Seismic Reflection Ground Roll Filtering

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Seismic Reflection Ground Roll Filtering

• Ted Bertrand
• SAGE 2004

2
Geology

• Basalt is visible in scarp, capped by alluvium
  and underlain by SF group

Structure

• Fault scarp is visibly eroded
• Most likely multiple faults exist, and possibly
  growth faults
• Need Geophysics!

3
Seismic Reflection

• 2km profile shot across strike of fault scarp
• Vibroseis sweep of 20-120Hz was shot at 5m
  increments along the line
• 60 channels each stacking 5 geophones recorded
  seismic energy for 3s
• RTK GPS coordinates were measured at 50m
  intervals and input into seismic processing
  software

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Ground Roll

• Vibroseis generates 3 types of seismic (pressure)
  waves
• Air waves
• Body waves
• Surface waves
• Surface waves are characterized by
• Low velocity
• Low frequency
• High amplitude

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Ground Roll

• Common problem
• To remove
• Stacking
• Muting
• F-k filtering
• Frequency Filtering

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Raw Data CMP Stack

• Only coherent signals sum during CMP stacking
  after NMO correction is applied
• Surface waves have relatively low velocity
  (600m/s) and are not hyperbolic.
• Therefore, stacking CMP data using reflection
  velocities to apply an NMO correction should
  attenuate surface waves

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Raw Data CMP Stack
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Surgical Muting

• Surface waves can also be removed from the data
  by removing them from the data!
• Disadvantage any coherent signal hidden beneath
  the surface waves is lost.

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Muted CMP Stack
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Frequency Filtering

• A 40Hz 120Hz Butterworth bandpass filter was
  applied to eliminate low frequency ground roll
  waves
• Disadvantage May limit our ability to image
  deeper layers

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40-120 Hz BP CMP Stack
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La Bajada Reflection Profile
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Conclusions

• Bandpass filtering was the most effective method
  for eliminating ground roll
• A faint reflector at 100m corresponds well with
  WT depth from electrical methods
• Location of the fault agrees well with a strong
  gravity gradient, as well as estimates from
  seismic refraction and EM methods