Archive of Boomer Seismic Reflection Data Collected on USGS Cruise 99LCA01, Crescent Beach Spring, Florida, 26 April - 27 April, 1999.

Seismic reflection profiles are acquired by means of an acoustic source (usually generated electronically), and a hydrophone or hydrophone array. Both elements are typically towed in the water behind a survey vessel. The sound source emits a short acoustic pulse, which propagates through the water and sediment columns. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor), and detected at the hydrophone. This process is repeated at intervals ranging between 100 ms and 1 s depending on the source type. In this way a two-dimensional image of the geologic structure beneath the ship track is constructed. Seismic data are stored in SEG-Y format, which is a standard digital format that can be read and manipulated by most seismic-processing software packages. The SEG-Y file format includes a 3200-byte descriptive header that contains detailed information regarding the data acquisition and processing parameters. All the data presented in this report are stored in SEG-Y, integer, Motorola format. The SEG-Y formatted trace data files are located in the SEGY folder at the top level of this disc and have a .tra extension. Additional recording parameters for each seismic data file can also be found in the .par file associated with each .tra file. However, the .par files and the trim_gga.pln file are only needed to replay the data with Triton Elics Delph Seismic software. WARNING: In order to comply with ISO 9660 naming standards tracklines d118_1806, d118_1812, and d118_1840 have been changed to d1806, d1812, and d1840 respectively on the trackline maps, navigation files, and the SEG-Y data files. The seismic source employed here was a boomer transducer towed on a sled at the sea surface providing 100 joules per shot. The reflected energy was received by an Innovative Transducer Inc.(ITI) solid core streamer and recorded by PC-based Triton-Elics Delph Seismic acquisition software. The ITI streamer contains 10 hydrophones evenly spaced over 6 meters however only data received by elements 5-8 where summed, resulting in a higher signal to noise ratio for the data. The streamer was positioned parallel to the boomer sled and laterally separated from it by 7 m. The sled was towed 20 m behind the GPS antennae. The sample frequency of the data was 16000 Hz and the total record length was 100 ms, except for lines d118_12 and d118_1840 which were recorded to 200 ms. The fire rate was every 0.5 sec, which resulted in a shot spacing of about 1 m based on survey speeds of 3.5-4 knots. GPS navigation was provided by a Trimble Centurian P-Code receiver directly fed to the acquisition system every second. The accuracy of this receiver is within 15 m. However, the SEG-Y format only allows for seconds of arc which is equivalent to 31 m. The shotpoint navigation provided in this report has not been corrected to reflect the 20 m offset of the source to the GPS antenna. All navigation data are stored in flat ASCII files containing latitude and longitude values that can be used to plot trackline maps of the seismic profiles at any scale or map projection desired. The trackline maps provided on this disc are geographic projections (no datum), and were created with ESRI's GIS software ArcView 3.2, exported to Adobe Illustrator and then saved as a GIF images. The images are viewable with your WWW browser. Also provided on this CD-ROM are Seismic Unix (SU) scripts that allow the user to strip off navigation fixes from the SEG-Y headers along with a fix for every 500th shot. The seismic data is then converted to SU format, filtered, gained and both Postscript and GIF formatted images are produced. These images can them be displayed using a variety of shareware programs such as ghostview (Unix, Linux, MAC, Windows), kview (Linux), or a web browser.