基于弱光栅阵列相位载波解调的分布式井中地震勘探技术研究

分布式光纤声波传感被认为是现有油气资源勘探中最有应用前景的技术，但传统光纤分布式地震勘探利用的光纤本征瑞利散射存在强度随机涨落、偏振衰落等问题，导致解调声场不稳定，系统噪声较高。利用在光纤中插入大量反射率不足1/1 000的全同弱光栅阵列替代瑞利散射，使后向光稳定性显著增强，并采用相位载波解调实现了光纤分布式声波传感(distributed fiber acoustic sensor, DAS)系统噪声的压制。实验室测试结果表明，基于全同弱光栅阵列相位载波解调的DAS系统频响范围可达0.1～320.0 Hz,本底噪声平均约为■,满足了地震检波的需求。现场井中VSP地震资料采集实验表明，全同弱光栅阵列采集到的地震数据信噪比约为传统单模光纤分布式采集数据信噪比的2.5～3.0倍，VSP资料质量有显著提升，为下一代新型光纤分布式地震勘探技术提供了一种有效解决方案。

A distributed well seismic exploration technology based on a weak grating array and phase carrier demodulation

Distributed acoustic sensing(DAS)is deemed the most promising technology for oil and gas resource exploration.However,intrinsic Rayleigh scattering caused by the optical fiber used in traditional DAS exploration results in random intensity fluctuations and polarization fading,leading to an unstable demodulation field and high system noise.A large number of identical weak grating arrays with a reflectivity<1/1000 were inserted into the optical fiber to eliminate the Rayleigh scattering,thereby significantly enhancing the backward light stability.Furthermore,phase carrier demodulation was used to suppress the noise in the DAS system.Laboratory test results showed that the frequency response range of the so-enhanced DAS system could reach 0.1~320Hz,with an average background noise of 50μrad/√Hz.This value is deemed adequate for seismic detection.Through an on-site VSP seismic data acquisition experiment,the signal-to-noise ratio of the seismic data collected by the identical weak grating array is about 2.5~3.0 times that of the traditional single-mode fiber distributed acquisition data.The quality of the VSP data has been significantly improved,and an effective solution is provided for a new generation of optical fiber distributed seismic exploration technology.