Real-time Feature Extraction in a Distributed Acoustic Sensor Based on Phase Demodulation With Fast Hilbert Transform
No Thumbnail Available
Date
2024-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Phase-sensitive optical Time Domain Reflectometry (Φ-OTDR) is the most common
implementation of a Distributed Acoustic Sensor (DAS) system. It employs
the observation of speckles resulting from Rayleigh Back-scattering from coherent
pulses in an optical fiber[1]. Since they are sensitive to local disturbances altering
the intensity and phase of light, perturbations induced by events cause changes
in the speckle pattern whose precise measurement provides information on the
amplitude and frequency of vibrations distributed along the fiber. Demodulation
of the local phase change is key to the precise measurement of events since it is
more linearly related to the strain applied to the fiber. One of the key issues in
distributed sensing is that phase demodulation schemes usually require additional
post-processing algorithm runs for each spatial location, which introduces delays,
and hence reductions in dynamic sensing capability when scaled along the whole
sensing distance.
In this research, we analyze the impact of the post-processing in different phase
demodulation techniques employing Phase-Generated Carrier (PGC) on the bandwidth
of distributed feature extraction in a typical DAS system by quantifying the
total computation time needed for a benchmark, 10-km sensing range at meterscale
and sub-meter-scale spatial resolutions. We then design, implement, and
analyze a signal processing scheme for phase extraction in Φ -OTDR enabling
real-time dynamic measurements based on a Fast Hilbert Transform (FHT). Particular
focus is given to the choice of this demodulation scheme for optimizing the
bandwidth of distributed feature extraction as it enables the use of parallel processing
of adjacent blocks in such a way that the overall throughput of spatially
resolved concurrent demodulation allows dynamic vibration sensing at speeds relevant
to most distributed monitoring applications. Our analysis shows that that on
average 3 orders magnitude reductions in computation times are achieved when
employing the Fast Hilbert transform for demodulation compared to the commonly
used PGC-arctan algorithm, while there is a three-fold reduction compared
to PG-DCM and PG-DMS algorithms.
Description
Keywords
Phase-Sensitive Optical Time Domain Reflectometry (Φ-OTDR), Phase-Generated Carried Demodulation, Fast Hilbert Transform (FHT), and Distributed Acoustic Sensing (DAS).