PSN-L Email List Message
Subject: Re: Digest from 02/02/2008 00:00:01
From: Randall Peters PETERS_RD@..........
Date: Sun, 03 Feb 2008 10:49:46 -0500
When you refer to the VolksMeter's response being flat from D.C. to 1 Hz, you
are correct; however, to say that the velocity response is narrow-band is not.
The difference between acceleration response (position sensor such as the
VolksMeter0 and velocity response (most seismometers) is summed up by the upper
right pair of graphs shown on John Lahr's page at
These illustrate (for perfect electronics if it existed) the difference
between an 'acceleration' detector (VolksMeter) and a 'jerk' detector
(conventional instruments that use a Faraday-law--magnet coil- detector) in terms
of their response to earth's motion. The only thing that causes any seismometer
to respond is acceleration (or tilt as a special case therof), and so the
conventional instrument is measuring the derivative of the acceleration, which
engineers call the 'jerk'.
For 'perfect' electronics, the acceleration response is superior for sensing
lower frequencies of earth motion, whereas the jerk response is superior for higer
frequencies. The limit of detectability, within the differing constraints of
their architecture, is the noise introduced by the electronics. My statement
about 'superiority' assumes equally effective electronics for the cases.
[ Top ]
[ Back ]
[ Home Page ]