JP2009007859A - Ground structure estimating method and ground structure estimating apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly accurate ground structure estimating method capable of easily performing measurement even at a narrow site and acquiring highly reliable results. <P>SOLUTION: A vibration generator 11 and detectors 12a-12c are installed on the surface of the ground 5. While changing its oscillation frequency, the vibration generator 11 adds vibration to the ground 5 to detect elastic waves at the detectors 12a-12c. On the basis of this, phase velocity characteristics and the dominant frequency of the ground 5 are computed. Then a hypothetical ground model is created to determine the S-wave velocity of a layer in the vicinity of the surface of the hypothetical ground model on the basis of a phase velocity in a frequency region between a phase velocity peak in the vicinity of the dominant frequency and a phase velocity in a region having a frequency higher than this in the phase velocity characteristics. This computation is repeated as altering parameters of the hypothetical ground model until a dominant frequency and phase velocity characteristics computed on the basis of the hypothetical ground model is approximately matched with those of the object ground 5. In the case of approximate matching, the hypothetical ground model is outputted as an estimated result ground model. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention mainly relates to a method for estimating an underground structure by analyzing elastic waves.

As one of methods for estimating the underground structure in the ground, a method called so-called microtremor survey is known. In this microtremor wave exploration, a plurality of detectors are installed at measurement points, and Rayleigh wave components are extracted from natural vibrations in the distance (waves and traffic vibrations in the distance). Then, the Rayleigh wave velocity at each frequency of 1 to several tens Hz (several hundreds Hz) is obtained by dividing the detection time difference of the Rayleigh wave between the detectors by the distance between the detectors. Next, the Rayleigh wave velocity is multiplied by a predetermined constant to obtain the S wave velocity (S wave structure) for each depth of the ground. This micromotion wave exploration is disclosed in, for example, Patent Document 1.
JP 2001-193046 A

  In addition, the underground ground structure is estimated by so-called PS logging. In this PS logging, a vibrator is installed on the ground surface, and a hole is dug vertically at an appropriate distance of several tens of centimeters to several hundreds of meters from the vibrator. Install detector at depth. Then, for example, by operating the exciter at several Hz to several hundred Hz and dividing the distance from the exciter to the detector by the vibration detection delay time of the detector, the S wave velocity for each depth of the ground is directly Ask for.

Furthermore, in recent years, a method called surface wave exploration is also known. In this surface wave exploration, an exciter is installed on the ground surface, and a plurality of detectors are installed on the ground surface at an appropriate distance of several tens of centimeters to several hundreds of meters from the exciter. For example, the vibrator is operated at several Hz to several hundred Hz, the vibration wave taken out by the detector is regarded as a Rayleigh wave, and the Rayleigh wave velocity is obtained by dividing the detector interval by the vibration detection time difference between the detectors. Ask for. Next, the Rayleigh wave velocity is multiplied by a predetermined constant to obtain the S wave velocity (S wave structure) for each depth of the ground. This surface wave exploration is disclosed in Patent Documents 2 and 3, for example.
JP 2005-127760 A JP 2007-120959 A

  The above-described microtremor wave exploration uses natural vibration as a Rayleigh wave, and is difficult to apply when a source of artificial vibration (for example, a factory or a construction site) is at a short distance. In addition, it is necessary to accumulate data for a long time and perform correlation processing, which takes time and man-hours.

  Further, the PS logging method needs to form a borehole or the like in the ground, and the place where it can be applied is limited. In addition, exploration requires large-scale equipment, which increases time and cost.

  On the other hand, surface wave exploration has been particularly widespread in recent years because it can solve the above-mentioned problems of microtremor wave exploration and PS logging, and can be measured in a short site in a short time.

  However, in general, when vibration is applied to the ground with an exciter, not only Rayleigh waves but also various elastic waves are included in the region close to the excitation point, and it is difficult to extract only Rayleigh waves from among them. . On the other hand, in the surface wave exploration, the detected vibration is uniformly regarded as a Rayleigh wave and is analyzed.

  In this regard, the inventors of the present application pointed out in a well-known paper that there is a large difference between the ground surface velocity and the Rayleigh wave velocity especially in the region close to the excitation point (Takanori Harada, Hiroshi Wang) Masato Saito, Norihiko Yamashita, Genji Mori, P-SV wave ground vibration and wave propagation characteristics due to harmonic vibration load, Vol.8, pp. 685-692, 2005). This paper describes the knowledge that the elastic wave velocity changes according to the distance from the exciter to the detector, and that the elastic wave velocity changes at different frequencies even at the same distance. .

  In order to actually confirm this knowledge, the present inventors conducted a verification test using an actual machine. Then, especially when the detector is installed at a distance of several tens of centimeters to several meters from the oscillation point, the results of the layer thickness and speed obtained by the measurement depend on the position of the detector even though it is the same formation. It was confirmed that it was very different. In addition, when comparing the ground structure obtained by the actual boring survey with the above measurement results, it was confirmed that there is a possibility of considerable errors in the ground structure estimation by the conventional surface wave exploration.

  As described above, the conventional surface wave exploration has left room for improvement in the reliability of the measurement results when the distance between the vibrator and the detector cannot be secured large. The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide a ground structure estimation method capable of easily measuring even in a narrow site and obtaining a highly reliable result.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above. Next, means for solving the problems and the effects thereof will be described.

  According to the first aspect of the present invention, the following ground structure estimation method is provided. That is, an exciter and a detector are installed on the ground surface of the target ground, and the ground is vibrated by the exciter, and the detector detects an elastic wave and outputs it based on a signal in the ground. A ground structure estimation method for estimating a ground structure, which includes the following first to fourth steps. That is, in the first step, the ground is vibrated by the exciter while changing the vibration frequency, the detector detects an elastic wave, and the phase velocity in the ground is determined based on the output signal of the detector. Calculate the frequency characteristics and the dominant frequency. In the second step, a hypothetical ground model is created, and in the frequency characteristics of the phase velocity obtained in the first step, the phase velocity peak near the dominant frequency and the frequency is larger than the dominant frequency. The S wave velocity of at least one layer of the hypothetical ground model is determined based on the phase velocity in the frequency region between the phase velocity peak in the region. In the third step, the difference between the dominant frequency calculated from the hypothetical ground model and the dominant frequency calculated in the first step is within a predetermined range, and the phase velocity calculated from the hypothetical ground model Until the difference between the frequency characteristic of the phase velocity and the frequency characteristic of the phase velocity calculated in the first step is within a predetermined range, while changing the parameters of the hypothetical ground model, the frequency of the dominant frequency and the phase velocity The calculation of the characteristics is repeated, and when the difference falls within a predetermined range, the hypothetical ground model is set as the estimation result ground model. In the fourth step, the estimation result ground model is output.

