A real-time distributed acoustic sensing system with ten thousand channels is proposed to detect dynamic signal along the fiber. The phase-sensitive optical domain reflectometry and phase-generated carrier algorithm are used to acquire the phase information of Rayleigh backscattering along the whole fiber. The sensing length of this system could be 10km with the sample resolution of 0.4m, which means that up to 25000 channels signal processing is realized in real time with Field-Programmable Gate Array module and host computer. The working principle of coherent Rayleigh backscattering interference, phase-generated carrier algorithm and the signal processing flow are introduced, and the experimental results are given in this paper.
Distributed acoustic sensing (DAS) is a new type of replacement technology for geophysical geophone. DAS system is similar to high-density surface seismic geophone array. In the stage of acquisition, DAS can obtain the full well data with one shot. And it can provide enhanced vertical seismic profile (VSP) imaging and monitor fluids and pressures changes in the hydrocarbon production reservoir. Walkaway VSP data acquired over a former producing well in north eastern China provided a rich set of very high quality data. A standard VSP data pre-processing workflow was applied, followed by pre-stack Kirchhoff time migration. In the DAS pre-processing step we were faced with additional and special challenges: strong coherent noise due to cable slapping and ringing along the borehole casing. The single well DAS Walkaway VSP images provide a good result with higher vertical and lateral resolution than the surface seismic in the objective area. This paper reports on lessons learned in the handling of the wireline cable and subsequent special DAS data processing steps developed to remediate some of the practical wireline deployment issues. Optical wireline cable as a conveyance of fiber optic cables for VSP in vertical wells will open the use of the DAS system to much wider applications.
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