Cross-correlation-based manhole location using ambient traffic and fiber sensing

Abstract

A system and method for locating manholes along deployed fiber optic cables using cross-correlation techniques and surrounding road traffic traveling near manholes containing fiber optic communication cables to detect the location of the manholes using distributed fiber optic sensing (DFOS). Advantageously, the location of the manholes is determined without laborious field surveys.

Description

[Technical Field]

[0001] This application relates to optical fiber communications. More particularly, this application relates to a distributed optical fiber sensing (DFOS) system, method, and structure used for cross-correlation-based manhole location using ambient traffic and fiber sensing. [Background technology]

[0002] Over the past 30 years, telecommunications carriers and service providers have deployed over 300 million miles (approximately 460 million kilometers) of optical fiber. As known to those skilled in the art, manholes are convenient locations for interconnecting deployed fiber and storing splicing trays. However, the exact locations of many manholes are not properly recorded, and paper records are often woefully inadequate and outdated. While telecommunications carriers and service providers have developed methods for locating manholes, these methods are often time-consuming and labor-intensive. Summary of the Invention

[0003] The above problems are solved and an advancement in the art is made according to aspects of the present disclosure directed to locating manholes along deployed fiber optic cables. In contrast to the prior art, the method of the present invention uses cross-correlation techniques and surrounding road traffic traveling near manholes containing fiber optic communication cables to detect the location of the manhole using distributed fiber optic sensing (DFOS).

[0004] In contrast to the prior art, the system and method of the present invention determines the location of manholes without employing laborious field surveys. [Brief explanation of the drawings]

[0005] A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings.

[0006] [Figure 1(A)] FIG. 1 is a schematic diagram illustrating an exemplary DFOS system according to an aspect of the present disclosure.

[0007] [Figure 1(B)] FIG. 1 is a schematic diagram illustrating an example architecture of a coherent detection Rayleigh OTDR according to aspects of the present disclosure.

[0008] [Figure 2] FIG. 1 is a schematic diagram illustrating an example workflow for manhole location according to aspects of the present disclosure.

[0009] [Figure 3(A)] FIG. 1 is a schematic diagram illustrating an exemplary system configuration and traffic for automatic manhole location according to an aspect of the present disclosure.

[0010] [Figure 3(B)] 3(B) shows a pair of waterfall plots as a vehicle passes near the manhole of FIG. 3(A), according to one embodiment of the present disclosure.

[0011] [Figure 4] FIG. 1 is a schematic diagram illustrating an illustrative example of an implementation of cross-correlation at a sensing point j in a waterfall with two neighboring points (p=2) according to an embodiment of the present disclosure.

[0012] [Figure 5(A)] 1 illustrates a set of plots used to detect and locate manholes using ambient traffic vibrations, according to aspects of the present disclosure. FIG. 1 illustrates an input waterfall showing vehicle trajectories with the locations of surveyed manholes indicated by dashed lines.

[0013] [Figure 5(B)] 10A-10C illustrate a set of plots used to detect and locate manholes using ambient traffic vibrations, according to an embodiment of the present disclosure, showing cross-correlation maps of waterfall data with 200 time step segments and 10 neighbors.

[0014] [Figure 5(C)] 10A-10C show a set of plots used to detect and locate manholes using ambient traffic vibrations, according to aspects of the present disclosure, with the resulting curve peaks shown in dashed lines coinciding with the locations of the surveyed manholes. DETAILED DESCRIPTION OF THE INVENTION

[0015] The following is merely illustrative of the principles of the present disclosure, and it will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the present disclosure and are included within its spirit and scope.

[0016] Furthermore, all examples and conditional language set forth herein are intended to be for educational purposes only to aid the reader in understanding the concepts contributed by the inventors to further the principles and techniques of the present disclosure, and should not be construed as being limited to such specifically recited examples and conditions.

[0017] Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, such equivalents are intended to include both currently known equivalents as well as equivalents developed in the future, i.e., elements developed that perform the same function, regardless of structure.

