PSN-L Email List Message
Subject: Re: Integrating in WinQuake
From: Brett Nordgren brett3nt@.............
Date: Mon, 23 Nov 2009 20:06:51 -0500
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
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
Watch our wiggles
or watch some very very good wiggles
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