Over the last ten years I have developed and simplified seismograph designs, originally from a 1979 Scientific American "Amateur Scientist" article, to the stage where they are reliable, easy and cheap to build, but most of all still do look like the diagrams in the textbooks. Even more important, they detect earthquakes from New Zealand, Papua New Guinea and Indonesia regularly and the seismograph printouts are nearly the same as the textbook drawings; easy to explain and easy for the kids to understand.
So far this year I have built several different "Horizontal Pendulum" type seismographs. These are also known as "swinging gate" designs because the hinges of the "gate" are nearly in a vertical line, with the top hinge just slightly forward of the bottom hinge so that the "gate" will swing shut. In the horizontal pendulum design the top pivot holding the wire support is about 1mm forward of the bottom pivot, making the seismograph horizontal bar swing very slowly back to the same resting position.
The picture shows three nearly identical horizontal pendulums on the vibration table that I have built: same 2 kg mass, same distance from pivot, same threaded horizontal rods, same support wire angle and same pivot positions. The table will allow me to alter one parameter at a time to determine how this effects the period of the pendulum and the sensitivity of the detector. I have since built a motion control device capable of moving the table 0.02 millimetres to again allow comparisons of different design modifications.
The monitor display was a real quake detected on the seismograph with the yellow painted 1 kg lead masses, with the same damping and detector.
Hardware stores seem to use both metric and imperial measurements. Sorry about the mixture of units; it won't matter if you use metric or similar sized imperial bolts.
|The top mounting point for the seismograph is with a "1/4" inch bolt through the side of the display cabinet about 35 cm up from the floor of the display cabinet. It should be half way between the back wall of the display cabinet and the glass front. Earlier photos show a ring bolt and an adjustable turnbuckle but I found that these were not needed. A large washer drilled through near one edge gave a connecting point for the wire. HOLD THE WASHER IN A VICE AND WEAR SAFETY GOGGLES WHEN DRILLING. A nut behind the washer stops it tilting too far. Nuts and washers on the inside and outside of the display case wall fix the washer wire hole about 1 to 2 cm in from the display case wall.|
One of the few CRITICAL adjustments in the whole design is that the lower mounting point for the bar MUST, MUST, MUST be vertically below the top mounting bolt. Do not assume that the wall and display case are vertical. I have found that the best way to ensure that things are vertical is to make a plumbob with fine fishing line and a lead bean sinker. Hang this from the hole in the washer and it will give you a true vertical on the inside of the display case. The Lower mounting bolt should be about 10 cm up from the inside floor of the display case. CHECK THE SIZE OF THE MASS/MASSES THAT YOU WILL USE TO MAKE SURE THAT WHEN THEY HANG FROM THE BAR THAT THERE WILL BE AT LEAST 1 CM CLEARANCE ABOVE THE FLOOR OF THE DISPLAY CASE.
A "half inch" bolt (about 5 cm long) screws tightly in through the side of the display cabinet. It is the lower pivot point for the "knife edge" end of the horizontal "5/16" threaded rod. Because the ½ inch bolt is wider than the knife edge there is actually a small amount of adjustment possible to get the lower pivot directly below the top wire pivot point. If you do end up with them out of vertical alignment you might be able to twist the washer slightly to one side so that the bar rests in the centre of the display case. The slight lip on the end of the bolt stops the knife edge dropping during adjustments.
The final adjustment of the seismograph needs you to screw in the ½ inch bolt until the lower pivot point is less than 1mm behind the top pivot point. This is important for the overall functioning of the seismograph because it effects the natural period (swing time in seconds for one complete swing from centre, across to one side, back to the centre, across to the other side and back to the centre again) of the seismograph. A longer period (e.g. 20 seconds) means that the seismograph is more sensitive to the long period Love waves and Raleigh waves that travel near the earth's surface from very distant (1000 km to 10000 km) earthquakes. A shorter period (e.g. 1 second) would make the seismograph more sensitive to the P and S waves from nearby (< 500km) earthquakes (but Australia has few nearby earthquakes unless you live in Tennant Creek, Meckering, Adelaide or Gunning) and so a long period seismograph is more likely to detect earthquakes regularly.
A period of about 5 seconds seems to work best with my seismographs in Sydney.
The 5/16 threaded rod must be cut to fit comfortably into one side of the display case. In the schools I have seen, this would mean that the bar would need to be about 800mm long. A longer bar does not make the seismograph more sensitive to all seismic waves. Remember that it is the display case that moves, not the bar, so a longer bar does not help.
A good trick is to wind a nut onto the threaded rod before you hacksaw the rod. File off the sharp edges and then unscrew the nut so that it evens out the thread of the rod. You can then get other nuts on easily. USE SAFETY GOOGLES WHEN CUTTING, FILING OR GRINDING THE KNIFE EDGE.
