JP2015145592A - Soil state monitoring system and soil state monitoring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a soil state monitoring system capable of predicting a landslide or the like with high accuracy.SOLUTION: A scattered wave is acquired for which pulse laser light incident to an optical fiber buried to be deformed together with soil is scattered within the optical fiber, and a Brillouin frequency shift distribution within the optical fiber and a Rayleigh frequency shift distribution within the optical fiber are measured. While using a coefficient peculiar for the optical fiber for associating pressure, temperature and distortion of the soil with the Brillouin frequency shift and the Rayleigh frequency shift, the distributions of the pressure, temperature and distortion of the soil along the optical fiber are calculated by analysis. Variance in a state of the soil is determined on the basis of temporal variance in at least one of the calculated distributions of the pressure, temperature and distortion of the soil.

Description

  The present invention relates to a ground condition monitoring system that detects early signs of ground condition fluctuations such as landslides using the Brillouin frequency shift and Rayleigh frequency shift phenomena of optical fibers.

  For example, Patent Document 1 discloses a conventional technique related to prediction of an earthquake or a landslide. In Patent Document 1, air vibration (noise) in an observation well is detected by a vibration sensor, a frequency spectrum of air vibration is created by processing of a CPU, and a spring judgment criterion and a frequency spectrum preset in a memory are obtained. The presence or absence of underground spring water is determined by comparison. As a result, it is possible to detect underground springs by identifying the time of occurrence at the same time as they occur, and to reliably detect springs that end in a short time, and to obtain spring monitoring data that can be used to predict natural disasters. Is disclosed.

  In Patent Document 2, as a method for detecting deformation and collapse of the ground or cliff, an optical fiber is wired so as to have a two-directional component in a two-dimensional plane in the ground, and pulsed light is incident from one end of the optical fiber. By detecting the Brillouin scattered light and Rayleigh scattered light based on the optical fiber distortion caused by the deformation of the ground after a time corresponding to the distance, the optical detection method detects the distortion of the optical fiber. It is disclosed that the deformation and collapse of the ground is detected by detecting the size of the ground and identifying the location where the distortion occurs from the time when the incident light returns.

  On the other hand, Patent Document 3 discloses a system that mainly measures pressure and temperature distributions using the Rayleigh frequency shift phenomenon in addition to the Brillouin frequency shift of an optical fiber. However, in this system, the purpose is to measure the distribution of pressure and temperature, and since the optical fiber is not fixed to the object to be measured, the strain measured by this is useless.

JP-A-8-285594 JP-A-11-166869 JP 2010-216877 A

  In the method disclosed in Patent Document 1, it is necessary to dig an observation well. However, particularly when sudden spring water is generated, a lot of rain falls, the well is blocked, and the well water is surrounded by rain or wells. It was not practical because it was affected by the effects of flowing into In addition, it is in a situation where the underground situation cannot be accurately determined only by observation of a well, and an apparatus capable of predicting landslides with higher accuracy has been required.

  Moreover, it can be said that it is difficult to detect the deformation and collapse of the ground and the cliff at an early stage by measuring only the strain according to Patent Document 2. In particular, the recent deformation and collapse of the ground and cliffs are due to concentrated torrential rain that occurs intensively in the region, and deformation and collapse occur due to the increase of groundwater flowing under the ground and cliffs.

  The present invention has been made in view of the above problems, and based on detailed examination of factors that cause landslides, it is possible to detect a sign such as landslides in various situations, and to predict landslides with higher accuracy. The purpose is to provide a monitoring system.

  The present invention provides a scattered wave acquisition unit that acquires a scattered wave obtained by scattering a pulsed laser beam incident on an optical fiber that is embedded so as to be deformed together with the ground, and a scattering wave acquired by the scattered wave acquisition unit. A Brillouin frequency shift measurement unit for measuring the Brillouin frequency shift from the wave in the optical fiber, and a Rayleigh frequency shift measurement for measuring the distribution in the optical fiber of the Rayleigh frequency shift from the scattered wave acquired in the scattered wave acquisition unit. A coefficient storage unit for storing a coefficient peculiar to an embedded optical fiber and a Brillouin frequency shift measurement unit for relating the pressure, temperature, and strain of the ground, the Brillouin frequency shift and the Rayleigh frequency shift. Brillouin frequency shift distribution measured by the Rayleigh frequency shift measurement unit Distribution along the optical fiber of ground pressure, temperature, and strain at the time when the scattered wave acquisition unit acquires the scattered wave using the distribution of the Rayleigh frequency shift and the coefficient stored in the coefficient storage unit. And a ground state determination unit that determines a change in the state of the ground based on at least one temporal variation of the distribution of pressure, temperature, and strain of the ground obtained by the analysis unit. Is.

