1. A non-transitory machine-readable medium storing instructions executable by a processing resource to cause the machine to: extrapolate an up-going pressure wavefield to a subsurface image level; extrapolate a down-going velocity wavefield to the subsurface image level; and determine an impulse response at the subsurface image level from a hypothetical seismic source by spectral division of the extrapolated up-going pressure wavefield by the extrapolated down-going velocity wavefield.