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.