| Borehole strain observation is an approach to precisely record and analyse crustal stress-strain condition and its variation, by installing strainmeter dozens to hundreds of meters deep down underground. It plays an important part in geophysical studies of crustal deformation and geostatic stress field, seismic wave and earthquake precursor, earth elasticity and free oscillation. It is a novel method to apply fiber optic sensors to borehole strain observation. Fiber optic sensors are more resistant to high temperature and pressure, not sensitive to electromagnetic interference, and can withstand the harsh environment in the field. The intrinsic defect of electrical measuring instruments can be solved by replacing them with fiber optic sensors.An elastic mechanics model of volumetric borehole strainmeter is established, and the borehole strain expression deducted. The history and development, principles and structures, merits and demerits of many kinds of borehole strainmeters are presented. Then a novel borehole strainmeter based on fiber optic sensing is developed, experimented and its feasibility verified. Instructions and precautions of borehole drilling and strainmeter installation are introduced in the end.The fiber borehole strainmeter developed is based on a Michelson interferometer, in which the earth strain is transformed to the change in length of an optical fiber, namely the measuring arm. Another one optical fiber, acting as the referencing arm, is used to reduce the influence resulted from temperature and environment. The optical fibers are rotated on the structure hundreds of turns for amplification. The optical path difference is demodulated using white light interferometry and the earth strain information determined. Experimetal result shows that its sensitivity is117nm, which has much room for improvement. This sensor has advantages over traditional ones, and can be further developed to meet the needs of geological observation and science research. |
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