There is no such problem in the dispersion-relation domain, since
triplications are impossible there.
Figure 3 shows how the three approximations shown
in Figure 2 work in the dispersion-relation domain.
Just like elliptical anisotropy,
Muir's four-parameter approximation is calculated
in the dispersion-relation domain
by simply using slownesses instead of velocities
in the same approximation formula.
Note in Figure 3 that the
double-elliptic approximation in the dispersion-relation domain
is nearly exact for this example.