JP7681550B2 - Strata thickness estimation method - Google Patents

Description

The present invention relates to a method for estimating the thickness of a layer of ground.

Patent document 1 discloses a method in which a vibration generator installed on the ground generates waves of a specified frequency, the propagation time of these waves is detected by multiple vibration detectors installed on the ground in a straight line from the vibration generator, and the propagation speed of the waves is calculated by a calculator from the time it takes for the waves to propagate between each vibration detector and the distance between each vibration detector, and the layer thickness and layer quality of each constituent layer of the ground structure are estimated from these values.

In this method, a cable that is freely inserted and removed from the vibration generator and has an open/close switch for an electric circuit is moved in and out in a straight line between the vibration generator and each vibration detector, and the open/close switch for the cable is closed at the position where each vibration detector is installed, and the distance between each vibration detector is calculated by a calculator from the electrical signal and the signal from an encoder attached to the vibration generator that detects the movement of the cable. This allows the distance between the vibration detectors to be measured accurately.

Japanese Patent Application Publication No. 11-101876

In the method described in Patent Document 1, in order to accurately measure the distance between detectors and estimate the ground structure in detail, vibrations are measured using a device equipped with various components such as multiple sensors and cables equipped with open/close switches for electrical circuits, making the device large-scale. In addition, it takes a great deal of work to measure the distance between vibration detectors.

On the other hand, when creating an analytical model of the ground structure, the properties of the surface layer of the ground are dominant. For this reason, a method for easily estimating the properties of the surface layer of the ground, especially the layer thickness, is desired.

The present invention aims to obtain the thickness of the surface layer of the ground in a simple manner.

The present invention is a method for estimating the thickness of a layer of ground, and includes the steps of: generating vibrations of multiple frequencies in the ground by a vibration source; measuring the vibrations at two measurement points on the ground at different distances from the vibration source; calculating the vibration propagation speed for each frequency from the phase difference of the vibrations at the two measurement points and the distance between the two measurement points, extracting a constant-speed frequency region in which the change in the propagation speed is equal to or less than a predetermined value, and acquiring the surface layer propagation speed based on the propagation speed in the constant-speed frequency region; calculating the displacement response per unit excitation force applied to the ground by the excitation source, acquiring the maximum value of the displacement response that satisfies a predetermined judgment condition as the local peak value of the displacement response, and acquiring the frequency at which the local peak value occurs as the peak frequency; and estimating the thickness of the surface layer of the ground based on the wavelength calculated by dividing the surface layer propagation speed by the peak frequency.

According to the present invention, the thickness of the surface layer of the ground G can be obtained in a simple manner.

1 is a schematic diagram of an estimation system according to an embodiment of the present invention; 2 is a flowchart showing the procedure of a layer thickness estimation method according to an embodiment of the present invention. FIG. 4 is a graph showing the relationship between the frequency of vibration propagating through the ground and the displacement response per unit excitation force according to the embodiment of the present invention. 1 is a graph showing the relationship between the frequency of vibration propagating through the ground and the phase difference between two measurement points according to an embodiment of the present invention. FIG. FIG. 4 is a graph showing the relationship between the frequency and propagation speed of vibration propagating through the ground according to the embodiment of the present invention. 10 is a flowchart showing the procedure of a layer thickness estimation method according to a modified example of an embodiment of the present invention. FIG. 11 is a graph showing the relationship between the frequency of vibration propagating through the ground and the displacement response per unit excitation force according to the modified example of the embodiment of the present invention. FIG. 11 is a graph showing the relationship between the frequency and the propagation speed of vibration propagating through the ground according to the modified example of the embodiment of the present invention.

Below, we will explain the layer thickness estimation method and layer thickness estimation system 100 according to an embodiment of the present invention with reference to the drawings.

