Seismometer Calibrator

September 1996

by Bob Barns
Email: <75612.2635@CompuServe.COM>

Download this article and drawing (15k ZIP file).

Figure 1

How would you like to know the sensitivity of your seismometer in absolute terms? Richter's book, "Elementary Seismology", 1958 p 210, gives this definition: "Seismometer, a seismograph whose physical constants are known sufficiently for calibration, so that actual ground motion may be calculated from the seismogram."

Apparently, amateur seismometers are calibrated empirically, that is, recordings of known quakes are used to relate the max. reading to magnitude. I live in New Jersey, not a location of wild and wonderful seismic activity, so I expect that doing a reasonably accurate calibration using known quakes might take a long time. This has the virtue of including how well the seismometer is coupled to the crust, etc.. However, in many kinds of measurement it is useful to know instrumental sensitivity in terms of well -defined quantities. I may have reinvented the wheel but if not, I offer the following scheme. My Lehman is now hooked up to Larry Cochrane's A/D board and is working nicely. It got a good recording of the Easter Is. event of 9/5. I was surprised that there were no nasty surprises in getting the Lehman adjusted and running. A box made of 2" Styrofoam greatly reduced the noise due to air currents and I solved the problem with transmissions from my amateur radio 2 meter rig getting into the electronics.

Operation of the Lehman raised the question of what is its sensitivity and what is the seismic noise level on my basement floor (which is surprisingly flexible). I devised this calibration scheme to get at these questions. The heart of the calibrator is a common 1ma (or practically any other current range) D'Arsonval meter movement. These are available in unlimited numbers at ham flea markets for $3 or less. I measured the force vs meter current constant by mounting the meter (after removing its case ) on edge so that the needle was horizontal with 0.5ma. A measured length of #30 copper wire (about 0.2") was hung on the end of the needle and the additional current required to bring the needle back to horizontal was measured. This was 0.44 ma. The weight of the piece of wire was calculated (from the wire tables) to be 2.64mg. This process gave the force calibration of the meter which is 6 milligrams/milliamp or 6 dynes/ma. This whole calibration took about 1 hr. The greatest uncertainly is measuring the length of the wire. Be careful not to suddenly apply more that about 5ma to the meter (without the extension) or you'll bend the needle. A strip of 0.003" shim stock about 0.08 X 1.25" was fastened (using wax and a little heat from a soldering iron) to the needle so that the end of the strip extended beyond the edge of the meter movement. (Be careful not to let bits of magnetic material such as iron filings get into the area around the moving coil of the meter. )

The meter was then mounted on a sheet metal bracket attached to a heavy base. The meter was mounted with its needle (and shim stock extension) hanging down vertically and at a height such that the tip end of the extension could push horizontally on the side of the boom of the Lehman. The base was positioned so that with zero current, the extension tip was about 1/16" from the boom.

A 555 timer circuit giving a pulse of 0.83 secs. was used with a variable resistance between its output and the meter. This time was chosen to be short compared to the period of the Lehman (18 secs.) A manual push-button triggered the pulse. Since the output voltage from the 555 is constant (with reasonable loads), the current of the pulse and hence the force-time product on the boom is easily calculated.

The calibrator pushed on the boom 11" from the pivot and the mass is 27.5" from the pivot so the equivalent force at the mass is 0.4 times the force given by the current. Also, the tip of the calibrator is twice as far from the meter pivot as was the calibration weight so the force-current factor is 3 mg/ma times 0.4 or 1.2 mg/ma. The force in dynes is 0.98 times this or 1.2 dynes/ma.

The velocity of the mass is (force[in dynes] * time) / mass[in grams]. The mass is 3,500 grams. The table shows the values from the A/D of the initial deflection and the calculated velocity of the boom in nanometers/sec due to the current pulse in ma. for 0.83 secs.

  ma       nm/sec     A/D value

 1.15        590           75

 1.7         860           120

 3.2         1650          230

 4.5         2310          300

 6.5         3340          450

You can plot velocity vs. value and see that it is nicely linear and goes thru zero. A least-squares fit gives nm/sec = p-p value * 7.46 - 2.5. This measurement also gives the sense, e.g., positive value corresponds to a compression arriving from the west. At 3340 nm/sec., the deflection was calculated to be about 8 microns.

My seismic noise level seems to be somewhat less than half the 590 mn/sec velocity so I conclude that my seismic noise level is about 300 nm/sec. ( With no input to the amp, the noise is < 1/20 of this.) The only hard number I have to compare this with is the PEPP noise specification for their seismometers of 100 mn/sec.

Another use for this thing is to make occasional checks of the performance of the whole system. Since the calibrator can be left in place (it does not touch the boom), a check is just a matter of powering up the 555 and pushing the button.

It should be possible to do a frequency response curve of the whole Lehman-amp system by driving the meter movement with variable VLF sin wave oscillator. I plan to try that.

Comments on this scheme are welcome.

Bob Barns

 Seismometer Calibration method as described by Bob McClure

Calibration is a tedious but recommended procedure for the serious amateur, Bob McClure

  I wrote the following note on my calibration method to the PSN List on 07/19/2003, a year ago. At this time, I am adding a dissertation on how to use current applied to the sensor coil and measurement of the resulting force exerted by the coil to achieve calibration. Be reminded of the fact that these calibration methods only determine the sentivity of the sensor for frequencies greater than the natural resonant frequency of the sensor. Sensor response falls very rapidly at frequencies less than the natural frequency.

Bob McClure
Locust Valley, NY
Web page: http://www.jclahr.com/science/psn/mcclure/
-----------------------
APPLIED MOTION METHOD:
-----------------------
1. Set up limit stops on the pendulum so that it can be displaced over a
known and fixed number of millimeters at its radius of gyration.

