Chapter 2, An Inexpensive Vertical Seismometer for Classroom Demonstrations: a Low Cost Model

Developed for the Mid America Earthquake Center Workshop

at Georgia Tech, June 22-25, 1998

 

 

Introduction

The design of the vertical seismometer can become complex and elaborate when great care is taken to eliminate high-frequency undamped vibrations and to maintain a low noise level with a high dynamic range. Typically, professional instruments start at over $2500 for the Guralp PEPPV seismometer and can go as high as $100,000. On the other hand, many amateur seismologists have discovered that with minimal expense they can build a seismometer that will record as well as many of the professional seismometers. The most common design is the horizontal swinging door seismometer for which there exists many designs (for example see George Averill, 1995, and the figure in the index page). The same is not true for vertical component seismometers. Vertical motion seismometers take special care because the achievement of a long period depends on the elastic properties and stability of a spring and the geometrical design of the instrument.

 

Design Objective

In designing the vertical demonstration seismometer described below, we have made a concerted effort to use only generally available supplies and to make the design as simple and safe to construct as possible. Most of the components do not require special equipment or extensive care to make. All parts should be able to be fabricated by anyone with basic skills in using workshop tools and acknowledge of recommended safety precautions.

 

The Instrument Platform

When a seismometer is used to record earthquakes, care must be taken to place the instrument on a platform that is tightly coupled to the ground, preferably on bedrock. The best seismic observatories have cement piers that connect directly to bedrock that may be 10 to 50 feet below the ground and are separated from any other structure by motion absorbing material. The next best base would be a large cement block, perhaps one cubic yard, on the surface and housed in a protective enclosure. Inside a structure, a recording seismometer should be located as close to the ground as possible and in an area away from moving equipment like furnaces. In a classroom environment, a heavy laboratory bench would be best. Light benches or wobbly tables move too much to allow measurements. The surface on which the instrument is placed should be flat and smooth.

 

 

General Description

The components of the vertical seismometer described here are mounted on a wooden (preferably hardwood) base that for stability rests on three points. Two points of the base are made at the U-bolt ends and the third point is formed at the leveling screw. The base is pre-drilled for the mounting of all the components. The U-bolt is used as two points of the base and the suspension of the lever arm. The spring support block has a set of holes drilled in the base so that it may be moved to adjust to spring strength. The travel-limit bracket prevents excessive movement of the lever arm. The sensor mount provides support for the sensor. The leveling screw on the end provides the third support point for the base. The layout of the base is apparent in our picture of the component parts of the seismometer. A top view of one assembled seismometer is shown on the cover page.

The lever arm support is a square U-bolt bracket, 6" tall by 3 2" wide and made from 3/8 diameter stock, usually used to fit 4x4 posts. The holes in the base are 4" from the end and the center to center separation of the U-bolt we found is 3 3/8", but each should be independently measured to assure a correct fit. The U-bolt is mounted with nuts above and below the board. The nuts are adjusted to level the board and tightened down to make the suspension rigid. On the bottom the U-bolt should not extend more than 1/8" beyond the nut.

The spring support is a block of wood bolted to the base. A set of holes is drilled to allow easy movement of the block for adjustment to accommodate variations in spring stiffness. The holes in the mounting block can be used as a guide to guarantee a close fit. The spring is attached to an eyebolt placed through the spring support block. The nut on the eyebolt allows adjustment of spring tension and the position of spring origin. The additional bolting holes in the base allow additional adjustment of the spring attachment position by moving the spring support block forward or backward. These adjustments are needed to locate the zero length point on the spring directly below the hinge. By moving the weight and spring tension, the period may be adjusted to be as long as desired. Fine level adjustments to the period are made with the level screw. The travel limit bracket is used to maintain movement within the sensitive range of the sensor. It is not actually needed for operation, but limits possible damage when moving the seismometer from one location to another.

The design of a damping mechanism is at this time left as an exercise. Passive damping of the motion of the lever arm can be achieved mechanically or electronically. Mechanical damping can be obtained by mounting a disk (washer, etc.) below the lever arm so that the disk can move in a container of liquid (water, oil, etc.). This combination forms a classical Adash-pot@ and produces a retarding force proportional to the velocity of the lever arm and to the viscosity of the liquid. In this mechanical model, the fluid can be changed to change the damping or the size of the washer can be increased to increase damping. We recommend this method for short-term classroom demonstrations. If oil is used for damping, a non-vegetable oil is preferred (mineral oil, etc.) to keep from spoiling.

One common electronic damping method uses a conducting plate in a strong magnetic field. For this to work, the conducting plate would be placed on the lever arm and the strong magnet on the base. Seismographs using a coil moving in a magnetic field can often damp the motion by placing a damping resistor across the coil. We are currently working on an electronic feedback motor for this seismometer that would have active electronic damping. Electronic damping can be accomplished by using a coil mounted on the base plate that can react with magnets mounted on the lever arm. A feedback circuit using a position sensor as input, can be used to drive the coil-magnet combination (which forms a small linear motor) to provide the damping force required.

The mounting position for the displacement sensor is at the end of the beam to give maximum sensitivity to movement of the lever arm. For the lever arm, we use aluminum edging for 2 inch plywood, which is available at most building supply companies. Any similar shaped beam would be sufficient. The crosspiece is bolted to the lever arm perpendicular to the lever arm at its position to the right of the suspension as can be seen in the top view on the cover.

We have examined the common screen door spring and found it to be nearly zero length and ideal for this application. Other springs may also be zero length, but we have tested only one so far. The screen door spring is much too long for the instrument. The spring is cut to the maximum length that will be accommodated between the end of the lever arm and the spring mount, directly below the hinge (see Figure 1). The last loop on each end of the spring is bent to form a hook to be used to attach the spring. In order to test for the zero length property, the spring was stretched with successively larger weights, chosen as equal weight increments. The extension of the spring was measured and plotted in Figure 4. Note that for the first 2 or 3 weight increments, the spring does not extend, but for additional equal weight increments, the spring extends uniformly. The slope of the extended portion of the plot can be projected back to the origin to determine the zero length position. For application to the long-period seismometer, the extended length should correspond to the distance R. Thus if the spring is not a pure zero length spring, it should be placed so that when hooked to the end of the rod, the zero length is directly under the hinge.

A critical element in the mechanism is the reduction of lateral movement. The lever arm must be constrained to move in the vertical plane only. Hence, the design of the hinge requires that the hinge must both be virtually frictionless and must allow movement only in the vertical position. The restriction to vertical motion can be achieved by using perpendicular sets of thin metal shims. Thin shims can bend easily, but offer considerable resistance to extension.

Averill, George (1995). Build your own Seismograph: an earth-shaking, in-class project, The Science Teacher, March, pg 48-52.