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.
A number of fiber optic sensors for geophysical applications have been developed over the past two decades at the
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography. These include: a strain sensor to
monitor ice flow in Antarctica, a strain sensor to track sediment creep on the ocean bottom, a borehole strain sensor to
monitor fault movement during earthquakes, a pressure sensor to detect low frequency acoustic waves, and a
seismometer. All of these sensors utilize one of two interrogation techniques. The first is a commercially made
electronic distance meter which, by measuring the transit time of light pulses through the sensing fiber, can track
changes in a 1000-m-long fiber with a precision of about 1 mm. The second technique is interferometry. For this
purpose, a quadrature fringe resolver based on a digital signal processor has been developed. It combines wide
dynamic range (centimeters) with high resolution (picometers). Continuous records spanning days to years have been
obtained with these instruments.
As part of a program to measure and model vertical strain in the West Antarctic ice Sheet, we developed a new sensor to accurately and stably record displacements. The sensors consist of optical fibers, encased in thin-walled stainless steel tubes, frozen into hot-water drilled boreholes, and stretched from the surface to various depths in the ice sheet ranging to 1000 m. An EDM (electronic distance meter) connected annually to the fibers read out their absolute lengths with a precision of about 2 mm. An initial elongation of about 0.15% of the optical fibers allowed them to follow an ice
thinning rate of 300 ppm per year for up to five years. Two sets of five sensors were installed in the 1997-1998 field season: one set was near the Siple Dome core hole (an ice divide) and a second set was on the flank 7 km to the north (the ice thickness at both sites is approximately 1000 m).