This Seismometer Is No Toy!

This is an image of the computer recording "drum" that shows four events recorded on the same day at my station in Denver, CO; two in Mexico and two on the opposite side of the world in Sumatra. The vertical short-period instrument that recorded these events can be made in a home shop.

Smart phone "earthquake" apps that use the built-in screen-tilt accelerometers can only detect gross movements that can be felt. The seismometer in this Instructable can detect ground motion of less than 50 microns/sec. (a human hair is about 100 microns), way below what can be felt or seen. This makes it sensitive enough to detect earthquakes from anywhere in the world greater than magnitude 6.5 and much smaller for closer events. Yet, mechanical and electronic filtering limits local signal noise.

This instrument rivals those of the USGS Mobile Seismic Array if placed in a suitably quiet and environmentally stable location like a basement and you will be able to gather data in the background through a USB port using free software that requires very few cpu resources while performing other tasks on your computer.

Note that like the professional instrument it nicely differentiates between Primary and Secondary body waves as well as the larger surface (L) waves allowing distance to an event and even magnitude to be accurately measured.

The seismometer consists of four basic components each of which I will describe in detail. The total cost will be between $300 and $350 U.S.. Free software to display and manipulate data is available online.

This design is a vertical short-period unit that is tuned to wave frequencies of about 1 1/2  - 2 sec which gives strong response to the  P and S waves of an earthquake.
There is room for some design latitude, but the arm dimensions, spring slope and especially spring tension characteristics are critical.

Wood base and post are sufficient in relatively stable moisture conditions, but pores should be sealed with at least two coats of paint.
Aluminum can be used also, but presents its own thermal expansion issues. The metal must be non-magnetic.

Major dimensions shown. Individual parts will be detailed in subsequent steps.

A box cutter blade is used for the contact point of the arm "hinge". It fits into a piece of the aluminum arm material with a V-shaped notch that allows free motion of the arm up and down. The arm is made of 1 1/4 in (3.2cm) strap aluminum 1/8 in (.32cm) thick. The arm is aluminum and not steel because it must be non-magnetic so as not to generate a secondary magnetic field with the horseshoe magnet.

The post is held in place by wood glue and a large lag bolt inserted through a pilot hole from the bottom. The leveling screws will need to protrude through the bottom far enough for the lag bolt head to clear the floor.

The spring parameters are CRITICAL. If they are too stiff the horseshoe magnet will not be heavy enough to lower the arm to horizontal without adding weights to the end of the arm. The springs below are what I used and are ideal. You will need three springs hooked end-to-end so you will need two packages.

I purchased these at Lowes Home Improvement in the U.S. Manufactured by The Hillman Group - Utility Extension Spring #208
Item #: 199346 |  Model #: 543004   $3.48 U.S.
http://www.lowes.com/pd_199346-37672-543004_0__?Ntt=199346&UserSearch=199346&productId=3115821&rpp=32

Adjust the arm to horizontal (using the bubble level) by adjusting where the upper spring hooks onto the fixed hook (see image in step 3).

Fasten the lower spring to the arm with a non-magnetic angle connector.

I used a 30 lb. pull Alnico horseshoe magnet from ALL Magnetics Inc. Item # 07272. $35 U.S.
http://www.allmagnetics.com/hardware/horseshoe.htm
Fasten the magnet to the arm with a non-magnetic brass or aluminum machine bolt and nut.

The 2 3/4 in (7 cm) coil discs are 1/8 in. (3 mm) hardboard because it is non-magnetic. It needs to be quite stiff so as not to spread at the edges when the magnetic wire is wrapped.
http://www.dickblick.com/products/hardboard-panels/The spacer core of the spool is a 1in (2.54cm) wood dowel 7/16 in (1.1cm) long. It is important to get your magnet first and calculate how wide the spool can be so it provides a small clearance inside the magnet gap. Add wood dowel "washers" to the outsides of the to the spool to give it more strength (they are not protruding through the hardboard as part of the inner spool). Drill a hole through the dowel for a non-magnet machine bolt to keep the spool together against the outward pressure of the magnetic wire windings.

