A Borehole Fiber-Optic Strainmeter

The SAFOD project provides an opportunity to study the basic mechanics of a fault that undergoes relatively frequent events. Magnitude 2 earthquakes within a few km of the borehole occur several times per year and produce strain signals on the order of a nanostrain. We installed a vertical fiber-optic strainmeter in the annulus of cement between the inner and outer casings of the borehole. The sensor consists of an optical fiber cable stretched from the surface to a depth of 864 m. The optical length of the fiber is monitored with a laser interferometer sampled at 50,000 samples per second. The strainmeter noise is about 0.01 nanostrain at 1 Hz, which provides clear records of coseismic offsets.