A wireline log is how a borehole describes itself: a tool is lowered on a cable and fires acoustic pulses into the rock. Travel time betrays the formation — dense rock answers fast, loose sediment answers slowly — and the travel-time curve kinks at every boundary.
Stack strata with the buttons below, then fire the pulse. Each layer rings at its own velocity as the pulse crosses it; the curve on the right is the log you just cut.