  According to the present invention, it is possible to obtain a ground condition monitoring system capable of quickly grasping ground condition fluctuations such as a sign of landslide by accurately and simultaneously measuring the pressure, temperature and strain distribution of the ground.

It is side surface sectional drawing which shows the outline | summary of the ground condition monitoring system by Embodiment 1 of this invention. It is an expanded side view in the ground of the ground condition monitoring system by Embodiment 1 of this invention. It is an expanded transverse cross section of the sensor cable of the ground condition monitoring system by Embodiment 1 of this invention. It is a block diagram which shows an example of a structure of DPTSS of the ground condition monitoring system by Embodiment 1 of this invention. It is a flowchart which shows an example of the process regarding the monitoring of the ground condition of the ground condition monitoring system by Embodiment 1 of this invention. It is a figure which shows notionally the measurement result of the normal time of the ground condition monitoring system by Embodiment 1 of this invention. It is a figure which shows notionally the measurement result when groundwater increases by the heavy rain of the ground condition monitoring system by Embodiment 1 of this invention. It is a figure which shows notionally the measurement result when the sign of the landslide which causes heavy rain of the ground condition monitoring system by Embodiment 1 of this invention appears. It is a figure which shows notionally the measurement result of the fluctuation | variation of the temperature in the ground condition monitoring system by Embodiment 1 of this invention when the sign of the landslide resulting from heavy rain appears. It is a figure which shows notionally the measurement result of the fluctuation | variation of the temperature in the ground condition monitoring system by Embodiment 1 of this invention when the sign of the landslide resulting from the earthquake appears. It is a flowchart which shows the operation | movement flow of the ground condition monitoring system by Embodiment 1 of this invention. It is side surface sectional drawing which shows typically an example of the ground to which the ground condition monitoring system of this invention is applied. It is the schematic which shows the outline | summary of the ground condition monitoring system by Embodiment 2 of this invention. It is the schematic which shows the outline | summary of another ground condition monitoring system by Embodiment 2 of this invention.

Embodiment 1 FIG.
1 is a side sectional view showing an outline of a ground condition monitoring system according to a first embodiment of the present invention, FIG. 2 is an enlarged side sectional view in the ground 100 of FIG. 1, and FIG. 3 is an enlarged view of a sensor cable of FIG. It is a cross-sectional view. In the ground 100 where landslide may occur, there is a clay layer 103 that is an impermeable layer under the topsoil layer 101, and groundwater 102 flows on the clay layer 103. Below the clay layer 103 is a bedrock 104, for example. An observation well 3 is installed in the ground 100.

  In the observation well 3, the sensor cable 2 is embedded in the layer of the cementing 34 in order to measure the distribution of the pressure P, the temperature T in the ground 100 and the strain ε of the formation. FIG. 3 shows an example of a cross-sectional structure of the sensor cable 2. The sensor cable 2 includes a first optical fiber 21 that is affected by pressure and a second optical fiber 22 that is cut off from the influence of pressure. The second optical fiber 22 is accommodated in the metal thin tube 24 for pressure blocking. A protective cover 23 may be provided around the first optical fiber 21. However, the protective cover 23 needs to be made of a material / structure that allows the first optical fiber 21 to be affected by ambient pressure and deformation. The 1st optical fiber 21 and the metal thin tube 24 comprise the sensor cable 2 as a strand wire with the some metal wire 25, for example. Further, in order to measure the ground strain ε, the first optical fiber 21 must be fixed to the cementing 34 layer.

  Since the sensor cable 2 is embedded in the layer of the cementing 34, it is affected when a change in state occurs in the surrounding ground 100. For example, when the ground 100 fluctuates and deforms, the sensor cable 2 is deformed integrally with the cementing 34 layer. In this case, the first optical fiber 21 receives the pressure and senses the pressure, but the second optical fiber 22 accommodated in the metal thin tube 24 is not affected.

  About each of such 1st optical fiber 21 and 2nd optical fiber 22, Brillouin measurement and Rayleigh measurement are performed by the measuring apparatus 1 installed on the ground, and distribution along the optical fiber of Brillouin frequency shift and Rayleigh frequency shift is performed. Is required. From these Brillouin frequency shift and Rayleigh frequency shift distributions, the distribution of pressure, temperature, and strain along the sensor cable 2 can be known simultaneously. Therefore, the inventors named the measuring device 1 as DPTSS (Distributed Pressure Temperature Strain System) 1.

