Seismic
Infrasound
Of all the aircraft in the sky, helicopters and the Doppler effect due to their rotor stand out exceedingly well. It appears that the pulses couple into the ground surprisingly well, especially (and not surprisingly) when the spherical wavefronts are directly over the ground.
It's easy to get low resolution frequencies of both the approach and departure, and I felt that there ought to be a pleasantly simple equation to convert them to the helicopter's speed.
It turns out there is.