Reconstruction of TTI (transversely isotropic with a tilted symmetry axis) velocity fields requires supplementing P-wave reflections with borehole or other data and applying appropriate regularization operators. Here, we discuss a tomographic algorithm that combines reflection data and VSP traveltimes and apply it to an offshore field data set. The stability and convergence of parameter estimation is improved by implementing a three-stage model-updating procedure that gradually relaxes the constraints on the spatial variations of the Thomsen anisotropy parameters, while the symmetry-direction velocity is updated on a fixed rectangular grid. Structure-guided regularization helps propagate along interfaces reliable parameter updates corresponding to well locations. The algorithm is tested on a 2D line from an ocean-bottom survey acquired at Volve field in the North Sea. Check-shot traveltimes in two nearby wells are used in estimating the initial parameters and constraining the tomographic inversion. The obtained TTI model, despite having a somewhat lower vertical resolution than an existing anisotropic model for the field (which incorporates more detailed borehole information), noticeably improves image quality. The accuracy of the depth scale of the migrated section is verified by comparing prominent imaged reflectors to the available well markers.


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