PSN-L Email List Message

Subject: Re: seismometer sensitivity--fundamental physics
From: "Geoff" gmvoeth@...........
Date: Fri, 9 Nov 2007 09:49:37 -0700

Then you got to worry about the Earths
Rotation affecting the direction
of the pendulum. ??

----- Original Message ----- 
From: "Randall Peters" 
Sent: Friday, November 09, 2007 6:35 AM
Subject: seismometer sensitivity--fundamental physics

> Ian,
>    Since you asked about period extension to increase sensitivity, let me explain something very few seem to understand.  The 
> sensitivity of a simple pendulum is just like that of every other seismometer; i.e., you can show that it is proportional to the 
> square of the period.  This can be proven from
> the equations of motion of the various mechanical oscillators in general, but for the pendulum, understanding is trivial.
>    As anybody would expect from 'horse-sense', the longer the 'quasi-rigid' pendulum, the greater the sensitivity of the 
> instrument if the sensor is placed at the bottom.  Because the period of the pendulum is given by 2 pi times the square root of 
> the ratio of length to earth field (little g), one
> sees immediately then, from this well known expression, that the sensitivity is proportional to the square of the period.
>   So then--to increase your sensitivity, hang as long a rod as you can find, consistent with your house size, and then place your 
> sensor at the bottom.  There are many possible sensor types to go with this incredibly cheap but probably very effective 
> earthquake detector.   They could be moire' pattern
> types with white light (incrediby simple) to function as a tsunami detector.  Or they might be greatly, greatly sensitive by means 
> of the Ronchi approach, who made optical testing famous by means of a coarse grating (much better than the classic knife edge 
> test).  The Ronchi ruling works with white
> light!  Or the sensor might be capacitive in nature like my SDC array.  The list of possibilities goes on and on, thus my interest 
> in the serendipty that is likely to come out of the listserve.
>   Randall


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