The picture above shows a 2 kg mass from the Science Store room held firmly by nuts and washers. The mass is about 10 cm from the end of the bar. The right hand nut holds the support wire in position. 1mm to 2mm steel wire is easily strong enough. You can put a match box under the mass to hold it 1 - 2 cm off the floor of the display case and guess at the required length of wire. Twist it with pliers. The support nut can be wound in or out until the bar is horizontal. Note how the wire strands go over the shoulders of the nut. If the knife edge is not quite vertical you can adjust it by winding the wire support nut to the left or right.
A small "screen" of aluminium screwed to a small bulldog clip half covers the light beam from the LED to the LDR (below the aluminium). During a quake the bar will stay still and the room, display case and detector will move back and forth about 0.1 mm. The different light levels hitting the LDR will be turned into a voltage and recorded by the data logger and computer.
Try to make sure that the mass and any clamps or bulldog clips don't twist the bar. The knife edge must remain vertical.
|Damping, to reduce the natural resonance of the bar could be either a "paddle" moving in oil or water as here or magnetic. This paddle shows how liquid damping can work; the piece of wood is attached to the bar with a small hose clamp. To reduce the length so that it would fit inside a display case the paddle could be attached to another bulldog clip and the liquid container could fit under the bar. It seems to work just as well on the pivot side of the mass.|
As the damping fluid, I have tried:
FINE TUNING. Most people start out wanting the seismograph to be extremely sensitive. After you have detected a few quakes try increasing the amount of damping (larger paddle area or more viscous liquid). This should reduce the "background noise" that the seismograph detects to a single straight line. Quakes will also be lower but will be more obvious against the background. With more damping you are also more likely to detect P and S waves.
A supermagnet (also called a rare earth magnet) attached to the bar could be set 1mm from a very fine wire coil with the two ends of the wire joined together. (Two magnets with the coil in between works even better.) The induced current in the coil as the bar and magnet move during a quake will tend to gently dampen the swing of the bar. Old fish tank air pumps have such coils of wire (make sure the wire isn't burnt out; the resistance should be 2 k - 10 k) but the magnets in the pumps are not strong enough to give sufficient damping. The optimum level of damping would stop the bar in one or two swings. As long as the bar movement is damped in less than about one minute the results are satisfactory.
Some computer hard drives have super magnets and it might be worthwhile trying to scavenge one or two but be careful.
SUPERMAGNETS ARE DANGEROUS. I HAVE BEEN BADLY CUT WHEN A PAIR DECIDED TO PLAY "NORTH ATTRACTS SOUTH" WITH MY HAND IN BETWEEN. Your credit rating might also head south if your cards get too close, your computer monitor might look like a rainbow, floppy disks get scrambled and anyone carrying a steel laboratory stool nearby is likely to be followed by the seismograph bar. The good thing about magnetic damping however is that once you get it right it will stay right.
Some very serious "amateur" seismograph designs use negative feedback taken from the detectors to electronically damp out all resonance but this is well beyond the level needed for this project. There are hundreds of amateur seismograph web sites that I have surfed through over the last five years from the original 1979 Lehman design (http://psn.quake.net/lehmntxt.html) to very, very advanced electronic designs on the Public Seismic Network in California (http://psn.quake.net/equip.html). These need to be very sensitive as even in California they rarely "see" magnitude 6 or 7 quakes. We are lucky in Australia where frequent large quakes around the edges of our continental plate mean that my seismograph design works very well even if it is not finally tuned. I have detected a magnitude 5 from New Zealand however so it is still very sensitive.
|This photo of a seismograph at my school, Turramurra High shows an early prototype in a laboratory display case. Improvements over this would include
I will be producing display posters for the seismograms and setup procedures for each brand of data logger. These could go on the back walls of the display case.
If you don't want to fit the seismograph into a display case then the picture shows how to make it free standing. The two vertical bolts near the pivot end allow the tilt to be adjusted. Cut a "screwdriver slot" in the end of each bolt and use a screwdriver to adjust the position of the bar from above. The light detector circuit is in the grey Jiffy Box. The seismograph could be positioned in a fish tank or cupboard where there is no air movement.
The sensitivity of the bar can be checked with the "blow test". From about 1 metre away, blow gently towards the mass for about three seconds. The force of the blow should be just enough to make a candle flame flicker if the candle was next to the mass but not enough to blow the candle out. Don't laugh; it has taken me a long time to think of a way to standardise blowing power. You should just be able to see the bar move by a fraction of a millimetre.
