CN111551639B - Method for measuring soil elastic wave dispersion curve - Google Patents

Method for measuring soil elastic wave dispersion curve Download PDF

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CN111551639B
CN111551639B CN202010403323.0A CN202010403323A CN111551639B CN 111551639 B CN111551639 B CN 111551639B CN 202010403323 A CN202010403323 A CN 202010403323A CN 111551639 B CN111551639 B CN 111551639B
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高艳
李新宏
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Changzhou Changgong Electronic Technology Co ltd
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Abstract

The invention belongs to the technical field of material or object detection, and particularly relates to a method for measuring a soil elastic wave dispersion curve, which comprises the following steps: designing a sound source frequency modulation signal according to the measurement requirement; selecting a proper arrangement mode of the earth surface sound source and the two sensors according to the fact that the measured elastic wave is a horizontal transverse wave or a surface wave; through sound source excitation, two sensors synchronously acquire ground vibration signals, and calculate the short-time cross-correlation coefficient of the two vibration signals; judging a main lobe of the short-time cross-correlation coefficient, and determining a cross-correlation delay curve; and calculating and drawing a dispersion curve of the elastic waves in the soil. The method reduces the influence of algorithm parameters on the dispersion curve, obtains the unique and stable dispersion curve, solves the problem of low signal-to-noise ratio of partial frequency bands, and has higher frequency resolution; under the condition of ensuring the frequency resolution of the dispersion curve, the short-time cross-correlation is used for time delay estimation, so that the calculation complexity and the calculated amount are reduced, and the difficulty in parameter selection is smaller.

Description

Method for measuring soil elastic wave dispersion curve
Technical Field
The invention belongs to the technical field of material or object detection (geophysical exploration/shallow buried object detection), and particularly relates to a method for measuring a soil elastic wave dispersion curve.
Background
The soil belongs to a multi-phase particle medium and has dispersion characteristics, namely the wave speed of elastic waves changes along with the frequency. The dispersion curve of the soil elastic wave is an important parameter in the shallow buried object detection technology, provides important information for shallow buried object detection, and particularly lays a foundation for eliminating wake wave influence, imaging ghost and the like.
The existing dispersion curve measurement technology can be divided into a frequency spectrum phase curve method and an iterative matching method:
(1) Spectral phase curve method: the method is widely applied to the measurement of the dispersion curve of electromagnetic waves and elastic waves, pulse or sweep frequency and other broadband excitation signals are mostly adopted, a vibration signal is collected by using a laser vibration pickup instrument or a vibration sensor, the time delay is estimated by calculating the slope of a phase-frequency curve between vibration signals at different positions, and the optical path or the acoustic path difference is divided by the time delay to obtain the dispersion curve. However, in the application of measuring the elastic wave dispersion curve of the soil, when the method is used for solving the slope of the phase curve, the slope result is influenced by the width of a curve fitting interval, namely the widths of curve fitting are different, and the dispersion curves are different; in addition, the method is limited by the frequency response of the excitation transducer and the spectral characteristics of the excitation signal, and the signal-to-noise ratio is low in some frequency bands.
(2) An iterative matching method: the method is applied to surface wave dispersion curve measurement, the generalized Hooke coefficient tensor of each frequency of the material is adjusted in the calculation process, and the forward modeling result is compared with the actual measurement result; and obtaining a dispersion curve through multiple iterations. The method adopts a frequency spectrum phase curve method when processing measured data, so that the method has the same problem as the frequency spectrum phase method, and relates to forward evolution and inversion of elastic waves, and has the advantages of large calculation amount, high calculation complexity and sensitivity to parameters.
In view of the problems of the two methods, the inventor of the present invention has actively researched and innovated based on the practical experience and professional knowledge that are abundant for years in engineering application of such products and with the application of theory, in order to create a method for measuring the elastic wave dispersion curve of soil, so that the method has higher practicability.
Disclosure of Invention
The invention aims to provide a method for measuring a soil elastic wave dispersion curve, thereby effectively solving the problems in the background art.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a method for measuring a soil elastic wave dispersion curve comprises the following steps:
designing a sound source frequency modulation signal according to the measurement requirement;
selecting a proper arrangement mode of the earth surface sound source and the two sensors according to the fact that the measured elastic wave is a horizontal transverse wave or a surface wave;
through the excitation of the sound source, the two sensors synchronously acquire ground vibration signals, and the short-time cross-correlation coefficient of the two vibration signals is calculated;
judging a main lobe of the short-time cross-correlation coefficient, and determining a cross-correlation delay curve;
and calculating and drawing a dispersion curve of the elastic waves in the soil.
