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Pacific Coastal & Marine Science Center

Coastal and Marine Earthquake Studies


Western Coastal and Marine Earthquake Studies.

 

Central California/San Francisco Bay Earthquake Hazards Project, 1998

Common Midpoint Method

Common Midpoint Method

The MCS method derives its benefits from data redundancy and multiple raypaths. The diagram shows four successive shots along a survey line, and the resulting raypaths for reflections from the seafloor. At each shot, a reflection from a common mid point on the seafloor is recorded on increasingly further channels. The increasing travel times can be corrected for geometrically, and the resulting traces stacked together with a resultant increase of signal to noise proportional to the square root of the number of traces stacked, also refered to as the fold. The MCS method also improves the lateral resolution. Note that the CMP distance is half of the distance between channels, and independent of the shot interval. The MCS data from the San Francisco Bay was collected with a 6.25 meter channel spacing, resulting in CMP's that are 3-1/8 meters apart. The shot interval varies between 6.25 and 12.5 meters, but that affects only the resulting fold.

Conventional MCS (research)

  • hydrophone streamer: 2400 to 4000 meters 48 to 240 channels
  • source array: 10 to 20 air guns 2000 to 8000 cubic inches
  • power required: 1000 SCFM
  • group interval: 12.5 to 50 meters
  • shot interval: 50 meters
  • CMP interval: 6.25 to 25
  • meters penetration: > 10 km
  • passband: 5 to 120 Hz
  • resolution (vertical): 20-70 meters

"High-resolution" MCS

  • hydrophone streamer: 100 to 250 meters 24 to 48 channels
  • source array: 1-2 air guns 40 to 80 cubic inches
  • power required: 50 SCFM
  • group interval: 5 to 10 meters
  • shot interval: 5 to 20 meters
  • CMP interval: 2.5 to 5
  • meters penetration: > 1 km
  • passband: 20 to 400 Hz
  • resolution (vertical): 2-7 meters

 

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