High-Resolution Seismic-Reflection and Marine-Magnetic Data from Offshore Central California—San Gregorio to Point Sur

Data Acquisition and Processing

Seismic-Reflection Data

Data were acquired on four separate offshore research surveys. Information on each cruise is available at: http://walrus.wr.usgs.gov/infobank/.

The first offshore survey, S-N1-09-MB (http://walrus.wr.usgs.gov/infobank/s/sn109mb/html/s-n1-09-mb.meta.html) was in November and December 2009, between Moss Landing and Año Nuevo, aboard the U.S. Geological Survey R/V Parke Snavely and the (National Oceanographic and Atmospheric Administration) SV 4107. Approximately 520 km of seismic-reflection profiles were collected at approximately 1,250 m line spacing using a SIG 2mille minisparker source.

The second offshore survey, S-15-10-NC, (http://walrus.wr.usgs.gov/infobank/s/s1510nc/html/s-15-10-nc.meta.html) was in August 2010, between Año Nuevo and San Gregorio aboard the U.S. Geological Survey R/V Parke Snavely. Approximately 190 km of seismic-reflection profiles were collected at approximately 1,250 m line spacing using a SIG 2mille minisparker source.

The third offshore survey, S-06-11-MB (http://walrus.wr.usgs.gov/infobank/s/s0611mb/html/s-06-11-mb.meta.html) was in May 2011, between Moss Landing and Point Sur, aboard the U.S. Geological Survey R/V Parke Snavely. Approximately 460 km of seismic-reflection profiles were collected at approximately 1,250 m line spacing using a SIG 2mille minisparker source; seismic-reflection profiles were collected simultaneously on 7 km of tracklines using the EdgeTech Chirp 512. Marine-magnetic data also were collected on all tracklines.

The fourth offshore survey, S-04-12-MB (http://walrus.wr.usgs.gov/infobank/s/s0412mb/html/s-04-12-mb.meta.html) was in May 2012, in outer Monterey Bay between Santa Cruz and Monterey, aboard the U.S. Geological Survey R/V Parke Snavely. Approximately 240 km of seismic-reflection profiles were collected at approximately 1,250 m line spacing using a SIG 2mille minisparker source; seismic-reflection profiles were collected simultaneously on 37 km of tracklines using the EdgeTech Chirp 512.

The SIG 2mille minisparker system uses a 500-Joule, high-voltage electrical discharge fired at intervals ranging from 0.5 – 2.5 seconds, depending on depth. The source signal frequency bandwidth is about 100 – 1,600 Hz. One-half second intervals were used where water depth was less than about 350 m, which characterizes most of the study area. The 0.5 – second firing rate and normal survey speeds of 4 – 4.5 nautical miles per hour yield a data trace about every meter. The data were recorded on a 15-m-long hydrophone streamer in standard SEG-Y 32-bit floating point format with Triton Subbottom Logger (SBL) software that merges seismic-reflection data with differential GPS navigation data. After the survey, a short-window (20 ms) automatic gain control (AGC) algorithm and a 160 – 1,200 Hz bandpass filter were applied to the minisparker data to make the JPEG images in this report. The SEG-Y files have been resampled to 8 kHz. Data quality varies with substrate, with penetration up to 200 – 300 m in inferred fine-grained sediments.

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 is 500 Hz – 4,500 Hz, and data are recorded by hydrophones located on the bottom of the fish. After the survey, a short-window (20 ms) automatic gain control (AGC) algorithm was applied to these data to create the JPEG images. The SEG-Y files have been resampled to 8 kHz. Data quality similarly varies with substrate, but penetration rarely exceeds a few tens of meters.

Seismic-reflection profile images can be viewed and downloaded from the Survey Map and Data Tables sections of this report. A link to the SEG-Y data files is provided in the Data Tables section.

Magnetic Data

Marine-magnetic data were collected on survey S-06-11-MB using the Geometrics G882 cesium vapor magnetometer. The magnetometer was towed approximately 50 m behind the vessel. Magnetic data were collected at a 10 Hz sample rate along NE/SW transects, simultaneously with the mini-sparker seismic-reflection data, at a line spacing of 1,250 m. Magnetic and seismic-reflection data also were collected along a number of shore-parallel tie lines (see Survey Map). During the survey, a magnetic base station was set up onshore in Moss Landing at Moss Landing Marine Labs (lat 36.7948°N., long 121.7879°W.) to measure the local magnetic field continuously 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.

Initial processing of the magnetic data involved smoothing, mainly to remove noise from the mini-sparker source. The data smoothing resulted in approximate along-track data point spacing of 10 m (boat speed 4 kts). Further processing included removal of the diurnal field variation by using the magnetic base-station readings and subtraction of the reference field defined by the International Geomagnetic Reference Field (IGRF) for 2011. The processed magnetic data are plotted in profile view above the seismic-reflection data. These images can be accessed from the Survey Map and Data Tables section of this report.

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