Fiber Bragg grating sensor combined with silicone compliant cylinder for orientation identification of three-component geophone

Three-component geophone rotates randomly during oil and gas exploration, which is difficult to orient. There is a severe impact on the subsequent stratigraphic inversion. By fixing the fiber Bragg grating (FBG) sensor to the three-component geophone, the twist angle is the same. The rotation angle of the three-component geophone can be obtained by measuring the twist angle of the FBG sensor. However, the three-component geophone below the FBG sensor stretches the FBG sensor. Therefore, avoiding the effect of tensile strain on the torsional strain, requires simultaneous inversion of the twist angle and stretch length of the FBG sensor. This paper introduces an FBG sensor with simultaneous inversion of torsion angle and tensile length. The sensor consists of two FBGs embedded in the spiral groove of the compliant cylinder. Firstly, the sensing principle of the FBG sensor is analyzed, and developed FBG sensing model for tensile–torsional strain, including FBG by tilting arrangement, the extension of the tensile strain range of the FBG sensor was realized. Secondly, it describes the fabrication process of the FBG sensor, including loading the FBG with pre-stress, making the FBG sensor realize the extension of the torsion angle range. Next, the convenience of inversion is realized by averaging the calibrated strain transfer coefficients. Finally, to verify the effectiveness of this bidirectional helical fiber design and sensing technology, simultaneous torsion and tension tests were conducted on FBG sensors. The experimental results show that the sensor achieves simultaneous torsion angle and tensile length inversion. The maximum torsion angle of the inversion is 180°. The maximum stretch length of the inversion is 10 mm.