Active geophysical monitoring involves detection and interpretation of subsurface changes in waves periodically excited by repeated controlled sources. There is little doubt that active monitoring of time-evolving structures and states in the tectonically active lithosphere of the Earth is very important. However, no systematic analysis of continuous geophysical monitoring has been done up to now, because no reliable technology systems have been developed other than those generating impulsive signals (e.g., chemical explosives, airguns, and seismic vibrators). Extensive technological and theoretical effort has been expended over the last few decades on developing continuously running sources (e.g., using electromagnetically controlled sources), but these efforts have generally not received much international attention.