  Here, the measurement principle of pressure, temperature, and strain distribution measurement using an optical fiber will be described. When light is incident on an optical fiber and the frequency of the scattered light is analyzed, Rayleigh scattered light having substantially the same frequency as the incident light, Raman scattered light having a frequency significantly different from the incident light, and incident light having a frequency of about several to several tens of GHz. Brillouin scattered light is observed.

The Brillouin scattering phenomenon is a phenomenon in which power moves through an acoustic phonon of an optical fiber when light enters the optical fiber. The frequency difference between the incident light and the Brillouin scattered light is called the Brillouin frequency, which is proportional to the speed of sound in the optical fiber, and the speed of sound depends on the strain and temperature of the optical fiber. Thus, by measuring the change in Brillouin frequency, the strain and / or temperature applied to the optical fiber can be measured. In addition, the present inventors have confirmed that the Brillouin frequency changes depending on the pressure applied to the optical fiber. Here, the change in Brillouin frequency is referred to as Brillouin frequency shift.

  The Rayleigh scattering phenomenon is a scattering phenomenon that occurs because light is scattered due to the fluctuation of the refractive index in the optical fiber. The frequency difference between the incident light and the Rayleigh scattered light is the Rayleigh frequency. This Rayleigh frequency also varies with strain and / or temperature applied to the optical fiber. Here, the change in the Rayleigh frequency is called a Rayleigh frequency shift.

Conventionally, the Rayleigh scattering phenomenon has been considered to be sensitive only to strain and temperature. In the patent document 3 which is the previous proposal by the present inventors, the system is proposed on the assumption that the Rayleigh scattering phenomenon is sensitive only to strain and temperature. As a result of the subsequent studies by the present inventors, it has been found that the Rayleigh scattering phenomenon has sensitivity not only to strain and temperature but also to pressure, similarly to the Brillouin scattering phenomenon. That is, the Brillouin frequency shift Δν _B and the Rayleigh frequency shift Δν _R can be expressed as in the equations (1) and (2) by the pressure change amount ΔP, the temperature change amount ΔT, and the strain change amount Δε, respectively.

ここで、Ｃ_ijは光ファイバ固有の係数であり、使用する光ファイバについて、予備試験などによりこれらの係数の値を求めておくことにより、下記のように圧力変化量ΔＰ、温度変化量ΔＴ、ひずみ変化量Δεの分布を求めることができる。このレイリー周波数シフトΔν_Rに圧力の項を導入することで、より精度が高い圧力、温度、ひずみ分布測定が可能となったのである。

Here, C _ij is a coefficient specific to the optical fiber. By obtaining the values of these coefficients for the optical fiber to be used by a preliminary test or the like, the pressure change amount ΔP, the temperature change amount ΔT, The distribution of the strain change amount Δε can be obtained. By introducing a pressure term into this Rayleigh frequency shift Δν _R , pressure, temperature, and strain distribution measurements with higher accuracy became possible.

Assume that Δν _B and Δν _R are measured. In order to separate the effects of pressure P, temperature T and strain ε in the measured values, three or more independent measurement quantities are required. Since one optical fiber can only obtain two independent measurement values, Δν _B and Δν _R , by using two types of optical fibers having different sensitivities to pressure P, strain ε, and temperature T, 4 Individual independent measurements are obtained. That is, the simultaneous equations of Expression (3) are obtained.

ここで、上付きの数字は、光ファイバの種類を示している。圧力および温度は光ファイバが存在する部分の場の圧力および温度であるため、２種類の光ファイバで同一の値を持つ。一方、ひずみの値は光ファイバが周囲の物質に固定されているか否かに依存する。ＤＰＴＳＳ１ではファイバ周囲の物質のひずみを計測する必要があるので、少なくとも一つの光ファイバは周囲の物質に固定されていなければならない。

Here, the superscript number indicates the type of optical fiber. Since the pressure and temperature are the pressure and temperature of the field where the optical fiber exists, the two types of optical fibers have the same value. On the other hand, the value of strain depends on whether the optical fiber is fixed to the surrounding material. Since DPTSS1 needs to measure the strain of the material around the fiber, at least one optical fiber must be fixed to the surrounding material.

  By solving the simultaneous equations of the above equation (3), the effects of pressure P, temperature T and strain ε can be separated. Therefore, by performing hybrid measurement of measurement of Brillouin frequency shift (referred to as Brillouin measurement) and measurement of Rayleigh frequency shift (referred to as Rayleigh measurement) and solving the simultaneous equations of Equation (3), the pressure change ΔP, temperature The distribution along the optical fiber of the change amount ΔT and the strain change amount Δε can be obtained.

