Pitfalls in seismic processing : The origin of acquisition footprint Sumit Verma, Marcus P. Cahoj, Bryce Hutchinson, Tengfei Lin, Fangyu Li, and Kurt J. Marfurt, The University of Oklahoma

Content 2 1.Introduction 2.Hypothesis and Methodology 3.Seismic Modeling i.Inadequate removal of groundroll ii.Incorrect velocity picking 4.Results 5.Conclusions IntroductionMethodologySeismic modelingResultsConclusions

Acquisition footprint refers to the imprint of acquisition geometry on the seismic amplitude time and horizon slices. Acquisition footprint can obstruct seismic amplitude/attribute interpretation. Commonly posed question: “How can one remove acquisition footprint?” Our focus: The illustration of the origin of footprint with seismic modeling. Practically: Studied a seismic dataset contaminated with acquisition footprint. Investigated possible causes of the generation of footprint on this dataset. 3 Introduction Introduction MethodologySeismic modelingResultsConclusions

4 Study area Sources Receivers Survey Area  9 mi Fold Fold Map Study area Texas USA Bin :110’ x 110’Introduction MethodologySeismic modelingResultsConclusions

5 Motivation Seismic Amplitude Positive Negative Time (ms) mi Curvature Positive Negative N 1 mi 250 Time slice at t = 420 ms Time (ms) 0Introduction MethodologySeismic modelingResultsConclusions

Inadequate removal of groundroll Improper velocity analysis causing the reflectors to be under/over corrected Not investigated in this study: far offset stretching of the wavelet Footprint – Hypotheses 8 IntroductionMethodology Seismic modelingResultsConclusions

Real seismic survey Simple 2D Seismic modeling 4 layers (acoustic) Real 3D survey geometry Model for dispersive groundroll (elastic) Synthetics with groundroll Inadequate suppression of groundroll Processing Velocity analysis Pick velocities 5% too fast Pick velocities 5% too slow Prestack Kirchhoff time migration Stack the synthetic data Compute geometric attributes Workflow 9 Hypothesis 1 Hypothesis 2 IntroductionMethodology Seismic modelingResultsConclusions Synthetics

Gridded a 2D geologic model and used the finite-difference method to solve the wave equation. We choose the velocity, density, and depth of the formations from well logs. 8 Modeling approach IntroductionMethodology Seismic modelingResultsConclusions

9 Survey geometry Trace-offset relationship from 2D model applied to the 3D survey geometry. Offsets from 2D model redistributed in a 3D geometry.

Inadequate removal of groundroll Improper velocity analysis causing the reflectors to be under/over corrected Not investigated in this study: far offset stretching of the wavelet Footprint – Hypotheses 8 IntroductionMethodology Seismic modeling ResultsConclusions

Offset (ft) Groundroll modeling Depth (ft) IntroductionMethodology Seismic modeling ResultsConclusions

V P =4000 ft/s V P =5000 ft/s V P =8000 ft/s V P =6000 ft/s Offset (ft) Time (ms) Offset (ft) Seismic Amplitude Positive Negative 0 Time (ms) Offset (ft) Groundroll modeling Depth (ft) Groundroll only Groundroll+ reflectors IntroductionMethodology Seismic modeling ResultsConclusions

13 Seismic Amplitude Positive Negative Time (ms) Offset (ft) Time (ms) Offset (ft) Groundroll modeling Groundroll+ reflectors After F-K filter IntroductionMethodology Seismic modeling ResultsConclusions

14 Time (ms) mi Curvature Positive Negative A A’ Stacked image – Groundroll Modeled data Time slice at t = 420 ms N IntroductionMethodologySeismic modelingResults Conclusions

Inadequate removal of groundroll Improper velocity analysis causing the reflectors to be under/over corrected Not investigated in this study: far offset stretching of the wavelet Footprint – Hypotheses 8 IntroductionMethodology Seismic modeling ResultsConclusions

Velocity picking - mute applied Time (ms) Semblance High Low Velocity *10 3 (ft/s) Offset ft/s IntroductionMethodology Seismic modeling ResultsConclusions

17 Velocity picking – 5% too fast velocity Semblance High Low Velocity *10 3 (ft/s) Offset ft/s Time (ms) IntroductionMethodology Seismic modeling ResultsConclusions

18 Velocity picking – 5% too slow velocity Semblance High Low Velocity *10 3 (ft/s) Offset ft/s Time (ms) IntroductionMethodology Seismic modeling ResultsConclusions

19 Stacked image – 5% fast velocity Seismic Amplitude 1 mi Time (ms) mi Curvature Positive N Time slice at t = 616 ms Negative Positive Negative IntroductionMethodologySeismic modelingResults Conclusions

20 Stacked image – 5% slow velocity 1 mi Time (ms) Seismic Amplitude Positive Negative 1 mi Curvature Positive N Time slice at t = 616 ms Negative IntroductionMethodologySeismic modelingResults Conclusions

21 Comparison Groundroll 5% too slow Real data IntroductionMethodologySeismic modelingResults Conclusions

Both improper removal of groundroll and incorrect velocity analysis can create artifacts in a seismic dataset Incorrectly picking velocities by a mere 5% can lead to significant effects in the data. Picking velocities too slow creates a close approximation of the undulatory feature seen in real data. Attributes can often exacerbate acquisition footprint. We believe that undulatory features in the Jean survey could be sourced by incorrect velocity analysis. 22 Conclusions IntroductionMethodologySeismic modelingResultsConclusions

Acknowledgement 23 Data courtesy Mike Burnett of TameCat LLC. AASPI consortium members. Thanks to AASPI consortium sponsors for financial support.

Thank you for your time. Questions? 24