Amateur Seismology

I told my Los Angeles cousin Rita that I was interested in seismology - Rita had gone to school with Tom Sanders of Palo Alto. Tom was interested in amateur seismology.

I contacted Tom, and found that he was a member of
     the "World Amateur Seismological Society" ( WASS )
and was the current treasurer and also editor/publisher of its quarterly journal.

Soon I was a member and contributing articles. I seemed to be the only member contributing articles. (There were about 180 "members" paying annual dues (mostly going into postage for the quarterly newsletter) but not saying or volunteering anything.)
Tom tired of publishing the WASS journal, and offered me the editorship - I seemed the only "victim".

After about two years of rather solitary work, as treasurer, author, editor, and publisher, I too tired of publishing the WASS journal- but could not find a victim - and closed WASS.

After a few years I "down sized" and gave the WASS notebooks of publications to Larry Cochrane of the Redwood City Public Seismic Network.

One of many fascinating articles in "Amateur Scientist" was how to build a seismograph sensitive in a horizontal direction to very low frequency waves, like a wave that takes 10 seconds. The output signals needed amplification to be seen an/or recorded. At the time, vacuum tube amplifiers were in, transistors flaky and expensive. The project seemed too much bother.

We moved to earthquake country, lots of earthquake activity, in 1972, just about the same year a lovely little integrated transistor amplifier, the 741, became available, for about $ 2 instead of say $300 for the previous hot, power hungry, troublesome, vacuum tube direct current amplifiers.

The drawing to the left, from the British Geological Survey. I had heard that making a long period vertical seismometer was is a major engineering challenge :-( , so lets see if we can build a longer period horizontal instrument sensitive down to 1/20 Hz, (wave per second).

I had studied the following "Amateur Scientist" articles in "Scientific American"

1. April, 1952. Larkin's Sprengnether Seismometer
   describes a Wood-Anderson torsion seismometer - owned and operated by Harry H. Larkin, Jr.
2. June, 1953. About an Ingenious Electronic Seismograph and ... see picture
3. July, 1957. Concerning Simple and Ingenious Devices to Record the Waves Made By Earthquakes
   see picture 1, picture 2
4. August, 1970. Devices for Listening to Sounds Both in Water and in the Solid Earth
   - w transistorized DC amplifier - 10 transistors -
   Note that C.L. Stong calls this type of seismometer "Galitzin" after its inventor.

The "garden gate" type horizontal seismometer ( the sensor of a seismograph system ) seemed promising, reasonably simple, and the "Scientific American" articles had shown examples.

For your background, you can view
     - http://en.wikipedia.org/wiki/Seismometer
     - http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.html
and for that lovely little DC amplifier, the 741 ( not cutting edge any more ;-)
     - http://www.ti.com/product/lm741

There is also the "style" of the sensor. One of the early seismometers used a mirror on a wire. The mirror would be an inertial mass twisted by the wire shaking with the earth. A beam of light bouncing from the mirror would darken moving photographic film to record the earthquake. This system was used by Richter to define the "Richter Scale".

Using a coil interacting with a magnetic field is widely used now, in part because of availability of convenient and stable electronic amplifiers. It is easy to get a signal from a coil near a moving magnet, * but * to get a reasonably linear calibrated output over a reasonable range of movement requires some engineering and care - well beyond the scope of this little paper.

Here is a nice, informative picture with text from http://www.bgs.ac.uk/downloads/start.cfm?id=659 (Local copy of "Building Your Own Simple Seismometer") which states that the effective top pivot will be about 2 mm from vertical over the effective bottom pivot for a 20 second period. As the restoring forces are very slight, maybe 0.5 percent of the effective arm mass, the pivots must work almost flawlessly ...

Seismometer Overview

The long dimension of the base is usually about 18 inches for convenience. Adjusting the Period Screw adjusts the period of the swing. The Leveling Screws help position the magnet in the coil assembly. At 15 second period, 1/32 of a turn makes a significant difference. The pivot details are not shown here, but are critical to successful performance. Another requirement is that all structures other than the pendulum have mechanical resonances damped and at a frequency well above the highest of interest.     I made my frame from 1.5 inch copper tubing using a 4 way junction - easy, nice - the pendulum L shaped of 1 inch copper tubing (no diagonal bar).

Pivots

This is not a construction article, but you will likely spend a lot of time thinking about and experimenting with pivots - knife edges sound nice for bottom pivots but the must be carefully aligned vertically. Points flatten, wires harden, and on and on ;-))

Sensing Coil - outputs the voltage to the amplifier -

Get a half pound of the finest enamel wire you can wind with your electric drill ( maybe # 38 ? ) onto a non-magnetic non-conducting bobbin. When winding feel the wire carefully as there may be a non-conductive knot connecting two wires in that pound spool of wire :-|

Magnet

The magnets and magnet housing from a defunct microwave oven magnetron work well ;-))

Leveling and Period screws and nuts

You will spend significant time      a) Leveling the seismometer to get the magnet and/or mass aligned      b) Adjusting the period of the pendulum. These adjustments interact somewhat, as the period gets longer the leveling gets touchier. It is very convenient that:      a) The adjustment knobs you glue onto the screw heads are large      b) Use fine thread screws for easier fine adjustments          - some professional machines used differential threads      c) Use flat smooth material (window glass?) under the screw points.

