Roger,

Maybe I can help confuse the issue a bit more.

You can not say that a ground motion is only a displacement, or a velocity, 
or an acceleration, it is always all three.  A ground motion can be 
perfectly described by recording its displacement or its velocity or its 
acceleration over a period of time.  If you know any one, you can exactly 
compute the other two by taking the integral or derivative of the recorded 
signal, as required.

Likewise seismic detectors don't just detect displacement or velocity or 
acceleration.  They always detect all three.  For example, what is actually 
meant by a Velocity Sensor is one which is "Flat to Velocity"  that is, it 
responds the same to sinusoidal ground motions having the same velocities, 
regardless of their frequencies.  To ground motions having the same 
accelerations, it will respond differently at different frequencies, and 
similarly with ground motions having the same displacements.

Some sensors are by their nature flat to velocity or to displacement or 
acceleration.  A capacitance sensor tends to be flat to displacement, a 
coil and magnet is generally flat to velocity, and a MEMS accelerometer is 
relatively flat to acceleration (within their respective operating 
frequency ranges).

You can even use one type of sensor to construct an instrument which 
responds like another.  In the feedback vertical, even though we start with 
a capacitance sensor whose response is flat to displacement, the feedback 
electronics can roughly be thought of as differentiating that signal to 
arrive at an instrument which has a response that is generally flat to 
velocity.

Regards,
Brett

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


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