From September 2 through 4, 2008, the U.S. Geological Survey and St. Johns River Water Management District (SJRWMD) conducted geophysical surveys in Lakes Cherry, Helen, Hiawassee, Louisa, and Prevatt, central Florida. This report serves as an archive of unprocessed digital boomer seismic reflection data, trackline maps, navigation files, GIS information, FACS logs, and formal FGDC metadata. Filtered and gained digital images of the seismic profiles are also provided.
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.
For more information on the seismic surveys see
http://walrus.wr.usgs.gov/infobank/j/j408fl/html/j-4-08-fl.meta.html
These data are also available via GeoMapApp (
http://www.geomapapp.org/) and Virtual Ocean (
http://www.virtualocean.org/) earth science exploration and visualization applications.
The data archived here were collected as part of the USGS Lakes and Coastal Aquifers (LCA) study.
The USGS Florida Integrated Science Center (FISC) - St. Petersburg assigns a unique identifier to each cruise or field activity. For example, 08LCA04 tells us the data were collected in 2008 for the Lakes and Coastal Aquifers (LCA) study and the data were collected during the fourth field activity for that study in that calendar year. Refer to
http://walrus.wr.usgs.gov/infobank/programs/html/definition/activity.html for a detailed description of the method used to assign the cruise or field activity ID. The naming convention used for each seismic line is as follows: yye##a, where 'yy' are the last two digits of the year in which the data were collected, 'e' is a 1-letter abbreviation for the equipment type (for example, b for boomer), '##' is a 2-digit number representing a specific track, and 'a' is a letter representing the section of a line if recording was prematurely terminated or rerun for quality or acquisition problems. 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. The power supply was a C-Products C-Boom Low Voltage Boomer (LVB) System, which provided 100 Joules per shot. Reflected energy was received by a 1.8-m-long Teledyne Instruments SDS-55 hydrophone array streamer and recorded by Triton Imaging, Inc. SB-Logger (FSSB) v. 1.6.500.90 and Chesapeake Technology, Inc. (CTI) SonarWiz.SBP acquisition software. The streamer contained 10 hydrophones evenly spaced every 0.2 m. All phones were turned on during data acquisition. Please refer to figure 1 included with this archive (Data Series 445) for a diagram of the acquisition geometry. The sample frequency of the data was 24 kHz, and the record length was 100 ms. Based on survey speeds of approximately 2.5 knots and a shot rate of every 0.500 s, the shot spacing was about 0.6425 m. The unprocessed seismic data are stored in SEG-Y, integer, IBM 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 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 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 CTI SonarWeb software to produce geospatial versions of the profiles, which 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 geospatial 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.
