U.S. Geological Survey Data Series 308
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 the receiver, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (for example, 0.5 s) and recorded for specific intervals of time (for example, 100 ms). The resulting profiles are two-dimensional (2-D) vertical images of the shallow geologic structure beneath the ship track.
A C System C-Boom power supply provided 105 Joules per shot. Reflected energy was received by a Benthos 1210 hydrophone streamer and recorded by Delph Seismic acquisition software. The streamer contains 10 hydrophones evenly spaced every 0.5 m. For lines 06b01 through 06b21 all phones were used. The streamer was positioned approximately 9 m behind the research vessel and laterally separated from the boomer sled by about 4 m. Refer to figure 1 included with this archive (Data Series 308) for a diagram of the acquisition geometries. The sample frequency of the data was 24 kHz, and record length was 50 ms. Based on survey speeds of 1.5 - 2 knots and a shot rate of every 0.500 s, shot spacing was about 0.386 - 0.514 m.
The Chirp system uses a signal of continuously varying frequency. The towfish is a sound source and receiver, which is typically towed 2 - 5 m above the seafloor. 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, 0.125 s) and recorded for specific intervals of time (for example, 50 ms). Likewise, the resulting profiles are two-dimensional vertical images of the shallow geologic structure beneath the ship track.
The seismic source employed for Chirp data collection consisted of an EdgeTech SB-424 towfish running Triton Imaging SB Logger (FSSB) V. 1.6.421 acquisition software and towed about 10 m behind the GPS antenna. The frequency range was 4 - 24 kHz. Sample frequency of the data was 25 kHz, the shot rate was every 0.250 s and the record length was approximately 50 ms. Shot spacing was approximately 0.193 - 0.257 m.
For each recorded shot of the Chirp data, three channels of trace data are collected. Channel 2 is the "real," or in-phase component of the signal, channel 1 is the "imaginary," or quadrature component of the signal, and channel 0 is the "envelope," or product of channels 1 and 2. Only channel 0 was used to produce the profiles presented here. However, all channels are included in the SEG-Y data files.
The unprocessed seismic data are stored in SEG-Y, integer, 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 .SEG or .TRA extensions. Additional recording parameters for each trace file can also be found in the .PAR file associated with each .TRA file. However, the .PAR files included here are only needed to process or display the data with Delph Seismic software. 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 and (or) 1,000 shots, and produce a filtered and gained GIF image of each profile.