High-Resolution Seismic-Reflection and Marine Magnetic Data Along the Hosgri Fault Zone, Central California

Data Acquisition And Processing

Magnetic Data

Marine magnetic data were collected by using a Geometrics G882 cesium-vapor magnetometer that was towed approximately 30 m behind the research vessel. Magnetic data were collected at a 10-Hz sampling rate on a line spacing of 800 m along shore-perpendicular transects simultaneously with the minisparker seismic-reflection data. Additional magnetic transects were run across segments of the Hosgri Fault Zone, resulting in a localized line spacing of 400 m. Also, seismic-reflection and marine magnetic data were collected along three shore-parallel tielines (see survey map). A magnetic base station was set up onshore in Morro Bay State Park on the south flank of Black Hill (lat 35.3536° N., long 120.8314° W.) to continuously measure the local magnetic field in order to remove diurnal field variations from the offshore survey. Base-station readings were recorded every minute, using a Geometrics G856 proton-precession magnetometer. Figure 2 illustrates the importance of operating a local magnetic base station, as opposed to relying on data from a regional magnetic observatory. The regional magnetic observatory in Fresno, Calif. (http://www.intermagnet.org/apps/dl_data_prel_e.php), not only failed to record data during one 28-hour period but also did not capture the maximum amplitude of diurnal variation in the Morro Bay area.

Initial processing of the magnetic data involved smoothing, mainly to remove noise from the minisparker source. The smoothed data resulted in approximate along-track data spacings of 10 m for lines where seismic-reflection data were also collected (boatspeed, 4 knots), and 25 m along the magnetometer-only lines (boatspeed, 10 knots). Further processing included removal of the diurnal field variation by using the magnetic-base-station readings and subtracting the reference field defined by IGRF2005 extrapolated to the survey month June 2008. Data were imported into the Geosoft Oasis montaj™ geophysical software package, where survey lines were leveled by using the tielines and then gridded (200 m) by using a minimum-curvature criterion (Briggs, 1974; Figure 3). In addition, the marine magnetic data were subtracted from the recently collected aeromagnetic data (Langenheim et al., 2008), resulting in a residual-magnetic-anomaly map of the shallow subsurface (<500 m below the sea floor, Figure 4). The aeromagnetic data were collected at a nominal flightline elevation of 300 m over the water, with a flight-track spacing of 800 m, and a flightline orientation parallel to the offshore marine magnetic data.

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 and an Edgetech SB-0512i chirp system. Water depths in the survey area ranged from 6 m near shore to 210 m at the northwest corner of the survey area.

The Edgetech 512 chirp subbottom-profiling system consisted 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 to 7,200 Hz with a 30-ms sweep length, recorded by hydrophones located on the bottom of the fish. The SIG minisparker system used a 500-J high-voltage electrical discharge that created a source with greater power and lower frequency than the chirp and was received by a towed 15-m-long hydrophone streamer. The minisparker source was fired at a rate of 2 times per second, which, at normal survey speeds of 4 to 4.5 knots, gave a data trace every 1 m. Record lengths were 0.35 s for the chirp and 0.5 s for the minisparker. 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 merged seismic-reflection data with differential GPS navigation data. Digital sampling was 12.5 kHz for the chirp data and 16 kHz for the minisparker 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. All the lines that were collected with the chirp system are indicated by the prefix “PBC”, and minisparker lines begin with the prefix “PBS”.

During initial deployment, the chirp system was unable to image deeper than 10 m subbottom depth and was quickly abandoned in favor of the lower-frequency minisparker system, which was able to penetrate as deep as 150 m. Subsequently, only four chirp lines were collected, and in the rest of the survey we used the minisparker sound source.

After the survey, all the SEG-Y files were read by using Seismic Unix software, and PostScript (PS)-format image files of all the profiles were generated. A short-window (30 ms) automatic-gain-control (AGC) algorithm was applied to both the chirp and minisparker data, and a 160- to 1,200-Hz bandpass filter was applied to the minisparker data. These data-processing steps were applied only for display purposes and have 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.

For more information, contact Ray Sliter.