| CPC G01H 9/004 (2013.01) [G01D 5/268 (2013.01); G01D 5/35361 (2013.01); H04L 27/2278 (2013.01)] | 6 Claims |
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1. A method for frequency-drift compensation in a chirp-pulse-based distributed acoustic sensing system (DAS) comprising:
a length of optical fiber sensor cable;
a DAS interrogator in optical communication with the length of optical fiber sensor cable, the DAS interrogator including:
a seed laser; and
a coherent receiver;
the interrogator configured to:
produce probe signals including chirped pulses at a given frame rate, chirp duration and chirp slew rate and launching the probe signals into the length of optical fiber sensor cable; and
recover Rayleigh backscatter from the optical fiber sensor cable using the coherent receiver;
wherein the seed laser exhibits a non-negligible frequency drift such that a frequency of a local oscillator is different from a frequency which generated the chirped pulses resulting in timing jitter of a Rayleigh impulse response determined from a correlation of received backscatter with known chirp;
the method comprising:
compensating timing jitter in an estimated Rayleigh impulse response by
dividing each received frame of Rayleigh backscatter into overlapping blocks wherein the first frame received is a reference frame from which timing jitter of subsequent frames are estimated by correlating amplitude profiles of the subsequent frames with an amplitude profile of the reference frame and;
re-aligning the frames by the estimated jitter.
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