  In this method, the wave is not considered as a Rayleigh wave but is analyzed as a composite wave including other waves, so that a constant estimation result can be obtained regardless of the distance from the excitation point, especially near the excitation point. A reliable estimation result can be obtained even when a detector is installed in the. Therefore, even when a large space cannot be secured for measurement, accurate estimation is possible. Moreover, since the S wave velocity of the hypothetical ground model layer at the time of creation is appropriately determined, the calculation time and calculation load for obtaining the estimation result ground model can be reduced. Furthermore, in this method, the ground in the ground is estimated using the phase velocity and ground vibration characteristics (only) observed between two points near the excitation point (or between two points including the excitation point). be able to. Accordingly, an exciter having a strong excitation force is not required, and the ground can be accurately estimated with a small number of detectors.

  In the ground structure estimation method, the following is preferable. That is, in the frequency characteristic of the phase velocity, the second step includes a frequency between a phase velocity peak near the dominant frequency and a phase velocity peak in a region where the frequency is larger than the dominant frequency. The maximum value of the phase velocity in the region is set as the S wave velocity of at least one layer of the hypothetical ground model.

  Thereby, since the S wave velocity of the layer of the hypothetical ground model at the time of creation is determined more appropriately, the calculation time and the calculation load for obtaining the estimation result ground model can be further reduced.

  In the ground structure estimation method, the following is preferable. That is, the third step includes Step A and Step B. In step A, a prevailing frequency is calculated in the hypothetical ground model, and the hypothetical ground model is used until the difference between the prevailing frequency of the hypothetical ground model and the prevailing frequency calculated in the first step is within a predetermined range. The calculation of the dominant frequency is repeated while changing the above parameters, and when the difference between the dominant frequencies falls within a predetermined range, the hypothetical ground model is set as the second hypothetical ground model. In step B, the frequency characteristic of the phase velocity is calculated in the second hypothetical ground model, and the frequency characteristic of the phase velocity of the second hypothetical ground model and the frequency characteristic of the phase velocity calculated in the first step are calculated. The calculation of the frequency characteristic of the phase velocity is repeated while changing the parameters of the hypothetical ground model until the difference is within the predetermined range, and when the difference in the frequency characteristic of the phase velocity is within the predetermined range, the second The hypothetical ground model is used as the estimation result ground model.

  As a result, in the hypothetical ground model, the ground frequency model of the phase velocity is first matched and then the frequency characteristics of the phase velocity are calculated so that the ground model of the estimation result can be easily obtained with a small amount of calculation. Therefore, calculation time and calculation load can be reduced.

  In the ground structure estimation method, it is preferable that an interval between the vibrator installed on the ground surface of the target ground and the detector closest to the vibrator is 10 m or less.

  That is, the method of the present invention is particularly useful in that a reliable estimation result can be obtained even when there is only a narrow space where the distance between the exciter and the detector must be narrowed.

  In the ground structure estimation method, it is preferable that, in the third step, the dominant frequency and phase velocity frequency characteristics of the hypothetical ground model are calculated by a stiffness matrix method.

  Thereby, the stability of the numerical calculation solution in the hypothetical ground model can be easily secured, and the calculation can be performed with a relatively simple program.

  According to the 2nd viewpoint of this invention, the ground structure estimation apparatus of the following structures is provided. That is, this ground structure estimation device is provided with a vibration generator and a detector on the ground surface of the target ground, and the vibration is applied to the ground by the vibration generator, and the detector detects and outputs an elastic wave. It is used in a ground structure estimation method that estimates the underground ground based on signals. The ground structure estimation device is based on the data when the ground is vibrated by the vibrator and the detector detects an elastic wave while changing the vibration frequency, and the frequency characteristics of the phase velocity in the ground. And calculate the dominant frequency. Then, the ground structure estimation device creates a hypothetical ground model, and in the obtained frequency characteristics of the phase velocity, a phase velocity peak near the dominant frequency and a region where the frequency is larger than the dominant frequency. Based on the phase velocity in the frequency region between the phase velocity peak and the S-wave velocity of at least any one layer of the hypothetical ground model is determined. Further, the ground structure estimation device has a difference between the dominant frequency calculated from the hypothetical ground model and the dominant frequency calculated in the first step within a predetermined range, and is calculated from the hypothetical ground model. Until the difference between the frequency characteristic of the phase velocity and the frequency characteristic of the phase velocity calculated in the first step is within a predetermined range, the dominant frequency and phase velocity are changed while changing the parameters of the hypothetical ground model. When the difference is within a predetermined range, the hypothetical ground model is set as the estimation result ground model. The ground structure estimation apparatus is configured to be able to output the obtained estimation result ground model.

  This device does not consider the wave as a Rayleigh wave, but analyzes it as a composite wave including other waves, so it can obtain a constant estimation result regardless of the distance from the excitation point, especially near the excitation point. A reliable estimation result can be obtained even when a detector is installed in the. Therefore, even when a large space cannot be secured for measurement, accurate estimation is possible. Moreover, since the S wave velocity of the hypothetical ground model layer at the time of creation is appropriately determined, the calculation time and calculation load for obtaining the estimation result ground model can be reduced. Furthermore, in this apparatus, the ground in the ground is estimated using the phase velocity and ground vibration characteristics (only) observed between two points near the excitation point (or between two points including the excitation point). be able to. Accordingly, an exciter having a strong excitation force is not required, and the ground can be accurately estimated with a small number of detectors.

  In the ground structure estimation device, in the frequency characteristics of the phase velocity, the frequency between a phase velocity peak near the dominant frequency and a phase velocity peak in a region where the frequency is larger than the dominant frequency. It is preferable to set the maximum value of the phase velocity in the region as the S wave velocity of at least one layer of the hypothetical ground model.

