The characteristics of a seismograph can be modified by the use of filters or by the application of negative feedback. Both methods provide the same basic signal to noise capabilities, and each has its particular advantages. The principal advantage of the feedback instrument is its flexibility and the possibility of linearity over a greater dynamic range.
The application of electrical feedback to a seismometer requires the creation of normally nonexistent electrical input terminals. By incorporating the seismometer into a balanced Maxwell impedance bridge, input terminals can be simulated and the feedback introduced through the bridge. With the use of such negative feedback, it is possible to control individually the effective mass, spring and damping constants of a seismometer. One instrument can thereby simulate seismometers of very different mechanical properties. For example, it is possible to increase the effective mass of a Willmore Mark I seismometer to well over one ton.
A feedback seismograph has been constructed using these principles and has been in continuous operation for nearly two years.