2. Connect the sensor directly to the A/D, and log data as you move the
pendulum gently back-and forth a few times between the stops.  Record at a rate
that gives a reasonable number of samples for the time taken to move between
stops. Do not move so fast that you exceed the voltage range of the A/D,
otherwise you won't get valid data, and you might even blow out the A/D. Also, verify
that the resistance of the sensor coil is low compared to the input impedance
of the A/D.

3. Make a WinQuake event file out of the data.

4. Use WinQuake to integrate the data. You should see the actual
displacement versus time, measured in counts.  Measure the peak-to-peak displacement in
counts, using the mouse readout when it is positioned on successive peaks.

5. Scale the count measurement to what you would have obtained for one
centimeter of motion.  If you used 5mm of displacement, for example, you would
multiply your count estimate by two.

6. Multiply the scaled counts by the voltage gain setting of the amplifier
you normally use.

7. Take the inverse of the number obtained in step 6.  This is the number you
should enter for "Sensitivity:" in the Sensor Information Dialog box.
------------

------------------------
APPLIED CURRENT METHOD:
------------------------
 Recall the following from your physics textbook:

Generator Law: Volts= B*L*(dx/dt), where B= magnetic field in Teslas, L= total length of wire in meters cutting flux lines, (dx/dt)= velocity of coil motion in meters per second.

Motor Law:     Force=B*L*I where F= force in newtons, B in Teslas, L in meters.

Combining the two laws, we obtain  V/(dx/dt) = F/I

so, if we measure F/I we know V/(dx/dt), the sensor output in volt-seconds per meter. We don't need to know anything about magnetic field strength or coil configuration.

One newton is the force required to accelerate one kilogram mass at one meter per second. In grams, it is 1000/9.8 = 102.04.

Suppose we apply a current of 5 milliamperes to the coil and measure a force of 40 grams.
I=.005 amperes, F= 40/102.04= .392,  F/I= 78.4 volt-seconds per meter.

~~~~~~~~~~~~~
  The equation is simple, but execution of the measurement is not. If you use a countertop digital scale, you have to figure out how to transmit the force from the sensor to the scale. This ususually would require rigging up levers and pushrods. I did this measurement, but using only the component parts -- the magnet and the coil. My coil is a flat pancake, positioned in a 4-pole Nd magnet assembly. What I did was to place the magnet on edge on the scale, and make up a rig to hold the coil in place from the counter top. I then measured the apparent change in the weight of the magnet as current was applied in either direction to the coil. If you use a heavy magnet, you will probably have to secure the coil to the scale, and the magnet to the counter top. Estimate coil current by dividing the applied DC voltage by the coil resistance in ohms.

  If you have a sensitive spring scale, that's great. You can probably use it on an assembled sensor. Just be sure that you pull the boom back to its normal rest position when the scale is being pulled upon by the current being applied to the coil. Measure how far from the boom pivot you took the force reading, and estimate the radius of gyration of the pendulum.
 
  By whatever means you measure force, you will have to figure out what the force actually would be at the radiu of gyration, which is not usually at the center of the coil. You will have to make lever arm corrections to convert the measured force at the point of measurement to the force at the radius of gyration, which in most cases is very near the center of the extra mass added to the boom.

  After you have made the F/I measurement, make the lever arm correction to find the equivalent value at the center of gyration. Divide this F/I by 100 to get volt-seconds per centimeter, and multiply that result by your amplifier's voltage gain. Next, divide your A/D's full scale volts by the full scale digital word value. Usually, you will be dividing 10 by 32,768, yielding 0.000305176 volts per bit. Divide the result by the amplified F/I to get the number to be entered for sensitivity in WinQuake.

  You also have the option of letting WinQuake do the calculations for you. Just enter the sensor output (volt-seconds per centimeter) in the "Output Voltage:" box, the amplifier voltage gain in the "Amp Gain:" box, the A/D full scale voltage in the "A/D Voltage:" box, and the A/D bit number in the "A/D Bits:" box, and then click on the "Calc Sens" box. (Note to Dataq users: Always enter 16 for A/D bits, even though the actual number is less. Dataq always scales 10 volts to digital value 32,768, regardless of the number of active bits in the device.)
~~~~~~~~~~~~~~~~~~~~~~
Afterthoughts:

  If you use shunt resistance across the coil to provide damping, you may have to reduce your measured sensor output calibration value. The reduction factor is (Rshunt/(Rshunt + Rcoil). The value of Rshunt must also include the contribution of the amplifier input impedance: 1/Rshunt = 1/Rexternal + 1/Ramplifier.

  I recommend the use of shunt damping whenever possible because it permits easy measurement of natural period (with the shunt removed) and precise control of the degree of damping (amount of shunt conductance applied). It is easiest to use if your sensor has high output combined with low pendulum mass and low coil resistance. The conventional massive Lehman design may not have such properties, however. My own horizontal sensors are at the other extreme. They have a coil resistance of only 340 ohms, a pendulum mass of around 70 grams, and an output of 0.8 volt-sec/cm. Critical damping requires only 30K of shunt resistance. My amplifier input impedance is 100K, and so I add another 90K across that to get proper damping. My vertical sensor is more like a Lehman, with much more mass and much more coil resistance. Even so, I use shunt damping on it, at the loss of some sensitivity.

  If you are building a sensor, consider the use of multiple magnets. If you place a horseshoe magnet on one side of the coil, place another with poles reversed on the opposite side. You will get twice the output, and much better linearity. My own preference is a 4-pole magnet assembly using powerful Nd block magnets, a narrow gap, and a pancake coil.

L-15B Mark Products Geophone

WinSDR setup