Deriving 4D velocity effects for the shearwater field with least squares pre-stack time-shift tomography

We present the development of a time-shift least squares tomography technique to directly invert pre-stack time-shift fields to 4D velocity perturbation. The basis of the technique is reviewed, which consists of ray tracing through the baseline velocity field to obtain ray path travel times, which are formulated in conjunction with the measured pre-stack time shift fields as a linear least squares equation system, enabling the calculation of the 4D perturbation field via a tomographic inversion. The incorporation of a Tikhonov regularization scheme allows for the stabilization of the inversion. The application of the technique to a synthetic and real data case for the Shearwater field is presented. The inverted 4D velocity field for the Shearwater dataset is interpreted in relation to a Geertsma modelled Ezz vertical strain section, where a conformance between the variation of the 4D velocity field and the geomechanical regime related strain variations is demonstrated. The pre-stack time-shift tomography method is shown to have benefits over conventional post stack approaches for the derivation of 4D velocity perturbation and offers the opportunity for the application of imaging correction techniques to resolve the influence of dynamic kinematic effects in the overburden on time-lapse measurements at the target reservoir.