PSN-L Email List Message

Subject: Re: seismometer sensitivity--fundamental physics
From: "Geoff" gmvoeth@...........
Date: Fri, 9 Nov 2007 09:50:47 -0700

More than 4 or 5 seconds is a waste of time
and money to deal with.

----- Original Message ----- 
From: "ian" 
Sent: Friday, November 09, 2007 6:46 AM
Subject: Re: seismometer sensitivity--fundamental physics

> Hi,
> thanks for the reply.  I did know that lengthening the pendulum also
> lengthens the period but to hang one from the top of my house has some
> practical problems(!).
> I had hoped there might be some other way of tackling the problem.  The
> Volksmeter II seems to have a pendulum of around 0.5 meters but It's
> response goes out to 10s of seconds -
> .  So I guess I'm looking
> for some cheap/easy way of lengthening the period.  I can't afford the
> $999 to buy it.  :-(
> Thanks
> Ian
> Randall Peters wrote:
>>    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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