Further, the frequency modulation frequency of the sound source frequency modulation signal is a function of time.
Further, the change rate of the frequency modulation frequency of the sound source frequency modulation signal is adjusted according to the intensity of the soil elastic wave dispersion.
Further, when the elastic wave is a surface wave, a vertical point sound source is used for excitation;
use the sound source is the original point, and the direction on perpendicular to ground is an axial construction cylindrical coordinate system, two the sensor is followed cylindrical coordinate system radially sets up, and the two with the sound path of sound source is different, signal acquisition direction with cylindrical coordinate system radial direction is unanimous.
Further, when the measured elastic wave is a horizontal transverse wave, a horizontal point sound source is used for excitation, and the excitation direction of the sound source is parallel to the ground surface;
use the sound source is the original point, follows sound source excitation direction constitutes a rectangular coordinate system, two the sensor is along the perpendicular to sound source excitation direction's earth's surface is laid, and the two with the sound journey of sound source is different, signal acquisition direction with sound source excitation direction is unanimous.
Further, the short-time cross-correlation coefficient is defined as follows:
Figure BDA0002490326000000031
wherein x is 1 (t) represents the near-end sensor signal, x 2 (t) represents the far-end sensor signal,
Figure BDA0002490326000000032
is x 2 (t) co-complex conjugation, h (t) is the selected window function, τ is the cross-correlation time, and e is the window time.
Further, the excitation signal is amplified through a power amplifier and fed back to the vibration exciter after being amplified, the vibration exciter is installed on the earth surface through a metal base, and the excitation time is adjusted according to the radiation efficiency of the vibration exciter.
Further, the estimated time delay between the two sensors is obtained, the estimated time delay is marked in a short-time cross-correlation coefficient graph to form a straight line, a curve closest to the straight line is a main lobe of the short-time cross-correlation coefficient, and a curve corresponding to a peak value of the main lobe is the cross-correlation time delay curve.
Further, the estimated time delay between the two sensors is obtained through a cross-correlation or spectrum phase method.
Further, the relationship between the cross-correlation time delay curve and the soil elastic wave dispersion curve is as follows:
Figure BDA0002490326000000041
where r is the difference in acoustic path between the two sensors, F -1 (f) And the time corresponding to the frequency modulation frequency f of the sound source is represented, and tau (t) is a cross-correlation time delay curve.
Through the technical scheme of the invention, the following technical effects can be realized:
according to the invention, the influence of algorithm parameters on a dispersion curve is reduced, the short-time cross-correlation coefficient between two sensors is calculated by designing and exciting by using frequency modulation signals, a cross-correlation time delay curve is determined, and a unique and stable dispersion curve is obtained; when frequency modulation signals are designed, the problem of low signal-to-noise ratio of partial frequency bands is solved by increasing the excitation duration of corresponding frequency bands, and the frequency modulation signals have higher frequency resolution; under the condition of ensuring the frequency resolution of the dispersion curve, the time delay estimation is carried out by using short-time cross-correlation, and compared with an iterative matching algorithm, the method reduces the calculation complexity and the calculation amount, and the difficulty in parameter selection is smaller.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the description of the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments described in the present invention, and it is also possible for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
FIG. 1 is a flow chart of a measurement method of the present invention;
FIG. 2 is a schematic diagram of a surface wave dispersion curve measurement;
FIG. 3 is a schematic diagram of horizontal transverse wave dispersion curve measurement;
FIG. 4 shows the short-term cross-correlation results of an embodiment;
FIG. 5 is a graph comparing dispersion curves obtained by the spectral phase method and the measurement method of the present invention in the embodiment, in which the solid line is the result of the method of the present invention, and the dotted line is the result of the spectral phase method;
reference numerals: a vertical point sound source 201, a direction 202 perpendicular to the ground, a radial direction 203 of a cylindrical coordinate system, a first vibration sensor 204, a second vibration sensor 205, a horizontal point sound source 301, a sound source excitation direction 302, a direction 303 perpendicular to the sound source excitation direction, a third vibration sensor 304, and a fourth vibration sensor 305.
Detailed Description
The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.
