| Fiber optic interferometric seismic geophones combine the advantage on anti-electromagnetic interference,intrinsic passive,small dimension and easy networking of fiber optic sensing technology with the superiority of interference technology on high sensitivity,large dynamic.At present,the ocean bottom seismic geophones have two mainly forms,dropped geophones and permanent geophones,however,the two types are usually traditional geophones,such as electromagnetic seismometer,piezoelectric type vibration detectors and so on.Fiber optical geophones have been applied in the ocean bottom seismic observation network as a new generation vibration detector in Japan,USA,Canada and other nations,moreover,the research on fiber optics vibration detecting technology in China also have made some good performance.The fiber optic geophones and the permanent seismic observation network in ocean bottom are mainly on experimental stage.This paper carried out fiber optic interferometric vibration measurement technology on seismic wave detection,and designed the sealed cabin for assembling the tri-component geophones on the ocean bottom,which has laid the foundation for the wide application of the fiber optic interferometric vibration measurement technology,and accumulated practical experience for new generation of seismic geophones based on fiber optic sensing technology.The main research contents are as follows:(1)The research on fiber optic interferometric vibration measurement theory.The equation of optical interference signal which is induced by the variance of optic phase in the fiber optic interferometer can be summed up by analysising the structure of the fiber optic interferometric geophones.Specific to the characterize on composition rich in low frequency of the earthquake,Comparative analysis were carried on three common fiber optic interferometric structures,and we chosed the fiber optic Michelson interference structure as the system measurement method.Furthermore,we also carred out the comparison between the disc type and mandrel type based Michelson fiber optic seismic accelerometer,which is the theory foundation for optic phase demodulation and acceleration measurement.(2)The phase generated carrier method for Michelson fiber optic inteferometric geophone system.The phase generated carrier method is used to demodulate the variable phase of the interferometer,we analysis the principle on signal modulation and demodulation,and carried out the detailed formulation derived on optic phase modulation method,the phase modulation depth is quantitative analyzed for the external modulation and direct modulation.Furthermore,we proposed a four quadrant arctangent demodulation method for low frequency vibration measurement by comparison between the differential cross multiplication(DCM)and arctangent methods.(3)The research on the sensitivity of the fiber optic interferometric geophone system.The axis direction sensitivity and transverse sensitivity of the compliant type Michelson fiber optic interferometric geophone were measured,and the results show that geophones have high axis sensitivity and high transverse depression ratio.The axis sensitivity of the tri-component compliant Michelson fiber optic interferometric accelerometers are 57.87 d B,51.68 d B,and 57.20 d B on X-axis,Y-axis,Z-axis respectively,the transverse depression ratio is large than 30 d B of them.And we also proposed a new method for resonance frequency measurement of the optic intereferometric vibration detectors,which is more efficient than the sensitivity curve method.(4)The minimum detectable level of the fiber optic interferometric acceleration detection system.The resolution of the system is measured by the minimum detectable level of the system,it determines the minimum detectable acceleration level for acceleration detection system.The geophones are demanded to detect the ultra-low amplitude of vibration in seismic applications,thus,we analysis the noise source of the system,and measured the noise level from electric module and laser noise level.The results show that the main noise source comes from laser phase noise fluctuations in the situation of the unbalance length of the interferometer is far less than the coherent length of the laser.(5)The dynamic range of the fiber optic interferometric acceleration detection system.The variation of optic phase of the interferometer is proportional to the detected acceleration amplitude,thus the dynamic range of the interferometer is usually very high in measuring application.Furthermore,we researched on the limited elements on the dynamic range of the system,which is consisted by three factors: the first,the coherence length of the laser;the second,the modulation frequency in the phase generated carrier method;and the last is the structure of the geophone.The measurement results show that the dynamic range of the system is larger than 130.68 dB frequency range from 0 Hz to 50 Hz.(6)Data analysis for the seismic event and sealed cabin design for ocean bottom seismic wave detection.Our system recorded the seismic event occurred at Song Yuan in Jinlin province of China with 4.9 Ms(July-23th-2017)and January 1st,2018 seismic event.The comparison measurement data from our system and CTS-1 which is an ultra-broadband seismometer based on the force-feedback-control theory show that the geophones we proposed can be used for locating the seismic source.The structure,seal method,compressive strength,tensile strength and the curved bear capacity of the sealed cabin are calculated and designed,and the sensing unit is innovative consisted by tri-component fiber optic geophone and fiber optic liquid level gauge.In this doctoral research project,several innovative approaches have been proposed and applied,they are outlined below:(1)A fiber optic interferometric vibration observation system has been developed,the operation characteristic of the system is high sensitivity,large dynamic,ultra-low noise level.(2)A four quadrant arctangent demodulation method is successful applied in demodulation for broadband vibration measurement;A new method of resonance measurement for the interferometric geophone is proposed,and a method for improvement on the dynamic is also proposed in the paper.(3)A sealed cabin for permanent ocean bottom seismic wave detection is developed and successfully applied in the second generation tsunami warning network in the south China ocean. |
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