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Does It Matter What Kind of

Does It Matter What Kind of. Vibroseis Deconvolution is Used?. Larry Mewhort* Husky Energy. Sandor Bezdan Geo-X Systems. Mike Jones Schlumberger. Outline. Introduction Description of Pikes Peak 141/15-06-50-23W5M VSP Filtering elements of the Vibroseis system

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Does It Matter What Kind of

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  1. Does It Matter What Kind of Vibroseis Deconvolution is Used? Larry Mewhort* Husky Energy Sandor Bezdan Geo-X Systems Mike Jones Schlumberger

  2. Outline • Introduction • Description of Pikes Peak 141/15-06-50-23W5M VSP • Filtering elements of the Vibroseis system • Down hole wavelets before and after deconvolutions • Conclusions • Acknowledgements

  3. Introduction I • Effective stratigraphic seismic interpretation is aided by having a constant and known phase in the final section. • Removal of the transfer function between the vibrator pilot sweep and the far-field velocity signature is needed to achieve such high quality seismic. • The downgoing wavefield extracted from a Vertical Seismic Profile (VSP) represents the far-field signature at the discrete depths sampled by the geophones.

  4. Introduction II • Vibroseis deconvolution attempts to remove the transfer function knowing only the pilot sweep; the impulse responses of the geophones and the recording instruments; and usually assuming white reflectivity in the Earth. • The VSP is an ideal tool to test the effectiveness of deconvolutions.

  5. Pikes Peak VSP Experiment • A 3C, five-level ASI tool was used to acquire data from 66 depths 27.0 to 514.5 meters (depth increment of 7.5 meters). • A Mertz HD18 Vibrator located 23 meters from the well head generated an 8 to 200 Hz linear sweep. • The weighted-sum estimate of the ground force (WSEGF) was used as the phase lock signal. • The WSEGF was maintained in phase with the pilot sweep as per the SEG standard.

  6. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation If these were all minimum phase then perhaps all that would be needed would be conventional spiking deconvolution?

  7. Convert the Klauder wavelet into its minimum phase equivalent with the Vibop operator Klauder Wavelet Minimum phase equivalent of the Klauder wavelet Vibop

  8. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  9. The recorded weighted-sum ground force estimates Wavelets 200 ms Amplitude 200 Hz 0 Hz 200 degrees Phase -200 degrees

  10. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  11. 50 Phase (degrees) 0 -50

  12. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  13. Standard Vibroseis Theory • The P-wave far-field particle displacement is proportional to the applied force. • Equivalently, the far-field particle velocity is the time derivative of the true ground force. • In the frequency domain the derivative filter boosts the amplitude spectrum 6 dB/octave and applies a +90 degree phase shift.

  14. Test of whether a differential filter is minimum phase Input Wavelets Derivative Wavelet Wavelet After Wiener Deconvolution Derivative Wavelet Wavelet

  15. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  16. Inverse Geophone Filter Filter Phase Amplitude

  17. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  18. Inverse Instrument Filter (phase has been removed by cross correlation) Amplitude spectrum in dBs

  19. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  20. Q from VSP Q - Spectral Ratios (blue) and Centroid Frequency (red) Q

  21. Theoretical effect of a constant Q of 50 Wavelet at 102 meters depth Wavelet at 514.5 meters depth 102 meter wavelet after applying a Q of 50 over a distance of 400 meters

  22. Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate Flexing Differential Filter Geophone Impulse Response Instrument Impulse Response Attenuation of the Earth ‘Q’ Reflectivity Scattering Attenuation

  23. Downgoing wavelets displayed in depth versus time Time Downgoing Multiple Depth

  24. Downgoing Wavelets 10 ms +200 degrees 90 degrees 0 Hz -200 degrees 200 Hz • wavelets have been compensated for the amplitude and phase effects of the geophone

  25. Finding the best fit Constant phase Wavelet Correlation Coefficient versus Constant Phase Blue is the best fit constant phase wavelet Red is the actual wavelet Green is the zero phase equivalent wavelet

  26. Downgoing Wavelets Average constant phase is 49 degrees 10 ms +200 degrees 0 Hz -200 degrees 200 Hz 90% 100% • wavelets have been compensated for the amplitude and phase effects of the geophone

  27. Spectra for the Downgoing Wavelets before and after Deconvolutions Geophone at 394.5 meters Amplitude (dB)

  28. Wavelets after zero phase deconvolution and geophone phase removal (80 ms operator 0.1% PW) Downgoing multiple 10 ms Average constant phase is 46 degrees Precursor +200 degrees -200 degrees 0 Hz 200 Hz Deconvolution operator designed on wavelets averaged over 400 meters 90% 100%

  29. Low Frequency Adjustments when computing the phase of the T5 Deconvolution Operator 22 dB Amplitude (dB) 48 dB/octave Applied to reduce the effect of low frequency estimation problems on the phase of the output

  30. Wavelets after T5 deconvolution (4 Hz frequency smoothing 0.01% PW) with geophone phase and amplitude removal Average constant phase is -75 degrees 10 ms +200 degrees -200 degrees 200 Hz 0 Hz Deconvolution operator designed on wavelets averaged over 400 meters 90% 100%

  31. Wavelets after T5 deconvolution (4 Hz Frequency smoothing 0.01% PW), low frequency filtering and Vibop Average constant phase is 41 degrees 10 ms +200 degrees Deconvolution operator designed on wavelets averaged over 400 meters -200 degrees 200 Hz 0 Hz 90% 100%

  32. Wavelets after T5 deconvolution (4 Hz frequency smoothing 0.01% PW) and spectral replacement Average constant phase is 3 degrees 10 ms +200 degrees 0 Hz -200 degrees 200 Hz Deconvolution operator designed on wavelets averaged over 400 meters 90% 100%

  33. Wavelets after T5 deconvolution (4 Hz frequency smoothing 0.01% PW) and low frequency filtering Average constant phase is -27 degrees 10 ms +200 degrees 0 Hz -200 degrees 200 Hz Deconvolution operator designed on wavelets averaged over 400 meters 90% 100%

  34. Conclusions I • T5 deconvolution gave the most consistent constant phase results. • Adjusting the Klauder wavelet to minimum phase resulted in wavelets that were less constant phase or compressed (but they appeared to be close to minimum phase). • Spectral replacement of the low frequencies resulted in the wavelets being less consistent than using low frequency filtering.

  35. Conclusions II • The amount of low frequency filtering changed the slope of the low frequency phase curve. • Zero phase deconvolution of course did not change the phase of the original data and did not remove the down-going multiple. • Removing the geophone impulse response was not desirable.

  36. Acknowledgements • Husky Energy • Geo-X (Xi-Shuo Wang and Mike Perz) • Schlumberger • Dr. Gary Margrave • Guillaume Cambois • AOSTRA and the CREWES sponsors

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