Listening to the Sounds of the Earth

A geophone is a specially built microphone for listening to the Earth. [JTAdams] found them at a reasonable price so bought some to play with. A geophone is used to detect vibrations from earthquakes, explosions, rumbling trucks, and vibroseis vehicles. To be useful it needs an amplifier and a recording device to capture the signals.

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[JTAdams] used a standard amplifier design for an LT1677 op-amp, fed the signal to an MCP3008 A/D converter, and read the output using a Raspberry Pi. A Python script records the data to a CSV file for processing. The Pi worked well because the entire setup needs to be portable to take into the field. Another Python script plots the data which is made available from a web page. A neat simple way of presenting the raw data. [JTAdams] promises more information in the future on post-processing the data. You don’t need a geophone to detect seismic waves if you build your own, but a real ‘phone will be more rugged.

Oh, what’s a vibroseis? It’s a truck with a big flat plate underneath it. The plate is hydraulically lowered to the ground until the weight of the truck is on it. The truck then causes the plate to vibrate, usually sweeping from around 10 hz to 100 hz. This infrasound pass through the ground until it is reflected back by underlying rock layers. A long string of geophones, think 1,000s of feet, detects the waves, which are recorded. In practice, many trucks are used to generate a synchronized signal of sufficient strength. Or, you can set off an explosion which is the technique used in water. Typically the information is used for oil and gas exploration.  A video of one of the trucks in action after the break.

17 thoughts on “Listening to the Sounds of the Earth

  1. explosions work fine for ground as well, using both a high and low speed explosive allows differential analysis of the geotypes below, it is also vastly cheaper than the equipment rental for a shaker.

    1. There may be some sweet spot between hobby 2D lines and commercial 3D surveys where explosives wins, but I’ve always understood that for any depth target in the 1000’s of feet, it’s cheaper, simpler (from the acquisition perspective), and faster to use vibroseis if you’re working in an area that supports it.

      1. you are probably correct, the work i have done was all as part of small scale research and not industry, we also used geoelectrical measurements.
        my point was that there isnt anything to prevent explosions from working on land.

        the explosives were handled by authorized personnel (that he was a part of our research group only made it convenient) and were of fairly low magnitude, 100g tnt equivalency.
        the hardest part was getting permission from the environmental ministry since the areas we wanted to work in are part of a national park, in the end it all worked out.

  2. I worked for a company that built these sensors, but they were fiber glass sensors, they even use them on submarines, is a patented technology, and the detected frequency range was a bit wider, from 1hz to 800 hz, I built them, and tested them, by conducting frequency response test on a shaker unit, basically a really big speaker driven by a frequency generator, after a while they built a custom software based on basic so the shaker could do an automated test, sweep test and point by point test, and let me tell you, those things are really sensitive, we could hear planes, trains, and trucks from miles away, if you dropped setting on the floor from tents of feet away, we could hear it and see it on the test, so the test room was acoustically isolated, they could detect the smallest vibrations so they used their own software to filter out unwanted noise, if you guys would like to know more about them checkout their website http://www.us-si.com

    1. *Many.
      For shallow (~100ft depending on locality) depths whacking a steel plate w/ a hammer is just fine. For deeper ‘shots’ they use hydraulic hammers or specialized 12ga cartridges (like in the opening of Jurassic park).

  3. Exactly the concept Tesla used (115 years ago), but instead of mechanical vibrations from tracks ( with 10 – 100 cycles), he used electrical DC current impulses in 20-35K cycles range, produced by massive coils with capacitance on one side (air) and good ground on the other side. HV dielectric breakdown was used for disturbance of the system.
    Clearly the mechanical method is hugely inferior to the electrically engineered one.

    1. Interesting, have you got a link? The only thing I know is that he predicted that Earth could be split by periodically (every few hours) detonating several tons of explosive. With proper timing you create the standing (reinforcing) waves, and presto :)

  4. Just a nit to pick, it is not the “rock layers” the sound waves bounce off, but the “boundary layers”. The difference between two difference rock strata (stratii ?) is where the reflections occur. B^)

    On another note, this time not a nitpick, we used explosives to determine the “weathered” depth. IOW the weathered layer is the stuff above the bedrock. Knowing that depth is important to calibrate the signal returning from the lower layers.

  5. Aren’t those moving coil-magnet geophone sensors ancient history? Isn’t the current practice to employ MEMS devices? Also, shouldn’t a moving magnet or coil sensor be in a bridge for best results? And why isn’t the OP raising these issues?

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