  Thereby, since the S wave velocity of the layer of the hypothetical ground model at the time of creation is determined more appropriately, the calculation time and the calculation load for obtaining the estimation result ground model can be further reduced.

  The ground structure estimation apparatus preferably has the following configuration. That is, when obtaining an estimation result ground model from the hypothetical ground model, a dominant frequency is calculated in the hypothetical ground model, and a difference between the dominant frequency of the hypothetical ground model and the dominant frequency calculated in the first step is calculated. Until the value falls within the predetermined range, the calculation of the dominant frequency is repeated while changing the parameters of the hypothetical ground model, and when the difference in the dominant frequency falls within the predetermined range, the hypothetical ground model is changed to the second hypothetical ground model. And Next, the frequency characteristic of the phase velocity is calculated in the second hypothetical ground model, and the difference between the frequency characteristic of the phase velocity of the second hypothetical ground model and the frequency characteristic of the phase velocity calculated in the first step is calculated. The calculation of the frequency characteristic of the phase velocity is repeated while changing the parameters of the hypothetical ground model until the value falls within the predetermined range, and when the difference in the frequency characteristic of the phase velocity falls within the predetermined range, the second hypothesis The ground model is used as the estimation result ground model.

  As a result, in the hypothetical ground model, the ground frequency model of the phase velocity is first matched and then the frequency characteristics of the phase velocity are calculated so that the ground model of the estimation result can be easily obtained with a small amount of calculation. Therefore, calculation time and calculation load can be reduced.

  In the ground structure estimation device, an estimation result ground model is based on data when an interval between the vibrator installed on the ground surface of the target ground and the detector closest to the vibrator is 10 m or less. It is preferable to obtain

  That is, the apparatus of the present invention is particularly useful in that a reliable estimation result can be obtained even when there is only a narrow space in which the distance between the exciter and the detector must be narrowed.

  In the ground structure estimation apparatus, it is preferable to calculate the frequency characteristics of the dominant frequency and phase velocity of the hypothetical ground model by a stiffness matrix method.

  Thereby, the stability of the numerical calculation solution in the hypothetical ground model can be easily secured, and the calculation can be performed with a relatively simple program.

  Next, embodiments of the invention will be described. FIG. 1 is a schematic diagram showing a state in which an exciter and a detector are installed on the ground and measurement is performed, and FIG. 2 is a flowchart for explaining measurement work. 3 and 4 are flowcharts for explaining the analysis work.

  The ground exploration apparatus 1 shown in FIG. 1 includes a vibration generator 11, detectors 12a, 12b, and 12c, a connection box 13, a signal processor 3, and a personal computer (calculation display) 4. As prepared.

  The exciter 11 is installed on the ground surface of the ground 5 to be investigated, and can apply vertical vibration to one point of the ground 5 (excitation point 6).

  The detectors 12a, 12b, and 12c are arranged side by side on a measurement line set so as to extend linearly from the excitation point 6 on the ground surface of the ground 5. The detector 12 a closest to the exciter 11 is installed at a distance within 10 m from the exciter 11. Each of the detectors 12a, 12b, and 12c includes a known servo type acceleration pickup including a pendulum and a servo amplifier, and can output a signal corresponding to the vibration at the detector installation point.

  The signal processor 3 includes a power amplifier 50, main amplifiers 51a, 51b, and 51c, a signal processing unit 52, and a communication interface 53.

  The output side of the power amplifier 50 is connected to the exciter 11 via a cable. The detection signals output from the detectors 12a, 12b, and 12c are sent to the connection box 13 via a cable, and are amplified by a preamplifier (not shown) built in the connection box 13. The amplified detection signal is sent to the main amplifiers 51a, 51b, 51c of the signal processor 3 via a cable.

  Then, the signal processing unit 52 of the signal processor 3 performs processing on the detection signals amplified by the main amplifiers 51a, 51b, and 51c. The processing result data is sent to the communication interface 24 of the personal computer 4 via the communication interface 53.

  The personal computer 4 includes a CPU 21, a memory 22, a hard disk 23, a communication interface 24, a display unit 25, and an operation unit 26.

  The operation unit 26 includes, for example, a keyboard and a mouse, and the user can perform various operations on the personal computer 4 via the operation unit 26. In addition, the display unit 25 includes a liquid crystal display, for example, and is configured to be able to display various calculation results and the like.

  Appropriate software is installed in the personal computer 4 using a storage medium. By this installation operation, an appropriate program is stored in the hard disk 23.

  When the user appropriately operates the operation unit 26, the program can be loaded into the memory 22 and executed. By executing this program, the exciter 11 is driven, the signal processor 3 is controlled, the data is received from the signal processor 3, etc. via the communication interface 24, and the received data from the signal processor 3 is processed. Arithmetic processing and the like are performed.

  Next, measurement work using the ground exploration apparatus 1 will be described with reference to the flowchart of FIG. Before the start of the measurement work, the operator of the ground exploration apparatus 1 installs the vibrator 11 and the detectors 12a, 12b, and 12c on the ground 5 to be measured as shown in FIG. 3 and the connection box 13 are set up in advance.

  In the measurement work, the operator first operates the operation unit 26 and inputs necessary measurement condition data to the personal computer 4 (S101 in FIG. 2). The measurement condition data includes a range in which the distance between the excitation point 6 and the detector 12a, the distances L1 and L2 (see FIG. 1) between the detectors 12a, 12b, and 12c, and the drive frequency of the exciter 11 are changed. And a magnitude (step frequency) for changing the frequency of the exciter 11 at a time. The frequency range is, for example, a range of 3 to 250 Hz. The input measurement condition data is appropriately stored in the memory 22.

  When the input of measurement conditions is completed, the first frequency is set in the signal processor 3 by the CPU 21 (S102). Subsequently, in response to an instruction from the CPU 21, a drive signal (sine wave signal) at the set frequency is transmitted from the signal processor 3 to the exciter 11, and the exciter 11 is driven (S103). As a result, the vertical harmonic vibration is applied to the ground 5 at the excitation point 6.

