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Anisotropic traveltime tomography for depth consistent imaging of PP and PS data

IFP, Rueil-Malmaison, France

Jan H. Kommedal

BP, Sunbury-on Thames, United Kingdom

Per G. Folstad

BP, Stavanger, Norway

Corresponding author: Karine.BROTO@ifp.fr

The first 20% of the full text of this article appears below.

P-to-S converted seismic waves have aroused significant interest in the oil exploration industry in recent years. When used in addition to PP data, they have proved valuable in reservoir characterization (e.g., lithology discrimination, determination of the fracture orientation) and in reservoir imaging (e.g., in case of the presence of gas, shale/sand discontinuities, multiple reflections).

Combining this information for reservoir imaging requires PP and PS depth-migrated images that are correctly focused (i.e., resulting from the stack of common image point gathers characterized by flat seismic events) and consistent (i.e., PP and PS depth events end up at the same lateral and vertical positions). Methods for velocity model determination have therefore to ensure cofocusing (first requirement) and codepthing (second requirement).

The velocity model determination method we propose is based on prestack traveltime tomography. In the following, we show that joint tomographic inversion of PP and PS data is very well suited to meet the two requirements, especially since anisotropy can be taken into account. Then, after describing the tomographic inversion methodology in some detail, we illustrate how we applied the joint PP and PS tomography to a 2D, 4C North Sea data set. Comparing isotropic and anisotropic inversion results reveals the necessity to take anisotropy into account for obtaining highly accurate depth-migrated images.

	Anisotropic prestack traveltime tomography

 
Prestack traveltime tomography aims at determining a subsurface model that minimizes misfits between calculated traveltimes (computed by prestack ray tracing in the current model) and observed traveltimes (interpreted seismic events in prestack data).

Traveltime information is generally not sufficient to ensure the uniqueness of the solution velocity model. Note that this underdetermination is not specific to traveltime tomography but to any kinematic methods, and is due to e.g., low-incidence angles at reflectors and to the nonillumination of some areas of the subsurface. Traveltime tomography offers the opportunity to find, . . . [Full Text of this Article]