As shown in fig. 1, a method for measuring a soil elastic wave dispersion curve includes the steps of:
s1: designing a sound source frequency modulation signal according to the measurement requirement;
s2: selecting a proper arrangement mode of the earth surface sound source and the two sensors according to the fact that the measured elastic wave is a horizontal transverse wave or a surface wave;
s3: through sound source excitation, two sensors synchronously acquire ground vibration signals, and calculate the short-time cross-correlation coefficient of the two vibration signals;
s4: judging a main lobe of the short-time cross-correlation coefficient, and determining a cross-correlation delay curve;
s5: and calculating and drawing a dispersion curve of the elastic waves in the soil.
As a preferred example of the foregoing embodiment, in step S3, the excitation signal is amplified by the power amplifier and fed back to the vibration exciter after being amplified, and the vibration exciter is installed on the earth surface through the metal base, wherein the excitation duration is adjusted according to the radiation efficiency of the vibration exciter, and specifically, the excitation duration of the corresponding frequency band should be increased in the frequency band with lower radiation efficiency of the vibration exciter.
As a preferred example of the above embodiment, in step S1, the frequency modulation frequency of the sound source frequency modulation signal is represented as F = F (t) as a function of time, wherein the change rate of the frequency modulation frequency of the sound source frequency modulation signal is adjusted according to the intensity of the soil elastic wave dispersion, and specifically, the frequency change rate of the frequency modulation signal should be reduced in a frequency band where the soil elastic wave dispersion is strong.
In the specific implementation process, in step S2, when the elastic wave is a surface wave, a vertical point sound source is used for excitation; a cylindrical coordinate system is constructed by taking a sound source as an original point and taking the direction perpendicular to the ground as the axial direction, the two sensors are radially arranged along the cylindrical coordinate system, the sound paths of the two sensors are different from the sound source, and the signal acquisition direction is consistent with the radial direction of the cylindrical coordinate system. Specifically, as shown in fig. 2, a cylindrical coordinate system is formed by using a vertical point sound source 201 as an origin and a direction 202 perpendicular to the ground as an axial direction; the first vibration sensor 204 and the second vibration sensor 205 are arranged in the radial direction 203 of the cylindrical coordinate system.
Or when the measured elastic wave is a horizontal transverse wave, a horizontal point sound source is used for excitation, and the excitation direction of the sound source is parallel to the ground surface; a rectangular coordinate system is formed along the excitation direction of the sound source by taking the sound source as the origin, the two sensors are distributed along the earth surface perpendicular to the excitation direction of the sound source, the sound paths of the two sensors and the sound source are different, and the signal acquisition direction is consistent with the excitation direction of the sound source. Specifically, as shown in fig. 3, a rectangular coordinate system is formed along a sound source excitation direction 302 with a horizontal point sound source 301 as an origin, and a third vibration sensor 304 and a fourth vibration sensor 305 are arranged on the ground in a direction 303 perpendicular to the sound source excitation direction.
As a preference of the above embodiment, in step S3, the short-time cross-correlation coefficient is defined as follows:
Figure BDA0002490326000000061
wherein x is 1 (t) represents the near-end sensor signal, x 2 (t) represents the remote sensor signal,
Figure BDA0002490326000000062
is x 2 Co-complex conjugate of (t), h (t) is the selected window function, τ is the cross-correlation time, and e is the window time.
Preferably, in step S4, the estimated time delay τ between two sensors is obtained predict . Will tau predict And marking the curve in the short-time cross-correlation coefficient graph to form a straight line, wherein the curve closest to the straight line is the main lobe of the short-time cross-correlation coefficient, and the curve corresponding to the peak value of the main lobe is a cross-correlation delay curve and is marked as tau (t). The estimated time delay between the two sensors can be obtained by means of cross-correlation or spectral phase, and can be excited by using noise or pulse signals.
As a preferable example of the above embodiment, in step S5, the relationship between the cross-correlation time delay curve and the soil elastic wave dispersion curve is as follows:
Figure BDA0002490326000000071
where r is the difference in acoustic path between the two sensors, F -1 (f) And the time corresponding to the frequency modulation frequency f of the sound source is represented, and tau (t) is a cross-correlation time delay curve.
In summary, in the technical scheme of the invention, before the dispersion curve is measured, the arrangement mode of the earth surface sound source and the sensor is selected according to the type of the elastic wave to be measured; for the excitation of an earth surface sound source, specifically, frequency modulation signal excitation is used, namely, a continuously and slowly-changed periodic signal is generated, so that in an earth surface wave field, the frequency modulation frequency of an elastic wave slowly changes, and due to the dispersion characteristic of soil, the time delay between two sensors also slowly changes along with time, and a time delay curve describing the time delay change along with time is formed.
