A method for depth positioning of buried non-metallic pipelines based on acoustic mutual information

Abstract

The present invention discloses a method for locating the depth of buried non-metallic pipelines based on acoustic mutual information, which relates to the field of buried pipeline detection. The method comprises the following steps: first, testing the wave velocity in a pipeline-free area, calculating the longitudinal wave velocity based on the ratio of the distance between longitudinal wave detectors to the received signal delay time and the Poisson's ratio of the soil; then, collecting acoustic wave signals in a pipeline-containing area; simultaneously, denoising the received signals using a collective empirical mode decomposition combined with a wavelet threshold algorithm; and extracting the reflected longitudinal wave signals; then, establishing a rectangular coordinate system, dividing the measurement surface, and calculating the longitudinal wave propagation time; further, calculating the frequency domain mutual information superposition value to obtain a superposition value matrix; and finally, imaging using the superposition value matrix to determine the pipeline depth. The present invention has a good effect in locating the depth of buried pipelines and is not restricted by geological conditions, providing a reliable technical means for the depth positioning of buried non-metallic pipelines.

Description

Technical Field

[0001] The present invention relates to the field of buried pipeline detection, and in particular to a method for depth positioning of buried non-metallic pipelines based on acoustic mutual information. Background Art

[0002] With the acceleration of urbanization and the expansion of underground infrastructure, non-metallic pipelines are widely used in critical pipeline networks such as gas and water supply due to their advantages such as light weight, corrosion resistance, and easy construction. However, due to their long service life, non-metallic pipelines may develop defects of varying degrees due to various factors. In particular, due to uncertainty about pipeline location, problems such as mis-excavation and damage frequently occur during construction. In serious cases, these problems can lead to safety accidents such as gas leaks and explosions. Traditional positioning methods such as electromagnetic induction, radar ground penetration, and radio frequency identification are significantly affected by pipeline material and soil moisture content, making them poorly suited for locating buried pipelines and difficult to accurately locate non-metallic pipelines. Therefore, the development of more efficient positioning technologies is crucial. In recent years, research on locating buried non-metallic pipelines has continued to advance. Among them, positioning methods based on acoustic wave reflection are more flexible and cost-effective. There are two main strategies for this approach: time-domain signal positioning and frequency-domain cross-correlation function positioning. The principles of these two methods are largely similar, differing in the selection of source signals and the extraction of reflected longitudinal wave signals. While these two methods can locate pipelines, the calculated pipeline depths can still deviate significantly from the actual depth.

[0003] Reference 1 (GE Liang, HE Jian, GAO Yan, XIAO Xiaoting, WU Jiaye, WANGTian. Acoustic imaging detection using time domain stacking for buried polyethylene pipelines [J]. ACTA ACUSTICA, 2024, 49 (3): 569-576.) proposed an optimized time domain stacking acoustic imaging method. However, the positioning depth of non-metallic pipelines with a tested burial depth of 0.4m and 1.0m was not accurate enough, and there was a lack of signal denoising. Reference 2 (Cui X, Gao Y, Muggleton J, et al. Superimposed imaging of acoustic wave reflections for the detection of underground nonmetallic pipelines [J]. Mechanical Systems and Signal Processing, 2024, 209: 111127.) used the strategy of "multi-point emission, multi-point reception and cross-correlation coefficient superposition" to draw the acoustic field map of underground non-metallic pipelines to locate buried non-metallic pipelines. However, this work required a lot of experimental equipment, complicated wiring, and lacked quantitative evaluation of the positioning results. Summary of the Invention

