Lehman Seismograph Construction Notes

{Work is in progress}

Because I occasionally get requests for information about the construction of the Lehman seismograph, I thought it might be helpful to add a page of notes with some of that information.

I should begin by saying that the purpose of this seismograph was for the pure enjoyment of observing earthquakes.  It is not calibrated in amplitude; I use a walk-up calibration described below.  The time calibration is somewhat more precise, but only within the timing accuracy of the internet time software that I use, the Windows Linux operating system, and the Amaseis recording software.  I have not analyzed any of the errors in the timing system.  Having said that the instrument has recorded a lot of earthquakes during the past nearly ten years of continuous operation and is capable of demonstrating a surprising range of seismic phenomena.

Dimensions and Construction Details

The basic layout is described in the Lehman Seismograph page.  The instrument is shown in the photo below.

The labels refer to the locations of various attachment points that have some importance in tuning the seismometer response and operation.

• S is the upper pivot and suspension for the entire boom
• O is the lower pivot formed by knife edge on the boom against a modified bolt.
• P is the magnetic pick-up (sensor) location
• B is the attachment point for the suspension wire on the boom
• M is the location of the seismic mass, roughly five pounds of lead
• D is the location of the damping vane

The rough dimensions are given below:

• Vertical distance between the upper and lower pivot points, OS, is 44.5 cm
• Distance along the boom from lower pivot to the sensor, OP, is 69 cm
• Distance along the boom from lower pivot to the suspension attachment, OB, is 75 cm
• Distance along the boom from lower pivot to the seismic mass, OM, is 83 cm
• Distance along the boom from lower pivot to the damping vane, OD, is 92 cm

The upright support is provided by a frame constructed from two eighteen inch lengths of ½ inch pipe attached to the aluminum base with pipe flanges and connected at the top with two 90º els and a six inch nipple. A steel bar spans the two uprights with its center about 2 inches above the base. A hole is tapped in the center to accept a bolt for the lower pivot point (knife edge) to rest against. The head of the bolt was filed flat.  A very shallow groove was filed across it to accept the knife edge that was ground on the mating end of the boom, a 3/8 inch diameter steel rod.  The photo below shows a close-up of the lower pivot.

The photo below shows the upper pivot point and suspension for the pendulum.  The suspension wire is a 0.012 inch nickel-steel guitar string.  The small ball on the guitar string passes through a hole in the bracket on the left and the wire slips into a slot to a shallow countersunk hole to center the ball.  The wire passes over a shallow groove in the bolt head on the right and on to the boom.  The bolts at the wire suspension and the knife-edge provide adjustment to offset the top of the line between them very slightly toward the the pendulum.  This adjustment is used to tune the period of the undamped pendulum to between 12 and 16 seconds.

Tuning and Final Adjustments

At this point the seismometer should be placed in the position where it will operate.  Mine is placed directly on the concrete floor of our basement.  In its normal operation it is covered with a box made of 1/2 inch blue foam building insulation to reduce the effects of thermal changes and air currents on the soon to be very motion sensitive boom.  The boom is usually oriented to swing either in an East-West direction or a North-South direction.  Mine swings EW.  The base is then leveled use the three screws in the base until the boom centers to its operating position. The first real adjustment is to tune the undamped period of the pendulum to 12 to 16 seconds as described above.  This is best done using whatever utility software came with the analog to digital converter (ADC) you are using, WinDaq in the case of the Dataq device that I am using, and assumes that you have performed whatever ADC calibration that is necessary.  Gain and offset adjustments should then be made to your amplifier to center the trace and to provide a good signal without overloading the ADC.  For the Dataq ADC the input range is ±10 volts  so you want to remove any DC offset from the signal and adjust the gain to provide a signal level of ±10 volts when the boom is oscillating. Then you just give the boom a little nudge and let it swing.  As shown below I measured the time it took for six cycles and divided by six to get the period.  These period adjustments take some time to do but I don’t recall that it was difficult once you develop a feel for how sensitive your particular instrument is to the adjustments.  The image below shows a typical undamped response and the period estimate of 16.5 seconds.

At this point I continued to use the WinDaq utility software to adjust the damping as well but you can probably use the AmaSeis software as well.  If AmaSeis is used you must adjust the center offset (zero setting) to get correct results.  The Help screens will guide you through those better than I can.

I use a walk-up test to adjust the gain and damping.  In my experience and for my location, I have found that the instrument gives acceptable performance if it shows a full-scale deflection when I walk up next to it.  Because of the slow period one must proceed slowly through the procedure.  I walk up to the covered instrument fairly quickly and then stand still for 30 seconds or more.  Then I quickly walk away from it.  When properly adjusted this procedure will show two full excursion peaks on the seismogram, one when you walk up to the instrument and one 30 seconds (or more) when you walk away.  Each peak will show a very small overshoot on its return to center, perhaps 10% of the peak value.  This amount of damping is obtained by an aluminum vane with a slight bend in it attached to the boom and swimming in a small dish of 90 weight gear oil.  Because of the inertia of the seismic mass, it takes a surprising amount of oil and even a little twist in the aluminum vane to adequately damp the oscillation.   I will try to find or recreate a sample seismogram that demonstrates my preferred result.

Ok, this is not the preferred result (not enough gain or damping) but it is what I found in my archives and demonstrates what the “walk-up test” should look like when done in the WinDaq utility software.  The first peak, at -2.668 volts, is negative going as I walked up beside the instrument.  The overshoot shows 0.481 volts. The positive going peak is when I walked away from the instrument followed by a small overshoot. Ordinarily, the larger peaks would be in the ±8 volts or more and the smaller peaks would be about the same as shown giving a ratio of less than 10% instead of the 18% shown here.

The photograph below shows the end of the business end of the boom, starting from the left side: the red horseshoe magnet (with brass counter balance across the boom from it) interacting with the coil of magnet wire, then a small hole with a brass screw next to it where the suspension wire for the boom is attached, approximately five pounds of lead cast on a pipe nipple to provide the seismic mass, and ,finally, the damping vane and oil bath.