## PSN-L Email List Message

Subject: Re: help-me
From: ChrisAtUpw@.......
Date: Thu, 25 Jan 2001 22:41:45 EST

```In a message dated 25/01/01, agustri@......... writes:

>  I am not yet clear about the difference of
>  1.displacementmeter, velocitymeter and accelerometer also
>  2.shortperiod, longperiod and broadband seismometer,

Hello Agus Tri Sutanto,

Consider a weight on the end of a very long pendulum. When an earthquake
happens, the ground will move a certain distance while the pendulum weight
stays nearly still. Measuring this will give the displacement, usually
measured in microns. If, instead of measuring the displacement, you use a
coil of wire on the ground and put a magnet on the weight, you will get a
signal which depends on the rate of movement of the ground - it's velocity.
If you connected the weight to the ground with a 'load cell', which measures
force, the weight will now have to follow the ground movement and you will
get an output which is proportional to the acceleration of the ground.

In a simple system like the pendulum, it will tend to have one particular
frequency or period at which it oscillates naturally, like in a clock. Now we
want to measure the ground movements, not the oscillation of a pendulum, so
the movement is 'damped', maybe by putting a paddle on the weight and letting
it dip into a container filled with oil, resting on the ground. The weight
can still move, but if you give it a light push, it deflects a small distance
and then moves back to the original position, but not past it. The name
short, or long, period seismometer refers to the natural period of the
apparatus when it is not damped. A Shakleford-Gunderson seismometer may have
a period of less than a second and this would be called short. The popular
Lehman seismometer may have a period of 10 to 20 seconds and this would be
called long. The response of a damped seismometer changes at the natural
oscillation frequency, but there is no peak output at any particular
frequency.

A broad band seismometer is constructed so that it gives a output which
does not depend on the frequency of the earthquake signal over quite a wide
range. Obviously there are both upper and lower frequency limits to the
behavoir.

Prof. Erhard Weilandt of the University of Stuttgart has written a very
full account in his paper "Seismic Sensors and their Calibration", which is
available on-line at

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