In Expression (3), if the superscript number 1 fiber is the first optical fiber 21 in FIG. 3 and the superscript number 2 fiber is the second optical fiber 22, the second optical fiber 22 is cut off from the influence of pressure. Therefore, Expression (3) is simplified and becomes Expression (4).

Also in the equation (4), since the pressure and temperature are the pressure and temperature of the field where the optical fiber exists, the two types of optical fibers have the same value. On the other hand, the strain ε ¹ received by the first optical fiber 21 fixed to the surrounding material is different from the strain ε ² received by the second optical fiber 22 housed in the metal thin tube 24. There are four unknowns, ΔP, ΔT, Δε ¹ , and Δε ² , but since there are four equations, these four unknowns can be obtained. However, a useful value as the strain is the strain ε ¹ of the first optical fiber 21 that directly receives the strain of the surrounding material.

Each coefficient C _ij in the equation (4) of the first optical fiber 21 and the second optical fiber 22 is obtained in advance by a preliminary test, etc., hybrid measurement of Brillouin measurement and Rayleigh measurement is performed, and simultaneous equations of the above equation (4) By solving the equation, the distribution along the optical fiber of the pressure change amount ΔP, the temperature change amount ΔT, and the strain change amount Δε can be obtained. Hybrid measurement of Brillouin measurement and Rayleigh measurement can be performed simultaneously at a certain point in time, so not only the distribution of one-dimensional pressure change ΔP, temperature change ΔT and strain change Δε along the optical fiber, but also the time axis Can also be obtained.

  Note that equation (4) is an incremental equation. That is, in order to obtain the Brillouin frequency shift and Rayleigh frequency shift on the left side, two measurements are required: a reference measurement in the initial state and a main measurement after the state changes. Further, what is obtained by solving the equation (4) is the amount of change in each of pressure, temperature, and strain based on the initial state. When absolute amounts of pressure, temperature, and strain are required, the absolute amounts of the respective distributions of pressure, temperature, and strain at the time of initial measurement are measured by some method.

  The initial state can be arbitrarily selected. Speaking of ground condition monitoring, before installing a cable in the observation well 3, initial measurement can be performed in a constant temperature room on the ground. In this case, a state in which the pressure and temperature distribution are uniform and constant can be set as the initial state.

  Alternatively, the state when the cable is installed in the observation well 3 can be set to the initial state. In this case, the amount of change in pressure, temperature, and strain due to changes in the ground condition can be directly obtained by solving Equation (4). When absolute amounts of pressure, temperature, and strain are required, the absolute amount distribution of pressure and temperature measured with an electric sensor or the like when the observation well 3 is installed may be used. If there is initial measurement data in the constant temperature room on the ground, the absolute quantity distribution of pressure and temperature can be obtained from the measurement when the observation well 3 is installed.

  By collecting changes in the absolute amount of pressure P, temperature T, and strain ε, or by collecting data on the amount of change, the state change and distribution of the ground 100 can be monitored. For example, the temperature change in the groundwater 102 and the deformation of the ground 100 Etc. can be monitored.

  FIG. 4 is a block diagram showing an outline of an example of DPTSS1. The scattered wave acquisition unit 11 acquires the scattered wave scattered by the optical fiber. The acquired scattered wave is analyzed by the Brillouin frequency shift measuring unit 12 to measure the Brillouin frequency shift. At this time, the Brillouin frequency shift is measured as a distribution along the length direction of the optical fiber. Similarly, the Rayleigh frequency shift measurement unit 13 measures the Rayleigh frequency shift. The Rayleigh frequency shift is also measured as a distribution along the length of the optical fiber.

The coefficient storage unit 14 stores in advance the coefficient C _ij of Expression (4) obtained by a preliminary test or the like. Using the measured Brillouin frequency shift and Rayleigh frequency shift and the coefficient stored in the coefficient storage unit 14, the analysis unit 15 uses the equation (4) to calculate the pressure change ΔP, the temperature change ΔT, and the strain change Δε. Is analyzed and stored in the distribution data storage unit 16. The above measurement and analysis are executed at predetermined time intervals, and are stored in the distribution data storage unit 16 as distribution data of changes in pressure, temperature, and strain for each time. In the initial measurement, when the absolute amount of the initial distribution of pressure, temperature, and strain is measured, if this value is stored in the distribution data storage unit 16, the absolute value at each time point is combined with the distribution data of the change amount. Quantity distribution data is obtained. The ground state determination unit 17 determines the state of the ground 100 based on the pressure, temperature, strain change over time, and monitors, for example, a sign of landslide.

