Yes, seems like it must be Doppler-shift.
Low frequencies (like in the mhz region) are used in ground-penetrating radar because the higher frequencies reflect off the surface. This causes a loss of resolution, but there might be a way to interpret the phases of the recovered signal from an transmitter/receiver array to get useful 3D mappings.
You could use a color screen and put the Doppler-shifting parts in red or something to indicate movement. You could write a software filter that only detects periodic motion in a frequency range that corresponds to the range of human heartbeats.