In a homogeneous medium, the seismic energy generated by a shot propagates along the straight rays that connect the shot position xs, reflector position (x,z), and receiver position xr. The total travel-time of this propagation can be formulated as
is the vertical two-way travel time. We can solve equation (1) for t0,
Equations (1) and (2) define the relations between two travel times and are the key equations of kinematic operators for imaging.