Detection of seismic signals using fiber optic distributed sensors

A method of detecting seismic signals, comprising:

• deploying a first fiber optic cable in a borehole extending from a surface into an earth formation, the first fiber optic cable configured to react along its length to incident seismic signals;
• deploying a second fiber optic cable in a borehole extending from the surface into the earth formation, the second fiber optic cable configured to react along its length to incident seismic signals;
• launching optical pulses into the first and second fiber optic cables during occurrence of a seismic event in the earth formation;
• and
• detecting backscattered optical signals generated by the first and second fiber optic cables in response to the optical pulses to determine characteristics of the seismic event, wherein the detecting backscattered optical signals comprises detecting coherent Rayleigh noise (CRN).

A method for conducting a seismic survey of an earth formation, comprising:

• launching a first optical signal into a first fiber optic cable disposed in a borehole extending from an earth surface into the earth formation;
• generating a seismic wave that propagates through the earth formation from outside of the borehole;
• and
• analyzing coherent Rayleigh noise (CRN) produced in response to the first optical signal while the seismic wave is incident along a length of the first fiber optic cable to determine characteristics of the earth formation.

A system comprising:

• a first optical fiber disposed in a borehole that extends from an earth surface into an earth formation, the first optical fiber configured to react to a seismic wave incident on the first optical fiber at any of a plurality of locations along its length;
• a seismic source to generate a seismic wave that originates outside of the borehole;
• an optical source to launch an optical signal into the first optical fiber to produce a backscattered optical signal while the seismic wave is incident on the first optical fiber;
• a receiver to detect coherent Rayleigh noise (CRN) in the backscattered signal;
• and
• a processing circuit to process the CRN to determine characteristics of the earth formation.