| Fiber optic sensing technology,especially fiber optic geophone,which can be used in petroleum geophysical engineering is widely studied in recent years.This dissertation focuses on fiber optic geophone technology and the key device – narrow linewidth distributed feedback fiber laser.π phase-shifted distributed feedback fiber laser(π-DFB-FL)that can be used in high sensitive geophone is researched.Two kinds of geophones are designed using π-DFB-FL as sensitive unit and light source respectively.And test experiments have been carried out in several oil fields.The main innovative work of this dissertation is as follows:1.Principle of π-DFB-FL and linewidth broadening are studied.Theoretical models and experimental method of the laser self-pulsation influence,environmental noise influence,amplified spontaneous emission noise influence,laser window bandwidth and others are set up.A self-injection locking method is proposed to compress laser linewidth.The compression effect of 3.5 times line width is realized.2.To achieve narrow linewideth laser output,π phase shift grating structure and manufacturing method are studied.An UV scanning mask exposure grating writing experimental and control system that based on argon ion laser is built up.π phase-shift grating is realized on a section of erbium-doped fiber.Anti-interference desensitization package is proposed.Laser power amplifier and the modularized packaging structure have been optimized.More than 35 m W laser power,1 k Hz line width,-115 d B/Hz@370k Hz RIN noise and-125 d B/Hz@1k Hz phase noise are realized.Comparative experiments with some of international top technical products such as NKT,NP are carried out.Most of technical indicators reach international advanced level.3.An optical fiber three component geophone with temperature self-compensation function is proposed and designed using π-DFB-FL as sensing sensitive element.Stability of sensors in different temperature environments of the oil well can be effectively solved.An optimized and improved system is proposed to eliminate suppression of Rayleigh scattering accumulation in long distance transmission.The coherent collapse and the multiple probe series engineering problem are eliminated.Three-component sensor high-capacity array is realized exploiting the advantages of π-DFB-FL’s wave division multiplexing ability.Furthermore,a pushing device to improve the effective coupling between seismic geophone and stratum are also be studied and designed in this dissertation to enhance the receiving sensitivity of acoustic vibration.The system and sensors had been tested in Changqing Oilfield,Shengli Oilfield and Xinjiang Oilfield.A very good seismic data was obtained after analyzed and processed in this dissertation.4.The distributed optical fiber geophone that based on optical fiber Rayleigh scattering technology is studied for the technology requirements of large capacity and high-density detection in oil exploration.The key technologies of signal synchronization measurement,real-time interference demodulation and signal sensitization are tackled.To solve the problem of polarization fading,the interference demodulation structure and method based on double Faraday rotator mirror are designed.The distributed optical fiber geophone is tested.The results shows that the spatial resolution of the system reaches below 4 meters,the long-range detection capability reaches over 40,000 meters,the frequency response range reaches 10-2000 Hz and the dynamic range reaches over 90 d B.The well tests were carried out in Shengli Oilfield using this system,and the seismic data of the corresponding blocks were restored and analyzed. |
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