The layer thickness estimation system 100 (hereinafter also referred to simply as "estimation system 100") has a vibration source 10 that generates vibrations in the ground G, a first sensor 20 and a second sensor 30 as multiple vibration measurement units that measure the vibrations of the ground G at measurement points a predetermined distance away from the vibration position caused by the vibration source 10, and a control device 40 that acquires the measurement results of the first sensor 20 and the second sensor 30 and estimates the layer thickness based on the measurement results.

The vibration source 10 may have a known configuration as long as it can generate vibrations of a predetermined frequency on the ground G. For example, in this embodiment, a portable impact hammer or banging machine is used as the vibration source 10. In FIG. 1, the vibration source 10 is illustrated as being installed on the ground G, but this is not limited thereto, and a handheld device such as an impact ball can also be used as the vibration source 10.

The excitation force and frequency of the vibration generated by the excitation source 10 on the ground G are measured by the vibration sensor 15, and the measurement results are input to the control device 40. The vibration sensor 15 may be provided on the excitation source 10, or may be installed near the excitation position on the ground G. For example, when an impact hammer is used as the excitation source 10, a sensor that is generally built into the impact hammer may be used as the vibration sensor 15.

The first sensor 20 and the second sensor 30 are vibration measurement sensors that measure vibrations. In this embodiment, the estimation system 100 has two sensors, the first sensor 20 and the second sensor 30, to measure vibrations at two measurement points. The first sensor 20 and the second sensor 30 are installed at measurement points (measurement positions) on the ground G that are different distances away from the excitation position. The first sensor 20 and the second sensor 30 are electrically connected to the control device 40 by wire or wirelessly. The distance between the measurement positions of the first sensor 20 and the second sensor 30 and the excitation position by the excitation source 10 is stored in advance in the control device 40. The first sensor 20 is a sensor installed at a measurement point relatively close to the excitation position, and the second sensor 30 is a sensor installed at a measurement point relatively far away.

The control device 40 is configured by a computer including a CPU (Central Processing Unit) that executes a control program, a ROM (Read-Only Memory) that stores the control program executed by the CPU, a RAM (Random Access Memory) that stores the results of calculations by the CPU, a communication device, and the like. In addition, an input device (not shown) that accepts operation inputs from an operator and a display device (not shown) that displays information output by the control device 40 are connected to the control device 40. The control device 40 executes various functions of the control device 40 described in this specification by the CPU executing the control program stored in the ROM. The control device 40 may be configured by one computer, or may be configured by multiple microcomputers and configured to distribute each control among the multiple computers.

Next, a method for estimating layer thickness using an estimation system will be described with reference to Figures 2 to 5. The method for estimating layer pressure in this embodiment is intended to estimate the layer thickness of the surface layer (first layer) of the multiple layers that make up the ground G.

Figure 2 is a flowchart showing the formation pressure estimation method of this embodiment. Step S10 is a process executed by an operator. Step S11 is a process executed by the estimation system 100. Steps S12 to S14 may be executed automatically by a program executed by the control device 40 of the estimation system, or may be executed by the control device 40 in response to an operation input by an operator inputted through an input device. A detailed explanation will be given below.

In step S10, the vibration source 10 generates vibrations in the ground G at a plurality of different frequencies. In step S10, the frequency is changed within a range of, for example, greater than 0 and less than or equal to 100 Hz to generate vibrations of different frequencies in the ground G. The frequencies of the vibrations generated by the vibration source 10 are measured by the vibration sensors 15 and input to the control device 40.

In step S11, the first sensor 20 and the second sensor 30 installed at the two measurement points measure the vibrations with different frequencies generated by the vibration source 10. Specifically, the first sensor 20 and the second sensor 30 measure the amplitude and frequency of the vibration of the ground G, and input them to the control device 40.

Next, in step S12, the displacement response per unit excitation force is calculated from the measurement results of the vibration sensor 15 and the first sensor 20, and the frequency at which the displacement response reaches its local peak value is obtained as the peak frequency.

