Abstract
The effects of hydrostatic pressure on strain measurements by using fiber optic distributed sensor system were elucidated through laboratory experiments. Strains of metallic plates and a cylindrical core of Berea sandstone were measured under hydrostatic pressure by using a system that measures strains by the frequency shifts of Brillouin and Rayleigh scattering in an optical fiber, together with simultaneous measurements by strain gauges. The measurements revealed that the radial strain of the optical fiber due to the hydrostatic pressure considerably affect the frequency shifts as well as the strain along the fiber line. The effects of two strains on the frequency shifts, Δf, are expressed as Δf=AΔεz+ BΔεr. where Δf, εz, εr are the Brillouin or Rayleigh frequency shifts, the fiber strain along the line and radial directions, respectively. A and B are the coefficients corresponding to the effects of the two directional strains for the Brillouin or Rayleigh frequency shifts. A and B are determined as 0.0497±0.0043, 0.0346±0.0014 (MHz/με) for the Brillouin and 0.130±0.0096, 0.0707±0.0032 (GHz/με) for the Rayleigh. Our experimental technique was also successfully applied for monitoring the deformation of sandstone under static pressure. The results assure accurate strain measurements by means of the fiber optic distributed strain measurements system.