  Immediately after driving the exciter 11, the CPU 21 instructs the signal processor 3 to start measurement (S104). Then, the signal processing unit 52 starts measurement processing (S105 to S107) of vibrations obtained from the detectors 12a, 12b, and 12c.

  Specifically, the signal processor 3 receives the signal of the detector 12a to acquire the amplitude W1 at that time, and the vibration wave generated by the exciter 11 is detected by the detector 12a. The delay time ΔA until is calculated and acquired. This is also performed in the other detectors 12b and 12c, and the amplitude W2 and the delay time ΔB in the detector 12b, and the amplitude W3 and the delay time ΔC in the detector 12c are obtained.

  The amplitudes W1 to W3 and the delay times ΔA to ΔC at the points of the detectors 12a, 12b, and 12c acquired in the above processing (S105 to S107) are transmitted from the signal processor 3 to the personal computer 4, 4 is stored in the memory 22 as needed.

  Next, the velocity of the wave between the two adjacent detectors 12a and 12b is calculated (S108). Specifically, the time difference Δt1 when the wave reaches the detectors 12a and 12b is obtained by calculating the difference in the delay time (Δt1 = ΔB−ΔA). Then, by dividing the interval L1 between the detectors 12a and 12b by the time difference Δt1, the wave velocity V1 between the detectors 12a and 12b is obtained (V1 = L1 / Δt1).

  Further, the obtained wave velocity V1 is substituted into a known equation of D1 = V1 / (K × F), and the depth D1 in the lower region between the detectors 12b and 12a is calculated (S109). In the above equation, F is a frequency and K is a conversion constant. The calculated speed V1 and depth D1 are appropriately stored in the memory 22 of the personal computer 4.

  Next, the velocity of the wave between the two detectors 12b and 12c is calculated (S110). Specifically, the difference Δt2 in which the waves have reached the detectors 12b and 12c is obtained by calculating the difference in delay time (Δt2 = ΔC−ΔB), as in the process of S108. Then, by dividing the interval L2 between the detectors 12b and 12c by the time difference Δt2, the wave velocity V2 between the detectors 12c and 12b is obtained (V2 = L2 / Δt2).

  Further, the obtained wave velocity V2 is substituted into the equation D2 = V2 / (K × F) to calculate the depth D2 in the lower region between the detectors 12b and 12c (S111). The calculated speed V2 and depth D2 are appropriately stored in the memory 22 of the personal computer 4.

  Next, it is determined whether measurement of all frequencies is completed (S112). When the measurement is not completed, a frequency obtained by adding the step frequency to the current frequency is set in the signal processor 3 (S113), the exciter 11 is driven at the new frequency (S103), and S104 is again performed. The process of S111 is performed and various data are calculated and acquired. Thus, by repeating the calculation of data while changing the frequency step by step, the amplitudes W1, W2, W3, wave speeds V1, V2, and depths D1, D2 are obtained for each frequency, and the memory 22 can be accumulated one after another.

  As a result of the repetition of the process, when it is determined that the measurement at all frequencies is completed in the determination of S112, the exciter 11 is stopped (S114), and the measurement conditions and calculation results stored in the memory 22 are changed. For example, the data file is stored in the hard disk 23 (S115).

  Note that the measurement conditions stored in this data file include the contents of the measurement condition data input in the process of S101. The measurement results stored in the data file include the relationship between amplitude and frequency (F-W1 characteristic, F-W2 characteristic, and F-W3 characteristic), and the relationship between speed and frequency (F-V1 characteristic and F-V2). Characteristics) and the relationship between speed and depth (D1-V1 characteristics and D2-V2 characteristics). After the saving is completed, the measurement work is finished.

  Next, the analysis work for analyzing the data obtained by the above measurement work will be described with reference to the flowcharts of FIGS. This analysis work may be performed by the personal computer 4 shown in FIG. 1 (personal computer that has executed the measurement process shown in FIG. 2), but this time, the case where it is performed by another computer will be described as an example.

  This computer (ground structure estimation device) includes a CPU as an operation unit, a memory and a hard disk as a storage unit, a display and printer as an output unit, a keyboard and a mouse as an operation unit, and the like. An appropriate analysis program is installed on the hard disk of the computer.

  When the user operates the computer and the analysis program is read into the memory and executed, the following analysis work is performed. In this analysis work, first, the data file saved in the process of S115 of the measurement work is read and stored in an appropriate memory of a personal computer (S201).

  Then, the phase velocity characteristic of the ground 5 is calculated from the relationship between the read velocity and frequency (F-V1 characteristic) (S202). Furthermore, the dominant frequency of the ground 5 is calculated from the relationship between the read amplitude and frequency (F-W1 characteristic) (S203).

  Here, the dominant frequency means a frequency that shows a peak in ground vibration. For example, if the relationship between the amplitude and the frequency is represented by a graph as shown in FIG. 7, since the amplitude shows a peak at a frequency near 5 Hz in this graph, the dominant frequency is determined to be 5 Hz.

  Next, the average value processing or the like is performed on the F-V1 characteristic data to remove noise, the frequency-depth characteristic is calculated again, and the D1′-V1 ′ characteristic is calculated based on this. (S204 in FIG. 3). Then, based on the point (inflection point) where the phase velocity V1 ′ shows the minimum value in the D1′−V1 ′ characteristic, the layer thickness of the ground and the S wave velocity of each layer are calculated to obtain the temporary ground structure (S205). .

  FIG. 5 shows an example in which the layer thickness of the ground and the S wave velocity of each layer are obtained from the D1′-V1 ′ characteristics. FIG. 5 is a display example of a display of a computer, and shows a state in which the D1′-V1 ′ characteristic (black dot) is displayed in a graph format. As shown in FIG. 5, the inflection points of the D1'-V1 'characteristic plotted by the user are displayed on the display as white circles. This inflection point may be automatically determined and plotted on the computer side.

  When the user performs an appropriate operation in this state, it is assumed that the plotted inflection point positions indicate the layer thickness and S wave velocity, and the layer thickness and S wave velocity (temporary ground structure) of each layer are obtained. Calculated. As shown in FIG. 5, the obtained depth-S wave velocity graph of the temporary ground structure can be superimposed on the D1'-V1 'characteristic graph and displayed on the display. The depth-S wave velocity graph depicted in FIG. 5 employs an arithmetic average of the velocities of two inflection points across the formation as the S wave velocity of the formation. However, the method is not limited to this calculation method, and the S wave velocity may be obtained by, for example, the mean square.