To explain step S12 in detail, first, the amplitude (displacement) of the vibration measured by the first sensor 20 is divided by the excitation force of the excitation source 10 measured by the vibration sensor 15 to obtain the displacement (displacement response) per unit excitation force for each frequency. A graph showing the relationship between frequency and the displacement response per unit excitation force is, for example, as shown in FIG. 3, and this graph is called a resonance curve. Here, it is known that one local peak value is observed in the resonance curve of a two-layered ground structure in which a relatively soft layer (a layer with a low propagation speed) is stacked on the surface side. In other words, in a two-layer structure, the displacement response becomes a maximum value at the resonance frequency of the two layers. Therefore, it is possible to estimate how many two-layered ground structures are combined in the ground G to be subjected to layer thickness estimation, depending on the number of local peak values of the resonance curve.

The displacement response is not limited to displacement, but may be velocity or acceleration per unit excitation force. The displacement response is calculated based on the measurement results of the first sensor 20, but may also be based on the measurement results of the second sensor 30.

For example, in FIG. 3, one local peak value is observed, so the ground G targeted in this embodiment can be approximated as a two-layered ground G in which a relatively soft ground G is layered on the surface layer side and a relatively hard ground layer is located below the soft ground layer. In other words, in this embodiment, the ground structure is approximated to a model in which soft and hard layers are layered alternately, and the layer thickness is estimated.

The local peak value of the propagation velocity is a maximum value that satisfies a predetermined judgment condition among multiple maximum values of the propagation velocity (values at which the tangent slope is zero on the graph of the resonance curve) where the propagation velocity changes from increasing to decreasing with a change in frequency. In this embodiment, the predetermined judgment condition is a condition in which the difference between the maximum value and the propagation velocity values at the frequencies before and after the frequency at which the maximum value occurs is equal to or greater than a predetermined value. This predetermined value is set so that a maximum value due to measurement error or the like is not unintentionally obtained as a local peak value. The predetermined value can be set arbitrarily by the operator, and the judgment condition is not limited to this and can be set arbitrarily.

In this way, once the local peak value of the resonance curve is obtained, in step S12, the frequency at which the local peak value occurs is obtained as the peak frequency.

In step S13, the propagation velocity of the surface layer of the ground G (surface propagation velocity) is obtained based on the measurement results of the first sensor 20 and the second sensor 30. Specifically, in step S13, the propagation velocity of the vibration is first calculated for each vibration frequency. The propagation velocity is calculated by first calculating the phase delay difference (phase difference) between the measurement points, that is, the phase difference between the measurement results of the first sensor 20 and the second sensor 30, based on the phase delay of the vibration from the excitation position at each measurement point for each vibration frequency from the measurement results of the vibration sensor 15, the first sensor 20, and the second sensor 30 (see FIG. 4). Then, the difference in the propagation time of the vibration between the measurement points (in other words, the time it takes for the wave to propagate between the measurement points) is calculated from the phase difference between the measurement points and the vibration frequency. The propagation velocity of the vibration can be calculated from the propagation time of the wave between the measurement points calculated in this way and the distance between the measurement points stored in advance in the control device 40. If the relationship between the vibration frequency and the calculated propagation speed is represented on a graph, it will look like the graph shown in Figure 5, for example, and this graph is called the dispersion curve of the vibration (wave).

Next, in step S13, the propagation speed (surface layer propagation speed) of the vibration propagating through the stratum is obtained based on the calculated propagation speed. To be more specific, in general, the shorter the wavelength and the higher the frequency of the vibration, the more likely it is to propagate through the surface layer of the ground G, which is shallower from the ground surface. Furthermore, if the propagation speed remains constant even when the frequency increases, it can be estimated that the vibration in the frequency band where the propagation speed is constant propagates through the surface layer. For this reason, in step S13, the average value of the propagation speeds in the constant speed frequency band where the propagation speed is approximately constant in the high frequency region among the generated vibrations is obtained as the surface layer propagation speed.