In the process of collecting ground surface vibration signals, two vibration sensors are used for collecting ground surface vibration signals, direct wave acoustic path difference exists between the two vibration sensors, when the ground surface sensors are close to a sound source, the radiation field is mainly direct waves, and the direct wave speed can be obtained by time delay of the signals collected by the two sensors and the sensor acoustic path difference.
In the process of solving the dispersion curve, calculating the frequency modulation frequency of the elastic wave under each window time; and calculating the wave velocity by using the cross-correlation time delay of the window time and the sound path difference of the direct wave, taking the wave velocity as the wave velocity corresponding to the corresponding frequency point, and drawing a wave velocity-frequency graph to obtain a dispersion curve.
The above examples are given below to demonstrate the measurement results obtained by the measurement method:
when the horizontal transverse wave dispersion curve of the soil is measured, the measuring frequency range is 40Hz to 200Hz, an accelerometer is used as a sensor to collect ground vibration signals, and the sampling frequency is 32768Hz.
The concrete implementation steps and results of the soil horizontal transverse wave dispersion curve measurement are as follows:
1. in the design process of the frequency modulation signal, a 16s frequency sweep signal is designed according to the measurement requirement, and the frequency sweep frequency interval is 40Hz to 200Hz;
2. in the course of selecting the ground surface sound source and sensor arrangement mode, the measurement scheme shown in fig. 3 is selected. The distances between the third vibration sensor 304 and the fourth vibration sensor 305 and the horizontal point sound source 301 are 0.5m and 3m respectively, the signal collecting direction of the sensors is the sound source exciting direction 302, and the signal sampling rate is 32768Hz;
3. in the short-time cross-correlation coefficient solving process, a 2048-point rectangular window is used, the window time step is 1024 points, and the corresponding resolution is 0.0312s, so that a short-time cross-correlation curved surface graph is obtained, as shown in fig. 4;
4. in the process of obtaining the time delay curve, using 800Hz low-pass band noise-limited excitation, obtaining the estimated time delay between the two sensors through cross-correlation, wherein the estimated time delay is about 0.01892s, and the estimated time delay is shown as a semitransparent plane in FIG. 4; as can be seen, the estimated time delay τ predict The curve is most consistent with a peak value curve with the time of 0.02s in the short-time cross-correlation curved surface graph, the curve is judged to be a main lobe of the short-time cross-correlation coefficient according to the peak value curve, and the curve is determined to be a cross-correlation time delay curve and is marked as tau (t);
5. in the dispersion curve obtaining step, if it is known that the acoustic path difference between the two sensors is 2.5m, the elastic wave velocity between the sensors is:
Figure BDA0002490326000000091
the known sound source frequency modulation frequency as a function of time is:
Figure BDA0002490326000000092
obtaining an inverse function t = F of F = F (t) -1 (f) And the dispersion curve is obtained by substituting the relationship between the wave speed of the elastic wave and the time:
Figure BDA0002490326000000093
the dispersion curve plotted by the above formula is shown in fig. 5, the solid line in the graph is the result of the method of the present invention, the "+" shape is the result obtained by the spectrum phase curve method, the spectrum phase curve method in this embodiment uses a linear least square fitting algorithm, when calculating the phase velocity of each frequency point, the data of 20 frequency points are used, the corresponding frequency bandwidth is 40Hz, and the abscissa of the "+" shape is the midpoint of the corresponding frequency band.
As shown in fig. 5, the result of the spectrum phase curve has large fluctuation and is deviated from the method related to the present invention. This is due to the fact that the signal-to-noise ratio of elastic direct waves in soil is low in certain frequency bands, resulting in a shift in the phase result, which causes large fluctuations in the dispersion curve. Compared with the frequency spectrum phase curve method, the frequency resolution is much higher in the measuring method of the invention, the fluctuation range of the dispersion curve is smaller, and a smoother dispersion curve of the soil can be obtained through multiple measurements.
The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are given by way of illustration of the principles of the present invention, but that various changes and modifications may be made without departing from the spirit and scope of the invention, and such changes and modifications are within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.

