Industry Article: Building velocity models for depth imaging in the presence of short-wavelength velocity variations

This case study encapsulates several issues that are highly relevant to the latest model building workflows for depth imaging. Firstly, because several datasets of different vintages and most importantly, different acquisition azimuths are input, multi-azimuth tomography is required. This is an increasing trend as more surveys are acquired with wide- and rich-azimuth technology. Secondly, because the chalk shows widespread strong velocity heterogeneity, there are many locations that nicely illustrate how the different azimuths actually contribute to solving short-wavelength velocity variations – much more so than any single azimuth could have done. In order to achieve this, the velocity picking needs to be dense and all the nonhyperbolic trends must be modelled by means of full multi-offset picking. Finally, because of the large velocity contrasts with both the chalk and salt layers, the model building needs to use a hybrid grid tomography technique. More routinely, the model is calibrated with a number of wells distributed across the model building area and VTI anisotropy is incorporated for several layers.

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