[0004] In order to solve the problems existing in the above-mentioned buried non-metallic pipeline positioning method, the present invention proposes a buried non-metallic pipeline depth positioning method based on acoustic mutual information. First, a test device is arranged on the ground in an area without pipelines. Acoustic wave signals are generated by source excitation. The sound waves propagate through the soil to the underground pipeline, are reflected back to the surface by the pipeline surface and received by multiple longitudinal wave detectors. The longitudinal wave velocity is calculated based on the ratio of the distance between the longitudinal wave detectors to the delay time of the received signal and the Poisson's ratio of the soil. Then, acoustic wave signals are collected in the area with pipelines. The received signals are denoised using the ensemble empirical mode decomposition combined with the wavelet threshold denoising method. The reflected longitudinal waves are extracted based on the energy intensity of various sound waves in the received signals and the time sequence of their arrival at the longitudinal wave detectors. A rectangular coordinate system is established, the measurement surface is divided, and the longitudinal wave propagation time is calculated. In addition, the mutual information function between the source signal and each reflected longitudinal wave signal is calculated. By calculating the frequency domain mutual information superposition value of the signal, a superposition value matrix is ​​obtained for each reflection unit based on the calculated longitudinal wave propagation time. Finally, the superposition value matrix is ​​used for imaging, and the pipeline position is obtained based on the maximum frequency domain mutual information superposition value.

[0005] A method for depth positioning of buried non-metallic pipelines based on acoustic mutual information specifically comprises the following steps:

[0006] S1. Arrange the P-wave geophones and the seismic source, calculate the P-wave velocity based on the ratio of the distance between the P-wave geophones and the corresponding received signal start time and the Poisson's ratio of the soil, and take the average value of multiple measurements;

[0007] The longitudinal wave geophones and the seismic source should be arranged above the soil area without pipelines, and four longitudinal wave geophones should be placed on the same side of the seismic source at equal intervals;

[0008] The longitudinal wave velocity is calculated based on the ratio of the distance of the longitudinal wave detector to the start time of the corresponding received signal and the Poisson's ratio of the soil, and the average value is taken after multiple measurements, specifically:

[0009] The distance of the longitudinal wave geophones refers to the distance between each geophone and the earthquake source;

[0010] Then, the ratio of the distance to the longitudinal wave detector to the start time of the corresponding received signal is calculated to obtain the surface wave velocity; the surface wave refers to the wave that the source sound wave signal reaches the detector along the soil surface;

[0011] Then, the longitudinal wave velocity is obtained by the relationship between the longitudinal wave velocity and the Poisson's ratio of the soil; the longitudinal wave refers to a wave propagated by the source sound wave signal in the soil;

[0012] Finally, the soil Poisson's ratio is used to obtain the longitudinal wave velocity of the site based on the relationship between the surface wave velocity and the longitudinal wave velocity. The soil longitudinal wave velocity can be accurately measured by taking the average value of multiple measurements.

[0013] S2. Rearrange the longitudinal wave geophone and the seismic source, excite the seismic source and collect the acoustic wave signal;

[0014] The rearrangement of longitudinal wave geophones and seismic sources refers to selecting soil near the top of the pipeline, arranging a survey line in a direction roughly perpendicular to the distribution of the pipeline, placing longitudinal wave geophones at equal intervals on the survey line, and placing the seismic source between any two geophones;

[0015] The acoustic wave signal is collected by setting the sampling rate and the number of sampling points by the host computer, and each longitudinal wave detector receives the acoustic wave signal and uploads it to the host computer;

[0016] S3, performing collective empirical mode decomposition and wavelet threshold denoising on the acoustic signal collected in step S2 to obtain a denoised received signal; the specific denoising process is as follows:

[0017] First, the signal to be processed is decomposed by collective empirical mode decomposition to obtain a set of natural frequency components of the intrinsic mode function, which are arranged from high frequency to low frequency, wherein the high-frequency intrinsic mode function mainly contains noise, while the low-frequency intrinsic mode function mainly contains the main components of the signal; then, the intrinsic mode function components that need to be denoised are selected, and by analyzing the frequency characteristics of each intrinsic mode function component, some high-frequency intrinsic mode function components that need to be denoised are selected; then, wavelet transform is applied to the selected high-frequency intrinsic mode function components to decompose the signal into approximate coefficients and detail coefficients of different scales, and the detail coefficients are thresholded to remove the noise component; the thresholding process is performed using a soft threshold; then, the denoised intrinsic mode function components are reconstructed by inverse wavelet transform; finally, the intrinsic mode function components that have been subjected to wavelet thresholding are recombined together with the unprocessed low-frequency intrinsic mode function components and the residual term to obtain the denoised signal;

