Locating passive seismic events in a wellbore using distributed acoustic sensing

A method of determining a location of a microseismic event within a formation, the method comprising:

• injecting, by a reflectometer of a distributed acoustic sensor, optical signals into a fiber optic cable of the distributed acoustic sensor within a wellbore;
• receiving, at the reflectometer, reflected optical signals from the fiber optic cable;
• identifying, by the reflectometer, strain detected in the reflected optical signals generated from seismic waves of a microseismic event;
• identifying, by the distributed acoustic sensor, a focal mechanism of the microseismic event and velocities of the seismic waves;
• and
• determining, by the distributed acoustic sensor, a position of the microseismic event using the strain detected in the reflected optical signals, the focal mechanism of the microseismic event within the formation, and the velocities of the seismic waves, wherein determining the position of the microseismic event comprises:
• estimating, by the distributed acoustic sensor, a source function of the strain by applying, to the strain, an adjoint imaging operator that is generated using the focal mechanism and the velocities of the seismic waves;
• and
• identifying, by the distributed acoustic sensor, a local maximum of the source function to locate the position of the microseismic event within the formation.

A non-transitory computer-readable medium having program code that is stored thereon, the program code executable by one or more processing devices for performing operations comprising:

• receiving a signal from a reflectometer representing a strain along a length of a fiber optic cable that results from seismic waves of a microseismic event within a formation surrounding a wellbore;
• and
• determining a location of the microseismic event using the signal representing the strain along the length of the fiber optic cable, a focal mechanism of the microseismic event, and velocities of the seismic waves, wherein determining the location of the microseismic event comprises:
• estimating a source function of the strain by applying an adjoint imaging operator to a data stream associated with the strain, wherein the adjoint imaging operator is generated using the focal mechanism and the velocities of the seismic waves;
• and
• identifying a local maximum of the source function to locate the location of the microseismic event.