Editing Seismometer (section)

== Basic principles ==
[[File:Seismographs.jpg|thumb|Basic horizontal-motion seismograph. The inertia of the round weight tends to hold the pen still while the base moves back and forth.]]
A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging from a spring, both suspended from a frame that moves along with any motion detected. The relative motion between the weight (called the mass) and the frame provides a measurement of the vertical [[ground motion]]. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a [[seismogram]].

Any movement from the ground moves the frame. The mass tends not to move because of its [[inertia]], and by measuring the movement between the frame and the mass, the motion of the ground can be determined.

Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic [[PID loop|negative feedback loop]]. The motion of the mass relative to the frame is measured, and the [[feedback loop]] applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally.

In other systems the weight is allowed to move, and its motion produces an electrical charge in a coil attached to the mass which voltage moves through the magnetic field of a magnet attached to the frame. This design is often used in a [[geophone]], which is used in exploration for oil and gas.

Seismic observatories usually have instruments measuring three axes: north-south (y-axis), east–west (x-axis), and vertical (z-axis). If only one axis is measured, it is usually the vertical because it is less noisy and gives better records of some seismic waves.{{citation needed|date=November 2017}}

The foundation of a seismic station is critical.<ref>[http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/index.html Erhard Wielandt's 'Seismic Sensors and their Calibration'] {{webarchive|url=https://web.archive.org/web/20100924231853/http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/index.html |date=2010-09-24 }}- Current (2002) reference by a widely consulted expert.</ref> A professional station is sometimes mounted on [[bedrock]].  The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides.  Other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. Originally, European seismographs were placed in a particular area after a destructive earthquake. Today, they are spread to provide appropriate coverage (in the case of [[weak-motion seismology]]) or concentrated in high-risk regions ([[strong-motion seismology]]).<ref name=Reitherman />