1. A non-transitory machine-readable medium storing instructions executable by a processing resource to cause the machine to: extrapolate, by the processing resource, an up-going pressure wavefield to a subsurface image level; extrapolate, by the processing resource, a down-going velocity wavefield to the subsurface image level; wherein the up-going pressure wavefield and the down-going velocity wavefield are based in part on a measurement by a receiver; determine, by the processing resource, an impulse response at the subsurface image level from a hypothetical seismic source, comprising a representation of a seismic source at the subsurface image level where a seismic source does not exist, by applying an inversion of a Fredholm integral that defines a seismic reciprocity relationship between an actual state and a hypothetical state; and generate a seismic image using the impulse response, the seismic image being indicative of a presence of hydrocarbons or a reservoir at the subsurface image level.