Low-velocity zone inversion by the Wiechert-Herglotz integral

Defining a p-Δ curve in terms of the Δ of the turning point of a ray rather than the point of emergence at the surface allows a p-Δ curve to be produced for a low-velocity zone. This curve can be explicitly inverted by performing three Weichert-Herglotz integrals, two of which are for the low-velocity zone contribution.