U.S. Geological Survey Data Series 620
The Chirp system uses a signal of continuously varying frequency. The towfish is a sound source and receiver, which is typically towed 1 - 2 m below the sea surface. 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-512i towfish running Triton Imaging SB Logger (FSSB) v. 1.6.500.9 acquisition software and towed about 16.5 m behind the GPS antenna. The frequency range was 0.5 - 6 kHz. Sample frequency of the data was 21 kHz, and record length was approximately 75 ms. Shot spacing was about 0.386 m.
For each recorded shot of the Chirp data, 3 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 archived trace data are in standard SEG-Y rev 1, IBM float format (Norris and Faichney, 2002), the first 3,200 bytes of the card image header is in ASCII format instead of EBCDIC format. The SEG-Y files may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). See the How To Download SEG-Y Data page for download instructions. The SEG-Y formatted trace files have a .sgy extension. Also provided are example SU 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 printable, gained GIF image of each profile. The processed SEG-Y data were exported to Chesapeake Technology, Inc. (CTI) SonarWeb software to produce an interactive version of the profile that allows 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: The only supported Web browsers that properly display all features of the interactive profiles are Internet Explorer 8 or greater for Windows and Firefox 3.6 or greater for all platforms. For both browsers, the status bar and JavaScript must be enabled. Firefox also requires "Change status bar text" to be checked under Advanced JavaScript Options. If using Internet Explorer 8, you must disable scripts/Active X controls. To do this, right click the information bar (found at the top of the page), left click "Allow Blocked Content...", and then select "Yes". 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.
All the processed SEG-Y data were then exported to CTI SonarWeb software to produce an interactive, geospatial version of each profile that allows the user to obtain a geographic location and depth for a cursor position on the profile. Refer to the Supplemental Information section above for details on how to use the interactive profiles.
