Ground sensing using active sources

Abstract

An apparatus and method for determining ground stability measurements utilizing an active source. In one example, distributed optical sensors detect strain along a length of a railroad track. One or more trains provide the active source. Static strain before and after the passage of the active source is used to determine ground stability. Measurements may be normalized based on characteristics of the active source.

Description

【Technical Field】 【0001】

[01] The present disclosure relates to distributed acoustic sensing that utilizes optical sensors, particularly fiber-optic based sensors. The following disclosure particularly focuses on determining ground stability using active sources detected using such sensors. 【Background Art】 【0002】

[02] Fiber-optic based sensors for detecting various parameters, including acoustic signals, through changes in strain within the fiber are known. Distributed optical sensors do not have pre-defined sensor positions, but use the analysis of return signals to infer phase changes along the length of the fiber and thus detect the fiber characteristics that affect those return signals. For example, Rayleigh backscattering may be used as the return signal. 【0003】

[03] FIG. 1 shows a schematic diagram of a conventional distributed optical sensor generally known as a distributed acoustic sensor (DAS). An interrogator emits a probe light pulse 11 into the first end of a measurement fiber 12. The measurement fiber 12 is laid in the area where sensing is required. The advantage of an optical sensor is that due to the low loss of the optical fiber, the interrogator can be relocated from the sensing location. Thus, there may be a fairly long measurement fiber 12 providing a lead-in from the interrogator to the measurement area. 【0004】

[04] As the pulse 11 propagates through the optical fiber, a portion of the light is scattered by scattering sites within the optical fiber. A portion of that scattered light is captured by the numerical aperture of the optical fiber and propagates back towards the interrogator 10. The main scattering mechanism of interest is Rayleigh scattering, which causes backscattering at the same frequency as the propagating light due to an elastic collision with the scattering site (the "scatterer"). 【0005】

[05] The backscattered pulse 14 is received by the interrogator. The arrival time at the interrogator is proportional to the round-trip distance from the interrogator to a point along the fiber. The pulse decays with time because the loss increases with distance. By sampling the return pulse 14 at a specific time, the backscatter from a specific location along the fiber can be determined. Interference in the optical fiber affects its physical structure (at a microscopic level) and therefore affects the backscattered pulse 14. Such changes can be used to infer the signal interfering with the fiber. 【0006】

[06] Figure 2 shows a schematic diagram of a typical interrogator 10. The transmitter 20 emits probe pulses, and the receiver 21 includes an optical sensor and sampling system for detecting backscatter pulses. The optical circulator 22 couples the probe pulses from the transmitter 20 to the measurement fiber 12 and the return backscatter pulses to the receiver 21. 【0007】

[07] An exemplary optical sensor is described in PCT application number PCT / EP2018 / 050793, published as International Publication No. WO2018 / 134137. 【0008】

[08] Conventionally, optical sensors have been used in a passive manner, detecting signals without any knowledge of the signal source. However, if the occurrence of a detectable signal can be predicted, additional information from the sensor may be obtained by correlating information about the signal source with the detected signal. [Overview of the Initiative] 【0009】

[09] This summary is provided in a simplified form to introduce a selection of concepts that will be further described in the following “Detailed Description.” This “Summary” is not intended to identify any major or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 【0010】

[10] A method for determining ground stability using a distributed optical sensor is provided, comprising the steps of: determining a first static strain at at least one location along the length of the distributed optical sensor; detecting a strain variation at at least one location due to an active source; determining a second static strain at at least one location after the strain variation due to the active source has finished; and determining the difference between the first static strain and the second static strain. 【0011】

[11] Detection of strain fluctuations may be performed by detecting high-frequency strain fluctuations at the site. 【0012】

[12] The relative intensity of the active source may be determined based on high-frequency fluctuations of strain. 【0013】

[13] The method may further include a step of normalizing the difference between a first static strain and a second static strain based on the intensity of the active source. 【0014】

[14] The method may further include a step of determining an index of ground stability at the site based on the difference between a first strain and a second strain. 【0015】

[15] An index of ground stability may be based on multiple differences between the first strain and the second strain. 【0016】

