The present disclosure describes methods and systems, including computer-implemented methods, computer program products, and computer systems, for generating a velocity model for a subsurface area. One computer-implemented method includes obtaining, at a data processing apparatus, source wavefields and residual wavefields, wherein the source wavefields and the residual wavefields are calculated based on seismic data associated with the subsurface area; decomposing, by the data processing apparatus, the source wavefields and the residual wavefields into a subsurface azimuth and reflection angle domain; calculating, by the data processing apparatus, a plurality of gradient direction components based on the decomposed source wavefields and the residual wavefields; scaling, by the data processing apparatus, the plurality of gradient direction components based on azimuths and angles associated with each of the plurality of gradient direction components; calculating a gradient direction based on the scaled gradient direction components; and generating the velocity model using the gradient direction.