Note that a constant-speed frequency band does not mean that the propagation speed is strictly constant, but rather that the change in propagation speed is equal to or less than a specified value. In other words, a constant-speed frequency band in a high-frequency range is a frequency band that is within the range from the maximum frequency to a specified frequency lower than the maximum frequency, and in which the difference between the maximum and minimum values of the propagation speed is equal to or less than a specified value.

Next, in step S14, the thickness of the surface layer of the ground (surface layer thickness) is estimated from the peak frequency acquired in step S12 and the surface layer propagation velocity calculated in step S13. Specifically, it is known that the thickness of the stratum (layer thickness) is correlated with the wavelength of the vibration propagating through the stratum, and for example, a method of estimating the layer thickness is known in which the layer thickness is ¼, ⅓, or ½ of the wavelength. Therefore, in this embodiment, the wavelength of the vibration propagating through the surface layer is calculated by dividing the surface layer propagation velocity by the peak frequency, and ½ of the calculated wavelength is estimated as the surface layer thickness. Note that the layer thickness may also be estimated as ¼ or ⅓ of the wavelength.

In this way, the surface layer thickness of ground G can be estimated. Estimating the surface layer thickness of ground G can be used to construct an analytical model of ground G.

Here, when calculating the local peak value in step S12, the judgment conditions are set by the operator, but in order to calculate the local peak value with high accuracy, the judgment conditions may need to be set with high accuracy. In such cases, it may be difficult to obtain a local peak value that satisfies the judgment conditions.

Therefore, in this embodiment, when the resonance curve reaches a maximum value at a frequency where the phase difference between the measurement points is π/2, the maximum value is acquired as a local peak value. Normally, in a vibration system, the phase difference when resonance occurs is π/2. Therefore, when the displacement response reaches a maximum value at a frequency where it is π/2, the maximum value is a local peak value caused by the resonance of two layers of earth, and the frequency at that time can be estimated to be the natural frequency of the vibration system of the ground. The acquisition of such a local peak value based on the phase difference may be performed only when a predetermined judgment condition is not met, or may be performed regardless of whether the predetermined judgment condition is met.

The above embodiment provides the following effects:

In this embodiment, since high-frequency vibrations propagate through the surface layer, the surface layer propagation velocity is obtained from the propagation velocity in the region where the velocity is constant in the high-frequency region. It is also known that in a structure in which a soft layer exists on a relatively hard layer, a local peak occurs in the displacement response per unit excitation force. Therefore, the highest frequency among the peak frequencies at which a local peak occurs can be considered to be the frequency of vibration that is likely to propagate through the surface layer of the ground G. It is also known that there is a correlation between the wavelength of vibration propagating through the layer and the thickness of the layer, so the thickness of the surface layer of the ground G can be estimated based on the wavelength calculated from the high-frequency peak frequency and the surface layer propagation velocity. In this way, in this embodiment, the thickness of the surface layer can be obtained quickly and simply with at least the vibration source 10 and a device that measures vibration at two measurement points.

In addition, if the resonance curve has a maximum value at a frequency where the phase difference between measurement points is π/2, the maximum value can be obtained as the local peak value, making it possible to obtain the local peak value even when it is difficult to determine the local peak value using specified determination conditions.

Next, we will explain a variation of this embodiment.

In the above embodiment, an example was described in which one local peak value appears in the resonance curve. In contrast, there may be two or more local peak values. Below, a case in which two local peak values appear will be described with reference to Figures 6 to 8. A case in which two local peak values appear is a case in which the target ground G can be approximated as a stratum structure consisting of a total of four layers, including a two-layer structure of a surface layer and a second layer that is harder than the surface layer, and a two-layer structure of a third layer that is softer than the second layer and a fourth layer that is harder than the third layer.