Claims (6)

1. A method for measuring a soil elastic wave dispersion curve is characterized by comprising the following steps:
designing a sound source frequency modulation signal according to the measurement requirement;
selecting a proper arrangement mode of the earth surface sound source and the two sensors according to the fact that the measured elastic wave is a horizontal transverse wave or a surface wave;
through the excitation of the sound source, the two sensors synchronously acquire ground vibration signals, and the short-time cross-correlation coefficient of the two vibration signals is calculated;
judging a main lobe of the short-time cross-correlation coefficient, and determining a cross-correlation delay curve;
calculating and drawing a dispersion curve of elastic waves in soil;
when the elastic wave is a surface wave, exciting by using a vertical point sound source;
a cylindrical coordinate system is constructed by taking the sound source as an original point and taking the direction perpendicular to the ground as an axial direction, the two sensors are arranged along the radial direction of the cylindrical coordinate system, the sound paths of the two sensors and the sound source are different, and the signal acquisition direction is consistent with the radial direction of the cylindrical coordinate system;
the short-time cross-correlation coefficient is defined as follows:
Figure FDA0003787699450000011
wherein x is 1 (t) represents the near-end sensor signal, x 2 (t) represents the remote sensor signal,
Figure FDA0003787699450000012
is x 2 (t) co-complex conjugation, h (t) is a selected window function, τ is cross-correlation time, and epsilon is window time;
calculating the estimated time delay between the two sensors, marking the estimated time delay in a short-time cross-correlation coefficient graph to form a straight line, wherein a curve closest to the straight line is a main lobe of the short-time cross-correlation coefficient, and a curve corresponding to the peak value of the main lobe is the cross-correlation time delay curve;
and solving the estimated time delay between the two sensors by a cross-correlation or spectrum phase method.
2. A method for measuring a soil elastic wave dispersion curve is characterized by comprising the following steps:
designing a sound source frequency modulation signal according to the measurement requirement;
selecting a proper arrangement mode of the earth surface sound source and the two sensors according to the fact that the measured elastic wave is a horizontal transverse wave or a surface wave;
through the excitation of the sound source, the two sensors synchronously acquire ground vibration signals, and the short-time cross-correlation coefficient of the two vibration signals is calculated;
judging a main lobe of the short-time cross-correlation coefficient, and determining a cross-correlation delay curve;
calculating and drawing a dispersion curve of elastic waves in soil;
when the measured elastic wave is a horizontal transverse wave, a horizontal point sound source is used for excitation, and the excitation direction of the sound source is parallel to the ground surface;
the sound source is taken as an original point, a rectangular coordinate system is formed along the excitation direction of the sound source, the two sensors are distributed along the ground surface perpendicular to the excitation direction of the sound source, the sound path of the two sensors is different from that of the sound source, and the signal acquisition direction is consistent with the excitation direction of the sound source;
the short-time cross-correlation coefficient is defined as follows:
Figure FDA0003787699450000021
/>
wherein x is 1 (t) represents the near-end sensor signal, x 2 (t) represents the far-end sensor signal,
Figure FDA0003787699450000022
is x 2 (t) co-complex conjugation, h (t) is a selected window function, τ is cross-correlation time, and epsilon is window time;
calculating the estimated time delay between the two sensors, marking the estimated time delay in a short-time cross-correlation coefficient graph to form a straight line, wherein a curve closest to the straight line is a main lobe of the short-time cross-correlation coefficient, and a curve corresponding to the peak value of the main lobe is the cross-correlation time delay curve;
and solving the estimated time delay between the two sensors by a cross-correlation or spectrum phase method.
3. The method for measuring soil elastic wave dispersion curve according to claim 1 or 2, wherein the frequency modulation frequency of the sound source frequency modulation signal is a function of time.
4. The method for measuring a soil elastic wave dispersion curve according to claim 1 or 2, wherein a change rate of the frequency modulation frequency of the sound source frequency modulation signal is adjusted according to the severity of the soil elastic wave dispersion.
5. The method for measuring the soil elastic wave dispersion curve according to claim 1 or 2, wherein an excitation signal is amplified by a power amplifier and fed back to a vibration exciter, the vibration exciter is installed on the ground surface through a metal base, and the excitation time is adjusted according to the radiation efficiency of the vibration exciter.
6. The method for measuring a soil elastic wave dispersion curve according to claim 1 or 2, wherein the correlation time delay curve is related to the soil elastic wave dispersion curve as follows:
Figure FDA0003787699450000031
where r is the difference in acoustic path between the two sensors, F -1 (f) And represents the time corresponding to the frequency modulation frequency f of the sound source, and tau (t) is a cross-correlation time delay curve.
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