Seismographs are used by seismologistss to record seismic waves. By studying seismic waves, geologists can map out the interior of the Earth.
When an earthquake occurs, seismographs near the epicenter are able to record both S and P waves, but those on the other side of the Earth can only record P waves.
Seismographs consist of a foundation, a suspension, a damper, a suspended mass, a motion sensor attached to the mass, an amplifying device, and a recording device.
Prefessional seismographic observatories usually have instruments for three axes, north-south, east-west, and up-down. One set of three instruments is tuned to oscillate at three seconds, and the other at thirty seconds. Seismologists generally prefer a vertical seismograph if only one instrument is possible.
A professional observatory is best when mounted on bedrock with an uncracked connection to a continental plate. Amateur and field instruments are mounted on small 30cm (1foot) buried concrete piers.
A popular modern design uses strain-beam accelerometers constructed as integrated circuits. This makes it easier to construct a three-axis observatory in a small field or amateur unit.
The classic recorder was once a drum, recording one week per sheet on special graph paper. Now, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection. Many observatories now use personal computers. For amateurs, a PC with a recording sound card provides a computer with an inexpensive analog-to-digital converter.
An algorithm often used to eliminate insignificant observations uses a short-term average and a long term average. When the short term average is statistically significant compared to the long term average, the event is worth recording.
One classic seismograph is a "gate" design. A heavy weight is mounted on the point of a long triangle, hinged at a vertical edge. The hinge has very low friction. As the ground moves, the weight stays unmoving, swinging the gate on the hinge. The gate is slightly tilted, so the weight tends to slowly return to a central position. The gate is adjusted to oscillate once per three seconds, or thirty seconds. Amateur devices usually oscillate once per ten seconds.
A pan of oil is placed on the ground, and a small sheet of metal mounted on the underside of the gate drags in the oil to damp oscillations in the gate.
An arrangement of levers and mirrors reflects a light beam to a direct-recording roll of film, or in modern systems, a pair of differential electronic photosensors. The recording device was traditionally paper on a drum.
Vertical accelerometers use a variation on this design, but use a spring to suspend the arm.
Another gate design has a vertical torsion wire several feet long, and attaches a copper flag to it. A mirror on the flag reflects a light beam. A magnet induces eddy currents in the flag to damp motion.
One of the best amateur designs was commissioned by Scientific American. Basically, the design is a gate. The weight is a large sense coil, moving in the magnetic field of a magnetron magnet (cheaply available from microwave oven repair shops). The damper is a one-megaohm variable resistance across the sense coil. The hinges are sheets of brass, held in clamps. The frame is square aluminum tubing. The device senses velocity rather than position, but requires very little care, and is easy to construct and tune.
Another design actually measures the distance between two piers, which will change when a wave passes the instrument.
See also: Lehmann, Inge, Oldham, Richard DixonDesign