A High Sensitive Borehole Strainmeter based on Fiber Bragg Grating

Crustal deformation measurement is one of the main means to obtain seismic information. To monitor crustal stress-strain, we develop a borehole strainmeter based on FBG (Fiber Bragg Grating). A curve-side-triangle structure used as the strain gauge is designed by finite element analysis software. The gauge is encapsulated by a stainless steel cylinder. The problem of cross-sensitivity of FBG sensor is resolved though the method of reference grating: two gratings are fixed on the gauge. One is used for temperature compensation while the other one measures the strain. In order to simulate the environment in the borehole, a microstrain platform is established, where the borehole strainmeter's feasibility is tested. Experimental results show that the strain sensitivity is 9.2209pm/με and can be improved greatly.