INSPIRE Phase 2 - Lithology Prediction From Refined Interval Vp and Vs Values

In this issue we take a closer look at the INSPIRE project where funding is now (early 2004) being sought for Phase 2. The research team is headed by Anton Ziolkowski ( Anton.Ziolkowski@ed.ac.uk ), leader of the IOR Research Group at Edinburgh University ably assisted by Guangpin Li ( Guangpin.Li@glg.ed.ac.uk ) and David Taylor ( D.B.Taylor@ed.ac.uk ). In Phase 1 the group has developed tools for P-P and P-SV travel time inversion applicable to OBC and 3C land data. This development was funded by Shell Exploration and Production, ExxonMobil and the DTI. In Phase 2 the tools which give a consistent P -P and P-SV horizons will be applied to predict lithology on the basis of interval P and S velocities. The travel time inversion can be refined in regions of interest and finite difference elastic modelling can be employed to confirm amplitudes. The team also plan to exploit cross-line information to identify shear wave polarisations, which in the context of the inversion will allow fracture zones to be identified. The ANISEIS anisotropic modelling package is available to assist in relating surface polarisation to crack geometry. The sponsor ticket price for Phase 2 is £30k pa, over three years. If you would like a copy of the JIP proposal, please email Professor Anton Ziolkowski ( Anton.Ziolkowski@ed.ac.uk ) or Dr David Taylor ( D.B.Taylor@ed.ac.uk ).

Joint Inversion of the Guillemot Data

The techniques developed in Phase 1 of INSPIRE are exemplified in their application to an OBC data set from the Guillemot field in the North Sea . In this phase of the project Guy Hall ( Guy.Hall@glg.ed.ac.uk ) played an important part. Statics are first removed from pre-stack data and then travel time curves are picked from shot gathers using computer enhanced interaction. From travel times from the vertical component (Figure 1) interval P velocities are calculated using an inversion scheme based on ray tracing and accommodating dip and curvature. From the in-line component, travel times are selected which represent dominant shear wave reflections (Figure 2). Application of this velocity inversion method progressively from the top down allows the location of interfaces from P-P reflections and P-SV reflections to be compared and a common-earth model with consistent interfaces to be identified. The P- and S-wave velocity model can be improved by migration velocity analysis and the accuracy of the interface geometry can be enhanced by depth migration.

Figure 1: Picked Travel Times From the Vertical Component of Shot 140 (Click for larger view)

Figure 2: Picked Travel Times From the In-Line Component of Shot 140 (Click for larger view)

From a range of shots, interface information can be combined to give a picture of the reflectors (Figure 3). There is good agreement between the P-P and P-S interfaces for the first three picked horizons and it appears that the assumption of a uniform interval velocity is adequate. The fourth horizon shows discrepancies which are attributable in part to the difficulties in picking this horizon and to a possible failure of the assumption of a uniform interval velocity in the fourth layer. These difficulties will be addressed in Phase 2 by introducing more across-shot automation into the picking process and extending the modelling to encompass variations of velocity in the in-line direction.

Figure 3: Interfaces at Depth (Km) (Click for larger view)

Inversion From Travel Time Curves

Underlying the processing of the travel time curves is an optimisation method which represents the curves as polynomials and constructs interfaces consisting of polynomial sections. The method can currently accommodate major dips and curvature but is limited to models with a layer structure. This was appropriate for the Guillemot data in the upper layers, but it is planned to make this method more general.

JIP Proposal: INSPIRE Phase 2 - Lithology Prediction From Refined Interval Vp and Vs Values

In Phase 2, the tools developed in Phase 1 will be applied to predict lithology from interval P and S velocities. The relationship of different lithologies to these velocities is represented pictorially by Berg (1997) (Figure 4). Interval velocities will impose constraints which will be tight for shallow layers.

Figure 4: Map of Lithologies for P Velocities Against Vp/Vs and Poisson's Ratio (after Berg 1997) (Click for larger view)

To achieve resolution in areas of interest, additional interfaces will be picked and the inversion process, the pre-stack depth migration and migration velocity analysis will be re-run to give greater detail. To this end the travel time picking will be improved to generate travel time surfaces over source and receiver data and introduce more automation. A more general ray-tracing model will also be employed to allow for more complex geometry. From the cross-line component the method can be used to identify where shear wave splitting occurs and hence identify fracture zones and fracture orientation.

The sponsor ticket price is £30k pa, over three years. If you would like a copy of the JIP proposal, please email Professor Anton Ziolkowski ( Anton.Ziolkowski@ed.ac.uk ) or Dr David Taylor ( D.B.Taylor@ed.ac.uk ).

References

1. Ziolkowski , A., Li, G., Hall, G., Taylor, D., and Mancini, F., 2003, New advances in processing P-waves and S-waves for a shared-earth model. , Proceedings of DTI's Improved Oil Recovery Research Dissemination Seminar, Aberdeen, 24th June 2003
2. Berg, E., 1997, 4C-2D or 3D sea bed seismic methodology , Proceedings of IIR Forum on Acquisition, Processing and Interpretation of Marine Seismic Data, 1274-1282

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