[0018] S4, extracting the reflected longitudinal wave from the received signal after denoising in step S3 according to the energy intensity of various sound waves in the received signal in step S2 and the time sequence of arrival at the longitudinal wave detector;

[0019] The various sound waves in the received signal mainly include surface waves, direct waves and reflected longitudinal waves. Since the reflected longitudinal waves are actually used for pipeline positioning, other interference waves need to be eliminated;

[0020] The extraction of the reflected longitudinal wave from the received signal after denoising in step S3 is performed by establishing a no-pipeline model as a reference model and subtracting the signal received under the no-pipeline model from the signal received under the pipeline model to obtain the reflected longitudinal wave signal of the buried pipeline;

[0021] S5. Establish a rectangular coordinate system, divide the measurement surface and calculate the propagation time of the longitudinal wave of each reflection unit;

[0022] The rectangular coordinate system is established with the earthquake source as the coordinate origin O, the horizontal rightward direction is the positive direction of the x-axis, and the vertical upward direction is the positive direction of the y-axis, and the measurement surface is divided into the third and fourth quadrants;

[0023] The division of the measurement surface refers to the division of the measurement surface located in the third and fourth quadrants into units. The measurement surface is a rectangular area symmetrical about the y-axis, whose height is approximately three times the buried depth of the pipeline and whose width is five times the rectangular area of ​​the longitudinal wave detector array length. The unit division is usually carried out with a length less than the diameter of the pipeline as the side length, and the measurement surface is discretely divided into a number of square units.

[0024] The longitudinal wave propagation time refers to the propagation time of the sound wave from the earthquake source through the soil to the surface of the pipe, then reflected by the pipe surface, and then transmitted to the ground surface and received by the longitudinal wave detector. Each reflection unit is assumed to be the reflection point at the center point of each unit, that is, each unit has a pipe. The specific method for calculating the propagation time is:

[0025] The height of the measuring surface is H, the width is W, and the side length of the division unit is dx. Then the measuring surface has a total of H / dx rows and W / dx columns. The four longitudinal wave geophones are arranged symmetrically about the source, with an adjacent spacing of l. Their coordinates can be recorded as (x k ,0), where x k = -1.5·l+l·(k-1), k=1,...,4, the coordinates of the earthquake source are (0,0), that is, located between longitudinal wave detectors 2 and 3. Assuming that the buried depth of the pipeline is h, that is, the distance from the upper surface of the pipeline to the ground, and the horizontal distance between the center of the pipeline and the earthquake source is d, the coordinates of the pipeline are (d,-h);

[0026] For each reflection unit (i, j), i = 1, ..., W / dx, j = 1, ..., H / dx, the geometric center coordinates of the unit need to be calculated, where cx is the horizontal coordinate of the reflection unit center and cy is the vertical coordinate of the reflection unit center. The calculation formulas are:

[0027]

[0028] The distance that the sound wave travels from the earthquake source to the surface of the buried pipeline and then reflected by the pipeline surface back to the kth longitudinal wave detector is:

[0029]

[0030] Let the longitudinal wave velocity be c, then the propagation time of the sound wave from the source to the pipe surface and then reflected by the pipe surface back to the kth longitudinal wave detector is:

[0031] t k =S k / c

[0032] Based on this, the longitudinal wave propagation time of each reflection unit can be obtained;

[0033] S6, calculating the mutual information function between the source signal and each reflected longitudinal wave signal; further calculating the frequency domain mutual information superposition value of the signal, for each reflection unit, based on the longitudinal wave propagation time calculated in step S5, obtaining a superposition value matrix;

[0034] The mutual information function between the source signal and each reflected longitudinal wave signal is calculated as follows:

[0035] The source signal is represented by x(t), and the signal received by the longitudinal wave geophone is represented by y(t). First, the continuous time signals x(t) and y(t) are sampled to obtain discrete signals x(n) and y(n). Then, x(n) and y(n) are regarded as random variables, and the joint probability distribution P(x, y) and marginal probability distributions P(x) and P(y) between the x(n) and y(n) signals under different time delays are estimated by statistical histogram data. The mutual information between the x(t) and y(t) signals is calculated based on the two obtained distribution results. The formula is as follows:

