Limitation of the ray-tracing method

From the equation of de-migration (1), it is noticeable that the observed time t₀ exists only if:  

$$\left( \frac{\partial t}{\partial x} \right)_m^2 + \left( \frac{\partial t}{\partial y} \right)_m^2 < \frac{4}{V_{rms}^2}$$ (2)

The equation (2) above expresses the degree of limitation of the method with respect to the V_rms velocity. This means that if the dip of an initial time migrated surface overflows a certain value imposed by the value of the V_rms velocity, the de-migration process will fail.

In practice, I observed that for a dipping plane with a V_rms velocity equal to 2000m/s, this limitation problem occurs for a dip angle equal to 45 degrees.