Archive of Boomer Seismic Reflection Data Collected on USGS Cruise 99SCE01, Little River Inlet to the entrance of Winyah Bay, South Carolina, 8 June - 16 June, 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 1s 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 data presented here are stored in SEG-Y, Integer, Motorola format. The SEG-Y formatted trace data files 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: The acquisition system used to collect the seismic data presented in this report, may have added 6 ms to the top of each seismic trace. However, since no bathymetric data was simultaneously collected with the seismic data we have no way of proving that the added 6 ms does exist for this data set. 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. The fire rate was every 0.25 sec, which resulted in a shot spacing of about 0.5 m based on survey speeds of 3.5-4 knots. Differential GPS navigation was fed to the acquisition system every second by the R/V G.K. Gilbert's onboard navigation system, a Trimble Nav Graphic receiver. The accuracy of this receiver is within 100 m, however, the data proved to be of poor quality and required extensive editing. A filter was designed to remove spurious data values and these edited results can be used to generate trackline maps at any scale or projection desired, however the data has not been corrected to reflect the 20 m offset between the source and the GPS antenna. Also the raw navigation files did not have year or Julian day written to the headers. This information was handwritten into the edited and shotpoint navigation files. Position fixes for every 500th shotpoint are also provided as an aide for easy registering of the data after projection. The trackline maps provided in this report are geographic projections (no datum) and were created using ESRI's GIS software ArcView 3.2 and exported to Adobe Illustrator for further editing, then saved as GIF images. These GIF images are viewable with your WWW browser. All navigation files are stored as flat ASCII text files.