FWI using reflections for deep velocity model updates

Full Waveform Inversion (FWI) utilizes refractions and reflections to improve the accuracy and resolution of the earth subsurface models. The use of refractions is limited by the maximum offset of the acquisition, up to their maximum penetration depth. In contrast, reflections can produce deeper updates with small offsets, but they demand robust and more sophisticated algorithms. FWI using reflections needs hard boundaries in the velocity/density models to simulate backscatter energy and generate the velocity sensitivity kernels. Alternatively, one can apply the wave-equation and first-order Born approximation to decompose the seismic wavefields into background and perturbations. Here, we utilize the acoustic wave-equation in terms of vector reflectivity to produce reflections in the modeling engine of FWI. The vector reflectivity wave-equation is derived by parametrizing the variable density acoustic wave-equation. The main advantages of its insertion in the FWI algorithm are the following: it does not require the construction of density/hard boundaries in the velocity model to generate reflections; it allows the use of reflected events without the need of solving two different wave-equations in the forward and backward propagation; it is more accurate than the method based on the first-order Born approximation and perturbation theory. We illustrate with synthetic and field data examples the use of deep reflections to produce FWI Updates