The preceding equations that relate phase velocities and elastic constants suggest several approaches to solve the inverse mapping that depend on the recording geometry, the recording aperture, and the wave types available. From SH-wave phase velocities near either axis, it is always possible to estimate W44 and W66 because the NMO velocities are equal to corresponding direct velocities. However, from P- and SV-wave phase velocities near either axis, the estimation of the corresponding elastic constants is not straightforward. This section explains what to do in such a case.