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Accelerometer with 0.2mG resolution - seismography??

Discussion in 'General Electronics Discussion' started by (*steve*), Dec 14, 2013.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I've just been gifted some old equipment. I grabbed it mostly for the very nice weather resistant metal case it was in.

    On taking it apart, I find that it has a sensor, an ADXL105, and whilst it's almost 15 years old, the technology seems similar to what is used in these things today.

    I wonder if the 0.2mG resolution makes it at all useful for a sensor in a seismograph?

    It has an analogue output and the (analog) sensor output is 250mV per G.

    I guess the output is noise limited and they quote a noise figure of 225uV/sqrt(Hz) but considering that you're going to be looking for seismic signals at the very low frequency end of the spectrum, 1/f noise is going to be far more significant isn't it?

    That noise figure suggests for a DC to 10Hz bandwidth, the noise figure (0.7mV) will be at about 2.8mG. That corresponds to being tilted by 0.2 of a degree (which is significant because the device is (was) also touted as an inclinometer).

    How do I calculate what the 1/f noise will be?

    Some further research suggests that an earthquake is noticeable to humans at about 10 to 20 mG, so (at best) 2 orders of magnitude better than this is probably not going to be really useful.
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hey Steve

    Not too familiar with how much better the ADXL105 is compared to the earlier ADXL05 ?
    the ...05 was very insensitive unless the quake was quite feelable

    here's my a fellow amateur seismologist and designer of the recorder and analysis software wrote about the ADXL05 back in 1998 .....

    http://psn.quake.net/adx05chip.txt

    the ADXL05 device and maybe to some extent the ADXL105 are primarily strong motion devices

    Dave
     
    Last edited: Dec 15, 2013
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
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    Jan 21, 2010
    Not so much better. My calcs for noise on the new one were 2.8mg, this one says 2.6mg -- the difference will be in the rounding.
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    it would be worth a try using a low noise preamp
    would be interesting to see if it recorded any quakes
    There's quite a few events to the east through to NE and ~ 200 - 300 km from Perth
    that range in the Mag 2.0 to 4.0. The M3 and greater events would be the ones to try and record.
    If you really wanna get interested I could send you a 8Hz geophone which would have no trouble recording M3+ events at ~ 200 km. It will even pick up the "P" wave arrivals of the large Indonesian quakes

    And if you REALLY wanna get interested, haha, I can give you the links to my mate in California to have a look at his preamp boards, A to D system, recording and analysis software.

    cheers
    Dave
     
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