Choice of moving magnet or moving coil ??

Many people put the magnet on the moving boom as part of the mass, and place the coil on the fixed frame. I worried that moving iron near by - such as an automobile if the seismometer is in a garage that is used. I put the coil on the moving boom, and the magnet on the frame, hoping that a moving car near by would not cause as much false signal. But you are faced with passing a pair of very fine wires near the upper pivot. If you are not worried about moving magnetic material near your seismometer, choose option "A" above ;-))

Seismometer Protection

Obviously your sensitive seismometer must be physically protect from children, curious adults, cows, hunters, ... And weather, water, air movements, spiders, ants, mould, heat sources like amplifiers, ... A tale - I had my seismometer mounted on concrete, protected by insecticide, under a foam box 1.5 inches thick. It was working well - *except* - that about 3 AM some mornings there would be LARGE slow swings of the pendulum recorded. First I figured some spider had odd hours, but no spider. Then I wondered if the air cooled outside the box there must be a temperature inversion in the box which moved to non-inversion - causing the large slow movement. I installed a 2 watt lamp inside the top of the box, and that problem went away. :-))

Seismometer Adjustment

This is the basic scheme, does not describe all possibilities, "an exercise for the student". Disconnect the seismometer from the amplifier Disconnect the seismometer's damping system Adjust a Leveling Screw to position the magnet where you want it in the coil area Adjust the Period Screw to make the period more desirable. Tap the pendulum with a little stick or straw to get it swinging Adjust a Leveling Screw to position the magnet where you want it in the coil area If the period is not as desired, go back to 4 above Connect the damping system Tap the pendulum with a little stick or straw to get it swinging If the seismometer is under-damped, reduce the resistance, go to 8 above Correctly damped is just a little overshoot - If the seismometer is over-damped, increase the resistance, go to 8 above Reconnect the seismometer to the amplifier, you are done :-))

The June 1953 Amateur Scientist a schematic of a vacuum tube amplifier with 30 ufd coupling capacitors and 5 meg grid resistors - but you don't want to deal with that !!

I used the 741 op amp in the 1970s, great in its day, but by current standards had large offset (to be manually adjusted out), high temperature drift, and high input current.

I used a non-inverting logarithmic amplifier circuit similar to this. Values are not shown as this is not a construction article.

A Logarithmic Amplifier

Input vs. Output curve

This non-linearity permits recording regional as well as distant earthquakes without going off the recording surface or throwing ink. The disadvantage is difficult calibration. Oddly, the interpretation does not suffer much. The peaks are somewhat flattened, but the general form is shown.

OH - How could I forget the battle I lost !! -- I live about 20 miles from the Pacific coast, where the (sometimes large) waves break upon the shore. This results in noise in the 1/7 Hz region. When there was a big storm in the Pacific within 1000 miles, the 1/7 Hz noise - called "microseisms" - was very noticeable. :-(( https://en.wikipedia.org/wiki/Microseism I tried to make a notch filter that would reduce the amplitude of signals in the 1/5 to 1/10 Hz region. In the analog world this requires high ohmage resistors and "large" non-polarized capacitors - along with amplifiers of much higher input impedance than the 741 op amps. At the time, Radio Shack was selling counterfeit FET input op amps (relabeled 741s) which gave me fits. When I went digital I didn't even try to make a notch filter program.

Currently there are two good ways to get useful outputs from your seismometer:
     - for analog output, very nice chopper stabilized amplifiers with negligible offset and drift for $1
     - for digital output, very high resolution A to D converters, don't need amplifiers for $10
In either case, pay attention to the application notes, and also use twisted pair wires from the seismometer to the electronics to reduce noise and hum pickup. Shielded twisted pair is even better ;-)) Using a connector at each end eases handling and transportation problems.

I am assuming that you don't want to become an electronics bug complete with soldering iron, so lets just buy an already made board with components already installed and tested, with output to your PC for easy recording and possible post-processing the seismic activity you collect.

Larry Cochrane of the Redwood City Public Seismic Network has designed and sold ready made electronics for years. He also links to free PC software that operates this equipment and records and presents the seismic information.

A note on these wonderful high resolution Analog to Digital Converters - say 24 bit ADCs - A 24 bit ADC can (should ;-)) resolve its legal input values into over 16 million different digital values. It can produce useful digital output from a 5 microvolt wave - plenty good enough for almost anyone ;-))
And it outputs to USB to interface to any computer made in the past 10 years ;-))

Finally I correlated this effect with the position of big Air-force antenna atop Mt Umunhum, about 10 miles away "as the crow flies". My best guess is that sometimes the Air-force would either:     - aim one of their "nodding" height finding radar at Cupertino (my area) or     - lower the rotating main beam of the big antenna to its probable minimum.

