A cross-equalization processing flow for off-the-shelf 4-D seismic data James Rickett Stanford University David E. Lumley Chevron Petroleum Technology Company

A cross-equalization processing flow for off-the-shelf 4-D seismic data James Rickett Stanford University David E. Lumley Chevron Petroleum Technology Company

Summary Gulf of Mexico dataset Motivation Cross-equalization issues Processing flow Results & conclusions

1979 survey

1991 survey

Motivation Cross-equalization important for 4D study –Remove processing/acquisition differences –Remaining differences are due to production Post-stack vs pre-stack –Pre-stack data not easily available –Post-stack analysis quicker and cheaper

Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

34  difference in azimuth Bin-size: 41 x 41ft vs. 247 x 82ft

Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

Before matched-filteringAfter matched-filtering

Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

Envelope

Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

1979 survey1991 survey

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

Survey co-alignment 1979 survey –Bandpass and gain correction –Resampled from 4 to 6ms 1991 survey –Remapped onto 1979 grid –Spatial anti-alias filter –Rotation with linear interpolation Common window and mask

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

difference After co-alignment...

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

Global corrections Matched-filter –Bandwidth and phase –Least squares –Bulk shift Amplitude scale

difference After co-alignment...

difference After global corrections...

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

Non-stationary matched filtering Separate filters for each trace Design window: –0.5 s to 2 s depth (reservoir at 3 s) –3 traces wide Short operator (23 points)

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

Amplitude balancing Corrects for –Different T.V. gain functions –Incorrect amplitudes from matched-filters Assume signal >> noise –Scale data based on R.M.S. energy

difference After global corrections...

difference After non-stationary filtering/gain correction...

Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

Different velocity fields –Effects positioning of imaged events –Need residual migration operator  v unknown –Need estimate operator from the data

Warping Algorithm –Calculate local 3-D cross-correlation functions –Pick maxima –Obtain smoothly varying warp function –Apply vector shifts by interpolation

Typical “warp function”... (magnified x2)

After non-stationary filtering/gain correction difference

difference After warping...

After realignment to common grid After global corrections

After non-stationary corrections After warping

"Normalized difference energy" = RMS(difference)  [ RMS(79 survey) + RMS(91 survey) ]

After global corrections After non-stationary corrections After warping After co-alignment Normalized difference energy

Conclusions Cross-equalization processing flow important for 4-D seismic monitoring Global operators not sufficient Spatially-variable operators required –Balance non-stationarity vs. degrees of freedom Warping –Residual map migration to correct for event mispositioning (e.g. due to migration velocity)

Conclusions Reduced non-reservoir differences using physical processing operators Enhanced reservoir differences –Now ready for 4-D interpretation

Acknowledgements CPTC’s 4-D team in La Habra Chevron for providing the data Sponsors of Stanford Exploration Project