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For $F(x,t)=t-\tau (x)$, the conormal to its zero set is $dF=(-D\tau ,1)$. The principal symbol of ${\partial }_t^2+L$ isA characteristic hypersurface requires $p(x,t;dF)=0$ on the surface. Thus $\tau$ satisfies the anisotropic eikonal equationThe first-order and zeroth-order terms do not affect this condition.

Assume the medium varies on scales much longer than the wavelength and period, so the leading-order WKB method gives the local eikonal equation. For the phase $\theta$ in the stated ansatz, defineThe local dispersion relation becomes the Hamilton--Jacobi equationDifferentiating it with respect to $x_i$, using equality of mixed derivatives, and choosing a ray whose velocity isgivesHere the dot, or $d/dt$, is the total derivative along a ray,and not merely a partial time derivative at fixed position.

Finally, differentiate $\omega =\Omega (\mathbf{k};\mathbf{x},t)$ along the same trajectory:because the last two terms cancel. The Hamiltonian ray-tracing equations are therefore

Hereso the Euler-Lagrange field equation is the variable-coefficient wave equationAt leading order, $\Phi =A(X,T)\cos \theta$ hasand henceThe leading part of the modulated-wave first integral isA nonconstant periodic wave therefore requires the local dispersion relationThis is usually called the eikonal equation, or equivalently the leading geometric-optics dispersion relation.

Three distinguished limits are needed.

In the origin layer, put $x=\epsilon X$ and $y(x;\epsilon )=Y(X)$. Since $\mathrm{sech}(\epsilon X)=1+O({\epsilon }^2)$, the leading equation isIt has the first integralSolving this first-order linear differential equation givesRegularity at $X=0$ requires $D=-C$, and $Y(0)=1$ then fixes $C=1$. Thus Region I, $x=O(\epsilon )$, hasIts matching limit is $Y\sim X^{-1}$ as $X\to \infty$.

In Region II, $x=O(1)$ with $x\gg \epsilon$, setting $\epsilon =0$ reduces the second-order equation toHence $y_0=C{e}^{-x}/x$. Matching $C/x$ with the inner limit $\epsilon /x$ fixes $C=\epsilon$, so

The correction generated by $\epsilon x^{3}y$ changes the local decay rate when $x=O({\epsilon }^{-1/2})$. For Region III setThe terms of order ${\epsilon }^{-1/2}$ give the eikonal equationand the next balance gives $Z{G}^{\prime }+G=0$. Matching to Region II selectsThereforeThis solution is already beyond all algebraic orders in $\epsilon$ in the distant region. A leading multiplicative composite expansion that contains all three balances and satisfies the boundary value exactly iswith its value at $x=0$ understood by continuity.

Finally, the coefficient of $y^{\prime \prime }$ vanishes at the origin. Taking the regular limit of the original equation there givesso the prescribed value also fixes $y^{\prime }(0)=-1/(2\epsilon )$. Equivalently, the second inner solution behaves as $1/X$ and is excluded by regularity. This regular singular point is why one boundary condition determines the unique regular solution.