An example of the process related to the monitoring of ground condition fluctuation in the system of FIG. 1 is shown in the flowchart of FIG. First, two types of optical fibers to be installed as the first optical fiber 21 and the second optical fiber 22 are prepared, the characteristics of each optical fiber are measured by an indoor test or the like, and the coefficients C _ij of the equation (4) are calculated. Determine (ST1). Each determined coefficient is stored in the coefficient storage unit 14 of DPTSS1, for example. As the first optical fiber 21 and the second optical fiber 22 of the sensor cable 2 with the two types of optical fibers for which the respective coefficients have been determined, first, the initial measurement 1 is performed under constant pressure and temperature conditions in a temperature-controlled room, The Brillouin reference spectrum and the Rayleigh reference spectrum, which are the reference for the Brillouin frequency shift and the Rayleigh frequency shift, are measured (ST2). Next, the sensor cable 2 is installed in the observation well 3 in the ground 100 with the configuration shown in FIGS. 1 to 3 (ST3).

  After the installation of the sensor cable 2 is completed, the Brillouin reference spectrum and the Rayleigh reference spectrum, which are references for the Brillouin frequency shift and the Rayleigh frequency shift, are measured as the initial measurement 2 (ST4). When absolute amounts of pressure and temperature are required, the Brillouin frequency shift and the Rayleigh frequency shift are obtained from the measurement data of the initial measurement 1 and the initial measurement 2, and the pressure change ΔP and the temperature change using the simultaneous equations (4) The distribution of the quantity ΔT is calculated. Subsequently, the absolute amount distribution of the pressure and temperature at the initial measurement 2 is obtained using the pressure and temperature of the temperature-controlled room at the initial measurement 1.

After the sensor cable 2 is installed, the Brillouin frequency shift measurement unit 12 and the Rayleigh frequency shift measurement unit 13 measure the Brillouin spectrum and the Rayleigh spectrum, and take the difference from the measurement data of the initial measurement 2 to obtain the Brillouin frequency shift Δν ^1. _B , Δν ² _B (ST5) and Rayleigh frequency shifts Δν ¹ _R , Δν ² _R (ST6) are obtained, and the pressure change amount ΔP, temperature change amount ΔT, and strain change amount using the simultaneous equation (4) in the analysis unit 15. The distribution of Δε is calculated (ST7). ST5, ST6, and ST7 are executed at predetermined time intervals in DPTSS1 as described above, and are stored in the distribution data storage unit 16 as data for each time. The pressure change amount ΔP, the temperature change amount ΔT, and the strain change amount Δε calculated here are the respective change amounts from 2 o'clock of the initial measurement. By collecting changes in the absolute amount of the distribution of pressure P, temperature T and strain ε, or data on the amount of change, it is possible to measure ground and cliff status changes in real time due to sudden environmental changes such as rainfall. In addition, the ground state determination unit 17 can monitor and determine the ground state variation (ST8), and can give warnings such as landslides at an early stage.

  Here, the mechanism by which landslide occurs will be described. The causes of landslides can be divided into predisposition and incentive. The predisposition is that the terrain, geology, geological structure, moisture geological conditions, etc. where the landslide occurs are in a state where the landslide is likely to occur. That is, there are some regulation conditions for regulating landslides in places where landslides occur. Incentives are triggers that cause landslides, and are divided into natural and artificial incentives. As natural causes, there are many landslides that are generally caused by an increase in groundwater pressure caused by rainfall or melting snow. Other causes include the loss of earth and sand at the end of the landslide due to small-scale collapse or scouring by rivers, snow loads and earthquakes. Artificial incentives include cuts and embankments on slopes, civil engineering such as tunnel excavation, and dam flooding. Thus, a landslide is generated when an action as an incentive is generated on a slope having a predisposition such as a regulation condition in which the landslide is originally generated.

  Therefore, first, the observation well 3 of the ground condition monitoring system according to the present invention is installed in a place having a predisposition to landslide. Furthermore, for each incentive, the precursor of the ground condition when the landslide occurs is analyzed, and a warning is issued when the ground condition monitoring system monitors the change of the ground condition and grasps the precursor.

  An example will be described in which groundwater rapidly increases due to heavy rain, etc., and the topsoil layer 101 slips and landslides occur. When the groundwater rapidly increases, the groundwater 102 layer shown in FIG. 1 increases, and the temperature of the groundwater 102 changes due to a rapid inflow of groundwater. This will be described below. An example of the distribution of underground temperature, pressure, and strain at time t0, which is normal, is shown in FIG. Regarding the temperature, the surface temperature is greatly affected by the outside air temperature. In this example, the surface temperature is about 30 ° C. In the topsoil layer 101, the temperature first decreases rapidly, then increases once, and then the temperature tends to decrease in proportion to the depth. On the other hand, in the portion of the groundwater 102 and the clay layer 103, it can be observed that the temperature changes in a complicated manner. From the time of entering the bedrock 104, the temperature seems to be substantially constant. In the winter, when the outside air temperature becomes low, the temperature near the ground surface is around the outside air temperature, but when entering the bedrock 104, no significant change occurs throughout the year.

