthos MESH streamer and recorded by Delph Seismic acquisition software. The streamer, which was positioned parallel to the boomer sled and laterally separated from it by about 7 m, contains 10 hydrophones evenly spaced about every 30.5 cm (every 12 inches). Refer to figure 1 included with this archive (Data Series 237) for a diagram of boomer acquisition geometry. Sample frequency of the data was 16-20 kHz, and record length was 99-100 ms. Based on survey speeds of 3.5-4 knots and a shot rate of every 0.25 s, shot spacing was about 0.5 m.
The chirp system uses a signal of continuously varying frequency. The sound source and receiver is a towfish, which is typically flown 2-5 m above the seafloor. For each recorded shot of the chirp data, 3 channels of trace data are collected. Channel 3 is the "real," or in-phase component of the signal, channel 2 is the "imaginary," or quadrature 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 seismic source employed for chirp data collection consisted of an EdgeTech SB-424 towfish running FSSB software. Refer to figure 2 included with this archive (Data Series 237) for a diagram of chirp acquisition geometry, which varied during this cruise. The frequency range was 4-24 kHz. Sample frequency of the data was 25 kHz, and record length was 42-49 ms. Based on survey speeds of 3.5-4 knots and a shot rate of 0.125-0.25 s, shot spacing was about 0.25-0.5 m.
The unprocessed seismic data are stored in SEG-Y, integer, Motorola 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 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 Delph Seismic software. The SEG-Y files will not fit on one disc. They are distributed onto three DVDs with all boomer lines (04b01-04b37) on disc 1, chirp lines 04c01-04c24b on disc 2, and chirp lines 04c25-04c37b on disc 3.
Also provided 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 (boomer) or every 1,000 shots (chirp), and produce a filtered and gained GIF image of each profile.
Although all data and software 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 these data, software, or related materials.