Detecting seismic waves with a piezo element






12 thoughts on “Detecting seismic waves with a piezo element

  1. For the same intent I’m collecting those piezo modules from electric lighters. If tapped with a finger, a contemporary high-impedance VOM already shows a few volts. The hard part seems to be mechanical. I intuitively understand that the weight must be able to freely oscillate, and a certain amount of damping is needed to flatten the resonance curve. As in the textbooks, the resonance frequency \omega = sqrt(k / m).

    A voice coil ensures more displacement. Damping could be adjusted by loading it with a resistor. Perhaps an entire bass speaker, after a mass is affixed to the center, would make a nice 1-axis seismometer. But they are expensive.

    A piezo, though, has no elastic elements and so exhibits virtually no displacement (k is very large). After stiff coupling to a weight the resonance frequency will be high. A mass that ensures a low enough \omega might be infeasibly large, and almost a short-circuit might be needed for damping.

    Therefore I’m currently planning two compression springs at opposite sides of the weight. A piezo could then be placed between one of the springs and its support. However, despite the fact that I’m addicted to salvaging electronic components and small mechanical parts, my collection of springs is still too small :/

    1. There was an article I read years ago in one of the editions of “The Engineer’s Mini Notebook” by Forest M. Mims that described a similar setup using a permanent magnet suspended in a long tube (affixed to the floor) and in close proximity to a telephone voice pickup (from an old handset phone). Simple op-amp provided the amplification.

      I’ve not been able to find the article for some time however..

      1. I remembered that article as well and still happen to have a copy. Pages 40-41 of the Science Projects min-notebook, RS # 276-5018. It used two 741 op-amps; a telephone pickup coil or 9-volt relay; three resistors and an LED indicator, or optional piezo buzzer.

    1. A weight stores kinetic energy and releases it after the initial force has gone.
      Since more weight => more energy needed to move it => more time needed to apply an amount of energy => more time needed to “discharge” the accumulated energy, then the more weight you add, the lower frequency it oscillates.
      (you can clearly see the similarity between a weight in mechanics and a capacitor in electronics)

  2. You know the ultimate goal doesn’t have to be early warning. If the sensors can detect the initial compression vs contraction (not sure if his chopped waveform does this), you could use an array of these to locate the earthquake epicenter and the orientation (in at least two possible planes) of the fault break.

    Although they have been documented to be near audible at the epicenter due to attenuation as the travel earthquake waveforms tend to be .5 to 20 Hz depending on the distance.

    All said this is cheaper than the 1hz geophones I have. :)

  3. For my thesis i actually used piezo film accelerometers to do a seismic reflection (subsurface imaging) study… Where pretty much all of that “sensor” stuff is built into a small, convenient package.

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