Editing Seismometer (section)

=== Teleseismometers ===
[[File:Seismometer kum hg.jpg|thumb|A low-frequency 3-direction [[ocean-bottom seismometer]] (cover removed). Two masses for x- and y-direction can be seen, the third one for z-direction is below.  This model is a CMG-40TOBS, manufactured by Güralp Systems Ltd and is part of the [http://www.mbari.org/mars/default.html Monterey Accelerated Research System].]]

The modern broadband seismograph can record a very broad range of [[frequency|frequencies]].  It consists of a small "proof mass", confined by electrical forces, driven by sophisticated [[electronics]].  As the earth moves, the electronics attempt to hold the mass steady through a [[feedback]] circuit.  The amount of force necessary to achieve this is then recorded.

In most designs the electronics holds a mass motionless relative to the frame.  This device is called a "force balance accelerometer".  It measures [[peak ground acceleration|acceleration]] instead of velocity of ground movement.  Basically, the distance between the mass and some part of the frame is measured very precisely, by a [[linear variable differential transformer]]. Some instruments use a [[variable capacitor|linear variable differential capacitor]].

That measurement is then amplified by [[electronic amplifier]]s attached to parts of an electronic [[PID loop|negative feedback loop]].  One of the amplified currents from the negative feedback loop drives a coil very like a [[loudspeaker]]. The result is that the mass stays nearly motionless.

Most instruments measure directly the ground motion using the distance sensor. The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground. The current to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus measuring directly the ground's acceleration (using f=ma where f=force, m=mass, a=acceleration).

One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses.  The uneven changes in pressure caused by wind blowing on an open window can easily change the density of the air in a room enough to cause a vertical seismograph to show spurious signals.  Therefore, most professional seismographs are sealed in rigid gas-tight enclosures.  For example, this is why a common Streckeisen model has a thick glass base that must be glued to its pier without bubbles in the glue.

It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves.  This is also why seismograph's moving parts are constructed from a material that interacts minimally with magnetic fields.  A seismograph is also sensitive to changes in temperature so many instruments are constructed from low expansion materials such as nonmagnetic [[invar]].

The hinges on a seismograph are usually patented, and by the time the patent has expired, the design has been improved.  The most successful public domain designs use thin foil hinges in a clamp.

Another issue is that the [[transfer function]] of a seismograph must be accurately characterized, so that its frequency response is known.  This is often the crucial difference between professional and amateur instruments.  Most are characterized on a variable frequency shaking table.