Optically-linked laser-interferometric tiltmeter for ocean borehole observations

We have developed a borehole tiltmeter based on laser interferometry. Its principle is based on a simple pendulum whose horizontal 2D motion is detected by laser interferometers with high sensitivity. The laser light is introduced from outside the borehole through an optical fiber, and the observed signals are also led through optical fibers out of the borehole; the borehole sensor is optically-linked with the outside. Moreover, a wavelength-stabilized laser makes it possible to reduce the drift of the detected signal. Our test observations in an 80 m deep borehole with a preliminary model showed that the data agreed well with that obtained by water-tube tiltmeters, which are located at the same site. We have also succeeded in observing teleseismic waves. This tiltmeter, with an intrinsic low drift performance, rapid response, and high sensitivity, is suitable for wideband observations in deep boreholes or ocean boreholes to detect secular plate subduction, slow earthquakes, or long-period seismic waves.