The ray-based method aims at a lesser degree of approximation than equation (2). The interaction is calculated
for all the pairs of emitted and received ray that compose the two fields. Plane-wave approximations are used to
calculate these interactions, but they are done for rays, not for entire field. Hence the following expression of the
echo is obtained, where the contributions are summed over the NEm and NRe rays that compose the emitted and
received fields: