Subsurface structure estimation method and subsurface structure estimation system

Abstract

To improve estimation accuracy of an underground structure.SOLUTION: In the underground structure estimation method, micromotion of the soil at observation points P1 to P6 is observed by a micromotion array 10, and at least an S-wave speed Vs1 in a base layer 1 and a depth H to an upper surface 1a of the base layer 1 are calculated by inverse analysis using a relationship between a frequency and a phase speed of a wave propagating through the soil G from the observed micromotion, thereby estimating an underground structure at the observation points P1 to P6. In the subsurface structure estimation method, predetermined observation points among the plurality of observation points P1 to P6 are grouped as group observation points Pg, inverse analysis is performed assuming that the S-wave velocities Vs1 in the base layer 1 at all the grouped group observation points Pg are the same, and the S-wave velocities Vs1 in the base layer 1 and the depth H to the upper surface 1a of the base layer 1 at each of the group observation points Pg are calculated.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and system for estimating underground structure. [Background technology]

[0002] Patent Document 1 discloses a method for estimating underground structures using a microtremor array (microtremor array exploration). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-287865 Summary of the Invention [Problem to be solved by the invention]

[0004] In the invention disclosed in Patent Document 1, microtremor meters are installed in a triangular or circular pattern, and correlation analysis is performed on the waves propagating through the observed ground to calculate the relationship between frequency and phase velocity. Furthermore, the calculated relationship between frequency and phase velocity is used to perform inverse analysis to determine the S-wave velocity in each layer and the depth of each layer.

[0005] When using such microtremors to estimate underground structures, the accuracy of the estimation can be determined by, for example, confirming that the S-wave velocity of the basement layer is approximately the same at nearby observation points, or by confirming that the depth to the top surface of the basement layer is approximately the same as the depth at which a clear change in the N-value occurs in a standard penetration test.

[0006] However, with microtremor array surveys, noise such as traffic vibrations in the surrounding area can limit the frequency band of the phase velocity that is targeted in the inverse analysis. Also, the large number of unknown parameters that need to be identified in the inverse analysis can lead to unstable solutions, resulting in large differences in the S-wave velocity of the bedrock at adjacent observation points, or the depth to the top surface of the bedrock deviating from the N-value obtained by the standard penetration test, which can reduce the accuracy of estimating the underground structure.

[0007] The present invention aims to improve the accuracy of estimating underground structures. [Means for solving the problem]

[0008] The present invention is a method for estimating underground structure that observes microtremors in the ground at an observation point using a microtremor array, and calculates at least the S-wave velocity in the basement layer and the depth to the top surface of the basement layer by inverse analysis using the relationship between the frequency and phase velocity of waves propagating through the ground from the observed microtremors, thereby estimating the underground structure at the observation point.Predetermined observation points from among multiple observation points are grouped as group observation points, and inverse analysis is performed assuming that the S-wave velocities in the basement layer of all group observation points in the group are the same, and the S-wave velocity in the basement layer and the depth to the top surface of the basement layer are calculated for each group observation point. [Effects of the Invention]

[0009] In this invention, the relationship between wave frequency and phase velocity at multiple observation points is used to simultaneously perform inverse analysis under the condition that the S-wave velocity of the basement layer is the same. Therefore, even if there is an observation point where phase velocity over a sufficient frequency range cannot be evaluated, the accuracy of estimating the underground structure can be improved by making efforts to reduce the unknown parameters in the inverse analysis. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a ground structure estimation system according to the present invention. [Figure 2]FIG. 2 is a diagram showing the arrangement of vibration sensors in a microtremor array according to the present invention. [Figure 3] FIG. 3 is a diagram showing an example of dispersion characteristics calculated by inverse analysis. [Figure 4] FIG. 4 is a diagram showing an example of an S-wave velocity structure of the ground. [Figure 5] FIG. 5 is a flowchart showing the flow of the method for estimating an S-wave velocity structure in the ground structure estimation system according to the present invention. [Figure 6] FIG. 6 is a diagram showing an example of dispersion characteristics calculated assuming that the S-wave velocities of the basement layer are the same at multiple observation points. [Figure 7] FIG. 7 is a diagram showing an example of an S-wave velocity structure of the ground estimated by assuming that the S-wave velocities of the basement layer are the same at multiple observation points. DETAILED DESCRIPTION OF THE INVENTION

[0011] The ground structure estimation system 100 according to the present invention will be described below.

[0012] FIG. 1 is a schematic diagram of a ground structure estimation system 100 according to the present invention. The ground structure estimation system 100 according to this embodiment simultaneously observes microtremors of the ground G using multiple microtremor arrays 10 installed on the ground surface S, and performs correlation analysis using the observed microtremors to determine the relationship between frequency and the phase velocity of Rayleigh waves. The S-wave velocity structure (underground structure) of the ground G is estimated based on the reproducibility of the relationship between frequency and phase velocity. For ease of explanation, this embodiment will be described using an example in which the ground G is composed of two layers: a basement layer 1 and a topsoil layer 2 located closer to the ground surface S than the basement layer 1, and the basement layer 1 is an engineered basement. The engineered basement refers to a hard ground that serves as the basis for seismic motion estimation when designing a structure, such as a stratum with an S-wave velocity Vs1 of 400 m / s or more.

[0013] As shown in FIG. 1, the ground structure estimation system 100 includes a plurality of microtremor arrays 10 installed on the ground surface S, and an analysis device 20 (computing device) that calculates the relationship between frequency and phase velocity using microtremor data measured by the microtremor arrays 10 and analyzes the S-wave velocity structure of the ground G.

[0014] 2(A), the microtremor array 10 is composed of, for example, four vibration sensors 11. The four vibration sensors 11 are arranged at the vertices of an equilateral triangle and at the center of the triangle. The microtremor array 10 observes the vertical component of the microtremors.

[0015] Multiple microtremor arrays 10 are installed at appropriate intervals within the site to be surveyed. In the example shown in Fig. 1, microtremor arrays 10 are installed at six observation points P1 to P6, but the number of installation locations and the intervals between the microtremor arrays 10 can be selected as appropriate.

[0016] The frequency range of the dispersion characteristics obtained by the microtremor array 10 is determined by the spacing of the vibration sensors 11 in the microtremor array 10. For this reason, in this embodiment, for example, the frequency range is divided into three or four based on the exploration depth of the ground G, and the spacing of the vibration sensors 11 is changed according to these frequency ranges to measure the microtremors respectively.

[0017] In this embodiment, the microtremor array 10 is configured with four vibration sensors 11, but the number of vibration sensors 11 is not limited to four and can be changed as appropriate. For example, as shown in FIG. 2(B), seven vibration sensors 11 may be placed at the vertices and centers of two triangles of different sizes, and observations may be performed simultaneously. Furthermore, the positions at which the vibration sensors 11 are placed are not limited to the positions corresponding to the vertices of an equilateral triangle. For example, seven vibration sensors 11 may be placed at the center of a circle of a predetermined radius, and six sensors 11 may be placed at 60° intervals on the circumference of this circle of the predetermined radius. Furthermore, multiple vibration sensors 11 may be placed on each of the circumferences of multiple circles of different radii.

[0018] The analysis device 20 receives waveform data of microtremors observed by the microtremor array 10. The analysis device 20 calculates the relationship between frequency and phase velocity based on the waveform data of the microtremors, and calculates the S-wave velocity structure of the ground G by inversion analysis based on the reproducibility of this relationship. This will be explained in detail below.

[0019] The analysis device 20 calculates the dispersion characteristics of Rayleigh waves by the spatial autocorrelation method (so-called SPAC method) using the microtremor data observed by the microtremor array 10. The dispersion characteristics show the relationship between frequency F and phase velocity PV as shown in Figure 3. "O" in Figure 3 indicates observed data. The theoretical dispersion characteristics Lr in Figure 3 are the theoretical relationship between frequency F and phase velocity PV calculated from the S-wave velocity structure of the ground G.

[0020] In the ground structure estimation system 100 of this embodiment, the S-wave velocity structure of the ground G is estimated by performing inverse analysis to find an S-wave velocity structure in which the theoretical dispersion characteristic Lr matches the observed dispersion characteristic Lo. In this embodiment, the inverse analysis is performed using the S-wave velocity Vs1 of the base layer 1, the S-wave velocity Vs2 of the topsoil layer 2, and the thickness of the topsoil layer 2, i.e., the depth H from the ground surface S to the top surface 1a of the base layer 1, as parameters. A genetic algorithm is used as the inverse analysis method. Since genetic algorithms are well known, a description thereof will be omitted here. Note that the inverse analysis method is not limited to this, and the least squares method may also be used.

[0021] The S-wave velocity structure of ground G obtained in this way is shown in Figure 4. The horizontal axis of Figure 4 is S-wave velocity Vs, and the vertical axis is depth H from the ground surface S. In the example shown in Figure 4, the depth H to the top surface 1a of the basement layer 1 is H1.

