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Open-File Report 2008-1246

High-Resolution Chirp and Mini-Sparker Seismic-Reflection Data From the Southern California Continental Shelf—Gaviota to Mugu Canyon

Data Acquisition and Processing

Single-channel seismic-reflection data for this survey were acquired along shore-perpendicular transects spaced 1.0 to 1.5 km apart, extending offshore to at least the 3-mile limit of California state waters, using the SIG 2Mille mini-sparker and Edgetech SB-0512i chirp systems. Water depths in the survey area ranged from 10 m near shore to 500 m near the offshore extent of Mugu and Hueneme submarine canyons.

The Edgetech 512 chirp subbottom profiling system consists of a source transducer and an array of receiving hydrophones housed in a 500-lb fish towed at a depth of several meters below the sea surface. The swept-frequency "chirp" source signal was 500 Hz to 4,500 Hz, 50 ms length for the 2007 lines and 500 to 7,200 Hz, 30 ms in length for the 2008 lines and was recorded by hydrophones located on the bottom of the fish. All lines that were collected with the chirp system are indicated by the letter C as the third letter of the line number prefix (for example, SBC). The SIG mini-sparker system used a 500 J high-voltage electrical discharge that creates a source with greater power and lower frequency than the chirp and was received by a towed 15 m-long hydrophone streamer. Depending on water depth, the sources for either system were fired at 1 to 4 times per second, which, at normal survey speed of 4 to 4.5 knots, gives a data trace every 2.0 to 0.5 meters. Record lengths vary from 0.16 to 1.20 seconds depending on water depth. The data from each system were digitally recorded in standard SEG-Y 32-bit floating point format with Triton Subbottom Logger (SBL) PC-based software that merges seismic reflection data with differential GPS navigation data. Digital sampling for the chirp data was at 10 kHz (2007) or 12.5 kHz (2008), and was 16 kHz for the mini-sparker data. Differential GPS position fixes were written into the trace headers of the SEG-Y files and are also available as an ASCII text file.

The chirp data image to subbottom depths of 60 m across the Santa Barbara shelf, with vertical resolution of approximately 20 cm. The lower frequency mini-sparker data show deeper penetration (> than 200 m along the profiles crossing the channel) but with less than half the vertical resolution. The chirp system was used in areas where the primary imaging goals are within the upper few tens of meters. Across much of the survey area, chirp lines were alternated with mini-sparker lines in order to provide both detailed chirp images of the shallow subbottom and the deeper penetration of the mini-sparker.

After the survey, all the SEG-Y files were read using Seismic Unix software and postscript (PS) format image files of all the profiles were generated. A short-window (20 ms) automatic gain control (AGC) algorithm was applied to both the chirp and mini-sparker data and a 160 to 1,200 Hz bandpass filter was applied to the mini-sparker data. These data processing steps were applied only for display purposes and have not been applied to the available SEG-Y data. The PS image files were converted to TIFF and smaller JPEG format image files. All of the SEG-Y data files, the navigation file, and the TIFF and JPEG images files are available for download from the Data Tables section of this report.

For more information contact: Ray Sliter

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