Design and Feasibility Study with the Long Period Seismometer for Volcano Observation, Using a Electrical Feedback Method

A method of enhancing the natural period of an electro-magnetic seismometer is presented. The output signal of the detector coil of a transducer is amplified and fed to the feedback coil through a current-differentiation circuit. By this arrangement, the force of inertia is given to the pendulum, and the moment of inertia of the pendulum is effectively increased, lengthening the natural period of the seismometer. To make a long-period seismometer which is useful for volcano observation, the above method was applied to the electro-magnetic seismometer which has a pendulum of 2-second period. The new seismometer has 10-second period and not only is portable but also gives high performance in the moderate operating temperature ranges. The constants of this seismometer are as follows: atural frequency : 0.1 Hz sensitivity : 1.5 volt/kine damping constant : 0.6 The seismometer thus devised seems very useful to permanent observation, and it has been operated for volcano observation at the Asosan Weather Station since October, 1982.