Removal of Overburden Velocity Anomaly Effects for Depth Conversion

The time correction method (Armstrong et al., 2001) compensates for the presence of overburden velocity anomalies during depth conversion of interpreted seismic time horizons (primarily using conventional, post-stack time-migrated 3D seismic data). For example, in the North Sea, velocity anomalies may take the form of erosive Quaternary canyons, shallow gas accumulations, Mio-Pliocene and Eocene channels.

Positive and negative time delays are estimated from the push-down or pull-up of reflectors directly beneath the anomalies or from the interpreted time thickness of the anomalous body and associated interval velocities (estimated from well data).

The critical steps are pre-stack simulation of seismic acquisition across the velocity anomalies, incorporating the effects of a Fresnel volume which changes its width as a function of depth, and simulation of common midpoint (CMP) stacking using a linear regression of time delay, Δt versus offset-squared, X².

The time correction method predicts the time distortion for any target horizon and the distortion is removed as a correction in time. Depth conversion is then performed using a background velocity function. The final average velocity map is calculated from the resulting depth structure and the raw times at the target horizon and shows steep lateral velocity gradients which are constrained by the interpreted boundaries of the velocity anomalies.

Benefits

• Removal of systematic time errors at target seismic horizons leads to more reliable depth conversion beneath velocity anomalies (currently restricted to marine surveys);
• Avoids misleading depth structures, especially where well ties lie within the anomalous time delay at the target horizon (this is greater than the width of the actual velocity anomaly) – this could be the difference between a successful infill target and a dry hole;
• Simpler, cheaper and quicker than seismic re-processing, especially pre-stack depth migration;
• Develops understanding of the overburden and helps to build velocity models for pre-stack depth migration;
• Encourages a coherent approach to dealing with velocity anomalies.

Consultancy & Project Work

We offer a consultancy service to the oil and gas industry. We would like to review your seismic overburden in order to help you identify velocity anomalies and advise you on their potential effects. Every reservoir has its overburden but not all overburdens are equal!

Under favourable conditions, we can quantify the time delays associated with velocity anomalies at any level and predict their magnitude and spatial extent at underlying reservoir time horizons. After correction for time delays, depth conversion proceeds with functions or layers appropriate for an anomaly-free overburden.

If you would like more information, please contact us at: info@oscorr.com

Software Development

We have written software to implement this technique and have already carried out projects for a number of oil company clients. We now seek oil-industry sponsors to promote and develop this software further into a robust, desk-top product – perhaps as a plug-in to existing interpretation and mapping packages.

We would also like to upgrade the software for land and OBC (ocean bottom cable) 3D seismic surveys. At the same time, we want to gauge the level of industry support for this development.

Please feel free to contact us at: info@oscorr.com

Bibliography

• Al-Chalabi, M. 1979. Velocity determination from seismic reflection data. In: Developments in Geophysical Exploration Methods-1, A.A Fitch (Editor), Applied Science Publishers Ltd, London, pp1-68. (A classic paper. Essential reading for those using stacking and imaging velocities for depth conversion).
• Armstrong, T. 2001. Velocity anomalies and depth conversion – drilling success on Nelson Field, Central North Sea. Presented at 63rd EAGE Conference & Technical Exhibition, Amsterdam, 11-15 June 2001.
• Armstrong, T., McAteer, J. and Connolly, P. 2001. Removal of overburden velocity anomaly effects for depth conversion. Geophysical Prospecting, 49(1), 79 - 99.
• Armstrong, T., McAteer, J. and Connolly, P. 1999. Removal of overburden velocity anomaly effects for depth conversion, Presented at the 61st EAGE Technical Conference and Exhibition, Helsinki, Finland, 7-11 June 1999.
• Blias, E., 2005. Determination of shallow velocity anomalies using deep reflections. SEG, Expanded Abstracts, 24(1), 2585-2588.
• Blias, E., 2005. Stacking velocities in the presence of shallow anomalies. Critique, analysis and improvement of understanding. SEG, Expanded Abstracts, 24(1), 2193-2196.
• Blias, E., 2006. Imaging in the presence of shallow velocity anomalies: Non-first-break technology. SEG, Expanded Abstracts, 25(1), 3071-3075.
• Blias, E., 2009. Stacking velocities in the presence of overburden velocity anomalies. Geophysical Prospecting, 57, 323-341.
• Cartwright, J. 1995. Seismic-stratigraphical analysis of large-scale ridge-trough sedimentary structures in the late Miocene to early Pliocene of the central North Sea. In: Sedimentary Facies Analysis, International Association of Sedimentologists Special Publication No. 22, pp. 285-303, Blackwell Science Ltd.
• Honeyman, W. 1983. Near-surface faulting effects on seismic reflection times. Geophysics 48, 1140-1142.
• Kunst, F. and Deze, J.F. 1985. The case history of a high-resolution seismic survey in the central North Sea. 17th Annual Offshore Technology Conference, Houston, Texas, 6-9, May, 1985. OTC 4968, 103-110.
• Musgrove, F.W. 1994. Time-variant statics corrections during interpretation. Geophysics, 59, 474-483.
• Pickard, J.E. 1992. Velocity modelling of a long-period static anomaly, West Cameron Block 225, a Gulf of Mexico case history. Geophysics 57, 420-430.
• Wingfield, R.T.R. 1990. The origin of major incisions within the Pleistocene deposits of the North Sea. Marine Geology 91, 31-52.

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