There is a clear trend in the geophysical industry towards the development of integrated seismic processing, in particular vertical integration from acquisition to reservoir characterization but also horizontal integration by use of well data and geological information. Whereas inversion formalisms provide a natural framework for the latter, adequate choices have to be made for ensuring the former.
Velocity models required for imaging PP and mode converted PS waves are built in a data consistent way by means of traveltime tomography. These velocity models are then used to obtain seismic data sorted by incidence angle. The following step is a stratigraphic inversion of limited angle stacks in order to quantitatively estimate P and S impedance variations at the reservoir level. The techniques involved in this global approach have been applied to data sets from the North Sea and from offshore West Africa.
Consistent PP and PS images have been obtained from tomographic inversion. P and S impedances have been estimated and interpreted in connection with the reservoir properties.
Velocity model Velocity model determination is the first step on the way from seismic field data to reservoir determination characterization. Our velocity model determination method is based on prestack traveltime tomography. This iterative inversion method searches for a model that minimizes an objective function that measures the misfit between the traveltimes computed by ray-tracing in the current model and the observed traveltimes (interpreted reflection events in the prestack data). Besides, regularization and other geological and geophysical a priori information can be introduced into the tomographic objective function.
Our traveltime tomography software jerry1,2 uses a blocky velocity model representation. Whereas no correlations between the (spatially varying) P and S block velocities are assumed, the P and S models share the same topology (i.e. the same block limits). This implies that the interpreter has to recognize and interpret PP and PS reflection traveltimes generated at the same subsurface interface.
Note that this joint and consistent interpretation ("pairing") can be sensibly facilitated in the depth domain3 and the corresponding PP and PS traveltimes can then be obtained by kinematic demigration4. Pairing in the prestack domain ensures that the PP and PS reflectors are imaged at the same depth positions ("co-depthing"). Since traveltime tomography allows the simultaneous processing of PP and PS traveltimes5 (contrary to most methods used in the industry which are intrinsically limited to a sequential approach) it is also guaranteeing that both the PP and the PS depth images are optimally focused ("co-focusing"). N