  The calculated temporary ground structure includes data on the number and thickness of the strata near the ground surface and the elastic constants in each stratum. This temporary ground structure can be used as a basis for parameter setting when a hypothetical ground model described later is first created.

  After obtaining the temporary ground structure as described above, a ground model as a hypothesis is created as a starting point for obtaining the ground estimation result (S206 in FIG. 3). Hereinafter, this ground model is referred to as a “hypothetical ground model”. The parameters of this hypothetical ground model are composed of the number of ground layers, the thickness of each layer, and physical property values (S wave velocity, P wave velocity, density, material damping constant).

  In the process of S206, the parameters of the hypothetical ground model are given as follows. That is, in the phase velocity characteristic obtained by the process of S202, the peak of the phase velocity appears in the vicinity of the dominant frequency obtained by the process of S203, and the second peak of the phase velocity appears in a frequency region larger than that. appear. For example, if the relationship between the phase velocity and the frequency is represented by a graph as shown in FIG. 8, a peak of the phase velocity appears in the vicinity of the dominant frequency (5 Hz), and the phase velocity at a higher frequency (38 Hz). It can be seen that the second peak appears. And in the area | region R pinched | interposed into the frequency which shows these two peaks, the phase velocity has shown the substantially constant value.

  In the present embodiment, a value based on the phase velocity in the frequency range R showing the substantially constant tendency (specifically, the maximum value of the phase velocity in the range R) is applied to the ground surface in the hypothetical ground model. It is set as a parameter of the S wave velocity of the nearest first layer. For other parameters of the hypothetical ground model (for example, P wave velocity of the first layer, P wave velocity of the second layer and after, S wave velocity, etc.), the value of the temporary ground structure obtained in S205 is set. To do.

This hypothetical ground model setting method is based on the following knowledge. That is, according to the wave field analysis method (see the above paper) of the Heisei layer ground using the stiffness matrix, the transfer function on the ground surface when the elastic wave propagates in the ground and appears on the ground surface is the excitation point. When the distance from 6 is x and the angular velocity is ω, it is expressed by the following equation (1).

  In the above formula (1), R is a real part and I is an imaginary part.

Further, when the phase velocity (speed of movement of the point where the phase angle θ is constant) is expressed as a function c (x, ω) of the distance x from the excitation point 6 and the angular velocity ω, the following equation (2) become that way.

  As can be seen from the above equation, in the ground structure estimation method of the present embodiment, the transfer function and the phase velocity are calculated by considering not only the angular velocity ω but also the distance x from the excitation point 6 as a parameter. . Therefore, according to this method, a constant result can be obtained regardless of the distance from the excitation point, and the ground structure with high reliability and reproducibility can be estimated.

  Further, as can be seen from the equation (2), the phase velocity c (x, ω) is given by differentiation with respect to the distance x from the excitation point 6. Therefore, if the phase angle θ is continuous with respect to the distance x, the phase velocity changes smoothly, but if the phase angle θ is discontinuous with respect to the distance x, the phase velocity at that point becomes very large. I can say that.

Here, the inventor of the present application provided a sample ground having a heisei layered ground structure (layer thickness) as shown in FIG. 6 and its physical properties (S wave velocity, P wave velocity, ground density of each layer, material damping constant). A model was set up. This sample ground model is a three-layer water Heisei ground model lying on semi-infinite ground. In FIG. 6, H is the layer thickness, C _S is the S wave velocity, C _P is the P wave velocity, ρ is the ground density of each layer, and Q is the material damping constant.

  In the sample ground model shown in FIG. 6, a vertical harmonic vibration load is applied to one point (excitation point) on the ground surface, and based on the equations derived from the stiffness matrix method (the above two equations), The transfer function and phase velocity of the ground in the vertical direction of the surface were calculated by simulation.

  7 and 8 show the results of the simulation calculation. FIG. 7 shows the calculation result of the transfer function at a point 1 meter away from the excitation point for each frequency. FIG. 8 shows the calculation result of the phase velocity at a point 1 meter away from the excitation point for each frequency.

  As shown in FIG. 7, the transfer function of the ground in the sample ground model shows a peak around 5 Hz, and it can be seen that the dominant frequency (primary natural frequency) of the ground is about 5 Hz.

  Further, as shown in FIG. 8, a peak appears in the phase velocity in the vicinity of 3 Hz, which is slightly smaller than the dominant frequency. Note that there is a slight deviation between the peak frequency and the dominant frequency of the ground. The vertical axis of the graph of FIG. 8 is the speed, and the slope (that is, the change per unit time) is large. However, this is because the phase velocity peaks.

Considering the graph of FIG. 8 further, in the frequency region where the phase velocity is higher than the peak frequency, the phase velocity shows a relatively constant value, while the phase velocity is extremely large at about 38 Hz. It can be seen from the graph that the value of the relatively constant phase velocity from the vicinity of the dominant frequency of the ground to about 38 Hz is about 80 m / s at the maximum. Furthermore, rate of the 80 m / s is found to be consistent with the S-wave velocity C _S of the ground (first layer) in the vicinity of the ground surface in the sample ground model of FIG.

  As described above, in the frequency characteristic of the phase velocity (FIG. 8), the phase velocity in the frequency region from the peak value near the dominant frequency of the ground to the extremely large second peak value is the S of the ground near the ground surface. It can be inferred that it is equal to the wave velocity. Such a relationship has not yet been explained theoretically and clearly at the time of filing of the present application. However, since this relationship is recognized not only in the case of the sample ground model described here but also in the case where the present inventor has calculated a plurality of other models, it is not a coincidence and is not publicly known. This is considered an inevitable characteristic of the species.