[16] The active source may be a train, and the distributed light sensor is associated with the tracks on which the train runs. 【0017】

[17] The method may further include the step of performing the method at multiple locations along the length of the distributed optical sensor. 【0018】

[18] The method may further include the step of averaging measurements from multiple locations. 【0019】

[19] An optical sensing system for determining ground stability, comprising an optical fiber and an interrogator optically connected to the optical fiber and configured to transmit an optical signal into the optical fiber and detect a return optical signal output from the optical fiber, and an interrogator configured to execute the method described herein. An optical sensing system is also provided. 【Brief Description of the Drawings】 【0020】

[20] Embodiments of the present invention will be described by way of example with reference to the following drawings. [Figure 1] A schematic diagram of an optical sensing system. [Figure 2] A schematic diagram of an optical sensing system. [Figure 3] A strain vs. time chart for a ground monitoring system. [Figure 4] A strain vs. time chart for an active source. [Figure 5] A strain chart over time. [Figure 6] A diagram of strain along a sensor. [Figure 7] A flowchart of a method for determining ground stability. 【Modes for Carrying Out the Invention】 【0021】

[21] Further details, aspects and embodiments of the present invention will now be described by way of example only with reference to the drawings. The elements of the figures are illustrated for simplicity and clarity and are not necessarily drawn to scale in the original text. Like reference numerals are included in each of the drawings for ease of understanding. 【0022】

[22] Optical sensors make it possible to determine changes in the properties of an optical fiber. As described above, distributed sensors make it possible to make such determinations continuously along the fiber (according to the resolution of the system) without requiring a predetermined discrete sensor location. Optical sensors are typically used to estimate the strain of an optical fiber. The strain on the fiber is an indicator of the mechanical environment around the fiber and can therefore be used to estimate the movement of the fiber or the force applied to the fiber. If the frequency response of the optical sensing system is given to drop down to DC, static strain can be determined in addition to the change in strain. 【0023】

[23] Strain change detection may also be used to detect movement of the sensing fiber, for example, by laying the sensing fiber over or in a section of land to detect a landslide. Figure 3 shows the strain of the fiber (at selected locations) over time. In regions 30 and 31, only relatively small changes in strain are observed, which may be related to small movements in the ground that occur naturally over time. In these regions, the strain is essentially static. However, in region 32, there is a large change over a relatively short period of time, indicating a landslide, which causes movement of the sensing fiber and therefore a change in strain. The actual movement can be detected during the change in region 32 using a gradient indicating the velocity of movement. The offset between regions 30 and 31 indicates a change in static strain before and after the movement, suggesting a change in the position of the fiber and therefore the ground around the fiber. Static changes can be inferred from dynamic changes, or they can be detected directly if the sensing system can detect frequencies down to very low frequencies, or if it can detect static strain (DC) (either absolute or relative to a reference). 【0024】

[24] In the present disclosure, the term "ground" is used to refer to the environment surrounding the sensing fiber. This may be actual ground, but may also include materials or structures on which / within which the sensing fiber is mounted or mechanically related. For example, in the context of a sensing fiber along a railway line, the ground may include ballast, rails, sleepers, and track clips, all of which move and have associated stability values. In the context of a road, the ground may be the surrounding ground, the foundation, and / or the tarmac or road surface. Thus, generally, "ground" includes any aspect where the sensing fiber can detect movement in response to an energy source. 【0025】

[25] An example of an optical sensing system is described in Patent Application Publication No. WO2018 / 134137, which details a Rayleigh backscattering-based system having a bandwidth up to DC and appropriate resolution. 【0026】

[26] In an example of the installation of an optical sensor, the sensing fiber may be laid along a railway line. For example, the sensing fiber may be placed within or on the ballast supporting the railway line, or within a related cable duct or channel. When attached in such a position, this sensing system can be used to detect a range of parameters related to the mechanical environment of the sensor. During steady state, the sensor can be used to detect overall environmental movement of the sensor by detecting changes in strain, although these changes in strain are small and tend to occur over relatively long periods. When a train passes along the railway line, the sensor can be used to detect high-frequency changes in the strain of the fiber caused by the vibration of the railway line and surrounding materials due to the train. Since these high-frequency changes move along the railway line with the train, they can be used to monitor the movement of the train along the railway line and determine the position of the train at a given point. 【0027】