[0036]

[0037] The maximum mutual information corresponds to the best estimate of the time delay of the signal y(t) relative to x(t);

[0038] The frequency domain mutual information superposition value of the reflection unit is calculated based on the determination of the maximum mutual information value, and the maximum mutual information value is used as the dividing point to intercept the mutual information function of one cycle backward; the one cycle length is the signal length of one cycle of the source signal; then, for each reflection unit, according to the propagation time of each unit corresponding to the different longitudinal wave detectors obtained in step S5, the intercepted mutual information functions of each path are shifted to the left by the corresponding longitudinal wave propagation time; finally, the intercepted mutual information functions of each path are added to obtain the frequency domain mutual information superposition value of a reflection unit; the remaining reflection units are calculated according to this method to obtain a superposition value matrix;

[0039] S7, using the superposition value matrix obtained in step S6 to perform color imaging, to obtain the buried depth information of the pipeline;

[0040] The two-dimensional imaging of the pipeline position is performed by color imaging using the superposition value matrix obtained in step S6, so as to obtain an imaging map of the detection area. The larger the superposition value, the brighter the color of the corresponding reflection unit. Therefore, the brightest area in the map is the location of the pipeline.

[0041] Due to the adoption of the above technical solution, the present invention has the following advantages:

[0042] 1. This paper proposes a method for locating the depth of buried non-metallic pipelines based on acoustic mutual information. Compared with traditional positioning methods, the mutual information method is based on statistical analysis and calculates the amount of information by analyzing the frequency domain distribution characteristics of the acoustic wave signal. It is not affected by soil dryness or humidity, can better adapt to the variability of the underground environment, and is applicable to a wider range of geological conditions.

[0043] 2. The proposed ensemble empirical mode decomposition combined with wavelet threshold denoising algorithm has adaptive decomposition capabilities and suppresses mode aliasing, and provides multi-scale denoising. It can effectively separate and remove noise while preserving the signal's detailed features, significantly improving pipeline depth positioning.

[0044] 3. The proposed method of establishing a rectangular coordinate system to calculate the propagation time of a signal can simplify the problem, clearly define the position of a point, and systematically calculate the distance between any two points, thus facilitating the calculation and solution.

[0045] 4. The present invention proposes a method for calculating signal delay using the mutual information function. For nonlinear and non-stationary signals collected by longitudinal wave geophones, it can effectively detect the time delay of the geophone output signal compared with the source signal, thereby achieving high-precision buried pipeline positioning. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a flow chart of a method for locating the depth of a buried pipeline based on acoustic mutual information according to an embodiment of the present invention;

[0047] Figure 2 This is a layout diagram of a longitudinal wave geophone according to an embodiment of the present invention;

[0048] Figure 3 The original signal collected by the longitudinal wave geophone in the embodiment of the present invention;

[0049] Figure 4 The received signal is denoised by using the collective empirical mode decomposition combined with the wavelet threshold algorithm in the embodiment of the present invention;

[0050] Figure 5 This is a delay estimation curve of the mutual information method according to an embodiment of the present invention;

[0051] Figure 6 This is a pipeline positioning diagram using the denoising mutual information function method according to an embodiment of the present invention;

[0052] Figure 7 This is the pipeline positioning map without denoising mutual information function method;

[0053] Figure 8 This is the pipeline positioning diagram using the denoising cross-correlation function method. DETAILED DESCRIPTION

[0054] To further illustrate the method for locating the depth of buried non-metallic pipelines based on acoustic mutual information proposed in the present invention, an embodiment of the present invention is described with reference to the accompanying drawings to enhance understanding of the present invention's objectives, method structure, and application effects. This embodiment is intended solely to further explain the present invention. Researchers in this field may make further non-fundamental technical improvements based on the present invention, which are also within the scope of protection of the present invention.

