Research On The Key Technologies Of Fiber Hydrophone Demodulation System Based Towered Streamer For Oceanic Oil Gas Seismic Exploration

A fiber optic seismometer based phase interferometric scheme has advantagessuch as high sensitivity, flat response in low frequency band and facility inmultiplexing.The characteristics of FOIS(Fiber Optic Interferometric Seismo-meter)provide a novel technology to the acquisition equipment in oceanic oil and gasseismic exploration. The dissertation is supported by the Project"Development ofFiber Seismometer System Based Towered Streamer",which is a sub one of NationalOceanic863Momentous Project.A novel integrated system based directly modulatedsemiconductor laser working in the wet-end is proposed.The important systemperformance specifications,such as total harmonic distortion, noise floor, responsecapability to wideband and transient signals and etc.,are deeply analyzed with respectto theoretical limit and optimization realization.The main contents are generalizedas follows:1. The phase relationship between intensity modulation (IM) and frequnencymodulation (FM) when using direct laser modulation to generate carrier signal isstudied experimentally.An optical homodyne measurement system based a Michelsonfiber optic interferometer is set up for obtaining the paremeter of phase differencebetween IM and FM. PGC (Phase Generated Carrier) interferemetric signal modelbased directly modulated semiconduct laser reported before has been modified in thedissertation. The influence of the phase difference between IM and FM ondemodulation result is presented and modified in the process of demodulation.2. Based on modified laser intensity modulation (LIM) model, the scheme ofdividing two channels of PGC interference detection signal is proposed respectinglow harmonic distortion and high stability.A novel demodulation method callednonlinear function scheme and realized by Lookup Table (LUT) technique ispresented in detail.Combined with the algorithm for array signal demodulation, therelation between parameter tolerances in multi-channels and harmonic distortion ofa demodulation result has been analyzed.Approximate analytic relationship between ademodulation error and parameter deviation has been deduced based on totaldifferential equation.Meanwhile,CORDIC(Coordinate Rotation Digital Computer)algorithm is proposed to demodulate the non-linear function.The scheme can not only realize array signal demodulation optimization with fewer memory resources,but canreduce technological requirements of the multi-channel parameter consistency as well.Besides, according to the assumption of two channels of PGC interference detectionsignals being ellipse, a variety of ellipse estimation methods have been tried toimprove an estimation accuracy of system parameters.Meanwhile the impact levelaffected by system factors has been compared among such methods.3. Noise transfer relation between output and input (including circuit noise, laserrelative intensity noise and laser frequency noise) based on laser intensity modulation(LIM) model has been established.Approximate analytic relationship has beenobtained.Simulations are presented to corroborate the theory. Carson Rule areproposed to introduce to PGC modulation bandwidth design as respect to capability toresponse input signal. Evaluation specificaitons of wideband signal demodulationhave been presented with regards to common signals in engineering applications.Thespecificaitons can verify the PGC wideband phase modulation bandwidth estimationaccuracy and applicability based Carson Rule and can help provide a design guidancefor demodulation system ability to recover input signal.Based on the fundation as above, three hundred-meter-length streamer towedarray prototype including48fiber optic seismometers has been completed.Seismicsignal acqusition based towed streamer has been experimented at sea.