The use of multi‐frequency acquisition to significantly improve the quality of fibre‐optic‐distributed vibration sensing

Abstract

Unlike conventional sensors that measure the passage of seismic waves at a single position, distributed vibration sensing systems, also known as distributed acoustic sensing systems, detect the passage of seismic waves by averaging a measurement of strain over a section of fibre‐optic cable. Distributed vibration sensing systems work by transmitting pulses of light down the fibre and measuring the phase of the Rayleigh backscatter. At random positions along the fibre, however, fading occurs; this is where the amplitude of the backscattered signal is very small due to cancellation of the scattered electric fields, resulting in anomalously noisy traces in a common source gather.This paper addresses the problem of fading in a particular form of distributed vibration sensors: a new optical arrangement of the instrumentation is described that allows the measurement to be carried out quasi‐simultaneously at multiple optical interrogation frequencies. The interrogation frequencies are chosen to be sufficiently different that their fading properties are distinct and the diversity thus obtained is used to aggregate the data obtained to substantially reduce the noise caused by fading. As well as reducing the effects of fading, the aggregation of the independent results can also help to reduce the overall noise of the measurement and improve the linearity of the distributed vibration sensing system.

Rayleigh Fading Suppression in One-Dimensional Optical Scatters

“…[9] proposed rotated-vector-sum method to aggregate backscattering signals with four different frequencies, which is proved to be effective. Besides, the relationship between the number of aggregated frequencies and SN R φ of acoustic signal is shown in [12]. Based on the simulation results, the SN R φ enhancement factor is closed to √ M, which is similar to M averages on a single frequency intensity traces [13].…”

Performance Enhancement Methods for the Distributed Acoustic Sensors Based on Frequency Division Multiplexing

“…When the fading problem occurs, the intensity of the backscattered light drops to the fading region. Meanwhile, SNR is insufficient for effective signal demodulation, resulting in a high false alarm rate [5]. The frequency hopping technique of the laser source is a way to overcome the fading phenomenon, which change the laser frequency and backscatter signal integration gradually.…”

“…Healey et al discussed that the speckle phenomenon cannot occur at rest. The effects of fading components can be limited by averaging various Rayleigh traces through direct or specific randomization processes [5,6], however, the unintentional distribution of strain and temperature along the fiber is spatially random, which remains relatively stable over time. Therefore, fading cannot be eliminated by simply averaging multiple scans through a single fixed probe frequency.…”

“…Nevertheless, in the case of seismic waves, it does not require long sensing times, and the monitored disturbances last only a few seconds. They pay more attention to the suppression of the fading effect of the amplitude, and have not discussed the phase signal behavior in the fading region [5].…”

Performance Improvement of Dual-Pulse Heterodyne Distributed Acoustic Sensor for Sound Detection

“…The fading effect is fatal in the DAS system because, at the fading positions, the acoustic signal is lost. To overcome this issue, Hartog et al used multiple optical interrogation frequencies with diverse fading properties to aggregate the data obtained to substantially reduce the fading noise [17]. However, this scheme greatly increases the system complexity, especially at the source part.…”