A VERTICAL COMPONENNT SEISMOGRAPH FOR EDUCATION
LONG, Leland Timothy1 and Frank Williamson2, 1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332 2Georgia Tech Research Institute (retired), Georgia Institute of Technology, Atlanta, GA 30332 (email@example.com, firstname.lastname@example.org)
The home and/or high school seismograph has largely been the domain of a few skilled hobbyists. Typically, these instruments have varied in their level of success, and few students (or their parents) generate a complete and practical recording system. Commercial seismometers are not a practical alternative because they are too expensive for most high school budgets, require interfacing with recording equipment, and are technically too advanced and sensitive for the high school environment. Most are black boxes, which provide only a stream of data. A seismometer, in which the moving parts are easily visible, is a more appropriate teaching tool. The horizontal component "garden-gate" suspension seismograph is the design used most often. A vertical component seismometer is difficult for an amateur to build because the long-period response requires a zero length spring, which is difficult to find. We have developed a dual-spring vertical-component seismograph that achieves the equivalent of a zero-length spring for small displacements. To a first order approximation, the dual-spring seismometer has a restoring force that can be set to zero at its equilibrium position, theoretically giving it an infinite period. The torque of the first spring counters the torque applied by the mass. The second spring is positioned to exert no torque in the equilibrium position, but negative torque out of equilibrium. Tension is adjusted on the second spring to adjust the period of system. We easily achieve stable free periods of 3 to 6 seconds for small motions. The seismometer can be constructed entirely from materials found in a hardware store. Also, the design is appropriate for high-school teachers and students to assemble, and to directly observe its operation. Electronic feedback and digitizing can be added for long-term recording. The sensor we use is a differential light system that is capable of sensitivities to displacement up to 40,000 volts/m. Electronic feedback can then be use to generate a response that is flat to velocity for periods of 20 to 0.1 second.