We constructed a three‐component combination seismometer and geodetic sensor suitable for borehole deployment. The instrument uses no electronics at depth in the borehole, rather it relies on optics. Two 40  cm pendulums suspended from two orthogonal axes with free periods slightly more than 1 s and a vertical mass‐spring suspension with a free period near 5 s are tracked with optical interferometers inside a borehole sonde. Laser light for the interferometers is provided by optical fibers from the surface; the interference fringe signals are transmitted from the sonde to photo detectors at the surface, also via optical fibers. The system was installed in a 60‐m‐deep borehole at the U.S. Geological Survey’s Albuquerque Seismological Laboratory (ASL) for 1 year. The seismic noise floor of the optical seismometer’s components compares well with those of a collocated KS‐54000 borehole seismometer. Because the optical sensors operate to zero frequency, they also provide useful geodetic records.

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