[0055] A method for depth positioning of buried non-metallic pipelines based on acoustic mutual information specifically comprises the following steps:

[0056] S1. Arrange the P-wave geophones and the seismic source, calculate the P-wave velocity based on the ratio of the distance between the P-wave geophones and the corresponding received signal start time and the Poisson's ratio of the soil, and take the average value of multiple measurements;

[0057] The longitudinal wave geophones and the seismic source should be arranged above the soil area without pipelines, and four longitudinal wave geophones should be placed on the same side of the seismic source at equal intervals;

[0058] The longitudinal wave velocity is calculated based on the ratio of the distance of the longitudinal wave detector to the start time of the corresponding received signal and the Poisson's ratio of the soil, and the average value is taken after multiple measurements, specifically:

[0059] The distance of the longitudinal wave geophones refers to the distance between each geophone and the earthquake source;

[0060] Then, the ratio of the distance to the longitudinal wave detector to the start time of the corresponding received signal is calculated to obtain the surface wave velocity; the surface wave refers to the wave that the source sound wave signal reaches the detector along the soil surface;

[0061] Then, the longitudinal wave velocity is obtained by the relationship between the longitudinal wave velocity and the Poisson's ratio of the soil; the longitudinal wave refers to a wave propagated by the source sound wave signal in the soil;

[0062] Finally, the soil Poisson's ratio is used to obtain the longitudinal wave velocity of the site based on the relationship between the surface wave velocity and the longitudinal wave velocity. The soil longitudinal wave velocity can be accurately measured by taking the average value of multiple measurements.

[0063] S2. Rearrange the longitudinal wave geophone and the seismic source, excite the seismic source and collect the acoustic wave signal;

[0064] The rearrangement of longitudinal wave geophones and seismic sources refers to selecting the soil above the pipeline, arranging the survey line in a direction roughly perpendicular to the distribution of the pipeline, placing longitudinal wave geophones at equal intervals on the survey line, and placing the seismic source between any two geophones. Figure 2 This is a diagram of the layout of P-wave geophones on a survey line. The spacing between adjacent P-wave geophones is l = 0.4m. The earthquake source coordinates are (0,0), located between P-wave geophones 2 and 3.

[0065] The acquisition of acoustic wave signals is accomplished by setting the sampling rate and sampling time of the host computer, which are set to f = 10000 Hz and t = 0.045 s respectively. Each longitudinal wave detector receives the acoustic wave signal and uploads it to the host computer. The original signal collected by the longitudinal wave detector is as follows: Figure 3 As shown;

[0066] S3, performing collective empirical mode decomposition and wavelet threshold denoising on the acoustic signal collected in step S2 to obtain a denoised received signal; the specific denoising process is as follows:

[0067] First, the signal to be processed is decomposed by collective empirical mode decomposition to obtain a set of natural frequency components of the intrinsic mode function. These intrinsic mode function components are arranged from high frequency to low frequency, where the high-frequency intrinsic mode function may contain noise, while the low-frequency intrinsic mode function mainly contains the main components of the signal; then, the intrinsic mode function components that need to be denoised are selected, and by analyzing the frequency characteristics of each intrinsic mode function component, some high-frequency intrinsic mode function components that need to be denoised are selected; then, wavelet transform is applied to the selected high-frequency intrinsic mode function components to decompose the signal into approximate coefficients and detail coefficients of different scales, and the detail coefficients are thresholded to remove the noise component; the thresholding process is performed using a soft threshold; then, the denoised intrinsic mode function components are reconstructed by inverse wavelet transform; finally, the intrinsic mode function components that have been subjected to wavelet thresholding are recombined together with the unprocessed low-frequency intrinsic mode function components and the residual term to obtain the denoised signal, as shown in FIG. Figure 4 As shown;

[0068] S4, extracting the reflected longitudinal wave from the received signal after denoising in step S3 according to the energy intensity of various sound waves in the received signal in step S2 and the time sequence of arrival at the longitudinal wave detector;

[0069] The various sound waves in the received signal mainly include surface waves, direct waves and reflected longitudinal waves. Since the reflected longitudinal waves are actually used for pipeline positioning, other interference waves need to be eliminated;

[0070] The extraction of the reflected longitudinal wave from the received signal after denoising in step S3 is based on the difference in energy intensity and arrival time of various sound waves in the received signal at the longitudinal wave detector. By establishing a no-pipeline model as a reference model, the signal received under the no-pipeline model is subtracted from the signal received under the pipeline model to obtain the reflected longitudinal wave signal of the buried pipeline.

