In this post I am not talking about the chopped audio method used in all my previous devices. This one is just straight audio – directly into the PC’s sound card.
Some time ago I had bought into the idea that “you can’t record earthquakes through a PC’s sound card because they don’t respond to low frequencies,” in fact I had repeated it myself. But recently I’ve found that this is not strictly true for all sound cards. Very long period waves from distant quakes will certainly not register much through any sound card, but on some cards, quite good results can be obtained with local quakes and P waves from strong, distant quakes. (The trace in the above photo shows P waves from a 7.3 quake in Japan, a little left of center. Ignore the other stuff to the right of it; that’s “student quakes”.)
(Update Jan 3, 2013) So far I have found that my Acer notebook and Acer desktop pc both pass sufficient signal around 1 – 3 Hz to make quite sensitive seismometers. Local quakes seem to have most of their activity in this band. A USB sound card that I tried dropped off too much below about 5 Hz, and an HP laptop was just a little better. Time will tell if we actually get useful data from those but I doubt that they would record the smaller quakes. However, they are still quite good for demonstration purposes.
It works best if one has a large sensing coil with a strong neodymium magnet; then the signal has a better chance of appearing over the background noise. My coil was a roll of 44 swg wire I got at a local electronics store. It has a resistance of 4000 ohms and probably has 6-8,000 turns or so. It’s the same one used in my earlier post on the pendulum seismometer, “Cheap seismometer using sound card and Amaseis – now running under Windows” construction details of which can be found there.
The electronics couldn’t get any simpler, a single transistor amplifier with some low-pass filtering:
And you can set it up for continuous recording using the popular free amateur seismology software “Amaseis.” See below.
Here is a local quake I recorded on it – a 4.2 out in the ocean, about 150 Km away
A USGS seismometer in Taipei, which is designed to record very long period waves, hardly shows a blip for the same quake (at about 16 hours, 49 minutes):
My original pendulum using chopped audio recorded it much the same as the direct audio version:
While my Lehman, also using chopped audio, recorded it a little smaller, but this one is oriented East-West, whereas the pendulums favour North-South, and the quake was almost due south.
The next day I noticed the 7.3 from Japan (to the left of center), shortly after my student had been shaking the sensor (right of center):
When I checked the Lehman, I saw the huge S waves and realized it was a big distant quake:
Educationally, this makes a good introduction to the world of seismology. It is dirt cheap and very hands-on. The bottle version shown in the photo is great for younger students. When they shake the bottle they soon find out that it’s not attached – it’s just resting on the base. The next thing they discover is that the signal is produced by the interaction of the coil and the magnet. A little more observation and they find out that the pendulum will not swing back and forth when it’s suspended over the aluminium damping sheet. Depending on their level, the phenomena they observe can lead on to a lot of further research and study.
Getting it running under Windows
1) Download com0com here: http://sourceforge.net/projects/com0com/files/com0com/188.8.131.52/ and set up a virtual serial port pair. Name them “COM7″ and “COM8″.
2) Download and install AmaSeisSetup.exe and under “Device” in the settings menu, choose “AS1″. Set it up for COM7. “audioseis” will send the data out to COM8 and the virtual serial port pair are there to link them together.
3) Download audioseis here – the software I compiled to turn the audio into serial data for Amaseis. Unzip it and put the executable and two .dll files into the Amaseis working directory (probably C:\Amaseis) Create shortcuts to Amaseis and “audioseis.exe” on the desktop.
If all has gone well, you should be able to start audioseis first, then open Amaseis. You should see noise from the sound card on Amaseis’ trace. When the sensor is plugged in, adjust the mic input level so you have a little background noise. You may also need to adjust the “zero level” in the Amaseis settings menu so it’s on the line for the current hour.
If you need assistance getting it running, leave a comment or contact me via the Contact page.
Update Jan 11, 2013
A few of my students are making this project. James was the first to finish:
(update Jan 18, 2013) James is continuing to record data and his traces show a fairly noise-free location: (click on images for larger picture)