  A situation such as a temperature change in the case of sudden heavy rain in such an environment will be described. FIG. 7 shows an example of distribution of underground temperature, pressure, and strain when heavy rain falls and groundwater increases. When the groundwater 102 increases and reaches the time ti, the temperature from the ground surface to the bedrock decreases compared to the time t0 before heavy rain as shown in the temperature distribution shown in FIG. The temperature gradient becomes steep. FIG. 7 shows a state in which no signs of landslide appear yet, and pressure and strain do not change significantly from normal.

When heavy rain continues and at time tc, as shown in FIG. 8, when pressure or strain increases around the groundwater 102, that is, the topsoil layer 101 on the clay layer 103, a sign of landslide appears. In FIG. 9, the temperature difference between the normal time t0 and the time ti when the groundwater 102 increased, the pressure difference at the time t0 when the landslide sign appears, and the landslide The strain at time tc when the sign appears is shown. As described above, when a temperature difference at a predetermined time interval ty is detected and the temperature difference at a location above the clay layer 103 changes by a predetermined temperature, the landslide danger flag is set as a landslide risk. The distribution of the pressure P is also measured at every predetermined time interval ty. When the landslide danger flag is set, the distribution is measured at every predetermined time interval tz shorter than ty, and the pressure difference at the predetermined time interval tz is greater than or equal to a predetermined value. In such a case, it is determined that the possibility of landslide has become very high, and a landslide warning is issued. Alternatively, the strain distribution may also be measured at a predetermined time interval tz with the landslide danger flag set, and a landslide warning may be issued when the magnitude of the strain exceeds a predetermined value. .

The above is the situation of monitoring regarding the occurrence of landslides due to heavy rain, but there are cases where earthquakes occur and landslides occur in situations where there is no rain. In this case, as shown in FIG. 10, at a time t0 during normal times and a time tc at which signs of landslide appear are shown.
1) Temperature change hardly occurs.
2) A large change in pressure or strain appears.
Therefore, the occurrence of landslide can be predicted when the value of pressure change, strain change, or strain itself increases.

  FIG. 11 shows a flow for predicting and issuing a warning for both a landslide caused by heavy rain and a landslide caused by an earthquake. First, the normal monitoring time interval ty until the danger flag is set and the monitoring time interval tz when the danger flag is set are set (step ST11). For example, ty is 1 hour, and tz is 1 minute, for example. When the measurement is started at the time t0 in the normal time, the distribution of the temperature T, the pressure P, and the strain ε is measured at the time t0, and the measured values are stored in the distribution data storage unit 16 as Tt0, Pt0, and εt0. . Further, the measurement number i is set to 1 (step ST12). Next, when the time becomes t1 = t0 + ty, T, P, and ε are measured again, and the measured values are stored in the distribution data storage unit 16 as Tt1, Pt1, and εt1. (Step ST13). Using the measured value, it is determined whether or not the absolute value of the difference between Tt1 and Tt0 of the temperature distribution data is smaller than a predetermined difference Ta (step ST14). Since the difference is obtained as a distribution in the depth direction, it may be determined whether or not the maximum value is smaller than the predetermined difference Ta.

  If the difference is smaller than Ta (YES in step ST14), it is next determined whether or not the absolute value of the difference between the pressure data Pt1 and Pt0 is smaller than a predetermined value Pa (step ST15). Similar to the temperature, the difference is obtained as a distribution in the depth direction. Therefore, it may be determined whether or not the maximum value is smaller than the predetermined value Pa. If the difference is smaller than Pa (YES in step ST15), it is next determined whether or not the absolute value of the difference between εt1 and εt0 of the strain data is smaller than a predetermined value εa (step ST16). Similar to the temperature, the difference is obtained as a distribution in the depth direction, so it is only necessary to determine whether the maximum value is smaller than a predetermined value εa.

  When the difference is smaller than εa (YES in step ST16), it is determined that there is no risk of landslide. When monitoring is not terminated (NO in step ST17), the measurement number i is incremented by 1 (step ST18), and the time The distribution of the temperature T, pressure P, and strain ε is measured at t2 = t1 + ty, and the measured values are stored in the distribution data storage unit 16 as Tt2, Pt2, and εt2. Using the measurement value at time t2 and the measurement value at time t1, the determinations in steps ST04 to ST07 are performed. In this way, measurement is performed at every predetermined time interval ty, and determination is performed by obtaining a difference from the previous measurement value.

