Abstract

Reflection seismic data recorded on the Atlantic Coastal Plain to examine suspected faulting of the shallow (200 m) basement were contaminated by severe ground roll noise with an apparent velocity of 587 m/sec, 25 Hz dominant frequency, and 23 m apparent wavelength. A receiver interval of 5 m and a total spread length of 285 m were used to obtain 24-fold coverage. Although the noise problem was severe, overlapping source and/or receiver arrays on the surface were not used for the recording geometry in order to avoid a decrease in resolution caused by the smoothing effect of overlapping subsurface coverage. Instead, the large-amplitude surface waves were attenuated by a process called Vibroseis whitening (VSW).VSW is based on the application of time-varying amplitude scaling before Vibroseis crosscorrelation. A conventional method of automatic digital gain control was found to be effective for this purpose. This scaling results in a signal-to-noise ratio improvement equal to the gain expected from crosscorrelation. The scaling window length and the length of sweep are the only parameters required to define the signal-to-noise improvement for a given swept-frequency band. A shorter scaling window and longer sweep length give better results because of the higher gain obtained by crosscorrelation.An increase in the quality of Atlantic Coastal Plain data clearly showed that VSW processing resulted in high resolution on stacked sections and made it possible to map shallow basement reflections at 0.2 sec, revealing faults with up to 10 m offset. Reflections from the sedimentary section above basement also became distinct.

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