magnetometer: n. [Magneto- + -meter]
An instrument for measuring the intensity of magnetic forces [1913 Webster]
precursor: One who, or that which, precedes an event, and indicates its
approach; a forerunner [1913 Webster]
Scientists, particularly in Japan, have for many years been gathering evidence of electromagnetic signals that come from under the ground before earthquakes. The bigger the signal, the bigger the quake is likely to be. This can occur hours before, and in some cases, days or even weeks before the quake itself. Why it happens is not well understood but current theory is that it has to do with rocks creating large electric currents as a result of crushing or grinding under high pressure. Several methods have been used to detect these reportedly erratic, electromagnetic pulses. The method described here is one of the simplest. Technically, it’s called a “search coil magnetometer.” This does not respond to very slow magnetic changes, but it can sense short-term ones, making it suitable for detecting earthquake precursors.
This project is a follow-up to the seismometer project at http://motivationtolearn.org/wordpress/?p=104 and is intended as a further way to get children interested in physics and electromagnetic phenomena by presenting it in connection with something relevant to their lives. And with the ever-present threat of earthquakes in Taiwan, the idea of predicting them is particularly interesting.
This sensor connects to the amplifier/chopper electronics used on the seismometer:
Both audio channels are being used on my prototype now; one for the seismometer and one for this magnetometer. The software records both channels and stores both sets of data in the format of Amaseis data files so that they can be viewed on the same software. There are two parts to this. The main one, and at TWD$300, the most expensive, being a roll of copper wire, size 38 SWG (Standard Wire Gauge). The one I am using weighs 250g and has a resistance of 1440 ohms. I don’t know how many turns that is but it’s a lot.
You have to make sure that both ends of the wire are sticking out so you can connect a length of shielded microphone cable onto them. It doesn’t matter which way, just one to the center wire and one to the shielding wire. The wires should all be stuck securely to the roll with electrical tape. Especially the one from the inside of the coil. Make sure the wire can’t move about or it might break off. If it did, you would have to unwind the whole coil to get the end connection back. Then put a plug on the other end to connect to the amplifier.
The next thing is the core. I used some lengths of galvanized mild steel wire that I found in the DIY shop. You could also use mild steel bar, though I haven’t compared the two. In theory a solid bar might not be as efficient, though at the low frequencies we’re interested in, the difference might not be noticeable.
Straightened out they are about 50 cm long. Push a few at a time through the coil until you can’t fit any more. Then cover the two ends with tape or lengths of plastic water pipe.
The finished sensor is as shown at the top of this page.
It should be placed as far as possible from sources of noise such as traffic and electrical wiring. I tried placing it straight up and down, east-west and north-south, but finally, because of the high noise level from the house electrical wiring, I had to find a direction that minimized that noise. It is currently propped up in a cupboard, tilted to the south-west 45 degrees from vertical. When plugged into the audio chopper interface with the software running, a typical day’s recording looks like this:
Most of those spikes are passing traffic. When magnified, they look similar to this passing bus:
But some signals are quite different and there are many for which I haven’t yet proven a definite source, such as these strange pulses:
Are these earthquake precursors or some other natural or man-made phenomena? I am slowly gathering more familiarity with the recorded signals. Most of the useful data is recorded in the middle of the night since in the daytime, traffic produces so much noise on the trace it’s easy to miss anything new. A busy Taipei street is a poor location for a magnetometer. However, it’s all educational and can be quite interesting trying to trace the source of new signals. This one for example had me puzzled for some time until I realized it was just my printer printing some pages of text….
Last Saturday, April 30th, there was a 5.7 earthquake only about 90 km from Taipei. I hoped that it was close enough and large enough to produce some observable electromagnetic signals. In the last few weeks there had been more than the usual number of pulses such as those shown two pictures above and I had wondered if it signified any seismic activity. Turning on my monitor, I saw first the usual traffic noise. This screen shot is about 12 minutes before the quake.
But just prior to it there were these sharp pulses again:
to be continued……..