U.S. Geological Survey Data Series 420
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, sediment column, or rock beneath. The acoustic energy is reflected at density boundaries (such as the seafloor, sediment, or rock 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). In this way, a two-dimensional (2-D) vertical image of the shallow geologic structure beneath the ship track is produced.
A C-Products C-Boom Low Voltage Boomer System contained a power supply, which provided 100 J per shot. Reflected energy was received by a Teledyne Instruments SDS-55 array and recorded by Chesapeake Technology, Inc. (CTI) SonarWiz.SBP and Triton Elics Delph Seismic v. 2.7 acquisition software. Lines 08b01 - 08b07 were collected using SonarWiz software, and all associated SEG-Y files have a .sgy file extension extension; lines 08b09 - 08b19 used Delph Seismic and have a .tra extension. The streamer contains 10 hydrophones evenly spaced every 1.5 m. During data acquisition, all phones were used. The streamer was positioned approximately 8 m behind the research vessel and laterally separated from the boomer sled by about 3 m. Refer to figures 1 and 2 included with this archive (Data Series 420) for a diagram of the acquisition geometries. The sample frequency of the data was 20 kHz, and record length was 100 ms. Based on survey speeds of approximately 2.5 knots and a shot rate of every 0.500 s, shot spacing was about 0.6425 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 3 m behind the GPS antenna. The CHIRP SEG-Y data files have a .sgy file extension. The frequency range for line 08c01 was between 4 and 16 kHz and 4 and 24 kHz for line 08c02. 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 0.3213 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, 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 .sgy or .tra extension. Additional recording parameters for each trace file can also be found in the .par file associated with each boomer .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 or 1000 shots, and produce a filtered and (or) gained GIF image of each profile.
CHIRP processing: The SEG-Y data were also processed with Seismic Unix to produce GIF images of the seismic profiles included in this report. A representative CHIRP data processing sequence consisted of (1) stripping off channel 0 for each shot and converting to SU format, (2) applying automatic gain control, (3) displaying PostScript, and (4) converting of PostScript image to a GIF image.
All the processed SEG-Y data were then exported to CTI SonarWeb software to produce an interactive version of each profile that allows the user to obtain a geographic location and depth from the profile for a cursor position. Refer to the Supplemental Information section above for details on how to use the interactive profiles.