  In the process of S206 (FIG. 3) of the present embodiment, in view of the above discovery, the phase velocity shows a peak near the dominant frequency as the parameter of the S wave velocity of the first layer closest to the ground surface in the initial hypothetical ground model. The maximum value of the phase velocity showing a certain tendency is set in the frequency region sandwiched between the vibration frequency and the vibration frequency that is higher than that and exhibits the peak (second peak) phase velocity. . Thereby, it is possible to satisfactorily reduce the amount of calculation for obtaining the estimation result ground model described later starting from the initial hypothetical ground model.

  Next, it is assumed that the detectors 12a, 12b, and 12c are installed in the hypothetical ground model so as to have the same positional relationship as when the actual ground 5 is measured (FIG. 1) (S208). Then, simulation calculation is performed on signal data of the detectors 12a, 12b, and 12c when the excitation point 6 of the hypothetical ground model is vibrated by a sine wave. In this simulation calculation, the conditions input in S101 during measurement work (interval between the exciter 11 and the detector 12a, intervals between the detectors 12a, 12b, and 12c, a range of frequencies for driving the exciter 11, etc.) ) Is used.

  Specifically, the same frequency as the first frequency in the measurement condition is set as the frequency of the excitation point 6 (S209). Then, signal data obtained by the detectors 12a, 12b, and 12c when the harmonic vibration at the set frequency is applied to the excitation point 6 is calculated (S210). Then, using the formulas derived from the stiffness matrix method (the above formulas (1) and (2)), the transfer function, the phase velocity, and the like are calculated from the calculation results in S210 (S211). The calculated result is appropriately stored in the memory of the computer.

  Then, the calculation of the transfer function and the phase velocity described above is repeated while changing the frequency step by step (S212, S213). Thereby, the frequency characteristic of the transfer function and the frequency characteristic of the phase velocity are obtained.

  Next, the dominant frequency of the ground model is obtained from the frequency characteristic of the transfer function (S214). Further, an approximate value of the S wave velocity near the ground surface is obtained from the frequency characteristic of the phase velocity (S215).

  Subsequently, the dominant frequency of the hypothetical ground model calculated in S214 is compared with the dominant frequency calculated from the actual ground 5 in S203 (S216). If the difference between the dominant frequencies is larger than the preset allowable range, at least one of the hypothetical ground model layer thickness or ground constant is changed (S217), and the process returns to S207. The processes of S207 to S215 are performed again for the hypothetical ground model. It should be noted that an appropriate calculation formula stored in advance in a computer is used to change the layer thickness and the ground constant in S217.

  If it is determined in S216 that the difference between the dominant frequency of the hypothetical ground model and the dominant frequency calculated in S203 is within the allowable range, the hypothetical ground model is adopted as the second hypothetical ground model (S218). ).

  Next, the calculation of S208 to S215 in FIG. 3 is similarly performed on the second hypothetical ground model to calculate the phase velocity characteristic (S219 in FIG. 4). Then, the phase velocity characteristic of the second hypothetical ground model calculated in the process of S219 is compared with the phase velocity characteristic calculated from the actual ground 5 in the process of S202 (S220). In this determination, if the difference in phase velocity characteristics is larger than the preset allowable range, at least one of the layer thickness or the ground constant of the second hypothetical ground model is changed (S221), and the process returns to S219. The process of S219 is performed again for the second hypothetical ground model. It should be noted that an appropriate calculation formula stored in advance in a computer is used to change the layer thickness and the ground constant in S221.

  If it is determined in S220 that the difference between the phase velocity characteristic of the second hypothetical ground model and the phase velocity characteristic calculated in S202 is within an allowable range, the second hypothetical ground model is used as the estimation result ground model. Adopt (S222). Then, the layer thickness and the ground constant of the obtained estimation result ground model are displayed on a computer display or printed from a printer (S223).

  Thereafter, the ground structure (layer thickness, S wave velocity of each layer, etc.) of the estimation result ground model is output according to the user's operation (S224). As an output format of this ground structure, for example, outputting as a list of parameters as shown in FIG. 6 can be considered.

  Also, various information useful for the ground structure analysis is output according to the user's operation (S225). Examples of information that can be output include the relationship between frequency and transfer function, the relationship between frequency and phase velocity, the relationship between depth and phase velocity, the relationship between depth and S wave velocity, and the relationship between depth and ground structure. Conceivable. The information output format may be a graph format or a table format. FIG. 9 shows an example in which the relationship between the depth and the ground structure (layer thickness and S wave velocity) is output in a graph format.

  Thus, a series of analysis processes is completed. The user can further analyze and consider the information on the ground model of the estimation result obtained in this way and various information based on the information, thereby providing useful information for the foundation of civil engineering and construction, structural design, seismic design, or earthquake countermeasures. Can be obtained.

  As described above, in the ground structure estimation method of this embodiment, the ground 5 is vibrated by the exciter 11 while changing the vibration frequency, and elastic waves are detected by the detectors 12a, 12b, and 12c. Then, based on the output signals of the detectors 12a, 12b, and 12c, the frequency characteristic and the dominant frequency of the phase velocity in the ground are calculated (measurement work flow in FIG. 2 and S202 and S203 in FIG. 3). And while creating a hypothetical ground model, in the frequency characteristics of the phase velocity, between the phase velocity peak near the dominant frequency and the phase velocity peak in the region where the frequency is greater than the dominant frequency. Based on the phase velocity in the frequency domain, the S wave velocity of the selected layer (the layer near the ground surface) among the plurality of layers of the hypothetical ground model is determined (S206 in FIG. 3). Furthermore, the difference between the dominant frequency calculated from the hypothetical ground model and the dominant frequency calculated in the first step is within a predetermined range, and the frequency of the phase velocity calculated from the hypothetical ground model Calculation of frequency characteristics of dominant frequency and phase speed while changing parameters of the hypothetical ground model until the difference between the characteristics and the frequency characteristics of the phase velocity calculated in the first step is within a predetermined range. Are repeated, and when the difference falls within a predetermined range, the hypothetical ground model is set as an estimation result ground model (S207 in FIG. 3 to S222 in FIG. 4). Thereafter, the estimation result ground model is output (S223).

