The pressure wave field is 
and the seismic source is
.
The inhomogeneous constant density two dimensional
wave equation is
This equation is hyperbolic and the inhomogeneity is due to the
variable model velocities. The seismic source 
is applied at or
near the surface. Sources are sometimes buried to improve the
signal strength and to reduce noise generated by the near
surface. Receivers are placed in a line parallel to the
expected principal dip of the subsurface.
This is done to reduce the energy scattering by out of plane
reflectors.
