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

Outline Introduction Description of Pikes Peak 141/ W5M VSP Filtering elements of the Vibroseis system Down hole wavelets before and after deconvolutions Conclusions Acknowledgements

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.

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.

Pikes Peak VSP Experiment A 3C, five-level ASI tool was used to acquire data from 66 depths 27.0 to 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.

Filters that Deconvolution must remove to recover the reflectivity of the Earth Klauder Wavelet Vibrator Electronic and Hydraulic Distortions Baseplate FlexingDifferential 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?

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

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

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

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

Phase (degrees)

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

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.

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

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

Inverse Geophone Filter Filter Phase Amplitude

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

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

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

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

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

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

Downgoing wavelets displayed in depth versus time DepthDepth Time Downgoing Multiple

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

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

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

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

Wavelets after zero phase deconvolution and geophone phase removal (80 ms operator 0.1% PW) Downgoing multiple Deconvolution operator designed on wavelets averaged over 400 meters 10 ms Precursor 0 Hz200 Hz +200 degrees -200 degrees Average constant phase is 46 degrees 100%90%

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

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

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

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

Wavelets after T5 deconvolution (4 Hz frequency smoothing 0.01% PW) and low frequency filtering Deconvolution operator designed on wavelets averaged over 400 meters 10 ms 0 Hz200 Hz +200 degrees -200 degrees Average constant phase is -27 degrees 100%90%

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.

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.

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