  In this method, not only Rayleigh waves but also complex waves (in general, elastic waves) including other waves are searched and analyzed, so that a constant estimation result can be obtained regardless of the distance from the excitation point 6. In particular, even when a detector is installed in the vicinity of the excitation point 6, unlike the conventional surface wave exploration, a highly reliable estimation result with few errors can be obtained. Therefore, even when a large space cannot be secured for the measurement work, it is possible to estimate with high accuracy. Similarly, accurate estimation is possible even when using vibrations in the low frequency region. In addition, since the S wave velocities of several layers are appropriately determined in the hypothetical ground model at the time of creation, it is possible to reduce the number of calculation iterations until the estimation result ground model is derived from the hypothetical ground model by inverse analysis. Accordingly, the calculation time and calculation load for obtaining the estimation result ground model can be reduced. Furthermore, in this method, the ground in the ground can be estimated using the phase velocity and ground vibration characteristics (only) observed between two points in the vicinity of the vibrator 11. Accordingly, an exciter having a strong excitation force is not required, and the ground can be accurately estimated with a small number of detectors.

  Further, in the present embodiment, in the processing of S206, in the frequency characteristics of the phase velocity, between the phase velocity peak near the dominant frequency and the phase velocity peak in a region where the frequency is higher than the dominant frequency. The maximum value of the phase velocity in the frequency region at is set as the S wave velocity of the hypothetical ground model layer (layer near the ground surface).

  Thereby, since the S wave velocities of several layers are more appropriately determined in the hypothetical ground model at the time of creation, the calculation time and the calculation load for deriving the estimation result ground model from the hypothetical ground model can be further reduced.

  In the present embodiment, when the estimation result ground model is derived from the initial hypothetical ground model, the prevailing vibration frequency is calculated in the hypothetical ground model (S214), and the prevailing vibration frequency of this hypothetical ground model and the processing of S203 are calculated. Until the calculated difference in the dominant frequency falls within the predetermined range, the calculation of the dominant frequency is repeated while changing the parameters of the hypothetical ground model (S207 to S217), and the difference in the dominant frequency falls within the predetermined range. If this is the case, the hypothetical ground model is set as the second hypothetical ground model (S218). Thereafter, the frequency characteristic of the phase velocity is calculated in the second hypothetical ground model (S219), and the frequency characteristic of the phase velocity of the second hypothetical ground model and the frequency characteristic of the phase velocity calculated in the first step are calculated. Until the difference in phase is within the predetermined range, the calculation of the frequency characteristic of the phase velocity is repeated while changing the parameters of the hypothetical ground model (S219 to S211), and the difference in the frequency characteristic of the phase velocity is within the predetermined range. If this happens, the second hypothetical ground model is used as the estimation result ground model (S222).

  In this way, from the initial hypothetical ground model, a second hypothetical ground model having the same dominant frequency is first derived, and the estimation result ground model is made to match the frequency characteristics of the phase velocity of the second hypothetical ground model. Thus, the estimation result ground model can be easily obtained with a small amount of calculation. Therefore, calculation time and calculation load can be reduced.

  Moreover, in this embodiment, the space | interval of the exciter 11 installed in the ground surface of the target ground 5 and the said detector 12a nearest to the said exciter 11 is set to 10 m or less.

  That is, the ground structure estimation method according to the present embodiment is reliable even when the distance between the vibrator 11 and the detectors 12a, 12b, and 12c has to be narrowed as described above due to circumstances such as work space. This is particularly useful in that a reliable estimation result can be obtained.

  In the present embodiment, the dominant frequency of the hypothetical ground model is calculated by the stiffness matrix method in the process of S214, and the frequency characteristic of the phase velocity is calculated by the stiffness matrix method in the process of S219.

  Thereby, the stability of the numerical calculation solution in the hypothetical ground model can be easily secured, and the calculation can be performed with a relatively simple program.

  Although the preferred embodiment of the present invention has been described above, the above configuration can be changed as follows, for example.

  In the above embodiment, the case where the stiffness matrix method is employed for numerical calculation of the phase velocity and the like is described. However, since the elastic wave field analysis method includes a transmission matrix method, a reflection / transmission matrix method, and the like in addition to the stiffness matrix method, it can be changed to perform numerical calculation using these methods. However, when using the stiffness matrix method, compared to the transfer matrix method and the reflection / transmission matrix method, there are various advantages such as easy stability of the numerical solution and the ability to calculate with a relatively simple program. This is preferable.

  In the above embodiment, the S wave velocity of the first layer (the layer closest to the ground surface) in the hypothetical ground model is set based on the phase velocity characteristic obtained in the processing of S202 in the processing of S206 in FIG. ing. However, not limited to the first layer of the hypothetical ground model, for example, S wave velocities (one or more layers arbitrarily selected in the hypothetical ground model) are set based on the phase velocity characteristics. You may make it do.

  In the above embodiment, in the process of S206 in FIG. 3, parameters other than the S wave velocity of the first layer in the hypothetical ground model are set based on the temporary ground structure obtained in the process of S205. However, the present invention is not limited to this. For example, a random number generated by a computer can be set as the parameter.

  The calculation of the phase velocity characteristic of the ground (the process of S202) can also be obtained by dividing the phase lag between adjacent detectors by the distance between the detectors. Or it can also obtain | require by dividing | segmenting the phase delay from the vibrator 11 to a sensor by the distance from the vibrator 11 to a sensor.

  In the analysis work of FIGS. 3 and 4, only the analysis of the data obtained from the two detectors 12 a and 12 b on the side close to the vibrator 11 has been described. For example, the data obtained from the detectors 12 b and 12 c Of course, the same analysis can be performed.

  In the measurement operation described with reference to FIG. 1, four or more detectors can be arranged, or only one or two detectors can be arranged.

