Recording Earth Strain With Optical Fibers

Optical fibers are well suited to measure Earth strain because they can be stretched over long distances to average strain over a large interval. This is important to reduce disturbances to the measurement from very local effects. We have installed optical fibers ranging in length from a few 10 s of meters to 2 km in vertical boreholes on land and in an icesheet, and horizontally along the sea floor. Due to the high sensitivity of optical fibers to temperature change, an environment of stable temperature is important-this is often available in boreholes or on the sea floor. Longevity of fiber cables and the means to protect the glass fibers from environmental effects and the rigors of deployment are critical issues. Our experiences cover a broad range of success in this regard, with some deployments lasting for more than four years and others failing immediately.