Results from a borehole optical fiber interferometer for recording Earth strain

We have installed a set of 250 m long borehole optical fiber strainmeters at Piñon Flat Observatory in southern California, USA. These instruments consist of an optical fiber stretched between two positions in the borehole – the upper position is at depth of 4 m and the lower is at a depth of 254 m. The tensioned optical fiber’s length is monitored interferometrically; vertical Earth strain is determined by recording the fiber length change over this interval. We achieve a strain sensitivity of about 0.01 n(epsilon) (10–11) at periods near 1 s. The noise in the tidal band is 300 n(epsilon)/√Hz (near 1cycle per day). After an initial settling period of about a year, the instrument drift is 1-2 μ(epsilon) per year.