[27] It has now been shown that high-frequency changes can also be associated with changes in static strain before and after the passage of a train. That is, there may be an offset in static strain before and after the passage of a train, which suggests ground movement caused by the train. This change in static strain may be used to identify ground movement caused by the train, which may indicate ground instability that requires attention. 【0028】

[28] Figure 4 shows an example plot of strain at a location along a sensor fiber against time while a train is passing along the track associated with the sensor. In this example, the train forms an active source, which is detected by the sensor in region 40 as high-frequency fluctuations in strain and an overall change in strain. The static strain before 41 and after 42 are before-and-after values ​​of the static strain, giving the change in static strain due to the passage of the train. The change in static strain may indicate movement of the environment (i.e., the track bed or the environment in which the sensor is mounted) caused by the passage of the train. 【0029】

[29] Figure 4 shows the high-frequency fluctuations of strain as a train passes. The magnitude and morphology of these fluctuations can provide information about the dynamic movement of the ground, as well as information about the active source. For example, it can indicate how many sets of wheels the train has, and thus determine the type of train. Larger, high-frequency changes may indicate low ground stability, even if there is little static change after the train has passed. An increase in the magnitude of high-frequency movement over a series of sensing events (e.g., a series of trains) may indicate changes or deterioration of the ground over time. The gradient of strain change between static values ​​while the train is passing may also provide information about ground stability. 【0030】

[30] In the above example, correlation with information about the active source can increase the amount of information that can be determined. For example, if the weight or relative weight of the train is known or can be determined, the magnitude of the change in strain can be normalized to the weight, since a heavier train is expected to produce more movement. 【0031】

[31] Figure 5 shows an example of measurements taken by distributed sensors associated with the track before, during, and after the passage of a train. The color scale shows the change in strain compared to the train at time=0. The vertical axis represents time, and the horizontal axis represents distance. The train starts at approximately 1250m at t=0 and moves to higher distance values ​​as time increases. 【0032】

[32] Changes in strain at specific locations along the track are indicated by changes in color along a vertical line descending from the top to the bottom of the chart. The strain from each sensor location (distance along the sensor) is normalized to zero (white in Figure 6) at time t=0. The arrival of the train at each point along the track is indicated by the peak of the color bar (indicated by ellipse 50). If there is no change in static strain after the train has passed, it is expected that the color will be dark while the train is passing and return to white below it. Thus, there is a diagonal line that follows the peak of the visible column in Figure 5. However, as shown here, changes in strain may persist after the train has passed, indicating long-term disturbance of the ground due to the train's passage. These long-term changes correspond to the height of the offset / ramp, as explained with reference to Figure 4. 【0033】

[33] Darker areas indicate greater ground movement. Thus, it can be determined that the static movement caused by each train is greater in the area around 2000m than in the area around 4000m, which may indicate that the ground in the 2000m area is unstable. Furthermore, for example, the red line around 2500m indicates static contraction of the fiber. Thus, it can be concluded that the train causes static extension of the fiber in one area and static contraction in another. As shown in Figure 6, monitoring this data for a series of trains (perhaps over a long period such as several months or several years) can be used to show long-term trends in ground stability. Similarly, large changes in a few active sensing events (passing trains) may indicate rapid changes in ground conditions, which is also noteworthy. 【0034】

[34] Figure 6 shows a plot of changes in static strain (ramp height in Figure 5) as the train passes through each point. Blue indicates small changes, and red indicates large changes. The data pertains to distributed sensors associated with the track running between the two plotted lines. These lines are aligned along the length of the track to correlate with the track position, but are offset laterally relative to the track for better visibility. The circled area indicates a decrease in ground stability, with relatively little displacement in 2018 and greater displacement in 2020 (i.e., an increase in ramp height between 2018 and 2020). This decrease was confirmed by a site visit that revealed the need for repair of the track support aggregate. Thus, the disclosed system is confirmed to provide an index of ground stability by relating changes in static strain to an active source event (in this case, a train passing through the location). 【0035】