[0018] Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.

[0019] Unless otherwise specified herein, the figures comprising the drawings are not drawn to scale.

[0020] As some additional background, distributed fiber optic sensing (DFOS) is an important and widely used technology for detecting environmental conditions (e.g., temperature, vibration, acoustic excitation, and strain levels) anywhere along a fiber optic cable connected to an interrogator. As is well known, modern interrogators are systems that generate an input signal into the fiber, detect and analyze the reflected / scattered, and then received signal. The signal is analyzed, and an output is generated that indicates the environmental conditions encountered along the fiber. These received signals may result from reflections within the fiber, such as Raman backscattering, Rayleigh backscattering, or Brillouin backscattering. DFOS can also utilize forward signals, exploiting the velocity differences between multiple modes. Without loss of generality, the following discussion assumes reflected signals, but a similar approach can be applied to forward signals as well.

[0021] Figure 1(A) is a schematic diagram of a typical prior art DFOS system. As will be appreciated, modern DFOS systems comprise an interrogator that periodically generates optical pulses (or any coded signal) and injects them into an optical fiber. The injected optical pulse signal is transmitted along the optical fiber.

[0022] At some locations along the optical fiber, a small portion of the signal is reflected back to the interrogator. The reflected signal carries information that the interrogator uses to detect, such as changes in power level indicative of mechanical vibrations. While not shown in detail, the interrogator can include a coded DFOS system employing a coherent receiver configuration known in the art, as illustrated in FIG. 1(B).

[0023] The reflected signal is converted to the electrical domain and processed in the interrogator. Based on the time of pulse injection and the time the signal is detected, the interrogator determines from which location along the fiber the signal is coming, and is therefore able to sense activity at each location along the fiber.

[0024] As will be shown and explained, manhole location is achieved using distributed fiber optic sensing (DFOS) technology and cross-correlation methods based on surrounding traffic data along the fiber optic cable path.

[0025] FIG. 2 is a schematic diagram illustrating an exemplary workflow for manhole location according to an embodiment of the present disclosure. Referring to FIG. 2, it can be seen that the method of the present invention uses DFOS sensor data. As one skilled in the art will understand and appreciate, DFOS provides multiple sensing points along the optical sensor fiber that generate a time series of vibration data that constitutes a waterfall trace. Cross-correlation of the time series according to the present disclosure detects manholes along the path of the optical sensor fiber. Finally, once a manhole is detected, its corresponding location along the optical sensor fiber is identified and reported to appropriate personnel.

[0026] FIG. 3(A) is a schematic diagram illustrating an exemplary system configuration and traffic for automatic manhole location according to one embodiment of the present disclosure.

[0027] FIG. 3(B) shows a pair of waterfall plots as a vehicle passes near the manhole of FIG. 3(A), according to one embodiment of the present disclosure.

[0028] These diagrams can be referenced simultaneously to observe the system configuration and resulting waterfall trace. Advantageously, the DFOS system used can be configured to operate as either distributed acoustic sensing (DAS) and / or distributed vibration sensing (DVS), conveniently installed in a central office (CO) for remote monitoring of the entire fiber optic cable route. As will be appreciated, such fiber optic cables include multiple individual optical fibers, any one of which can function as an optical sensor fiber when optically connected to a DFOS interrogator. When DFOS is activated, surrounding traffic traveling near the optical sensor fiber affects the backscatter signal received / collected by the DFOS system for further analysis.

[0029] As shown in Figure 3(B), a visualized plot of waterfall data (received via DFOS operations) representative of a situation with a vehicle trajectory shows a vehicle traveling near an optical sensor fiber interrogated by the DFOS system interrogator. Two scenarios are further illustrated in this figure. The first (right) plot shows a continuous, unbroken slope, indicating the absence of a manhole along that path segment. In contrast, the left plot shows a horizontal deviation in the vehicle trajectory, indicating the presence of a manhole along that path segment.

