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High-Resolution Seismic-Reflection Data Offshore of Dana Point, Southern California Borderland

Data Acquisition And Processing

Seismic-Reflection Data

Single-channel seismic-reflection data for this survey were acquired along shore-perpendicular transects spaced 800 m apart, extending beyond the 3-mi limit of California State waters, using a SIG 2Mille minisparker. Water depths in the survey area ranged from 15 m near shore to 825 m at the southwest corner of the survey area.

The SIG ELC50 minisparker system used a 500-J high-voltage electrical discharge creating a signal received by a towed SIG ELC820 12-m-long single-channel hydrophone streamer containing 12 elements. The minisparker source was fired at a rate of 0.5 to 1.5 times per second (depending on water depth), which, at normal survey speeds of 4 to 4.5 knots, gave a data trace every 1 to 3 m. Record lengths varied from 0.5 to 1.5 s. The data were recorded digitally in standard SEG-Y 32-bit floating-point format with Triton Subbottom Logger (SBL) PC-based software that merged seismic-reflection data with differential GPS navigation data. Digital sampling was done at a rate of 8 kHz. Differential GPS position fixes were written into the trace headers of the SEG-Y files and also are available as an ASCII text file. The minisparker system was able to image reflectors up to 500 m below the seafloor.

After the survey trace ranges were edited, a seafloor mute was added, and a full trace balance was applied using DISCO seismic-processing software. The edited SEG-Y files were then manipulated by Seismic Unix software to create PostScript (PS)-format image files. A short-window (30 ms) automatic-gain-control (AGC) algorithm was applied for display purposes only and has not been applied to the available SEG-Y data. The PS-format 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-format image files are available for download from the Data Tables section of this report.