The magnet wire should be 26 or preferably 30 guage.
http://www.amazon.com/b?ie=UTF8&node=310354011

Drill a small hole through the spool core to the outside and feed a1 ft. (30.5cm) length through it to the outside. Alternatively you can tape the beginning end to the inside of the spool and let a 1 ft "tail" drape over the edge to the outside. Curl up the tail and tape it to the outside of the spool so as not to get it tangled and broken during the winding process. (You can see two ends coming from the coil). The wire is coated with lacquer so any final connections must be sanded.

After securing the spool with the bolt and nut, insert the end of the bolt into a drill press chuck and wind the wire from the purchased spool to the seismometer spool on the slowest speed. A hand drill can be used, but take care not to let the wire jump the spool and begin winding around the bolt. This can be tricky, but if you take your time and go slow there shouldn't be a problem.

If the arm is set in motion without damping it would continue to move up and down for several seconds or even minutes. Since earthquake waves are in the 1 sec. to 25 sec. range, different arriving waveforms would be obscured by the arm's continued reaction to the initial wave. Therefore the arm must be dampened to return to its resting state rather quickly. This can be done with oil, but it is messy and viscosity varies with temperature.

The magnetic damper consists of a copper blade that moves through a strong magnetic field produced by 4 very powerful neodymium magnets. The blade and brass bolt are non-magnetic, but the housing IS MAGNETIC so the magnets can stick to it as are the spacer bolts and nuts.

Since the damper simply sits on the base so it can be moved in and out of the blade for adjustment, the housing must be heavy enough to prevent it from moving in the strong magnetic field produced by its interaction with the blade. You can buy 1/4 in 6.5mm) nickel-plated mild steel but I bought an inexpensive steel carpenter's square 2 ins (5.1cm) wide and cut it into six 2 3/4 in (7 cm) lengths with a hacksaw - three for each side of the housing. See the next step.

I clamped all six pieces together and drilled the holes for the 1/4 in (6.5mm) bolts in 3 corners.
Neodymium magnets were purchases from K&J Magnetics, Inc.
http://www.kjmagnetics.com/

Four 1 in x 3/4 in x 1/4 in (Part No. BX0C4)  $5.63 U.S. ea.
They are arranged in opposite polarity on the same side and on facing sides.
S | N
N | S
Be careful with these rare earth magnets. They can raise a blood blister if you get skin caught between them.

The blade is made of 24 guage copper sheet. Heavier gauge can be used, but not lighter. It is 1 3/4 in (4.5 cm) long not including the shoulder and neck and 1 1/4 in (3.2 cm) wide. The blade can be soldered or glued into a notch in the bolt.
Clearance  for the blade should be about an 1/8 in (3 mm) on each side.

I have tried several amplifier configurations for my seismometers, but the one below designed by Andy Loomis is by far the best.
It is very stable and features a commutating auto-zero (CAZ) op amp (Maxim MAX420CPA) which protects against low frequency noise resulting in baseline drift common in most other op amps. There are several online sources for the CAZ chip. They aren't cheap, but you will be making a mistake by going with a non-auto zero chip. TL082 JFET op amp can be purchased at Radio Shack 


You can see a detailed discussion of his amp and a slightly more complex version (Fig. 3.2) that I did not find necessary at
http://jclahr.com/science/psn/epics/reports/folded/

The time signal input terminal is optional and unnecessary when outputting to a pc as in my design. The connection from the 100k pot and the TL082  with the 68k resistor is still necessary.

I created my amp on a solderless breadboard and stuck it to the bottom of a plastic project box. This setup is fine since the amp is going to be in an undisturbed location near the seismometer. In fact, it should be right next to the seismometer in any case. I added connectors to the back and the 100k pot as a panel pot to the front.

