Laser-Interferometric Broadband Seismometer for Ocean Borehole Observations

We have developed and demonstrated a borehole seismometer using a laser interferometer. With a low-frequency pendulum and feedback circuits, broadband response for long-period seismic waves can be obtained. Test observation data of the seismometer installed in a 10-m-deep borehole on land showed good agreement with data from a conventional land-based broadband seismometer above ~30mHz. At lower frequencies, electric noise of the instrument dominates the background noise level. By reducing the electric noise, the laser seismometer will extend broadband seismic observations based on conventional vaults on land into deep or ocean-bottom boreholes.