1. Attenuating the ice flexural wave on arctic seismic data David C. Henley

2. Summary Introduction—What is the ice flexural wave, how is it excited? Characteristics of the flexural wave Noise attenuation methods R-T domain techniques—spectral clipping, Gabor deconvolution Example—Hansen Harbour Conclusions

3. What is the ice flexural wave? Flexural wave is often observed on floating ice in the Arctic Flexural wave motion is similar to that of a drum membrane Flexural wave can be described as P-SV internally reflected modes (Ewing et al) Flexural wave is excited by surface vertical source or internal compressional source in a hard layer bounded by fluids

4. Characteristics of the flexural wave Very powerful—usually the strongest wavefield on a shot record by orders of magnitude 2-D wave—attenuates as 1/r, not 1/r 2 Highly dispersed—high frequencies can move at ice P-wave velocity; low frequencies slower than air velocity Confined to the ice—wave does not propagate past edge of floating ice, but reflects efficiently from shore

5. Noise attenuation methods Model noise in R-T domain and subtract in X-T domain—linear Attenuate noise directly in R-T domain using spectral whitening—linear, or spectral clipping—nonlinear

6. Hansen Harbour CREWES 3-C Receiver spread—50 3-C single phones 15 metres apart Colinear shot line centred on receiver spread—203 shots 30 metres apart Dynamite and Vibroseis used as sources to record separate profiles Vertical component Vibroseis data used for ice wave attenuation study

7. 0.0 1.0 2.0 397 7711131 Unscaled raw shot gather: source point on floating ice shoreline Source-receiver offset--m sec

8. 0.0 1.0 2.0 397 7711131 Scaled raw shot gather: source point on floating ice shoreline Source-receiver offset--m sec

9. 0.0 1.0 2.0 -1671-1296-937 Scaled raw shot gather: source point on land Source-receiver offset--m sec Shoreline

10. Dispersion and aliasing Dispersion provides unique separation of ice wave frequency components in R-T domain Spatial aliasing compromises effectiveness of frequency separation by moving components up into seismic band Ideal acquisition would sample ice wave with no aliasing at any frequency

11. Aliasing of the ice flexural wave 1000 m/s 300 m/s No NMO—Ice wave aliased at all frequencies

12. Aliasing of the ice flexural wave 1000 m/s linear NMO—lower frequencies still aliased

13. Aliasing of the ice flexural wave 250 m/s linear NMO—higher frequencies aliased

14. 0.0 1.0 2.0 397 7711131 Unscaled raw shot gather: source point on floating ice. R-T trajectories encounter monochromatic noise, due to dispersion of ice wave shoreline Source-receiver offset--m sec R-T trajectories

15. 0.0 1.0 2.0 200 15003000 R-T fan transform of raw shot gather—ice wave on radial trace is monochromatic R-T velocity—m/s sec Ice wave Reflections Radial trace

16. 0 -40 -80 dB 020406080100120 Hz Spectrum of radial trace with ice wave noise Ice wave peaks

17. Editing the spectrum Gabor deconvolution—linear, more effective on weaker, less monochromatic noise Spectral clipping—nonlinear, most effective on the strongest, most monochromatic noise

18. Seismic trace contaminated with monochromatic noise is transformed to frequency domain

19. Median spectrum computed from raw spectrum; threshold curves placed parallel to median; peaks in raw spectrum exceeding threshold are flagged

20. Flagged raw spectral amplitudes are replaced with median values, phase is unaltered

21. Edited spectrum is transformed back to seismic trace, sans monochromatic noise

22. 0 -40 -80 dB 020406080100120 Hz Spectrum of radial trace with ice wave noise Ice wave peaks

23. 0 -40 -80 dB 020406080100120 Hz Spectrum of radial trace with ice wave noise after spectral clipping

24. 0.0 1.0 2.0 200 15003000 R-T fan transform of raw shot gather after spectral clipping—monochromatic noise greatly attenuated R-T velocity—m/s sec Ice wave Reflections Radial trace

25. 0.0 1.0 2.0 397 7711131 Spectral clipping applied in R-T domain shoreline Source-receiver offset--m sec

26. 0.0 1.0 2.0 397 7711131 Gabor deconvolution applied in R-T domain shoreline Source-receiver offset--m sec

27. 0.0 1.0 2.0 397 7711131 Gabor deconvolution applied in X-T domain—no R-T processing shoreline Source-receiver offset--m sec

28. 0.0 1.0 2.0 397 7711131 Gabor deconvolution applied in X-T domain—R-T domain Gabor deconvolution shoreline Source-receiver offset--m sec

29. 0.0 1.0 2.0 397 7711131 Gabor deconvolution applied in X-T domain—R-T domain spectral clipping shoreline Source-receiver offset--m sec

30. 0.0 1.0 2.0 3.0 sec 1450 CDP Brute stack of Hansen Harbour line with no ice wave filtering

31. 0.0 1.0 2.0 3.0 sec 1450 CDP Brute stack of Hansen Harbour line after ice wave filtering

32. Conclusions Stacking alone is not sufficient to attenuate ice wave noise Ice wave should be properly spatially sampled during acquisition R-T domain more effective than X-T domain for ice wave attenuation Spectral clipping marginally more effective than Gabor decon for strong ice wave

33. Acknowledgements Thanks to Sponsors and staff of CREWES for support and assistance Thanks also to Natalia Soubotcheva for focussing my attention on Hansen Harbour and the ice wave problem