[35] Thus, this data indicates that the train acts as an active source for probing the surrounding ground, and the changes resulting from this probing are detected by sensors to provide an indicator of ground stability. The sensor system measures the longitudinal strain present in the optical fiber, which is typically induced by the lateral deflection of the environment around the optical fiber. However, each deflection can be in a different direction, and it is not possible to determine the direction from the strain measurement, so the changes in strain over time cannot be summed up to obtain the overall displacement. Thus, strain is an indicator of ground stability, not a measure of absolute ground displacement. 【0036】

[36] Passive measurements of the ground may not provide an indicator of ground stability due to cross-sensitivity to other parameters. For example, temperature changes may affect strain measurements. Furthermore, a decrease in ground stability may not be detected by passive sensors because it may not directly lead to ground movement. Only when the ground is probed by an active source such as a train may a decrease in stability lead to actual ground movement. A short period of the active source means that changes in other parameters that may affect static strain measurements are expected to be negligible. The disclosed system therefore enables the measurement of parameters that cannot be detected by passive sensors. 【0037】

[37] In addition to ground stability, ramp height can also be affected by other parameters, particularly those of the active source. For example, as the train becomes heavier or larger, ground movement is expected to increase, and therefore strain changes will be larger. Normalizing ramp height based on the size or weight of the train eliminates such dependencies and thus improves the ability to compare various active source events to determine ground stability. 【0038】

[38] High-frequency fluctuations of strain generated as a train passes may be used to estimate information about the weight or size of the train used to normalize the ramp height. For example, the number of wheelsets of a train may be determined from the frequency of strain at a given location. As a general indicator, static changes are static changes at frequencies <0.2 Hz, and dynamic changes or high-frequency changes are changes at frequencies >0.2 Hz. The number, spacing, and relative intensity of the wheelset signals can be correlated with the configuration and type of the train, and thus with the estimated weight. The amplitude of high-frequency changes may also indicate the weight of the train. Determining absolute weight may not be necessary, as it may be simpler to determine the relative weight between trains. Relative weight allows for normalization of the height of a series of ramps so that fluctuations are attributed to changes in ground stability rather than to larger probes (the weight of the trains). 【0039】

[39] Figure 7 shows an example of a method for determining ground stability. This method is for applications that utilize distributed sensors positioned to detect changes in strain due to ground movement along the length of a railway track. In step 70, the static strain at a point of interest along the length of the region of interest is determined. In step 71, the passage of a train passing through the point of interest is detected by the presence of high-frequency fluctuations in the detected strain. In step 72, the passage of the train passing through the point of interest is completed, and the static strain at the point of interest is determined. In step 73, the change in static strain between steps 70 and 72 caused by the passage of the train is determined. In step 74, the change in static strain is normalized based on the relative weight of the train, which may be determined from the high-frequency fluctuations in step 71. In step 75, the change in ground stability over time is determined by comparing multiple normalized changes in static strain at the point of interest. Thus, the method in Figure 7 makes it possible to determine the change in ground stability over time. 【0040】

[40] Data from multiple distances (sensors / channels) may be accumulated and processed as a set to provide further information. For example, the average of a set of static strains may be taken to determine the overall displacement of the region. Summing or averaging alternating positive and negative strains to approximately zero may indicate that there is no overall problem with the ground stability of the region, even if individual measurements are high. Furthermore, newly laid sensing fibers may initially slacken in the physical environment, indicated by net contraction over time. Data processing may be used to compensate for this change, which is not attributable to changes in ground stability. 【0041】

[41] In the method of Figure 7, the passage of the train may be determined by any suitable method, without utilizing the high-frequency strain change described in step 71. Similarly, the relative weight of the train may be determined by any suitable means, or the static change may not be normalized by the relative weight. In this specification, weight is used as an example parameter for normalizing the change in static strain, but any parameter indicating the strength of the active source may be used, depending on the nature of the active source. Other examples include kinetic energy (proportional to the square of the velocity) or, if the train is accelerating or decelerating, impulse (mass × change in velocity over a constant time interval). 【0042】

