= Ray-tube conservation for a P-wave jump
{title2=$\rho\alpha(a^{[k]})^2J=\text{constant}$}
A tube of neighboring <elastic rays> follows the <wavefront> normal. If $J$ is its infinitesimal transverse area, its expansion is $d\log J/ds=\nabla\cdot\mathbf n$ along arclength. The <P-wavefront discontinuity transport> law therefore gives the displayed conserved flux. Geometrical spreading, <mass density> and <wave speed> determine the leading jump <wave amplitude> together; the rule is valid before the ray tube reaches a <caustic>.
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