COORDINATED AND CORRELATED PROBE SIGNALS IN FIBER OPTIC SENSING SYSTEMS AND METHODS
A fiber-optic network is used to interrogate a geographic area for propagating waves initiated by events external to the fiber-optic network. The probe signals are frequency modulation aligned and/or carrier frequency aligned with one another. Scattered energy indicative of probe signals' interactions with the propagating wave are time aligned with one another, amplitude normalized, cross correlated and/or added together to improve signal-to-noise and spatial resolution.
1 . A method of using a fiber-optic network as a probe, the method comprising:
transmitting a first probe signal on a first fiber optic strand of the fiber-optic network;
receiving, at a detector, first scattered energy from the first probe signal's interaction with a propagating wave initiated by an event that is external to the fiber-optic network to generate a first data set;
transmitting a second probe signal on a second fiber optic strand of the fiber-optic network;
receiving, at the detector, second scattered energy from the second probe signal's interaction with the propagating wave to generate a second data set;
analyzing the first data set and the second data set to identify a time differential between the first probe signal's interaction and the second probe signal's interaction; and
removing the time differential to time align the first data set and the second data set.
2 . The method of claim 1 , wherein the first probe signal and the second probe signal are transmitted on a same optical carrier frequency, such that they are carrier frequency aligned.
3 . The method of claim 2 , wherein the first probe signal and the second probe signal are modulated at a same frequency, such that they are frequency modulation aligned.
4 . The method of claim 1 , wherein the first probe signal and the second probe signal are modulated at a same frequency, such that they are frequency modulation aligned.
5 . The method of claim 1 , wherein the step of time aligning the first data set and the second data set comprises adjusting the first data set to have an effective angle of incidence with the propagating wave equal to an actual angle of incidence of the second data set.
6 . The method of claim 1 , wherein the step of time aligning comprises adjusting data points in one or both of the first data set and the second data set to make highest peaks in each data set coincident in time.
7 . The method of claim 1 , wherein the time differential is caused by one or more of (i) interference from a ground path, (ii) interference from a fiber path, (iii) an angle of incidence between the first probe signal and the propagating wave, and (iv) an angle of incidence between the second probe signal and the propagating wave.
8 . The method of claim 1 , wherein the first probe signal and the second probe signal transmissions are coordinated to ensure the first probe signal and the second probe signal interact with the propagating wave at the same time.
9 . The method of claim 1 , further comprising:
identifying a frequency shift between the first probe signal and the first scattered energy; and
re-transmitting the first probe signal on the first fiber optic strand at an adjusted time until a negligible frequency shift between the first probe signal and the first scattered energy is identified, thereby detecting a position where the first probe signal and the propagating wave have perpendicular vectors.
10 . The method of claim 1 , wherein the first scattered energy and/or the second scattered energy comprise one or both of backscatter and forward scatter from the probe signals.
11 . The method of claim 1 , further comprising identifying a false positive for existence of the event when both the first scattered energy and the second scattered energy are expected to produce a signal greater than noise at the detector but only one of the first scattered energy and the second scattered energy produces the signal greater than noise.
12 . The method of claim 1 , further comprising:
normalizing amplitudes of the first data set and the second data set to generate normalized data sets;
determining a correlation between the normalized data sets as being acceptable or unacceptable based on a predetermined threshold; and
adding together the normalized data sets that are determined to be acceptable to create a final data set with increased signal-to-noise.
13 . The method of claim 1 , wherein at least one of the first data set and the second data set comprises a series of peaks defined by a Doppler effect.
14 . The method of claim 1 , further comprising determining a direction of propagation for the propagating wave.
15 . The method of claim 1 , further comprising transmitting telecommunication signals on the first fiber optic strand of the fiber-optic network such that the first probe signals and the telecommunication signals coexist on the first fiber optic strand.
16 . The method of claim 15 , wherein the first probe signals and the telecommunication signals are co-propagating.
17 . The method of claim 15 , wherein the first probe signals and the telecommunication signals are counter propagating.
18 . The method of claim 15 , further comprising transmitting telecommunication signals on the second fiber optic strand of the fiber-optic network such that the second probe signals and the telecommunication signals coexist on the second fiber optic strand.
19 . The method of claim 18 , wherein the probe signals and the telecommunication signals are co-propagating.
20 . The method of claim 18 , wherein the probe signals and the telecommunication signals are counter propagating.