| Hydrocarbon exploration methods are crucial to the exploration and development of oil and gas reservoirs.Traditional geophones used for in-well seismic exploration in China mainly rely on imported coil-type and MEMS geophones.However,they are susceptible to electromagnetic interference,and difficult to use in harsh downhole environments with high-temperature,high-pressure,and strong corrosion.In addition,the key technologies are dominated by foreigners,and the maintenance is low efficiency but high prices.Therefore,it is urgent to develop new detection technology of high sensitivity,multi-component,high-temperature and high-pressure resistance,and new multiplexing technology of dense array.As a new "passive" technology,optical fiber sensing technology is an important hydrocarbon exploration method in future.It has made great breakthrough and development in multi-component and three-dimensional(3-D)seismic,and has replaced some electrical devices.Nevertheless,there are still a lot of problems that need to be solved urgently.Among them,in the detection of narrow space with strict requirements on the size of the fiber optic seismometers,there are bottleneck problems such as complex device structure,large size,insufficient multi-dimensional detection capabilities,and insufficient network multiplexing capabilities,which need to be researched and resolved.In order to solve the key scientific and technical problems of micro-integration and multi-dimensional precision detection of fiber optic 3-D vector detectors for seismic exploration in boreholes,this dissertation carries out research on 3-D vector detection technology based on multi-core fiber Bragg gratings(MCFBGs),which is inscribed by femtosecond laser(FL).Connected seismic wave theory and optical fiber detection mechanism,an optical fiber 3-D vector seismometer is developed.The research of this technology has important significance and application value for downsizing the structure,improving the detection accuracy and broadening the application range of the fiber-optic seismometers for seismic exploration in borehole.The research contents are mainly as follows:1.The research background and significance of the 3-D vector fiber seismometer are analyzed,and the development status of borehole seismic exploration technology,fiber seismic survey technology and MCF are studied;combined with the sensing theory of FBG,the mechanism and method of FBG inscription by FL are researched.Subsequently,the inscription method is optimized according to the structural characteristics of MCF.2.The mechanism of 3-D vector detection based on MCF is researched,it contains that the elaborated principle and method;the characteristics of MCF bending sensing are researched,and two types of bending sensors based on interference structure and FBG are fabricated to verify the characteristics;moreover,the relationship between MCF bending and vibration acceleration is established,which provides a theoretical basis to realize 3-D vector detection by MCFBGs.3.A 3-D vector vibration accelerometer based on MCF have been proposed and fabricated.Including the design and optimization of the structure of the accelerometer,and the encapsulation technology of the accelerometer have been studied and improved.Ultimately,it is realized that the detection sensitivity of ? in cylindrical coordinate plane is 355 pm/g,the minimum recognition error vibration azimuth ? resolution is 0.269°,and the Z-axis detection sensitivity is 195 pm/g under the range of 20 Hz-200 Hz of low-frequency vibration signals.4.The application systemization of 3-D vector vibration accelerometer based on MCF is researched,and a seismometer based on a compliant cylinder is developed,thus,the cascade multiplexing of seismometers is realized preliminarily;The application scenario of seismometer in borehole is researched,and a complete detection system based on MCF for borehole seismic wave exploration is designed,which includes laser source and signal demodulation system on the ground and seismometers array in the borehole. |
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