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Seismic Sensors

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EEVblog:
BTW, yes, the test tank was for precision measurements on all sorts of active and passive underwater sonars and hydrophones etc.
We also has "flow tanks" that were more suited to towed arrays.

There is only so much you can in a test tank, even a large one, as the wall effects dominate very quickly.
Larger scale tests were always preferred but were logistically harder, so the test tank was handy for testing out new ideas and revisions to validate numbers etc.
We had a lake test facility in Jarvis Bay.

radioactive:

--- Quote from: diegogmx on January 11, 2019, 06:37:31 am ---One interesting method i know modern seismometers use is to have a test mass as an lvdt moving element, and using a control loop and a coil to keep the lvdt output at 0, the seismometer output waveform would be the output of the control loop, they use one of these for each axis, buying one of these would most likely be prohibitively expensive yet it would be an interesting (and probably damn hard) project for summer (unless you are in the north hemisphere) :)

--- End quote ---

From what I've read so far, this is the approach that interests me the most.  It appears you get similar low-frequency performance to the long-stroke type sensors without the larger mass/coils in the sensor.  @mark03 mentioned this type of sensor earlier in the thread  (e.g. Yuma design).  FBA (force balance accelerometer).  The Yuma design appears to use a differential capacitive sensor for the PID control loop in vertical configuration (FBV).  Sounds like capacitive is probably the best way to do this.

The reason I brought up the video with the laser interferometer measurement earlier in the thread, is that it seems like it might be another way to pick up the displacement.  I was thinking that a voice-coil (speaker) with retro-reflective tape (like in the video) could be used for the correction.  I have no idea if this could be made to work as well as the capacitive type sensor (probably not I'm guessing), but it seems like it would also be fun to play with.  I also ran across something describing Michelson interferometer involving a couple of mirrors / beam splitter to get higher resolution measurements, but that looks like it would be a bit much for a first attempt.

rhb:
A few comments:

Take a look at this thread:

https://www.eevblog.com/forum/projects/suggestions-for-high-resolution-tiltmeter-(inclinometer)-sensor/msg1531160/#msg1531160

in particular the differential capacitance sensing.  The results achieved are quite amazing.  You could sense the movement of a dielectric core.  So basically a capacitance version of an LVDT.

Making an LVDT would not be hard if you can wind a coil and allow generous clearance for the core.  There are lots of plans around for coil winding machines.

Dead CD/DVD drives are an excellent source of beam splitters and first surface mirrors for a Michelson interferometer.  There are laser diodes which have enough coherency length to serve for a seismometer.  IIRC these are the ones with a photodiode in the back and it requires a circuit to stabilize the frequency of the diode.

A cheap He-Ne laser will have a coherency length of 8-12" which is enough for a small ring gyro.

Finally there is the La Coste suspension.  There's a picture of the first LaCoste seismograph here:

https://web2.ph.utexas.edu/utphysicshistory/LucienLaCoste.html

I recall seeing a version made using 3 screen door springs.

diegogmx:
the problem i see with the interferometer approach (at least the open air one) is that you may very well measure air temperature fluctuations too, just thinking out loud but maybe if you use a REALLY high resolution and dynamic range sensor you could send the beam into a reasonably short fiber and measure a "linear sagnac effect" also using an interferometer
also if you want to sacrifice some resolution i figure maybe a laser diode / photo diode like the one in the applied video would suffice
the other disadvantage i see from a design like that would be that the low frequency response would be pretty pretty low since the sagnac is a consecuence of velocity, and at low frequency and low amplitude velocity is really low

i figure the highest resolution approach would be a michelson interferometer in vacuum with the mirror in a suspended test mass, pretty much like LIGO but without seismic isolation

rhb:
Once you start playing around measuring motions at sub-micron levels, *everything* becomes an issue.

One of the interesting aspects of a seismograph is that an amateur can build a research grade instrument.  It's challenging project because there are so many factors which must be taken into account.  But the cost is low if you have a suitable location and the necessary skills.

Where I live the traffic noise would be pretty bad, so I'll probably content myself with measuring deflection of the ground as the water level in the lake a mile from me changes.  No real value to that, but I *am* a retired geophysicist.  But then there are probably very low magnitude earthquakes associated with sudden loading from major rain events. So if I ever get my workshop organized who knows what might happen.

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