[0022] In this embodiment, the S-wave velocity structure is estimated at each of observation points P1 to P6 (see FIG. 1) within the site to be investigated. Note that, hereinafter, when the observation points P1 to P6 do not refer to a specific observation point, they will simply be referred to as "observation point P."

[0023] At nearby observation points P (e.g., observation points P1 and P2 in Figure 1), the S-wave velocity Vs1 in the basement layer 1 is likely to be similar. The depth H of the basement layer 1 is also likely to roughly correspond to the depth of the bearing layer determined by a standard penetration test. However, due to factors such as the influence of traffic vibrations in the vicinity, it may be impossible to evaluate the phase velocity over a sufficient frequency range at either observation point from the observed microtremor waveform data. Furthermore, when estimating the S-wave velocity Vs1 of the basement layer 1, the S-wave velocity Vs2 of the topsoil layer 2, and the thickness of the topsoil layer 2 (depth H from the ground surface S to the top surface 1a of the basement layer 1) at observation points P1 and P2, a total of six unknown parameters are required. In this way, when data are available only at limited frequencies and the number of unknown parameters is large, the S-wave velocity Vs1 of the basement layer 1 estimated at observation points P1 and P2 may differ significantly. Furthermore, the thickness of the topsoil layer 2 estimated at the observation points P1 and P2 may not correspond to the bearing layer depth based on the N-value obtained by the standard penetration test.

[0024] In this way, when the S-wave velocity structure is estimated based on the observation data obtained at each observation point P, there is a risk that problems may arise in the estimation accuracy of the S-wave velocity structure in the ground G.

[0025] Therefore, in the ground structure estimation system 100 of this embodiment, a plurality of observation points P are grouped, constraint conditions are set, and inverse analysis is simultaneously performed for the grouped observation points P. The method for estimating an S-wave velocity structure in the ground structure estimation system 100 of this embodiment will be specifically described below with reference to the flowchart shown in FIG.

[0026] First, in step S1, an inverse analysis is performed at each of the observation points P1 to P6. Specifically, the relationship between frequency and phase velocity is calculated based on the microtremor data observed by the microtremor array 10, and the above-mentioned inverse analysis is performed to calculate the S-wave velocity Vs1 of the basement layer 1, the S-wave velocity Vs2 of the topsoil layer 2, and the depth H to the top surface 1a of the basement layer 1 at each of the observation points P1 to P6.

[0027] In step S2, the observation points P1 to P6 are grouped. Specifically, the observation points P1 to P6 whose S-wave velocities Vs1 of the basement layer 1 calculated in step S1 are within a predetermined range (for example, the observation points P whose difference in the S-wave velocities Vs1 of the basement layer 1 is within a range of approximately 100 m / s to 200 m / s) are grouped. Note that, hereinafter, the grouped observation points P are referred to as "group observation points Pg."

[0028] In step S3, the S-wave velocity Vs1 of the basement layer 1, the S-wave velocity Vs2 of the topsoil layer 2, and the depth H to the top surface 1a of the basement layer 1 at each group observation point Pg are recalculated. Specifically, the S-wave velocity Vs1 in the basement layer 1 at all group observation points Pg is set as a common value (S-wave velocity Vs1com), and inverse analysis is performed simultaneously for all group observation points Pg to calculate the S-wave velocity Vs1com of the basement layer 1, the S-wave velocity Vs2 of the topsoil layer 2, and the depth H to the top surface 1a of the basement layer 1 at each group observation point Pg.

[0029] In the ground structure estimation system 100 of this embodiment, the underground structure (S-wave velocity structure) of the ground G is estimated in this manner.

[0030] For example, Figs. 6 and 7 show examples of the results of inverse analysis performed according to the flowchart shown in Fig. 5, with the observation points P1, P2, P5, and P6 in Fig. 1 grouped together. If the observation points P1, P2, P5, and P6 were inversely analyzed separately in step S1 of the flowchart shown in Fig. 5, the number of unknowns would be 12. However, if the S-wave velocity Vs1 of the basement layer 1 is set to a common value (S-wave velocity Vs1com) in step S3 of the flowchart shown in Fig. 5, the number of unknown parameters can be reduced to 9. Furthermore, at observation point P2, phase velocities on the low-frequency side are not available compared to the other observation points P1, P5, and P6, and this missing data can be supplemented at observation points P1, P5, and P6.

