1887

Abstract

Summary

The seismic quality factor, Q, is generally treated as frequency-independent, yet theory and field evidence suggest it may not be. We measure constant-Q and frequency-dependent Q(f), from downgoing direct-P arrivals in VSP data, over a 1000 m-thick granite interval in Cornwall, SW England, and a 700 m-thick interbedded shale-carbonate sequence in the Barents Sea, and find two significant results.

First, we obtain consistent constant-Q and Q(f) values, although Q(f) values vary smoothly, with frequency-specific ‘attenuation peaks’. In the granite, over 25–90Hz, constant-Q=7547, and Q(f)=35–100. For the sediments, over 15–110Hz, constant-Q=15240, and Q(f)=70–200. We conclude that our Q(f) workflow (using logarithmic decrement of individual frequencies’ amplitudes after geometric spreading corrections, so not imposing an analytic Q(f) form) is workable.

Second, attenuation in this granite is comparably high to that in these sediments. The granite has only naturally-occurring fractures, but they appear sufficient to increase attenuation above an intuitively-expected level. Well-log-based predictions of 1-D scattering contributions to attenuation, for both datasets, could clarify the origins of our measured Q(f).

Overall, we urge that more Q(f) measurements are made, to support of studies of subsurface petrophysical properties and application of signal-processing tools that account for attenuation in seismic imaging.

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2017-06-12
2019-12-14
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