[42] As is evident, the method of Figure 7 can be applied to multiple points along the length of the sensor to form a measure of ground stability along that length, such as the data shown in Figure 6. The resolution of the data is determined by the resolution provided by the sensor system when detecting static strain. 【0043】

[43] The above disclosures relate primarily to ground movement in relation to railways and the use of trains as active sources. However, the principle of detecting changes in ground stability using an active source equipped with distributed sensors can be applied to a wide range of other environments. For example, a vehicle may provide an active source, with sensors installed along a road or runway. Similar to the train example, changes in static strain before and after the passage of a vehicle can be used to monitor ground stability over time. The disclosures envision the use of any active source that disturbs the ground and causes changes in static strain. Other examples include temperate changes or tidal movements. 【0044】

[44] The above description is made primarily with reference to purely distributed sensors. However, these techniques and devices may also be applied to systems that have reflectors for generating specific return signals at defined locations along the fiber. This may be desirable to increase the sensitivity of the measurement fiber at defined locations along the measurement fiber. 【0045】

[45] As should be understood, the methods described herein may be performed by an interrogator unit, as set forth herein above. The interrogator unit may be implemented in hardware, or in hardware and software as appropriate. 【0046】

[46] Although the present invention has been described in relation to several embodiments, it is not intended to be limited to any particular form described herein. Rather, the scope of the present invention is limited only by the appended claims. Furthermore, although features may appear to be described in relation to a particular embodiment, those skilled in the art will recognize that various features of the described embodiments can be combined in accordance with the present invention. In the claims, the term “comprising” does not preclude the presence of other elements or steps. 【0047】

[47] Furthermore, the order of features in a claim does not imply a specific order in which the features must be performed, and in particular, the order of individual steps in a claim for a method does not imply that the steps must be performed in that order. Rather, the steps may be performed in any suitable order. Furthermore, singular references do not exclude plurals. Thus, references to “a,” “an,” “first,” “second,” etc., do not exclude plurals. In the claims, the terms “comprising” or “including” do not exclude the presence of other elements.

Claims

[Claim 1] A method for determining ground stability using a distributed optical sensor, The steps include determining a first static strain at at least one location along the length of the distributed optical sensor, A step of detecting a change in strain at at least one location caused by an active source which is an object moving on the ground, The steps include determining a second static strain at at least one location after the strain fluctuation caused by the active source has finished, A step of determining the difference between the first static strain and the second static strain, A step of determining an index of ground stability at the location based on the difference between the first static strain and the second static strain, Methods that include... [Claim 2] The method according to claim 1, wherein the detection of strain fluctuations is performed by detecting high-frequency fluctuations of strain at the location, and the high-frequency fluctuations have a frequency greater than 0.2 Hz. [Claim 3] The method according to claim 1 or 2, further comprising the step of normalizing the difference between the first static strain and the second static strain based on the weight, kinetic energy and impulse of the active source, wherein the step of determining the index is performed based on the normalized difference. [Claim 4] The method of claim 2, further comprising the step of normalizing the difference between the first static strain and the second static strain based on the amplitude of the high-frequency fluctuation, wherein the step of determining the index is performed based on the normalized difference. [Claim 5] The method according to claim 1 or 2, wherein the steps of determining the first static strain, detecting the variation in the strain, determining the second static strain, and determining the difference are performed at a plurality of locations along the length of the distributed optical sensor, and the step of determining the index of ground stability is performed based on a plurality of differences determined at the plurality of locations. [Claim 6] The method according to any one of claims 1 to 5, wherein the active source is a train, and the distributed optical sensor is associated with the railway tracks on which the train runs. [Claim 7] The method according to any one of claims 1, 2, and 6, further comprising the steps of performing the steps of determining the first static strain, detecting a variation in the strain, determining the second static strain, and determining the difference at a plurality of locations along the length of the distributed optical sensor. [Claim 8] The method according to claim 7, further comprising the step of averaging the first static strain, the second static strain, or the difference determined at the plurality of locations.

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