[0030] As one skilled in the art will understand and appreciate, when a vehicle passes over a manhole, the loose fibers located within the manhole will collectively vibrate, thereby providing a vibration signature to the method of the present invention which further employs cross-correlation techniques to detect the location of the manhole along the path of the optical sensor fibers.

[0031] As will be appreciated by those skilled in the art, when a vehicle passes close enough to a manhole (whether covered or not), vibrations are generated in the manhole's underground housing, which causes the entire slack optical fiber loop located within the manhole to vibrate simultaneously. According to aspects of the present disclosure, such synchronous vibrations along the slack optical fiber loop within the manhole identify adjacent sensing points along the optical fiber loop, and thus cross-correlation of the vibration time series at these adjacent points can be used to determine location.

[0032] There are several design parameters that need to be considered in order to effectively use the cross-correlation method to detect cable loops and therefore manholes. These parameters are:

[0033] (1) What time segments should be used to calculate the cross-correlation of time series, and how frequently should the cross-correlation be calculated?

[0034] (2) Over how many adjacent sensing points should these cross-correlations be calculated? For each sensing point, how many sensing points on either side along the optical sensing fiber should be considered for the cross-correlation calculation?

[0035] For this approach to work effectively, an estimated minimum cable loop of 5 m is required, which could be one or two adjacent points on either side of the sensing point, depending on the spatial resolution used in the DFOS system.

[0036] Generally,

number

[0037] where C j represents the vibration time series segment (waterfall sequence) at sensing point j, and XCorr j is the product of the cross-correlation values ​​of p adjacent sensing points on the left and right.

[0038] FIG. 4 is a schematic diagram illustrating an illustrative example of an implementation of cross-correlation at a sensing point j in a waterfall with two neighboring points (p=2) according to one embodiment of the present disclosure.

[0039] (3) Depending on the generated cross-correlation map, different statistical indicators such as sum, median, mean, and maximum are used to consider the combined effect of the cross-correlation values ​​and isolate the midpoint of the cable loop to estimate the manhole location.

[0040] 5(A), 5(B), and 5(C) show a set of plots used to detect and locate manholes using ambient traffic vibrations, according to an embodiment of the present disclosure. FIG. 5(A) is an input waterfall showing a vehicle trajectory with the location of the investigated manhole indicated by a dashed line. FIG. 5(B) shows a cross-correlation map of the waterfall data with 200 time step segments and 10 neighbors. FIG. 5(C) shows the resulting curve peak, which corresponds to the location of the investigated manhole, indicated by a dashed line.

[0041] Figure 5(A) shows an example of field traffic data for a road section. The vertical dashed lines in the figure indicate the manhole survey locations. When processed using the method of the present invention, the midpoints of the cable loops corresponding to the manhole locations form peaks, as exemplarily shown in Figure 5(C).

[0042] From the waterfall plot illustrated in Figure 5(A), it is clear that the vehicle trajectory detected by DFOS is accentuated when passing near a manhole containing a loop of slack optical sensing fiber, generating a wide horizontal area proportional to the length of the slack fiber located within the manhole through which the vehicle passed.

[0043] While the present disclosure has been described using several specific examples, those skilled in the art will recognize that the present teachings are not limited thereto. Accordingly, the present disclosure is limited only by the scope of the claims appended hereto.

Claims

1. A cross-correlation based manhole location method using ambient traffic and fiber sensing, comprising: using a distributed fiber optic sensing (DFOS) system to collect sensing signals including ambient traffic noise and generate sensing test data from the collected sensing signals; using cross-correlation of the sensing test data to identify DFOS locations indicative of manhole and non-manhole locations; and outputting an indication of the manhole location and the non-manhole location.

2. The method of claim 1 , wherein the cross-correlation is applied to one or more neighbors of a sensing point along an optical sensor fiber in optical communication with the DFOS system.

3. The method of claim 1 , further comprising identifying a DFOS location in the sensing test data that indicates a fiber loop location.

4. The method of claim 3 , further comprising estimating a length of the identified fiber loop.

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