Seismic Reflection Ground Roll Filtering Ted Bertrand SAGE 2004

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!

Seismic Reflection 2km profile shot across strike of fault scarp Vibroseis sweep of Hz 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

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

Ground Roll Common problem To remove: –Stacking –Muting –F-k filtering –Frequency Filtering

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

Raw Data CMP Stack

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.

Muted CMP Stack

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

Hz BP CMP Stack

La Bajada Reflection Profile

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