Time-reversal methods in geophysics

Before the 20th century there were few seismometers. So Earth's dynamic geophysical processes were poorly understood. Today the potential for understanding those processes is enormous: The number of seismic instruments is continually increasing, their data are easily stored and shared, and computing power grows exponentially. As a result, seismologists are rapidly discovering new kinds of seismic signals in the frequency range 0.001-100 Hz, as well as relatively large nonseismic displacements, monitored by the global positioning system, occurring over days or weeks. © 2010 American Institute of Physics.