1. New ProMAX modules for reflectivity calibration and noise attenuation
David C. Henley

2. New ProMAX modules Introduction
1) The spectral ratio reflectivity calibration method
SPECRAT module examples
2) The spectral clipping attenuation method
CLIPPER module examples
Commentary

3. Introduction
Spectral ratio technique delivers r(t,a) for sea floor P-P reflections, over some range of a. Useful for OBS, OBC data inversion?
Spectral clipping provides non-linear attenuation of monochromatic noise components (e.g. 60 Hz), and reduction of reverbs and other periodic phenomena.

4. Spectral ratio technique
Limited use in some form for ~25 years.
Used in experimental work by Shell in Gulf of Mexico (W.L. Walters, 1975) and offshore Canada (D.C. Henley, 1984).
Applicable only to marine hydrophone streamer data.
Often (incorrectly) applied to X-T domain.

5. Source
Hydrophones
Sea surface R R a
Sea floor
Primary and multiple sea floor reflection observed along a single raypath

6. Mathematics 1
Primary sea floor reflection
Single bounce multiple

7. Mathematics 2

8. Mathematics 3
Calibrated reflectivity spectrum
Scaled wavelet spectrum

9. Source
Hydrophones
Sea surface R R a
Sea floor
Primary and multiple sea floor reflection observed along a single raypath (more than one hydrophone required)

10. Source Hydrophone Sea surface R R a b Sea floor2 R 1 a b
Sea floor
Primary and multiple sea floor reflection observed at a single offset (only one hydrophone)

11. Geometry issues
Acquisition geometry does not match single-raypath geometry implied by math. Approximate solution--NMO and stack.
Exact single-raypath geometry simulated by Radial Trace transform of shot gather.
Sea floor primary and multiple usually spatially aliased on marine shot gathers, complicating R-T transform.

12. Model
Gated primary reflection from White Rose model Timing line spacing = 20 ms.

13. Model
Gated first order sea floor multiple from White Rose model. Timing line spacing = 20 ms.

14. Model
Calibrated reflectivity functions. Trace spacing = 0.2, timing line spacing = 20 ms

15. Model
Estimated source wavelet. Timing line spacing = 20 ms.

16. Real data
Primary sea floor reflection from R-T transform of real marine shot gather. Timing line spacing = 20 ms.

17. Real data
First order sea floor multiple gated from R-T transform of real marine shot gather. Timing line spacing = 20 ms.

18. Real data
Calibrated sea floor reflectivity functions. Trace spacing = 0.2, Timing line spacing = 20 ms.

19. Real data
Estimated source wavelets. Timing line spacing = 20 ms.

20. Comments--spectral ratio method
Technique can be used on other primary/multiple pairs, but with lower resolution and accuracy.
Higher order sea floor multiples can be used in the formulae, as long as scale factor is adjusted for ratio of path lengths. Resulting reflectivity bandwidth is narrower the higher the multiple order.

21. Spectral clipping technique
Practical (but non-linear) solution to a difficult noise problem
Well suited to strong monochromatic noises
Handles all monochromatic noises in single pass
Very effective in R-T domain against dispersive noise

22. Spectral clipping concept
F.T. f + f -
Seismic trace (left) transforms to amplitude and phase spectrum (right)

23. Spectral clipping concept
Monochromatic noise
Upper threshold A
Wings
(dB)
Peak width
Median spectrum
Lower threshold f
Running median spectrum computed from trace amplitude spectrum, thresholds equidistant above and below median spectrum.

24. Spectral clipping concept
Edited region--peak + wings A
(dB)
Median spectrum f
Raw spectrum edited by replacement with median spectral values

25. Spectral clipping concept
F.T. f + f -
Edited amplitude spectrum and unchanged phase spectrum transform to noise-free trace

26. Example 1
Raw shot gathers from Okotoks field school line

27. Example 1
Okotoks shots after 60 Hz notch filter

28. Example 1
Okotoks shots after spectral clipping.

29. Example 2
Raw shot gathers from Okotoks field school line

30. Example 2
Shot gathers after 60 Hz notch filter

31. Example 2
Okotoks shot gathers after spectral clipping

32. Comments--spectral clipping
Spectral clipping is most reliable on the strongest noise
Phase is undisturbed by spectral clipping
R-T transform of dispersed linear noise yields traces with strong single-frequency noise that can be greatly attenuated with spectral clipping.

33. Acknowledgements
Sponsors of CREWES
CREWES staff