  If the difference between Tti and Tti-1 is greater than or equal to Ta in step ST14 (NO in step ST14), if the difference between Pti and Pti-1 is greater than or equal to Pa in step 15 (step ST15 NO), or step 16 If the difference between εti and εti-1 is equal to or greater than εa (NO in step ST16), a danger flag is set because there is a risk of landslide. Moving to step ST19, the monitoring time interval tz is set, and the distribution of T, P, and ε is measured at every tz interval (step ST20). The measured value is compared with the previous measured value when the danger flag was set. If the measurement number when the danger flag is set is i, it is determined that the possibility of landslide has increased greatly due to the difference from the measurement value at time ti-1, and an alarm is issued.

  In order to execute this specifically, first, the time ti when the danger flag is set is set to tc (step ST19), and when the time becomes tc + tz, the time at this time is set to tc again. The distribution of T, P, and ε is measured (ST20). If the absolute value of the difference between the pressure Ptc at time tc and the pressure Pti-1 at time ti-1 is greater than or equal to a predetermined value Pb (step ST21 NO), an alarm is issued. If the absolute value of the difference between the pressure Ptc and the pressure Pti-1 is smaller than a predetermined value Pb, the absolute value of the difference between the strain εtc at time tc and the strain εti-1 at time ti-1 is evaluated, and the difference is a predetermined value. If εb or more (NO in step ST22), an alarm is issued. If the strain difference is also smaller than the predetermined value εb (YES in step ST22), tc is increased by tz for a predetermined time tb after setting the danger flag, and step ST20 to step ST22 are repeated every time interval tz.

  If tc has reached the time when the predetermined time tb has elapsed from the time ti at which the danger flag is set (YES in step ST23), it is determined that there is no possibility of landslide, and the normal monitoring is resumed. That is, in step ST24, the time tc is set to an initial value, that is, t0, the measurement number i is set to 1, and monitoring is started from step ST13.

FIG. 11 shows a flow for predicting and issuing a warning for both a landslide triggered by heavy rain and a landslide triggered by an earthquake. However, a plurality of similar flows are operated in parallel. By making at least one value of tz and tb different, prediction accuracy such as landslide can be further improved.
Further, in the above description, the clay layer 103 is provided in one place, and the ground on which the groundwater 102 flows is described as an example. However, as shown in the schematic diagram of FIG. There may be multiple layers. The ground water 102 flows over each clay layer 103, and even in such a ground, as described above, temperature, pressure, and strain distribution measurement can be performed and the same ground state determination can be performed. it can.

  As described above, temperature, pressure, and strain distributions are measured at predetermined time intervals, and ground state fluctuations are determined and alerted based on at least one of the pressure, temperature, and strain distributions over time. As a result, the landslides triggered by heavy rains and landslides triggered by earthquakes other than heavy rains can be detected at an early stage. Therefore, measures to prevent landslides, such as driving piles, can be made earlier.

Embodiment 2. FIG.
FIG. 13 is a schematic diagram showing an overview of the ground condition monitoring system according to the second embodiment of the present invention. As shown in FIG. 13, a plurality of observation wells 3 are installed on the ground having a landslide predisposition, sensor cables 2 are embedded in each, and the temperature along each sensor cable 2 is changed while switching with an optical switch 18 or the like. The distribution of pressure and strain may be measured by the measuring device DPTSS1.

  A plurality of observation wells 3 may be provided in the lateral direction of the slope as shown in FIG. 13, or a plurality of observation wells 3 may be provided in the vertical direction of the slope as shown in FIG. Further, a plurality of them may be provided obliquely, and the direction of the hole of each observation well 3 may be dug obliquely in the ground. Thus, what is necessary is just to determine the arrangement | positioning of the observation well 3 and the direction of a hole according to topography.

In FIGS. 13 and 14, separate sensor cables 2 are embedded in the respective observation wells 3 and switched by an optical switch 18 or the like, and distribution of temperature, pressure, and strain along each sensor cable 2 is measured. However, one continuous sensor cable may be folded and embedded in the plurality of observation wells 3, for example. In this case, the position of the sensor cable 2 and the position of each observation well 3 in the depth direction may be related, and the measurement value may correspond to the distribution in the depth direction of each observation well 3.

  As described above, by installing a plurality of observation wells 3 and measuring the distribution of temperature, pressure, and strain in each observation well 3, it is possible to monitor the ground condition with higher accuracy.

