Frequency-drift compensation in chirped-pulse-based distributed acoustic sensing
Aspects of the present disclosure directed to frequency drift compensation for coded-DAS systems that use chirped pulses as a probe signal. Our inventive approach estimates timing jitter by correlating the amplitude of the estimated Rayleigh impulse response of every frame with a reference frame, and then re-aligns each frame by the estimated timing jitter. As the amount of timing jitter varies within a frame, every frame is divided into blocks where all samples have similar timing jitter, and perform timing jitter estimation and compensation on a block-by-block, frame-by-frame basis using an overlap-and-save method. Tracking of a slowly changing channel is enabled by allowing the reference frame to be periodically updated.
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.
2. The method of claim 1 wherein a timing jitter of the reference frame is periodically updated to track a slowly changing channel.
3. The method of claim 2 wherein the timing jitter is estimated to an accuracy equal to an integer multiple of signal samples such that a time re-alignment corresponds to a barrel shift of the samples in a register.
4. The method of claim 3 wherein the timing jitter is estimated to an accuracy equal to a fraction of a signal sample.
5. The method of claim 4 wherein the fraction of the signal sample is obtained using a lookup table indexed by a ratio between a peak correlation value and correlation values of its neighbor peaks.
6. The method of claim 5 wherein resampling in time is performed by convolving each block of each frame of the Rayleigh impulse response by interpolation filter.