From: Brett Nordgren 
To: psnlist 
Sent: Wed, 27 Mar 2013 12:29
Subject: Re: Seismic Sensors

Hi Geoff,

    The amplitude of Microseisms may be from ~0.5 micron to a maybe
20 microns. For Earthquakes, you may want to measure movements
of ~10 nano metres upwards.
     The size of the pixels in a good photo detector maybe ~10 microns
minimum. This is a 'difficult / expensive technique' for trying to=20
measure
earthquake signals. You may require costly optical components.
    But you can use a couple of BPW34 photocells (87 cents each) for
measuring movements down to ~10 nano metres OK, with a movement
range of ~+/-1mm.
    This is both cheap and quite easy to do. Have a look at
http://jclahr.com/science/psn/chapman/photo_detect/index.html
A good free filter design program is FilterPro from Texas Instruments
    Regards,
    Chris Chapman

The only problem I can see is that the sensitivity you typically find
in a good instrument may be somewhat higher than what you are
considering.  For example, with the FBV instruments being operated by
Dave Nelson, a 2 Hz sinusoidal ground motion which registers 10
counts peak-peak on Larry's A/D would represent a ground motion of
~8nm p-p or about 18x the 0.44nm diameter of a large atom
(Potassium).  So one A/D count at 2Hz represents a motion of about
1.8x an atomic diameter.  And most seismic observatories using 24-bit
A/Ds are about 5x more sensitive.

For a really good instrument, one needs to be thinking in terms of
devices which can detect very small motions, though, in practice, an
instrument's true sensitivity will be determined by its internal
noise, not simply its bit-resolution.

Regards,
Brett

At 06:47 PM 3/26/2013, you wrote:
>On 26/03/13 23:56, Geoff wrote:
>>I like the idea of using a WEBcam or something to look at the red dot
>>then use a custom program to read the video and turn the graphic data
>>into rectangular coordinates of a standard four quadrant graph
>>with the point resting in the center of the graph
>>But to do this in near real time may not be possible for myself to do.
>>The max rate will be like 30Hz to 60Hz which is a video frame rate.
>>Video is usually encoded and needs to be uncompressed.
>>Uncompressing video takes lots of time to do so like a
>>series of BMP captures at whatever rate would be the thing to do.
>>You would take screenies at like 30 per second in a modulo fashion
>>then save all after an alarm and the series of like
>>108000 BMP images would be decoded into a series of (X,Y) coordinates
??
>
>This sounds a bit like a modified optical mouse.



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