Ray tracing

Ray tracing follows the paths along which a slowly modulated wave packet propagates. For a local dispersion relation $\omega =\Omega (\mathbf{k};\mathbf{x},t)$, the ray velocity is the group velocity ${\nabla }_{\mathbf{k}}\Omega$.

A path followed by a slowly modulated wave packet under a local dispersion relation. Its velocity is the group velocity, while its wavevector evolves as $d\mathbf{k}/dt=-{\nabla }_{\mathbf{x}}\omega$. In a uniform stationary medium both are constant, so the ray is straight even when phase velocity and group velocity differ.

A wave's travel time along a elastic ray is the integral of local slowness against arc length. Its variation with source and receiver position describes the arrival branches, including transmitted and reflected paths. Different paths can have different travel times between the same endpoints.

In a smooth isotropic medium, an elastic ray is the path followed by a local wavefront normal, with the appropriate P or S wave speed $c$. Arclength $s$ gives the displayed trajectory and travel-time equations. Its neighboring ray paths determine geometrical spreading, while the eikonal equation determines the normals. A caustic occurs where the neighboring paths focus and the elementary ray wave amplitude description ceases to be uniform.

For an isotropic radial wave speed $\alpha (r)$, the elastic ray equations imply conservation of $\mathbf{x}\times \mathbf{y}$, where $\mathbf{y}$ is the slowness vector. Indeed, its derivative is $\alpha \mathbf{y}\times \mathbf{y}+\mathbf{x}\times \nabla (1/\alpha )=0$. Thus each nonradial elastic ray lies in a fixed plane through the centre and $p=|\mathbf{x}\times \mathbf{y}|$ is constant. The angle $i$ is measured from the radial direction; tangential slowness continuity preserves $p$ across spherical interfaces.

For a spherically symmetric speed with strictly increasing $\eta (r)=r/v(r)$, a turning ray of parameter $p$ has $\Delta (p)=2p{\int }_p^{{\eta }_a}(d\log r/d\eta )({\eta }^2-p^2{)}^{-1/2}d\eta$. Integrate this against $(p^2-{\mu }^2{)}^{-1/2}$ and interchange the triangular integration region. The inner kernel integral is $\pi /2$, giving the displayed radius and then $v(r(\mu ))=r(\mu )/\mu$. Complete turning-ray branch data are required.

Where $\eta =r/v$ decreases outwards, it increases as a surface ray travels inward, so no surface ray turns inside that interval. Penetrating rays may traverse it and turn deeper. The radius is then not a single-valued function of $\eta$, and angular ray data constrain combined branch contributions rather than the single function inverted by Herglotz–Wiechert inversion. Additional structural or observational information is needed to locate the hidden profile.

In a constant-speed spherical shell with outer radius $R$ and inner radius $a$, a symmetric single reflection from the inner boundary has the displayed travel time. The boundary lies on the angular bisector of source and receiver. The branch exists only for $0\le \Delta \le 2\arccos (a/R)$, since the incident line must remain in the shell. At the endpoint it joins the mantle-only chord branch with the same slope $a/c$. Continuing the formula beyond that endpoint would make the line intersect the inner boundary before its supposed reflection point.

A solid-to-fluid interface can turn spherical source P waves into parallel vertical rays if its height $Z(R)$ satisfies the displayed travel-time identity, with $r=\sqrt{R^2+Z^2}$. The positive branch through $Z(0)=z_0$ is a hyperboloid when $\alpha >{\alpha }_f$. Differentiating the travel-time identity enforces Snell law for elastic and acoustic waves. Travel time above the interface is then $z_0/\alpha +(z-z_0)/{\alpha }_f$, independent of cylindrical radius. The leading wavefront is planar although its wave amplitude need not be uniform across that plane.

A local dispersion relation $\omega =\Omega (\mathbf{k};\mathbf{X},T)$ yieldsThe spatial and temporal derivatives on the right hold the wavevector fixed. Translation symmetries conserve the corresponding wavevector components, while time independence conserves frequency.

A simple turning point for a stratified wavespeed occurs at $c(Y_s)=\omega /k$, where $Q={\omega }^2/c^2-k^2$ has a nonzero derivative. If $c_Y(Y_s)>0$, write $q=-Q^{\prime }(Y_s)>0$; then $\ell \sim [q(Y_s-Y){]}^{1/2}$ below it. The vertical group velocity vanishes and the stationary wave amplitude in a stratified wavespeed diverges as $(Y_s-Y{)}^{-1/4}$.

In ${\epsilon }^2{\Psi }^{\prime \prime }+2{\epsilon }^2(c^{\prime }/c){\Psi }^{\prime }+Q(Y)\Psi =0$, with $Q\sim -q(Y-Y_s)$ and $q>0$, set $Y-Y_s=({\epsilon }^2/q{)}^{1/3}\eta$. The leading equation is ${\Psi }_{\eta \eta }-\eta \Psi =0$, the Airy ordinary differential equation. The first derivative and nonlinear coefficient corrections are smaller by $O({\epsilon }^{2/3})$.

The local inner solution is a combination of Airy functions $\mathrm{Ai},\mathrm{Bi}$. Decay in the forbidden region removes $\mathrm{Bi}$. The remaining Airy turning-point connection formula is oscillatory on the allowed side and represents an incident/reflected pair, not a single upward branch. Without the forbidden-side condition the local differential equation alone leaves both coefficients free.

A simple ray turning point has outer amplitude proportional to $|Y-Y_s{|}^{-1/4}$. At the Airy scaling at a variable-speed wave turning point, $|Y-Y_s|=O({\epsilon }^{2/3})$, this gives an inner wavefield of size $O({\epsilon }^{-1/6})$ when the incident action normalization is order one. The enhancement is finite for nonzero $\epsilon$ and is resolved by the Airy function.

For the zero-frequency dispersion relationwrite $\mathbf{k}=\kappa (\cos \varphi ,\sin \varphi )$. Thenwhere $\psi$ is the direction of the group velocity. Maximizing the angular slope gives a downstream wake wedge of semi-angle