An Optical Seismometer without Force Feedback

We are developing a new vertical seismometer, motivated by a desire to have an instrument whose performance is similar to that of observatory sensors yet can operate within a borehole without electronics. This has led us to an all-optical seismometer consisting of a spring-suspended mass whose position is monitored interferometrically. We use a Michelson interferometer illuminated with a 1 mW laser that can be linked to the seismometer with optical fibers only. A digital signal processor samples the interference fringe signal and produces a 400 samples/sec record of the seismometer mass displacement with a root mean square noise per octave band that varies from about 4 x 10(-12) m at 0.001 Hz to 4 x 10(-13) m at 1 Hz. The maximum displacement is limited by mechanical issues to a few millimeters at present, providing a dynamic range of at least 109, equivalent to 30 bits (180 dB). Experiments to test this idea have been performed on a modified STS1 vertical seismometer whose electronics have been replaced with an optical system. Comparisons with other seismometers show that, in terms of both noise and signal fidelity, the optical approach is quite viable.