Optical Seismometers: Borehole and Vault Applications

We have developed an interferometric seismometer which uses optics instead of electronics to infer ground motion. The sensor, assembled exclusively from glass and metal materials, could be deployed into deep boreholes where temperatures often exceed 150 °C. Our first prototype consists of a leaf-spring suspension and an optical-fiber-linked interferometer, which monitors vertical displacement of the seismic mass. Several years of testing and improvements have increased its performance at both low (e.g., tidal) and high (tens of Hz) frequencies. The prototype sensor performs as well as or better than most observatory grade seismometers and has an overall observed dynamic range of 109 or 30 bits of resolution (based on its observed noise floor and its maximum mass velocity). We have also built a simple horizontal component prototype which consists of a mass suspended from a vertical pendulum whose flexure is fabricated from a single block of material. Just as our vertical seismometer can serve as a gravity meter, the horizontal prototype can serve as a tiltmeter (both of their responses are flat to DC). Tests are currently being conducted with the new sensor in our Piñon Flat Seismic Test Facility (California). One advantage of our optical displacement transducer is its dynamic range, which relaxes the requirement that the horizontal component sensor be level, simplifying borehole installations. We have already achieved a dynamic range of ±5° and we expect that a range of ±10° is possible with some effort.