[0071] S5. Establish a rectangular coordinate system, divide the measurement surface and calculate the propagation time of the longitudinal wave of each reflection unit;

[0072] The rectangular coordinate system is established with the earthquake source as the coordinate origin O, the horizontal rightward direction is the positive direction of the x-axis, and the vertical upward direction is the positive direction of the y-axis, and the measurement surface is divided into the third and fourth quadrants;

[0073] The division of the measurement surface refers to the unit division of the measurement surface located in the third and fourth quadrants. The measurement surface is a rectangular area symmetrical about the y-axis. The height of the measurement surface is approximately three times the buried depth of the pipeline. The approximate depth of the pipeline is 0.9m, so the height of the measurement surface is 3m. The width of the measurement surface is approximately five times the rectangular area of ​​the longitudinal wave detector array length. The longitudinal wave detector array length is 1.2m, so the measurement surface width is 6m. The unit division is usually based on a length less than the diameter of the pipeline. The diameter of the pipeline is 0.1m, so the measurement surface is discretely divided into a number of square units with a side length of 0.01m.

[0074] The longitudinal wave propagation time refers to the propagation time of the sound wave from the earthquake source through the soil to the surface of the pipe, then reflected by the pipe surface, and then transmitted to the ground surface and received by the longitudinal wave detector. Each reflection unit is assumed to be the reflection point at the center point of each unit, that is, each unit has a pipe. The specific method for calculating the propagation time is:

[0075] It is known that the measurement surface is 3m high and 6m wide, and the side length of the division unit is 0.01m. The measurement surface has a total of 300 rows and 600 columns. The four longitudinal wave geophones are arranged symmetrically about the source, with an adjacent spacing of l = 0.4m. Their coordinates can be recorded as (x k ,0), where x k = -1.5·l+l·(k-1), k=1,...,4, the coordinates of the earthquake source are (0,0), and the buried depth of the pipeline is 0.9m (the distance from the upper surface of the pipeline to the ground);

[0076] For each reflection unit (i, j), i = 1, ..., 600, j = 1, ..., 300, the geometric center coordinates of the unit need to be calculated, where cx is the horizontal coordinate of the reflection unit center and cy is the vertical coordinate of the reflection unit center. The calculation formulas are:

[0077]

[0078] The distance that the sound wave travels from the earthquake source to the surface of the buried pipeline and then reflected by the pipeline surface back to the kth longitudinal wave detector is:

[0079]

[0080] Let the longitudinal wave velocity be c, then the propagation time of the sound wave from the source to the pipe surface and then reflected by the pipe surface back to the kth longitudinal wave detector is:

[0081] t k =S k / c

[0082] According to this formula, the longitudinal wave propagation time of each reflection unit can be obtained;

[0083] S6, calculating the mutual information function between the source signal and each reflected longitudinal wave signal; further calculating the frequency domain mutual information superposition value of the signal, for each reflection unit, based on the longitudinal wave propagation time calculated in step S5, obtaining a superposition value matrix;

[0084] The mutual information function between the source signal and each reflected longitudinal wave signal is calculated as follows:

[0085] The source signal is represented by x(t), and the signal received by the longitudinal wave geophone is represented by y(t). First, the continuous time signals x(t) and y(t) are sampled to obtain discrete signals x(n) and y(n). Then, x(n) and y(n) are regarded as random variables, and the joint probability distribution P(x, y) and marginal probability distributions P(x) and P(y) between the x(n) and y(n) signals under different time delays are estimated by statistical histogram data. The mutual information between the x(t) and y(t) signals is calculated based on the two obtained distribution results. The formula is as follows:

[0086]

[0087] The maximum mutual information value corresponds to the best time delay estimate of signal y(t) relative to x(t), and the calculation result is as follows Figure 5 As shown;

[0088] The frequency domain mutual information superposition value of the reflection unit is calculated based on the determination of the maximum mutual information value, and the maximum mutual information value is used as a demarcation point to intercept a mutual information function with a period length backward; the one period length is the length of one period of the source signal; then, for each reflection unit, according to the longitudinal wave propagation time obtained in step S5, the intercepted mutual information functions of each path are shifted to the left by the corresponding longitudinal wave propagation time; finally, the intercepted mutual information functions of each path are added to obtain the frequency domain mutual information superposition value of the reflection unit; and the remaining reflection units are calculated in this way to obtain a superposition value matrix.

