A signal processing method for a distributed acoustic sensing system, the method comprising: receiving, at a detector stage, a set of scattered signals, wherein each scattered signal in the set of scattered signals corresponds to a respective spatial channel and was scattered at a respective location along an optical path; receiving, at the detector stage, a local oscillator signal; for each of the spatial channels: generating, based on an interference of the local oscillator signal and the scattered signal corresponding to that spatial channel, a first complex carrier signal associated with that spatial channel, the first complex carrier signal being modulated by a phase difference between the local oscillator signal and the scattered signal corresponding to that spatial channel; processing the first complex carrier signal to generate a second complex carrier signal associated with that spatial channel, the second complex carrier signal being modulated by a spatial differential of the phase difference, the spatial differential being taken along a length of the optical path; storing an initial second complex carrier signal generated for that spatial channel as a reference complex carrier signal for that spatial channel; and generating a third complex carrier signal associated with that spatial channel, the third complex carrier signal being modulated by a phase difference between the second complex carrier signal and the reference complex carrier signal; summing two or more third complex carrier signals associated with two or more respective spatial channels, to generate a fourth complex carrier signal; determining, based on the fourth complex carrier signal, a value representative of an instantaneous frequency of an acoustic modulation at locations on the optical path corresponding to the two or more respective spatial channels; determining if the value representative of the instantaneous frequency meets a predetermined condition and, if the predetermined condition is met: for each of the two or more respective spatial channels, saving the second complex carrier signal associated with that spatial channel as the reference complex carrier signal for that spatial channel. receiving, at a detector stage, a set of scattered signals, wherein each scattered signal in the set of scattered signals corresponds to a respective spatial channel and was scattered at a respective location along an optical path; receiving, at the detector stage, a local oscillator signal; for each of the spatial channels: generating, based on an interference of the local oscillator signal and the scattered signal corresponding to that spatial channel, a first complex carrier signal associated with that spatial channel, the first complex carrier signal being modulated by a phase difference between the local oscillator signal and the scattered signal corresponding to that spatial channel; processing the first complex carrier signal to generate a second complex carrier signal associated with that spatial channel, the second complex carrier signal being modulated by a spatial differential of the phase difference, the spatial differential being taken along a length of the optical path; storing an initial second complex carrier signal generated for that spatial channel as a reference complex carrier signal for that spatial channel; and generating a third complex carrier signal associated with that spatial channel, the third complex carrier signal being modulated by a phase difference between the second complex carrier signal and the reference complex carrier signal; generating, based on an interference of the local oscillator signal and the scattered signal corresponding to that spatial channel, a first complex carrier signal associated with that spatial channel, the first complex carrier signal being modulated by a phase difference between the local oscillator signal and the scattered signal corresponding to that spatial channel; processing the first complex carrier signal to generate a second complex carrier signal associated with that spatial channel, the second complex carrier signal being modulated by a spatial differential of the phase difference, the spatial differential being taken along a length of the optical path; storing an initial second complex carrier signal generated for that spatial channel as a reference complex carrier signal for that spatial channel; and generating a third complex carrier signal associated with that spatial channel, the third complex carrier signal being modulated by a phase difference between the second complex carrier signal and the reference complex carrier signal; summing two or more third complex carrier signals associated with two or more respective spatial channels, to generate a fourth complex carrier signal; determining, based on the fourth complex carrier signal, a value representative of an instantaneous frequency of an acoustic modulation at locations on the optical path corresponding to the two or more respective spatial channels; determining if the value representative of the instantaneous frequency meets a predetermined condition and, if the predetermined condition is met: for each of the two or more respective spatial channels, saving the second complex carrier signal associated with that spatial channel as the reference complex carrier signal for that spatial channel.