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Lehman Seismometer Page

Figure 1. Lehman Seismometer
The boom is a horizontal pendulum. It has a period of 14.2 seconds, equivalent to a natural frequency of 0.071 Hz. A sensor at the free end measures the displacement. The boom length is 64 inch. The total frame height is 35 inch. The boom has a knife edge that pivots against a bolt head in the lower cross-beam of the frame.
The boom is suspended from the frame by a wire cable. The cable is attached to the top cross-beam of the frame. The other end of the cable is attached to the boom, about two-thirds of the distance from the pivot to the free end of the boom. The pivot point is offset from the top cable attachment point. Thus, the boom oscillates as if it were a "swinging gate."
The plate supporting the frame has three adjustable mounting feet. The feet can be adjusted to tune the pendulum to the desired natural frequency. Furthermore, the wire cable has a turnbuckle which is used to adjust height of the free end of the boom.
The detached frame in the center of the figure is used for assembly and to limit the displacement during tuning.

Figure 2. Sensor and Damping Method
The sensor is the metal cylinder mounted in the L-bracket at the left, center of the image.
The sensor is a non-contact inductive linear displacement transducer. The model is Omega LD701-5/10  with a 5-10 mm range. The sensor outputs a voltage proportional to the distance from the sensor head to the steel target, which is suspended underneath the wooden rod section of the boom.
The inductive sensor was my third, and by far most successful, measuring solution. I highly recommend this transducer.
The first sensor was a linear variable displacement transducer (LVDT) with its spring removed. The LVDT, however, had too much friction which increased the natural frequency of the boom to an unacceptable value.
The second sensor was a magnetic coil attached to the boom. The coil moved between the poles of a magnet, thus inducing a voltage proportional to the velocity. This is the classic sensor method used with most Lehman seismometers. The signal requires tremendous amplification. Regretfully, my coil, magnet, and amplifier set-up missed the Parkfield, California earthquake. This setback led me to the inductive displacement sensor, which has proven to be successful.
The lead disk is used for ballast. The pan contains canola oil which is used for viscous damping, both in the vertical and horizontal axes. A horizontal plate is connected to the disk and is fully submerged in the oil. Furthermore, the bottom side of the disk dips into the oil. The resulting viscous damping is 9.8%.
The thin aluminum plate at the free end of the beam was used for an alternative damping method that was abandoned in favor of the oil method.
Figure 3. Pivot End of the Boom
This end of the boom is actually a chisel blade. Its edge butts up against a chrome plated bolt head. Ballast weights are located on the plate below the pivot point.

Figure 4. Nicolet Vision Data Acquisition System
The trace is the displacement time history of the Solomon Islands earthquake on October 8, 2004 as measured by the Lehman seismometer via the inductive sensor. The magnitude was 6.8. The seismometer is located at home in Mesa, Arizona.
I use this same acquisition system for measuring pyrotechnic shock in launch vehicle stage separation tests. I also use it for rocket nozzle frequency response tests.
The Nicolet is a completely gold-plated Cadillac solution for the Lehman seismometer, but I wanted to do this science project with some style.
The Nicolet sample rate is set to 50 samples per second. Its lowpass filter is set to 5 Hz.
A Krohn-Hite filter, model 3343, is used to highpass filter the displacement signal at 0.03 Hz prior to its input to the Nicolet system. It also provides a 20 dB gain.
The following table lists of some of the notable quakes measused on this seismometer.
Date  (Year 2004)
 November 20

 Costa Rica
 November 15

 November 2

 Vancouver Island
 October 15

 October 8

 Solomon Islands
 November 2004 

Tom Irvine
Email: tomirvine@aol.com

Recommended Books
1. A. Chopra, Dynamics of Structures: Theory and Applications to Earthquake Engineering, 2000.
2. Bruce A. Bolt Earthquakes (Earthquakes, 4th Ed) 1999.

3. Levy and Salvadori, Why Buildings Fall Down, 1994.

4. T. Lay and T. Wallace, Modern Global Seismology, Academic Press, New York, 1995.

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