Multi-span distributed acoustic sensing from the shore with different sensing frequency ranges and sensing span lengths

A system for distributed acoustic sensing comprising:

• a first distributed acoustic sensing (DAS) station, located at a first end of a multi-span link, and configured to launch a plurality of DAS signals in a first direction;
• a second distributed acoustic sensing (DAS) station, located at a second end of the multi-span link, and configured to launch a plurality of DAS signals in a second direction;
• and
• a loopback array, comprising:
• a first set of proximate loopbacks, located nearest to the first DAS station;
• a first set of intermediate loopbacks;
• a second set of proximate loopbacks, located nearest to the second DAS station;
• and
• a second set of intermediate loopbacks, located closer to the second DAS station than the first set of intermediate loopbacks,
• wherein a first wavelength and a first interrogation rate of a first set of DAS signals routed through the first set of proximate loopbacks differs from a second wavelength and a second interrogation rate of a second set of DAS signals routed through the first set of intermediate loopbacks,
• and
• wherein a third wavelength and a third interrogation rate of a third set of DAS signals routed through the second set of proximate loopbacks differs from a fourth wavelength and a fourth interrogation rate of a fourth set of DAS signals routed through the second set of intermediate loopbacks.

A method for distributed acoustic sensing comprising:

• launching a first DAS signal from a first interrogator unit of a first DAS station at a first interrogation rate and a first wavelength;
• launching a second DAS signal from a second interrogator unit of the first DAS station at a second wavelength and a second interrogation rate, less than the first interrogation rate;
• routing back the first DAS signal to the first DAS station through a loopback of a first set of proximate loopbacks nearest to the first DAS station;
• and
• routing back the second DAS signal to the first DAS station through a loopback of a first set of intermediate loopbacks, located further from the first DAS station.