|August 2005 Update|
|I have continued to try different types of damping for the seismograph bar. By far the best, seems to be a supermagnet actually inside a coil of reasonably heavy gauge (0.5 to 1mm) enamelled or cotton covered copper wire. As the bar swings, the moving magnet produces a current in the coil that creates a magnetic field countering the movement of the magnet. The swing of the bar is damped out in five to ten swings. (If you pull the coil away from the magnet then the bar actually moves.)|
|The photo shows a cylindrical supermagnet (from Oatley Electronics http://www.oatleyelectronics.com/) inside a coil of wire wound onto a 'plumbing tape' plastic former. The coil has about 100 grams of wire wound reasonably smoothly. Drill two fine holes so that the ends of the wire can be fed out from the coil, scrape off the insulation and tightly wind or solder them together. The magnet holds itself onto the vertical face of a pair of nuts locked tightly against each other on the threaded rod. A flat edge could be filed in the rim of the former to make it sit more stably.|
|Most schools have large, 500 gram rolls of 0.71 mm insulated wire. If the former is not made of iron and you can make a connection to the inside turns of wire then this will be a perfect damping coil. I have one of these; it completely damps out the motion of the bar in 3 swings.|
|These two quakes were less than 5 hours apart on the same seismograph recording. The different shapes of the patterns were determined by the several factors including distance and direction from the seismograph in Sydney and the response of the seismograph. L waves in vibrate at right angles to the direction of the other waves. If a long period seismograph bar is pointing directly at the epicentre then the L waves will cause it to swing the most. If the bar is at right angles to the epicentre direction then L waves will be much less obvious. P waves are higher frequency and are more often detected by short period (1-10Hertz) vertical seismographs. P waves are attenuated more quickly than other wave types.|
Because of this, professional seismograph stations use two long period seismographs mounted North-South and East-West and also a short period Vertical seismograph. These should collectively detect all of the waves but it is rare that one seismograph detects all of the waves in a single recording.
I have built N-S, E-W and Vertical detectors both at the University and my home. I have yet to detect a quake on all 3 axes but have detected a small quake on the two horizontal sensors. The Geoscience Australia data from their Riverview station in Sydney also did not give show vertically detected waves so I am in good company. The vertical waves are much smaller than the horizontal waves; particularly the P waves.
Some school data loggers can log 4 channels at once and I recently purchased two DATAQ DI-194-RS data loggers that can record and graph up to four channels at once over several weeks. They cost about $60 + GST and delivery from Total Turnkey Solutions http://www.turnkey-solutions.com.au/ and are excellent for logging sensors on all three axes at the same time. I am building a 3-channel detector amplifier using an LM 324 quad op-amp to avoid the need for 3 separate plug packs, and detectors. I will include the circuit when it is complete.
|September 2005 Update: Vertical Seismograph|
|A vertical seismograph design that is worth a try. (Especially for those close to the action in Gunning, Tennant Creek, PNG or the Shaky Isles)|
The photo shows one of my attempts at building a vertical seismograph. I have not yet recorded a large or close earthquake using the design but have just detected the Raleigh waves of magnitude 6 quakes in PNG. The basic concept has the weight (= mg) of a 1.25 kg mass being balanced by the downwards force of the strong spring. When the earth accelerates upwards during an earthquake, the value of g increases from 9.8m/s2 to more than 10m/s2. (50km from a magnitude 7 quake the value of g can vary by 10% ie from 8 to 12 m/s2.) The weight of the mass increases but the downwards force of the spring does not change; therefore, the bar holding the mass will tilt downwards. As the earthquake wave passes, the ground accelerates downwards and the bar will tilt upwards.
Amateur designs have the spring on the same side as the mass but pushing upwards towards a complex low friction "hinge". The design in the photo has the bar (channel aluminium that was part of our shower screen) hinged over one end of a strong bracket (about 150mm x 150mm x 30mm x 3mm thick). The hinge end of the bracket is filed or ground to make a round edge. A large washer sits between the round edge of the bracket and the aluminium bar to reduce friction and to reduce the chance of a corrosion reaction between the steel and the aluminium. The spring must be at least partly stretched so that the coils are not touching. The threaded hook allows us to level the bar. Since the bar is just sitting on the bracket it can be moved slightly (to the left or right in the photo) to help level the bar as well. The aluminium bar was about 300 mm long and the light screen extension was about 150 mm. The two vertical threaded rods allow the positions of the two super magnets to be easily altered. (Don't have the magnets too close together; note the chip in one the magnets) The large coil damps the bounce of the bar and the small coil (from an aquarium air pump) feeds to a magnetic amplifier circuit. Maybe some of the dimensions are critical, but as yet I do not know. Please let me know if any changes make the design work better
As well as possibly detecting vertical waves I will use the design to compare the results for the light detector and the magnetic detector. STAY TUNED.
|November 2005 Update|
|As a means of assessing the effects of minor changes on the performance of the seismographs a motion control device was built for the vibration table. This used the long screw thread of a hooked bolt that was tapped through the right angle bracket to push a 470mm long lever. A 300 turning of the screw thread pushes the end of the lever 0.1 mm and pushes 0.01 mm against the vibration table. This allowed design changes such as bar length, wire angle or mass to be accurately assessed.|
|A hole was drilled through the aluminium bar 53mm from the pivot. A long bolt pushed against the vibration table to give a known horizontal movement for any specified rotation of the bolt.|
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