[0031] In the S-wave velocity structure estimation method of this embodiment, as shown in FIG. 7, the depth H of the top surface 1a of the basement layer 1 at observation point P2 is 13 m, which is shallower than the other observation points P1 (depth H = 16 m), P5 (depth H = 21 m), and P6 (depth H = 22 m). The S-wave velocity Vs1com at the basement layer 1 is 545 m / s, which satisfies the engineering foundation requirement of 400 m / s or greater. The depths H of the top surface 1a of the basement layer 1 at observation points P1, P2, P5, and P6 were confirmed to correspond to existing ground investigation data such as standard penetration tests and soil columnar profiles, and are therefore considered appropriate. Furthermore, when the S-wave velocity structures at observation points P1, P2, P5, and P6 were separately estimated (the results of step S1 in the flowchart shown in FIG. 5), the depth H of the top surface 1a of the basement layer 1 was estimated to be deeper at observation point P1 than at observation point P2, which did not correspond to the existing ground investigation data. In addition, there was a difference of approximately 150 m / s in the S-wave velocity Vs1 of basement layer 1 between observation points P1 and P2.

[0032] As described above, in the method for estimating an S-wave velocity structure according to this embodiment, inverse analysis is simultaneously performed at each group observation point Pg, assuming that the S-wave velocity Vs1 of the basement layer 1 is the same. By sharing data acquired by the microtremor array 10 at each group observation point Pg, the number of unknown parameters can be reduced, enabling stable inverse analysis. As a result, even if there is a group observation point Pg (observation point P2 in the examples shown in FIGS. 6 and 7 ) for which phase velocities in a sufficient frequency band have not been acquired, inverse analysis is performed taking into account low-frequency data from other group observation points Pg. This reduces the effect of errors due to a lack of low-frequency data, thereby improving the estimation accuracy of the S-wave velocity structure (underground structure) of the ground G.

[0033] In the above embodiment, first, an inverse analysis is performed for each of the observation points P1 to P6 (step S1), and then the observation points P1 to P6 are grouped based on the results (step S2). However, this is not limiting. For example, the observation points P1 to P6 may be grouped based on the distance between them. In this case, step S1 can be omitted.

[0034] Furthermore, in the above embodiment, the dispersion characteristics of the frequency F and the phase velocity PV are calculated by the spatial autocorrelation method, but instead, they may be calculated by the frequency-wavenumber spectrum method.

[0035] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments.

[0036] In the above embodiment, an example was described in which the ground G has two layers, but in reality, the number of layers is determined, for example, through a boring survey, and the S-wave velocity Vs of each layer is included as a parameter to perform an inverse analysis.

[0037] In addition, in the above embodiment, the case where the basement layer 1 is an engineered basement (where the S-wave velocity Vs1 is 400 m / s or more) has been described as an example, but this is not limiting, and the basement layer 1 may be a stratum where the S-wave velocity Vs1 is less than 400 m / s. In other words, the underground structure estimation method of the above embodiment can also be applied to a stratum where the S-wave velocity Vs1 of the basement layer 1 is less than 400 m / s. [Explanation of symbols]

[0038] 100···Ground structure estimation system 1. Foundation layer 1a...Top surface 2... Topsoil layer 10. Microtremor array 11. Vibration sensor 20...Analysis device (computation device) G···Ground H: Depth P, P1, P2, P3, P4, P5, P6... Observation points Pg Group observation point S...Surface

Claims

1. A method for estimating an underground structure at an observation point by observing microtremors of the ground at an observation point using a microtremor array, and calculating at least an S-wave velocity in a basement layer and a depth to an upper surface of the basement layer by inverse analysis using a relationship between the frequency and phase velocity of waves propagating through the ground from the observed microtremors, grouping predetermined observation points among the plurality of observation points as a group observation point; An underground structure estimation method in which the inverse analysis is performed assuming that the S-wave velocities in the basement layer of all the group observation points are the same, and the S-wave velocities in the basement layer and the depth to the top surface of the basement layer of each of the group observation points are calculated.

2. 2. The underground structure estimation method according to claim 1, performing the inverse analysis at each of the plurality of observation points to calculate the S-wave velocity in the basement layer; An underground structure estimation method in which, among the plurality of observation points, the observation points whose S-wave velocity in the basement layer is within a predetermined range are grouped as group observation points.

3. a microtremor array for observing ground tremors at an observation point; and a computing device that uses a relationship between the frequency and phase velocity of waves propagating through the ground from the microtremors observed by the microtremor array to calculate at least the S-wave velocity in the basement layer and the depth to the top surface of the basement layer by inverse analysis, thereby estimating the underground structure at the observation point, The computing device An underground structure estimation system that groups certain of the observation points into group observation points, performs the inverse analysis assuming that the S-wave velocities in the bedrock layer at all of the group observation points are the same, and calculates the S-wave velocity in the bedrock layer and the depth to the top surface of the bedrock layer at each of the group observation points.

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