Pacific Section AAPG Convention

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Modern Industrial Technology And Methodology Used In The Acquisition Of High-Resolution 3D Pcable Seismic Reflection Data, Offshore Central California


Extensive three-dimensional (3D) low-energy offshore seismic reflection surveys were undertaken in 2011 and 2012 for Pacific Gas and Electric Company (PG&E) to image faults and piercing points for constraining slip rates of the Shoreline and the Hosgri Fault Zones. Interpretations of the 3D/2D datasets are being used by PG&E in its ongoing efforts to characterize seismic hazards at the DCPP. The major components of the 3D multichannel data acquisition system were a digital recording system (CNT-2 Controller), 12 to 14 PCable-configured GeoEel solid digital streamers, CSP-D power supplies, and a triple-plate boomer acoustic source. The streamers were connected to a single PCable that was pulled taut from each end by port and starboard paravanes. Very high-resolution data were collected using state-of-the-art high-precision navigation, acoustic sound source, receivers, and recorders. The low-energy (1.5 kJ), high-resolution 3D PCable seismic-reflection profiling system has a frequency range of 100-700 hertz [Hz], with a 200-225 Hz fundamental frequency. A 3D navigation binning system was used. The streamer separation on the P-Cable was 6.25 m; each streamer consisted of eight active channels, with a group interval of 6.25 m. This allowed for tight control of bin spacing by using individual channels or summation of adjacent channels to increase the signal-to-noise ratio. With this configuration, a total of 96 channels (12 streamers) or 112 channels (14 streamers) of data were acquired at a sampling rate of 250 microseconds and a record length of 750 milliseconds (ms). The GeoEel streamer configuration used in this investigation provided a Nyquist frequency of 2 kilohertz. Survey sail-lines for 3D data collection in all surveyed areas used a trackline spacing of 37 to 40 m. The 3D swath width of this spread was 37.5 m or 43.75 m, depending on the number of streamers deployed. Shot spacing was at 3.125 m, with a CDP interval of 3.125 m and a 0.25 ms sample interval. Vertical resolution for all the seismic data is estimated to be 2 m; horizontal resolution is approximately 3 m. Processed 3D seismic-reflection amplitude and attribute volumes provide imaging of faults, stratigraphic horizons and geomorphologic features in unprecedented detail that were used to map fault geometry and evaluate slip-rates. The slip rate for the Shoreline fault zone is estimated to be less than 0.1 mm/yr., an order-of-magnitude less than that for the Hosgri Fault Zone.