Data Series 119
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 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 water surface and when discharged emits a short acoustic pulse, or shot, that propagates through the water and sediment column. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor), detected by the receiver, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (e.g., 0.25 s) and recorded for specific intervals of time (e.g., 100 ms). In this way, a two-dimensional vertical image of the shallow geologic structure beneath the ship track is produced.
A C-Products C-Boom power supply provided 100 joules per shot. Reflected energy was received by a Benthos MESH streamer and recorded by Triton Elics International, Inc. (TEI) Delph Seismic acquisition software. The streamer contains ten hydrophones evenly spaced about every 30.5 cm (every 12 inches). The sample frequency of the data is 24 kHz. Record length for all tracklines is 100 ms, and the shot rate was every 0.25 s. Shot spacing varied and should be calculated for each line of interest.
Coherent noise was detected on most of the profiles. Attempts to mitigate the noise had varying degrees of success and included frequent changes in the acquisition geometry and grounding of the power supply. The source of the noise remains uncertain, but a new generator or the engine propeller might be the origin. Figure 1 included with this archive (Data Series 119) displays acquisition geometries for this field activity. For more information about the acquisition geometry, refer to the science logbook and handwritten FACS logs also included with this archive.
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 and .PLN files included here are only needed to process or display the data with TEI 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.
Although all data and software published on this DVD have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and (or) the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data, software, or related materials.