Multi-frequency coherent distributed acoustic sensing (DAS) with single transmitter / receiver pair using offset Tx / Rx frequency framing scheme

1. A multi-frequency coherent distributed acoustic sensing (DAS) system comprising:

a length of optical fiber; and

an optical interrogator unit that generates optical pulses, introduces them into the optical fiber and receives Rayleigh reflected signals from the fiber;

the multi-frequency coherent DAS system CHARACTERIZED BY:

a coherent transceiver unit configured to extract information from the Rayleigh reflected signals, said transceiver unit including a single transmitter/receiver pair that employs an offset Tx/Rx frequency framing scheme.

2. The multi-frequency coherent DAS system of claim 1 FURTHER CHARACTERIZED BY:

a single IQ modulator (IQM) generates interrogation pulse(s) and local oscillator (LO) signal(s) for the coherent receiver.

3. The multi-frequency coherent DAS system of claim 2 FURTHER CHARACTERIZED BY:

the IQM is driven by one of an arrayed waveguide grating (AWG), digital to analog converter (DAC), or combined radio frequency (RF) sources to drive the IQM for the multi-frequency DAS operation.

4. The multi-frequency coherent DAS system of claim 2 FURTHER CHARACTERIZED BY:

a pulse carver for generating multi-frequency interrogation pulses in a first assigned frame.

5. The multi-frequency coherent DAS system of claim 4 FURTHER CHARACTERIZED BY:

a second assigned frame is used to generate multi-frequency LOs for the coherent receiver.

6. The multi-frequency coherent DAS system of claim 5 FURTHER CHARACTERIZED BY:

the system is configured to adjust frequency spacing of DAS interrogation.

7. The multi-frequency coherent DAS system of claim 6 FURTHER CHARACTERIZED BY:

a frequency offset is assigned such that baseband signals fit within a receiver analog to digital converter (ADC) bandwidth.

8. The multi-frequency coherent DAS system of claim 6 FURTHER CHARACTERIZED BY:

the system is configured to adjust frequency spacing of DAS LOs such that baseband signals fit within the bandwidth of an analog to digital converter (ADC).

9. The multi-frequency coherent DAS system of claim 8 FURTHER CHARACTERIZED BY:

the system is configured to perform parallel channel processing in a digital signal processor (DSP) such that static and dynamic DAS fading is mitigated.

10. The multi-frequency coherent DAS system of claim 9 FURTHER CHARACTERIZED BY:

DC phases from each measured channel are employed for alignment during recombination periods.