U.S. Geological Survey Data Series 259
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 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). In this way, a two-dimensional vertical image of the shallow geologic structure beneath the ship track is produced.
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 4-20 kHz. Sample frequency of the data was 25 kHz, and record length was 30 ms. Shot spacing was about 0.668 m. During data collection, a vertical striped pattern was observed in the profiles. This anomaly was caused by an acquisition software malfunction that resulted in zero values being recorded for traces at miscellaneous intervals.
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 printable profiles provided here are GIF images that were gained using Seismic UNIX software. The processed SEG-Y data were exported to Chesapeake Technology, Inc. (CTI) SonarWeb software to produce interactive versions of the profiles that allow the user to obtain a geographic location and depth from the profile for a cursor position. This information is displayed in the status bar of the browser. Note that the only supported Web browsers that properly display all features of the interactive profiles are Internet Explorer 6 for Windows and Firefox 1.5 for all platforms. For all browsers, the status bar and JavaScript must be enabled. Firefox also requires "Change status bar text" to be checked under Advanced JavaScript Settings. Use of other browsers may result in spurious or no information given in the status window. Firefox may be downloaded at http://www.mozilla.org/products/firefox.
Post-processing of the data also revealed the first shot value to be something other than one. The year and date were also omitted from the SEG-Y headers. A conversion program called FixPing V.1.0.0 was run to correct these three errors.The unprocessed seismic data are stored in SEG-Y, 4-byte integer 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 .SEG extension. 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 1,000 shots and produce a gained GIF image of each profile.
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