| Seismic survey is one of the most important means of oilfield engineering exploration in the petroleum industry,whether on land,marine seismic exploration will be a large number of geophone use.With the rapid development of optoelectronics and fiber optic sensing technology,fiber optic class geophones are promising for application in oil and gas wells by virtue of their anti-electromagnetic interference,high temperature resistance and passive characteristics.However,there is currently a lack of high precision,high speed and low cost demodulation instruments for fibre optic geophones.Among the existing demodulation methods,phase demodulation technology has the advantages of high accuracy,strong anti-interference capability,large dynamic range and good linearity compared to other demodulation methods.In order to meet the high precision demodulation of optical fibre geophones in practical engineering applications,this paper investigates the phase generation carrier(PGC)technique.Traditional PGC demodulation algorithms are divided into PGC-DCM algorithm based on differential cross product and PGC-ARCTAN algorithm based on inverse tangent.The demodulation effect of these algorithms is easily affected by the carrier modulation depth and light intensity perturbation.Based on the above problems,two PGC improvement algorithms are proposed in this paper.Through subsequent simulations,it is found that the signal frequency demodulated by the second improvement algorithm is two times that of the sensed signal.This paper is divided into three parts.The paper is divided into three main parts.The first part introduces the status of research on in-well geophones and the status of research on phase demodulation technology,introduces the measurement principle of in-well fibre-optic geophone systems,introduces the PGC modulation and demodulation method,and provides a mathematical derivation of the traditional algorithm,laying the foundation for the subsequent improved algorithms proposed.In the second part,two improved PGC algorithms are proposed,namely the improved PGC algorithm to suppress harmonic distortion and the improved PGC algorithm to eliminate modulation depth.The final expressions are derived after detailed theoretical derivation,and the results of the four different PGC algorithms are compared and analysed,and then the advantages of the improved algorithms are verified through simulations on the Labview software platform.The selection of the main parameters and the anti-harmonic distortion and anti-light intensity interference performance of the first improved algorithm are analysed in detail during the simulation study,highlighting the superiority of the first improved algorithm.The third part designs and builds an experimental test system for the demodulation algorithm.The experimental test system mainly includes the optical path building part,the signal modulation part,the signal detection and acquisition,the pre-processing part and the subsequent digital signal processing part.The experimental verification of the improved PGC algorithm and the traditional algorithm is carried out separately to confirm the advantages of the improved algorithm through experiments.The background noise and signal-to-noise ratio of the system are measured.The background noise reaches-120 d B,the signal-to-noise ratio reaches 51.16 d B,the signal-to-noise ratio reaches 44.71 d B and the total harmonic distortion reaches 0.582%,enabling complete measurement of the medium and high frequency signals(100Hz-2k Hz)detected by the geophone in the well.Problems arising during the experiments are studied and analysed,and improvement solutions are proposed in terms of both the algorithm and the optical path. |
