Читать книгу Distributed Acoustic Sensing in Geophysics - Группа авторов - Страница 58

Fiber optic DAS is an optoelectronic system that records the true acoustic or seismic signal continuously along the sensing fiber that can be tens of kilometers long. As described in Figure 3.1, when a pulse of light travels down an optical fiber, the backscattering light generates and then returns to the sensor unit. The optical fiber, affected by the localized acoustic or seismic signal along the fiber axis, will deform due to the photoelastic effect. The fiber length changes from L to L±ΔL, resulting in the phase change of the scattering light transmitted in the optical fiber, which is essentially a sensing of the strain change along the optical fiber. Then the acoustic or seismic signal along the wave‐affected optical fiber could be retrieved from the phase demodulation. Further, by recording the returning backscattering signal against time, a measurement of the acoustic or seismic wave field all along the fiber can be determined.

In general, although the fiber optic DAS system has achieved good performance and wide practical applications, most techniques utilize the SMF as the sensing element, which is mainly based on various spontaneous scattering effects in the fiber. However, the intensity of backscattering light is very low. For example, the Rayleigh backscattering light makes up 98% of the backscattering light while its scattering coefficient is only −55 dB, and the Brillion scattering coefficient is about 30 dB less than Rayleigh scattering. The weak Rayleigh light greatly lowers the SNR, and the interference between different scatters induces fading points randomly along the optical fiber, which prevents the ultrahigh precision and accuracy of fiber optic DAS measurement. And also, the distance‐dependent SNR degradation leads to nonuniform performance along the long‐distance fiber. Hence, high SNR and high stability are two important issues in the fiber optic DAS system.

Figure 3.1 Schematic principle of the fiber optic DAS system.