Figure 6 is a flowchart showing the steps of the method for estimating layer thickness when two local peak values are obtained. Steps S20 and S21 are similar to steps S10 and S11 in the above embodiment, and therefore will not be described.

In step S22, similar to the above embodiment, a local peak value is obtained from the resonance curve shown in FIG. 7. When two local peak values are obtained, in step S22, the higher (maximum) of the frequencies at which the local peak values are obtained is obtained as the first peak frequency.

In step S23, the dispersion curve shown in FIG. 8 is obtained, as in the above embodiment. When approximation is performed using two sets of two-layer structures (when two local peak values are obtained), as shown in FIG. 8, two constant-speed frequency regions occur on the dispersion curve corresponding to the number of local peak values. In step S23, the surface layer propagation velocity is obtained based on the constant-speed frequency region with the higher frequency (in other words, the highest). The extraction of the constant-speed frequency region and the acquisition of the surface layer propagation velocity are the same as in the above embodiment.

In step S24, similar to the above embodiment, the surface layer thickness is estimated from the first peak frequency obtained in step S22 and the surface layer propagation velocity obtained in step S23.

After estimating the surface layer thickness in this manner, the thicknesses of the second and third layers are estimated in steps S24 to S28.

In step S25, the third layer propagation velocity is obtained. In the dispersion curve, the constant speed frequency region on the low frequency side corresponds to a two-layer structure consisting of the third and fourth layers. Therefore, in the same manner as in obtaining the surface layer propagation velocity, the propagation velocity calculated from the constant speed frequency region on the low frequency side (second) is obtained as the third layer propagation velocity.

In step S26, the frequency of the local peak value on the lower frequency side of the resonance curve (Figure 7) is obtained as the second peak frequency. Then, similar to the case of the surface layer thickness, the third layer thickness is estimated from the third layer propagation speed and the second peak frequency.

In step S27, the propagation velocity of the second layer is estimated. As described above, when two local peak values are obtained, it is possible to approximate a model that combines two sets of two-layer structures consisting of a relatively soft layer and a hard layer. In other words, in a model that is approximated by two sets of two-layer structures, the second layer is a harder layer than the first and third layers. For this reason, in step S27, the propagation velocity of the second layer is estimated to be a velocity greater than both the propagation velocity of the first layer (surface layer) and the propagation velocity of the third layer. The extent to which the propagation velocity of the second layer is to be greater than the propagation velocity of the first and third layers can be arbitrarily set by the operator based on empirical rules, etc.

Then, in step S28, the second layer thickness is arbitrarily set based on the first peak frequency obtained in step S22 and the second peak frequency obtained in step S26. For example, the second layer thickness can be arbitrarily set based on empirical rules or the like in accordance with the magnitude relationship between the local peak value corresponding to the first peak frequency and the local peak value corresponding to the second peak frequency.

In this manner, the propagation speed and layer thickness of the first to third layers can be obtained. Based on the propagation speed and layer thickness of the first to third layers obtained, an analytical model of the ground G can be constructed.

In addition, when three or more local peak values appear, the layer thicknesses of the fifth layer and onward can be estimated by executing the processes corresponding to steps S25 to S28. In other words, when two or more local peak values are obtained, the surface layer propagation velocity is obtained based on the propagation velocity in the constant speed frequency region with the highest frequency, and the thickness of the surface ground is estimated based on the wavelength calculated by dividing the surface layer propagation velocity by the highest peak frequency among the peak frequencies obtained from the local peak values. When three or more local peak values appear, the layer thickness measurement method includes step S25 of acquiring the third layer propagation velocity based on the propagation velocity in the constant speed frequency region with the second highest frequency among the constant speed frequency regions, step S26 of estimating the thickness of the third layer ground based on the wavelength calculated by dividing the third layer propagation velocity by the second highest peak frequency among the peak frequencies acquired from the local peak values, step S27 of estimating a velocity greater than both the surface layer propagation velocity and the third layer propagation velocity as the propagation velocity in the second layer ground between the surface layer ground and the third layer ground, and step S28 of estimating the thickness of the second layer based on the wavelength calculated by dividing the propagation velocity in the second layer by the highest first peak frequency.