Schematic which shows a mode that a vibrator and a detector are installed in the ground and it measures. The flowchart explaining measurement work. The flowchart explaining the first half of analysis work. The flowchart explaining the latter half part of analysis work. The figure which shows a mode that a temporary ground structure is calculated | required from the depth-speed characteristic. Explanatory drawing which shows the sample ground model which this inventor used for calculation. The graph which shows the transfer function calculated from the sample ground model. The graph which shows the frequency characteristic of the phase velocity calculated from the sample ground model. The figure showing the example of an output of the ground structure of an estimation result model.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ground exploration apparatus 3 Signal processor 4 Personal computer 5 Ground 6 Excitation point 11 Exciter 12a, 12b, 12c Detector

Claims (10)

An exciter and a detector are installed on the ground surface of the target ground, the ground is vibrated with the exciter, and the ground structure in the ground based on a signal output by the detector detecting an elastic wave In the ground structure estimation method for estimating
The ground is vibrated by the exciter while changing the vibration frequency, and the detector detects an elastic wave. Based on the output signal of the detector, the frequency characteristics of the phase velocity and the dominant vibration in the ground A first step of calculating a number;
In addition to creating a hypothetical ground model, the phase velocity peak in the vicinity of the dominant frequency in the frequency characteristic of the phase velocity obtained in the first step, and the phase velocity peak in the region where the frequency is greater than the dominant frequency. A second step of determining an S-wave velocity of at least one layer of the hypothetical ground model based on a phase velocity in a frequency region between
The difference between the dominant frequency calculated from the hypothetical ground model and the dominant frequency calculated in the first step is within a predetermined range, and the frequency characteristics of the phase velocity calculated from the hypothetical ground model are as follows: The calculation of the dominant frequency and the phase velocity frequency characteristics is repeated while changing the parameters of the hypothetical ground model until the difference between the phase velocity frequency characteristics calculated in the first step falls within a predetermined range. , A third step in which the hypothetical ground model is used as an estimation result ground model when the difference falls within a predetermined range;
A fourth step of outputting the estimation result ground model;
A ground structure estimation method comprising: The ground structure estimation method according to claim 1,
The second step includes
In the frequency characteristics of the phase velocity, the maximum value of the phase velocity in the frequency region between the phase velocity peak near the dominant frequency and the phase velocity peak in the region where the frequency is greater than the dominant frequency is A ground structure estimation method, characterized in that it is set as an S wave velocity of at least one layer of the hypothetical ground model. The ground structure estimation method according to claim 1 or 2,
The third step includes
Calculate the dominant frequency in the hypothetical ground model, and change the parameters of the hypothetical ground model until the difference between the dominant frequency of the hypothetical ground model and the dominant frequency calculated in the first step is within a predetermined range. While repeating the calculation of the dominant frequency, and when the difference of the dominant frequency is within a predetermined range, the hypothetical ground model as a second hypothetical ground model,
A frequency characteristic of the phase velocity is calculated in the second hypothetical ground model, and a difference between the frequency characteristic of the phase velocity of the second hypothetical ground model and the frequency characteristic of the phase velocity calculated in the first step is within a predetermined range. Until the value is within, repeat the calculation of the frequency characteristics of the phase velocity while changing the parameters of the hypothetical ground model, and if the difference in the frequency characteristics of the phase velocity falls within the predetermined range, Step B as the estimation result ground model,
A ground structure estimation method comprising: A ground structure estimation method according to any one of claims 1 to 3,
A ground structure estimation method, wherein an interval between the vibrator installed on the ground surface of the target ground and the detector closest to the vibrator is 10 m or less. A ground structure estimation method according to any one of claims 1 to 4,
In the third step, a ground structure estimation method, wherein the frequency characteristics of the dominant frequency and phase velocity of the hypothetical ground model are calculated by a stiffness matrix method. A vibrator and a detector are installed on the ground surface of the target ground, the ground is vibrated by the vibrator, and the ground is grounded based on a signal detected and output by the detector. In the ground structure estimation device used for the ground structure estimation method to estimate,
Based on the data when the ground is vibrated by the vibrator and the detector detects an elastic wave while changing the vibration frequency, the frequency characteristics and the dominant frequency of the phase velocity in the ground are calculated. ,
In addition to creating a hypothetical ground model, in the obtained frequency characteristics of the phase velocity, between the phase velocity peak near the dominant frequency and the phase velocity peak in the region where the frequency is greater than the dominant frequency. On the basis of the phase velocity in the frequency region of at least any one layer of the hypothetical ground model,
The difference between the dominant frequency calculated from the hypothetical ground model and the dominant frequency calculated in the first step is within a predetermined range, and the frequency characteristics of the phase velocity calculated from the hypothetical ground model are as follows: The calculation of the dominant frequency and the phase velocity frequency characteristics is repeated while changing the parameters of the hypothetical ground model until the difference between the phase velocity frequency characteristics calculated in the first step falls within a predetermined range. , If the difference falls within the specified range, the hypothetical ground model is the ground result ground model,
A ground structure estimation device configured to be capable of outputting the estimation result ground model. The ground structure estimation device according to claim 6,
In the frequency characteristics of the phase velocity, the maximum value of the phase velocity in the frequency region between the phase velocity peak near the dominant frequency and the phase velocity peak in the region where the frequency is greater than the dominant frequency is A ground structure estimation apparatus, wherein the ground structure estimation device is set as an S-wave velocity of at least one layer of the hypothetical ground model. The ground structure estimation device according to claim 6 or 7,
When obtaining the estimated ground model from the hypothetical ground model,
Calculate the dominant frequency in the hypothetical ground model, and change the parameters of the hypothetical ground model until the difference between the dominant frequency of the hypothetical ground model and the dominant frequency calculated in the first step is within a predetermined range. While repeating the calculation of the dominant frequency, when the difference in the dominant frequency falls within the predetermined range, the hypothetical ground model is made the second hypothetical ground model,
A frequency characteristic of the phase velocity is calculated in the second hypothetical ground model, and a difference between the frequency characteristic of the phase velocity of the second hypothetical ground model and the frequency characteristic of the phase velocity calculated in the first step is within a predetermined range. Until the value is within, repeat the calculation of the frequency characteristics of the phase velocity while changing the parameters of the hypothetical ground model, and if the difference in the frequency characteristics of the phase velocity falls within the predetermined range, A ground structure estimation device characterized by using a ground model as an estimation result. The ground structure estimation device according to any one of claims 6 to 8,
A ground model for obtaining an estimation result ground model based on data when an interval between the vibrator installed on the ground surface of the target ground and the detector closest to the vibrator is 10 m or less Structure estimation device. The ground structure estimation device according to any one of claims 6 to 9,
A ground structure estimation device characterized by calculating the frequency characteristics of the dominant frequency and phase velocity of the hypothetical ground model by a stiffness matrix method.

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