The spherical shells work well with radial quadratic gradients — the ray paths are circular arc segments. But the solutions are computed under that assumption that the coefficient on r^2 is negative, and the constant term positive. (Meaning: velocities increase downwards.)
In some earth models, there a regions where velocity decreases as you go downwards. Although these can be approximated with stair-steps, it would be nice if the ray paths for downward-decreasing gradients could be handled directly, allowing much more modeling freedom.
The spherical shells work well with radial quadratic gradients — the ray paths are circular arc segments. But the solutions are computed under that assumption that the coefficient on r^2 is negative, and the constant term positive. (Meaning: velocities increase downwards.)
In some earth models, there a regions where velocity decreases as you go downwards. Although these can be approximated with stair-steps, it would be nice if the ray paths for downward-decreasing gradients could be handled directly, allowing much more modeling freedom.