The problems associated with overturned waves and initial conditions can be overcome by calculating the Green's functions in polar coordinates. Van Trier and Symes 1991 use polar coordinates for similar reasons in their finite difference solution to the eikonal equation.
I will follow exactly the same steps in deriving the algorithms in polar coordinates as I did in rectangular coordinates. In a 2-D polar coordinate frame, the wave equation is,
When this equation is Fourier transformed over time and we obtain, If we assume a solution of the form,P(r) = P(r_{0}) e^{ i kr ( r- r0 ) }, | (2) |
The initial conditions are not supplied at r=0, but rather at some finite initial radius r_{0}. The region between the origin and r_{0} is assumed to be homogeneous. A suitable initial condition is,
Figure shows the amplitude and phase plots for a mono-frequency Green's function calculated for a constant velocity medium. The polar coordinate data has been re-interpolated onto the same rectangular grid as the rectangular coordinate data. The amplitude is now more uniform for all dips. Figure shows one time slice through the full Green's function in the (x,z,t) domain. The result is a much more accurate representation of the wavefront.