U.S. Geological Survey Data Series 421
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 lake 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 lake bottom, sediment, or rock layers beneath the lake bottom), 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 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 Joules per shot. Reflected energy was received by a Teledyne Instruments SDS-55 hydrophone array streamer and recorded by Triton Elics Delph Seismic v. 2.7 acquisition software. The streamer contains 10 hydrophones evenly spaced every 0.2 m. All phones were turned on during data acquisition. The streamer position varied each day, please refer to figure 1 included with this archive (Data Series 421) for a diagram of the acquisition geometries. Three boomer acquisition geometries were used during this survey. The second method was used because windy weather conditions hindered steerage, and driving the boat in reverse actually helped maintain course and prevented the possibility of the streamer cables becoming entangled in the boat propellers. The third method was used to help attenuate propeller and generator noise. The sample frequency of the data was 24 kHz, and record length varied between 50 and 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 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 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 shots, and produce a filtered and gained GIF image of each profile.
The printable profiles provided here are GIF images that were filtered and 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: The only supported Web browsers that properly display all features of the interactive profiles are Internet Explorer 6 or greater for Windows and Firefox 1.5 or greater 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 Options. If using Internet Explorer 7, you must disable Active Content/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 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.
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