  In the above, the ground condition monitoring system has been described by taking an example of grasping a sign of landslide. However, according to the present invention, the ground condition monitoring system is a discontinuous surface in the ground, such as an uplift or subsidence of the ground surface, or a displacement on a fault surface. It can also be applied to status monitoring. It can also be applied to monitoring ground changes at deep underground locations, such as plate changes at the plate boundary.

  1 DPTSS, 2 sensor cables, 3 observation wells, 11 scattered wave acquisition unit, 12 Brillouin frequency shift measurement unit, 13 Rayleigh frequency shift measurement unit, 14 coefficient storage unit, 15 analysis unit, 16 distribution data storage unit, 17 ground condition determination Part 21, 21 first optical fiber, 22 second optical fiber, 23 protective cover, 24 metal thin tube, 25 metal wire, 34 cementing, 100 ground, 101 topsoil layer, 102 groundwater, 103 clay layer (impermeable layer), 104 bedrock

Claims (8)

A scattered wave acquisition unit that acquires a scattered wave that is scattered in the optical fiber by a pulsed laser beam incident on an optical fiber embedded so as to be deformed together with the ground;
A Brillouin frequency shift measurement unit that measures the distribution in the optical fiber of the Brillouin frequency shift from the scattered wave acquired in the scattered wave acquisition unit;
A Rayleigh frequency shift measurement unit for measuring the distribution in the optical fiber of the Rayleigh frequency shift from the scattered wave acquired in the scattered wave acquisition unit;
A coefficient storage unit for storing a coefficient specific to the optical fiber for relating the pressure, temperature, and strain of the ground to the Brillouin frequency shift and the Rayleigh frequency shift;
The scattering using the Brillouin frequency shift distribution measured in the Brillouin frequency shift measurement unit, the Rayleigh frequency shift distribution measured in the Rayleigh frequency shift measurement unit, and the coefficient stored in the coefficient storage unit. An analysis unit for obtaining a distribution along the optical fiber of the pressure, temperature, and strain of the ground at the time of acquiring the scattered wave in the wave acquisition unit;
A ground state determination unit that determines a change in the state of the ground based on at least one temporal variation among the distribution of pressure, temperature, and strain of the ground obtained by the analysis unit. Ground condition monitoring system.   The said optical fiber is comprised by the 1st optical fiber hold | maintained so that it may receive to the influence of pressure, and the 2nd optical fiber hold | maintained so that it may be interrupted | blocked from the influence of pressure, The optical fiber is comprised. The ground condition monitoring system according to 1.   The optical fiber is embedded so as to include at least a formation through which groundwater flows in the ground, and the ground state determination unit includes at least one temporal variation among pressure, temperature, and strain distribution obtained by the analysis unit. The ground condition monitoring system according to claim 1 or 2, wherein a sign of a landslide of the ground is grasped based on the ground.   The ground condition monitoring system according to claim 3, wherein a plurality of the optical fibers are embedded at different positions on the ground.   The ground state monitoring system according to claim 4, wherein the scattered wave acquisition unit switches scattered waves from the plurality of optical fibers to acquire scattered waves of the respective optical fibers.   The ground state monitoring system according to any one of claims 3 to 5, wherein the ground state determination unit outputs an alarm signal when grasping a sign of a landslide of the ground. A scattered wave acquisition step in which a pulsed laser beam incident on an optical fiber embedded so as to be deformed together with the ground acquires a scattered wave scattered in the optical fiber;
A Brillouin frequency shift measurement step of measuring the distribution in the optical fiber of the Brillouin frequency shift from the scattered wave acquired in the scattered wave acquisition step;
Rayleigh frequency shift measurement step of measuring the distribution in the optical fiber of the Rayleigh frequency shift from the scattered wave acquired in the scattered wave acquisition step;
Distribution of Brillouin frequency shift measured in the Brillouin frequency shift measurement step, distribution of Rayleigh frequency shift measured in the Rayleigh frequency shift measurement step, pressure, temperature, and strain of the ground, Brillouin frequency shift and Using the coefficient specific to the optical fiber for relating the Rayleigh frequency shift, the pressure, temperature, and strain of the ground at the time of acquiring the scattered wave in the scattered wave acquisition step, to the optical fiber Analysis process to find the distribution along the line,
A ground condition determination step for determining a state of fluctuation of the ground based on at least one temporal variation among the distribution of pressure, temperature and strain of the ground determined in the analysis step. Status monitoring method.   The optical fiber is embedded so as to include at least a formation through which groundwater flows in the ground, and in the ground state determination step, at least one of the pressure, temperature, and strain distribution obtained in the analysis step changes with time. The ground condition monitoring method according to claim 7, wherein a sign of landslide of the ground is grasped based on the ground.

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