One cannot ordinarily record earthquakes via a computer sound card as they don’t respond much to the slow vibrations experienced in earthquakes. (Update: not strictly true. See my latest on this: Simplest Seismometer – Experiments with direct recording through PC sound card) However, with some cheap electronic trickery and clever software, it can be done, and very adequately for the purposes of study. The seismometer above, made by a high school student, in its first night of testing recorded this local 3.8 quake – a magnitude 1 in Taipei:
(click on the pictures for larger versions)
and as a surprise bonus, some filtering of background noise revealed this distant quake as well. It was a 5.9 from 1000 km away in the Philippines.
This blog post is a follow-up to the original post on this subject “Recording Earthquakes for Beginners“. My quest for the simplest and cheapest seismometer design that a student could make, and with a computer, record real earthquakes, has been as successful as I could have hoped. There are software improvements to make one day that will improve the way data is displayed, and experiments are being done on a Windows version, but that’s going to need more study. (Update: The first Windows version has been done. See “Cheap seismometer using sound card and Amaseis – now running under Windows“) The design as it stands has proven quite workable and very educational, plus it can be built by high school students and even younger if they possess some basic handcraft skills. My first student to make one was only in elementary grade four, although he’s very advanced for his age. Following are some photos showing construction details and my students who made this project.
Two channel recording
It turned out to be a very simple matter to also connect the magnetometer described in this post: http://motivationtolearn.org/wordpress/?p=396 It was just a matter of adding two LEDs and a solar cell, then connect it to the second audio channel. This gives us a great way to research “earthquake precursors” – in this case, electromagnetic phenomena which can precede a big quake by days, weeks or even more. You can find more information in this very informative report on current research: http://www.quakefinder.com/research/pdf/Bleier_2010.pdf
The magnetometer traces recorded look much like the following 24 hour recording taken between 00:00 and 24:00 GMT on August 6th, 2011. These were recorded by two different units constructed almost identically and situated about 5 km apart in Taipei. The traces also show a distinct correlation with USGS magnetometers and others.
Construction of the mirror/magnet assembly of the magnetometer is described in the original post at http://motivationtolearn.org/wordpress/?p=396 .
The LEDs and solar cell were attached to a single piece of circuit board as shown here:
And from the other side, you can see that the light is adjusted to fall on the edge of the solar cell – half on. half off. That way we can record changes in either direction. This picture shows the experimental version. It still has the the indicator dial at the far end, no longer used here. A later one was built on a much shorter wooden base.
This drawing shows how it works:
Note: A piece of card was attached beside the LEDs to block light shining sideways onto the solar cell. Also, a box was placed over it to keep out other light sources, and it was lined with black paper to reduce internal reflections. Simon painted all his wooden pieces black as well.
The first thing to do is make the base and arm assembly. The seismometer usually took around 12 hours, but keep in mind that the students who made this were not building a kit set. They did everything, including sawing the wood….
Being a kind of educational experiment, I tried to avoid touching the students’ work myself as I wanted them to learn as much as possible, plus I wanted to find out how much they could do unassisted. I sometimes had to check for accuracy, and there were a few times I had to straighten out some mistakes such as in winding the coil. Safety was also an issue – some close supervision was needed when they used the electric drill and other tools. Other than that, they did practically everything themselves.
The pickup coils were made by hand. This required some concentration since there were a thousand turns of fine copper wire. A pen body was used as a coil former with side pieces from plastic bottle caps glued on. This wasn’t an easy way to make it – later I found the plastic center of a spool of ribbon was about the right size for a coil former. Luckily Shirley didn’t object too much to winding the ribbon around something else.
The picture above shows the piece of lead sheet wrapped around the arm which is simply for weight – about 1 Kg. The magnets, 15mm dia. x 4mm thick neodymium magnets were placed as close as possible to the coil and are adjusted to sit over one side of the windings. The larger damping magnet has a piece of aluminium in front of it. This was glued to a block of wood with brass sides extending down the sides of the frame, which allow it to be slid closer to or further from the magnet, for more or less damping of the arm movement.
A piece of iron wire connects the arm to the turnbuckle, then a piece of guitar string is used to connect it to the top of the frame, where it passes over a small steel plate with a “V” filed in it. The turnbuckle is used for leveling the arm. The screws in the side arms are used for adjusting the angle of the base. These allow us to set the arm so that it hovers about half way between the two wooden stops.
The interface electronics was built inside a small plastic box which now sits on top of the computer: (click to see a larger version)
And here is a circuit diagram: (click to see a larger version)
For those who are already familiar with electronics, here is a brief description of how it works:
The signal from the seismometer coil is amplified by a common LM386 audio amplifier I.C.. It has a gain of 200 with some simple low pass filtering to suppress signals above about 5 Hz. The signal is then chopped at 5 KHz by a simple 555 I.C. so that the sound card will process it as an audio signal. Next, software filters the signal, retrieves the original low frequency signals from the LM386 and stores it in hourly data files. These files can be read by Amaseis, a popular software package used by amateur seismologists.
The magnetometer simply uses the output of the 555 I.C. to drive the LEDs on and off at 5 KHz. The solar cell picks this up as an audio signal and passes it directly to the computer where the software processes and stores it the same way. Two LEDs were used so that the signal would be large