Topside interrogation using multiple lasers for distributed acoustic sensing of subsea wells
A distributed acoustic system (DAS) may include an interrogator that includes two or more lasers, a pulser module disposed after and connected to each of the two or more lasers, a wavelength division multiplexer (WDM), wherein each of the pulser modules are connected to the WDM as inputs, and a downhole fiber attached to the WDM as an output and wherein the downhole fiber includes at least one sensing fiber. A method for increasing a sampling frequency may include identifying a length of a downhole fiber connected to an interrogator, generating and launching a light pulse from each of the two or more lasers the pulser module, and delaying an output from the pulser module into the downhole fiber by k N seconds, where k is a pulse repetition interval of the pulser module and N is equal to the two or more lasers.
1 . A method comprising:
generating and launching one or more light pulses of a first optical frequency from at least one laser into a fiber optic cable, wherein the light pulses traverse the fiber optic cable to a sensing fiber;
forming a backscatter signature based on at least two combinations of the light pulses in the sensing fiber, wherein the backscatter signature traverses from the sensing fiber to a photodetector assembly;
determining one or more phases of the backscatter signature, wherein determining the one or more phases comprises:
shifting the first optical frequency to a second optical frequency with an acousto-optic modulator;
taking one or more measurements of the backscatter signature and the second optical frequency; and
comparing the one or more measurements with an information handling system to find a heterodyne beat signal,
determining a seismic energy of the backscatter signature based at least in part on the one or more phases; and
identifying a strain along the fiber optic cable based at least in part on the seismic energy of the backscatter signature.
2 . The method of claim 1 , further comprising amplifying the light pulse with an Erbium doped fiber amplifier (EDFA).
3 . The method of claim 2 , wherein the EDFA is disposed between the at least one laser and the fiber optic cable.
4 . The method of claim 1 , wherein the acousto-optic modulator is disposed in an interferometer.
5 . The method of claim 1 , further comprising detecting the backscatter signal and the second optical frequency with the photodetector assembly.
6 . The method of claim 5 , wherein the information handling system is connected to the photodetector assembly.
7 . The method of claim 1 , further comprising identifying one or more regions of the fiber optic cable for which backscatter is received with the information handling system.
8 . The metho of claim 1 , further comprising amplifying the backscatter signal with an Erbium doped fiber amplifier (EDFA).
9 . The method of claim 8 , wherein the EDFA is disposed between the sensing fiber and the information handling system.
10 . The method of claim 1 , further comprising multiplexing the light pulse with a wavelength division multiplexer (WDM) as the light pulse traverses through the fiber optic cable.
11 . A distributed acoustic system (DAS) comprising:
two or more lasers;
a wavelength division multiplexer (WDM) disposed after and connected to each of the two or more lasers;
one or more pulser modules directly connected to the WDM as inputs,
wherein each pulser module is configured to launch two or more combinations of the light pulses;
a distal circulator connected to the pulser module by a fiber optic cable;
a downhole fiber attached to the distal circulator as an output and wherein the downhole fiber comprises at least one sensing fiber, wherein a backscatter signature associated with the two or more combinations of light pulses traverses the at least one sensing fiber; and
a parse device configured to separate the backscatter signature into one or more phase components for calculation of a seismic energy of the backscatter signature, wherein the calculated seismic energy is indicative of a strain along the at least one sensing fiber.
12 . The DAS of claim 11 , further comprising an Erbium doped fiber amplifier (EDFA) disposed between the pulser module and the distal circulator.
13 . The DAS of claim 11 , wherein the pulser module delays a light pulse for each of the two or more lasers into the downhole fiber by
k
N
seconds, where k is a pulse repetition interval of the pulser module and N is equal to a number of lasers that are at least a part of an interrogator.
14 . The DAS of claim 11 , further comprising a photo detector assembly connected to the distal circulator by a second fiber optic cable.
15 . The DAS of claim 14 , further comprising a second EDFA disposed between the distal circulator and the photo detector assembly.
16 . The DAS of claim 15 , further comprising an interferometer disposed between the second EDFA and the photo detector assembly.
17 . The DAS of claim 16 , further comprising a second WDM disposed between the second EDFA and the photo detector assembly.
18 . The DAS of claim 17 , further comprising the interferometer disposed between the second EDFA and the second WDM.
19 . The DAS of claim 18 , further comprising two or more parse devices disposed between the second WDM and the photo detector assembly.
20 . The DAS of claim 19 , further comprising an information handling system connected to the photo detector assembly.