  The nearly-incessant free oscillations of the Earth (not the larger,
long-lived normal modes seen following intense quakes) were first observed by
accident in the record of a tilt-sensitive instrument designed to study surface
physics. Later tiltmeter studies demonstrated the usefulness of autocorrelation
for the routine study of these normally short-lived eigenmodes. More recent
studies suggest that many tilt-sensitive instruments are capable of observing
these oscillations--if the low-frequency response of their electronics is not
suppressed, as is customary with conventional designs.
