Seismic Reflection Ground Roll Filtering

1. 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

4. 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

5. Ground Roll • Common problem • To remove: • Stacking • Muting • F-k filtering • Frequency Filtering

6. 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

7. Raw Data CMP Stack

8. 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.

9. Muted CMP Stack

10. 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

11. 40-120 Hz BP CMP Stack

12. La Bajada Reflection Profile

13. 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