The amplifier requires a regulated +12/-12v power supply. There are several designs you can find on the web and other options for this including two 6v lantern batteries. Here is the schematic for the power supply I used. Note that the PIN numbers on the positive and negative regulator chips are different. All of these parts can be purchased at RadioShack.

I use the Dataq DI-158U Analog/Digital converter, but it is an obsolete model. It has12 bit resolution.
Both the DI-145 ($29) and DI-149 ($59) have 10 bit resolution (2^10 divisions = 1024 divisions over the +/- 10V input range which would give a 19.4 mV resolution over the 20V). This might produce some noticeable "stair-stepping" in the display trace and unwanted noise in the signal.

The DI-155 is pricy at $159 but it is >13 bit (2^13 divisions of the input voltage) and voltage programable. So at +/- 5V you would get 10V /8192 divisions = 1.2mV resolution, 16 times better than the less expensive models and will also produce less signal noise. Sampling rate doesn't much matter in low frequency earthquake data, but resolution does. It is up to you, but if you can afford it, go with the DI-155. Alternatively you could buy the DI-145 starter kit and see if you can live with the resolution.
Dataq website.
http://www.dataq.com/data-acquisition-starter-kits/data-acquisition-starter-kits.htm

As mentioned above, you can use the Dataq Acquisition software that comes with the A/D converter kit. It will definitely collect seismic data, but there are programs dedicated specifically for seismic data collection, display, mainipulation, filtering and storage.

I use the free program called AmaSeis AS-1 developed by Alan Jones for IRIS.
I notice that he has not updated it to use the Dataq DI-149 A/D or the DI-155, but I am sure he would if he were contacted.
http://bingweb.binghamton.edu/~ajones/AmaSeis.html

It has recently been replaced by a Java-based program called JAmaSeis which is still in beta development. I haven't tried it yet and I don't know if it has all of the filtering capability as Alan Jones's original version. It is compatible with the DI-145 and DI-149 so far. I am sure they will be adding new A/D converters as requests come in.
http://www.iris.edu/hq/programs/education_and_outreach/software/jamaseis

Your mechanical unit MUST BE COVERED and sealed against air currents which would create a great deal of signal noise.
I used part of a styrofoam insulation sheet taped at the four corners and used a heavy piece of particle board for the lid which holds the cover tightly to the floor.

Once you are getting a signal display on your screen it is time for the "lift" test to adjust the magnetic damper.
Cut out a small piece of file card 1/2in x 3/4in (1.3cm x 2cm) and attach it to a thread or fine string about 1 meter long and attached the other end of the thread to a stick or dowel of equal length.

Open the lid of the unit cover and lower the file card cutout onto the arm just behind (closer to the post) the damper attachment bolt. Make sure the string does not tangle in the spring. Drape the string over the edge of the box and close the lid. Stand quietly for a minute or two to let it settle down. Then raise the stick or dowel suddenly so the card lifts off the arm without disturbing the box.

If the lift test looks like this image, then the arm is properly dampened. If the initial deflection goes up instead of down, reverse the leads from your unit to the amplifier. If the deflection:rebound ratio is between 12:1 and 15:1, it is properly dampened.

If the ratio is less than 12:1 move the damping housing with the magnets to cover more of the blade. If the ratio is greater than 15:1 or if there is no rebound, move the damping housing back to cover less of the blade. Damping can also be controlled by changing the distance between the copper blade and the neodymium magnets.

Once you have adjusted the unit's damping you are ready to troll for earthquakes.
Be patient, it may take several days to a week or more for you to record your first earthquake. It takes one close enough or large enough if it is far away to move the floor literally a hair's bredth.  Depending on where you live you can expect a event every 3 to 10 days on average. More often if you live on a tectonic plate boundary.
Maybe you will get lucky and record the "Big One" like I did with the Great 9.0 magnitude Japan earthquake March 11, 2011 that caused a devastating tsunami. I recorded waves from this earthquake for more than four hours. The earth rang like a bell.

Good Luck and Good Hunting!