Multi-span sensing using wavelength / frequency multiplexing, synchronized receiver, and coherent signal processing

A multi-span distributed acoustic sensing (DAS) system, comprising:

• a first DAS transmitter, operative to transmit a first optical sensing signal over a first path, the first optical sensing signal comprising a plurality of wavelengths;
• a second DAS transmitter, operative to transmit a second optical sensing signal over a second path, the second optical sensing signal comprising a plurality of wavelengths;
• an optical cable, arranged over a plurality of spans to conduct the first optical sensing signal and the second optical sensing signal, a given span of the plurality of spans extending for a distance of 10 kilometers or more;
• a plurality of repeaters, distributed over the plurality of spans, wherein a given repeater of the plurality of repeaters comprises a pair of loopbacks, wherein a first loopback of the pair of loopbacks in a given repeater is arranged to conduct a first Rayleigh backscattered signal to the second path, wherein the first Rayleigh backscattered signal is derived from the first optical sensing signal, wherein a second loopback of the pair of loopbacks in a given repeater is arranged to conduct a second Rayleigh backscattered signal to the first path, wherein the second Rayleigh backscattered signal is derived from the second optical sensing signal;
• and
• a first receiver, collocated with the first DAS transmitter, and arranged to receive the first Rayleigh backscattered signal, and to process the first Rayleigh backscattered signal, wherein a time delay is applied to a first electrical signal derived from the first Rayleigh backscattered signal according to a wavelength of the first Rayleigh backscattered signal, and
• a second receiver, collocated with the second DAS transmitter, and arranged to receive the second Rayleigh backscattered signal, and to process the second Rayleigh backscattered signal, wherein a time delay is applied to a second electrical signal derived from the second Rayleigh backscattered signal according to a wavelength of the second Rayleigh backscattered signal.