The effect could be considered my bad - my little DC amplifier, with diodes, was completely unshielded. Even worse, the construction was not compact - the wire leads of the 741 were unclipped, just tack soldered onto what ever components it was connected to. The diodes were in a chain, I didn't want to clip any lead so that I could make changes, on the cheap.
After say 1980, there was no more of this type of interference - apparently the threat from Russian ICBMs was deemed greater than from bombers that could be seen with these radars, and the Mt Umunhum Air-force station deactivated.

Now fancy GPS equipment, accurate to nanoseconds, is available for about $80 -

And speaking of time - since seismology is world wide folks like to use what used to be called Greenwich Mean Time ( GMT ). Time turns out to be rather complicated - in the 1970s, a friend gave me a 40 page document document describing time used in earth satellites :-|

Recording onto Paper

Not trying to tell you how to lead your life ;-))      But given modern alternatives, you don't want to do this for the following reasons: - expensive - either to purchase or build ( I actually did - once - ) - must change paper after every recording day - dealing with ink can be interesting :-(( - - - - and on a close earthquake, the pen can throw ink about :-(((( - driving a galvanometer movement is interesting - - - - a combination of inductance and back EMF due to movement - - - - - think of it as a current driven, not voltage driven, device -

But the professional is interested in more:
     - the amplitude and frequency of the signals with some precision
     - the timing to seconds and milliseconds
Science is all about measurement.
     - how big, how fast, what is the energy and where did it come from, ...
     - this wave must have reflected off of something to arrive at this time, ...
Professional papers need facts to help support theories.

Missing from most (?all?) amateur reports is calibration data

1. velocity per inch or volt or ..., from a magnetic detector or displacement from a photo detector
2. calibration of the instrument, such as
   - natural period - of the swinging arm
   - damping of the swinging arm
3. calibration of the electronics - frequency limits and quirks
   RC coupled circuits are especially suspect.

Local earthquakes can easily sent these horizontal instruments we have been talking about off their physical scale.
Lets say that the linear range of our seismometer is one inch, and the local earth jumps 10 feet -
The boom of the instrument has been pushed into its stops, the instrument has probably slid and been knocked about a bit.
For this event, you need a "strong motion" machine -

Guessing at the dimensions, I estimate 3 inches diameter, 5 inches long. I forget if the magnetic structure or the the wire wound solenoid was attached to the boom. I do remember the wire was a larger diameter than I expected - maybe #32 enameled wire.

Note that for most of the travel of the solenoid in and out of the magnetic structure, you can expect a rather linear change of magnetic flux per millimeter of movement. I imagine that the pole pieces could be tapered to improve linearity.

Most amateurs (certainly myself) did not try for this level of sensor linearity. If using magnets, we tend to use circular magnets and circular coils and use an alignment similar to the figure on the left. (click on the figures to enlarge)

The equation (not including fringing effects) for the magnetic flux change through the coils per millimeter of offset from this position is unpleasant, but definitely non-linear. If we calibrate carefully with the components in say this position, and the components shift due to changes in leveling, then the calibration is no longer accurate. This limits the usefulness of our data for serious seismic work, such as velocity or displacement estimates.

However, lets make the coil narrow, with straight sides. A person would guess that the elongated coil would provide quite good linearity (and relatively constant resistive damping) over quite a range of positions. :-))
(Next time ;-)

How are you? I am 63 and retired. Over the years I have tried spring and weight for seismograph.... magnets and coils etc.

But I wanted smaller and more sensitive... So, I took a 2 inch piece of lab rubber tube, plugged at one end with bleeder hole and with an electret microphone capsule at the other end. The rubber tube provides spring and damping all in one. The end of a cylindrical 400 gr weight rests horizontally across on it.

The signal is amplified with stereo preamp and then fed to a DI-145 Dataq data logger. I have been running this rig for now 3 years...

There are a few problems with my station. One is the city noise. The other is baroms or infra-sound. The detector being a pressure sensor, I have to distinguish what is due to ground movement from infra-sounds . So I have a second tube and microphone without the weight that I record on a second channel.

Attached is a trace for a 4.2 M earthquake 200 km away. (second channel removed)

I wouldn't say boring.... But definitely the loneliest :-)

Thanks,

Marcel,
Ottawa, Canada

Marcel's trace

DI-145 picture & data sheet Newer unit available

I am an amateur seismologist. Retired, I have been quite active for the last three years building a seismometer, and programming a web page.

http://alainmichaud.ca

or just google: alainmichaud.ca

Please feel free to have a look or even link to it.

Sincerely

Alain Michaud

info@alainmichaud.ca

p.s. I am not (yet) a member of the facebook seismology group. I find the amateur seismology community is very small.

[ Ed Thelen here, I don't do facebook. ]