But this is much the same problem seismologists solve - finding the material properties based on what signals arrive where from a source. And seismologists were doing this long before convolutional neural nets were invented. So I thought I'd try something closer to what they do. It's also more insightful than just using AD which is something of a black box.
Anyway, here's the result of solving the same problem but getting the focus to happen much later. There's a sort of fractally refractive index now...
whitequark