Next, we will explain other variations.

In the above embodiment, vibration is measured at two measurement points, but vibration may be measured at three measurement points. For example, when it is difficult to provide a vibration sensor 15 at or near the excitation source 10, measurements may be taken at three measurement points far from the excitation source 10, and the measurement result of the measurement point closest to the excitation source 10 may be regarded as the same as the result of the vibration sensor 15 in the above embodiment, and the method of the above embodiment may be executed.

This embodiment can also be applied to ground G that is composed of multiple strata and that becomes harder with depth. For example, if the strata become harder in the order of the first to fourth layers, the strata are approximated as a set of two-layer structure, with a soft layer and a hard layer separated at a location where the difference in hardness between the strata is relatively large.

In the above embodiment, the vibration source 10 is portable, and vibration is applied to the ground G manually by an operator. In contrast, the vibration source 10 may be an electromagnetic type that is installed on the ground G and generates vibrations of a predetermined frequency in the ground G in response to a control signal from the control device 40.

Although the embodiments of the present invention have been described above, the above embodiments merely show some of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments.

100 Layer thickness estimation system 10 Excitation source G Ground

Claims (4)

A method for estimating a thickness of a stratum of ground, comprising:
A step of generating vibrations of multiple frequencies in the ground by an excitation source;
Measuring the vibration at two measurement points on the ground that are different distances from the vibration source;
calculating a propagation velocity of the vibration for each frequency from a phase difference of the vibration at the two measurement points and a distance between the two measurement points, extracting a constant-speed frequency region in which a change in the propagation velocity is equal to or smaller than a predetermined value, and acquiring a surface layer propagation velocity based on the propagation velocity in the constant-speed frequency region;
calculating a displacement response per unit excitation force applied to the ground by the excitation source, acquiring a maximum value of the displacement response that satisfies a predetermined judgment condition as a local peak value of the displacement response, and acquiring a frequency at which the local peak value occurs as a peak frequency;
and estimating a thickness of the surface layer of the ground based on a wavelength calculated by dividing the surface layer propagation speed by the peak frequency.
A method for estimating a layer thickness comprising: The method for measuring a layer thickness according to claim 1,
When the displacement response at the frequency at which the phase difference at the measurement point is π/2 is the maximum value, the maximum value is acquired as the local peak value regardless of whether the judgment condition is satisfied.
A method for estimating a layer thickness comprising: The method for measuring a layer thickness according to claim 1 or 2,
When two or more local peak values are obtained,
In the step of acquiring the surface layer propagation velocity, the surface layer propagation velocity is acquired based on the propagation velocity in the constant speed frequency region having the highest frequency,
In the step of estimating the thickness of the surface layer of the ground, the thickness of the surface layer is estimated based on a wavelength calculated by dividing the surface layer propagation speed by the highest peak frequency among the peak frequencies obtained from the local peak values;
The method for measuring a layer thickness comprises:
obtaining a third layer propagation velocity based on the propagation velocity in the constant speed frequency region having a second highest frequency among the constant speed frequency regions;
and estimating a thickness of the third layer of the ground based on a wavelength calculated by dividing the third layer propagation velocity by a second highest peak frequency among the peak frequencies obtained from the local peak values.
A method for estimating a layer thickness comprising: The method for estimating a layer thickness according to claim 3,
estimating a propagation velocity in a second layer between the surface layer and the third layer that is greater than both the surface layer propagation velocity and the third layer propagation velocity;
and estimating a thickness of the second layer based on a wavelength calculated by dividing the propagation velocity in the second layer by the highest peak frequency.
A method for estimating a layer thickness comprising: