UC San Diego

Motivated by the desire to have instruments whose performance rivals that of the best observatory sensors, two new seismometers have been developed that are designed to operate in borehole environments free of electronics and force-feedback. This is accomplished by the development of optical sensors consisting of leaf-spring suspended masses whose positions are monitored interferometrically. A digital signal processor (DSP) samples fringe signals created by the interferometric sensor and produces a displacement record of the seismometer mass with a resolution of less than 1 pm (10⁻¹² m). Maximum mass motion is limited by practical issues to about 15 mm, providing a dynamic range of 10¹⁰, equivalent to 33 bits. For a moderate-sized mass with readily achievable free period and damping, the mass- spring sensor's fundamental thermal noise is less than ambient noise at the seismically quietest sites. Hence, there is no need to abandon this basic, simple, mass- spring design. Elimination of electronics, however, means elimination of force feedback - the paradigm in seismometry for the past several decades. We have tested our non-fedback optical seismometers against standard fedback observatory-quality seismometers (STS-1) and found that they provide equivalent seismograms for signals ranging in frequency from tides to at least 10 Hz. We examine the effects of thermal, magnetic, and barometric noise observed on the interferometric sensors and compare with the STS-1