A new generation of DAS with greatly improved gauge length and spatial resolving function from the same measurement

Abstract

Fiber-optic distributed acoustic sensor (DAS) is attractive in measuring vibrations with acoustic frequency due to its unique advantage of continuous sensing over long fiber. In the past few years DAS illustrated excellent abilities in applications like structural health monitoring, intrusion detection, and geophysical exploration, hence, it has become a hot topic and attracted lots of researches and investments. However, challenges still exist in improving the interrogator's performance. As is known, the traditional DAS suffers many problems, such as interference fading, low signal-to-noise ratio (SNR), and the trade-off between spatial resolution and the sensing distance, which limit its expansion in wide applications. The time-gated-digital (TGD) technology can overcome those obstacles and leads a new member into the commercial DAS family. The TGD-DAS provides a continuous phase measurement with the gauge length as short as 0.2 m, which is one order smaller than OTDR types. Several experiments of the TGD-DAS interrogator demonstrate that its SNR exceeds 25 dB, and the noise floor of power spectral density (PSD) is as lower as -38 (dB nε2/Hz) measured in a standard SM fiber. An amazing ability is that the measurement data, so-called raw data, can be decoded by different chirp filter bands at the post-processing stage, and thus realize changeable spatial resolution. This is a unique, and very useful function when measuring frequency velocity dispersion curve, as well as in subsea measurement when different wavelength has different physics meaning. These properties of TGD-DAS have been demonstrated in several applications.