[0089] S7, using the superposition value matrix obtained in step S6 to perform color imaging, to obtain the buried depth information of the pipeline;

[0090] The two-dimensional imaging of the pipeline position is performed by color imaging using the superposition value matrix obtained in step S6, so as to obtain an imaging map of the detection area. The larger the superposition value, the brighter the color of the corresponding reflection unit. Therefore, the brightest area in the map is the location of the pipeline.

[0091] In order to verify the effectiveness of the proposed method of calculating signal delay by using the collective empirical mode decomposition combined with the wavelet threshold denoising algorithm and the mutual information function method, different positioning methods were selected for non-metallic pipelines with a buried depth of 0.9m to compare the positioning effects. In order to verify the effectiveness of the denoising process for pipeline depth positioning, the longitudinal wave geophone received signal was processed by the denoised mutual information function method and the non-denoised mutual information function method respectively; in order to verify the effectiveness of different time delay calculation methods for pipeline depth positioning, the longitudinal wave geophone received signal was processed by the denoised mutual information function method and the denoised cross-correlation function method respectively. All the methods to be compared in the experiment were based on the experimental environment constructed above. The depth of pipeline positioning is an important indicator for measuring the quality of the positioning method. Therefore, the positioning depth was selected as the evaluation indicator for the comparative experiment in the present invention.

[0092] The results of the pipeline depth positioning method and the comparative method are shown in Table 1: As can be seen from Table 1, the pipeline depth positioning method of the present invention achieves a more accurate positioning depth than the comparative method. Compared with the positioning method of processing the detector receiving signal by the non-denoising mutual information function method, the positioning error of the present invention method is 0.61m lower than that; compared with the positioning method of processing the detector receiving signal by the denoising cross-correlation function method, the positioning error of the present invention method is 0.19m lower than that. The positioning results of different pipeline positioning methods are shown in Table 1. Figures 6 to 8 As shown in the figure, the dotted circle represents the pipeline positioning position, and the solid circle represents the actual position of the pipeline. Figures 6 to 8 It can be seen that the positioning depth of the method of the present invention is closer to the actual depth of the pipeline, and the positioning depth effect is better.

[0093] Table 1 Performance comparison of different pipeline positioning methods

[0094]

[0095] The above simulation experimental results fully demonstrate that the method of the present invention is more accurate in locating buried pipelines, provides technical support for accurately locating the buried depth of pipelines, and has good application prospects.

[0096] The above describes the method and system of the present invention, and researchers in this field can reproduce the present invention based on these descriptions. Based on the content of the present invention, other embodiments and results obtained by researchers without making creative work should fall within the scope of protection of the present invention.

Claims

1. A method for deep positioning of buried non-metallic pipelines based on acoustic mutual information, characterized in that: The following steps are involved: S1. Arrange the seismic source and P-wave geophones, calculate the P-wave velocity based on the ratio of the distance between the P-wave geophones to the start time of the received signal and the Poisson's ratio of the soil, and take an average value of multiple measurements; the seismic source and P-wave geophones are arranged above the soil area without pipelines, and four P-wave geophones are placed at equal intervals on the same side of the seismic source; the surface wave velocity is obtained based on the ratio of the distance between the P-wave geophones to the start time of the corresponding received signal; and the P-wave velocity is then obtained based on the relationship between the P-wave and the Poisson's ratio of the soil; S2. Rearranging the seismic source and longitudinal wave geophones, generating acoustic wave signals by excitation of the seismic source, and collecting the acoustic wave signals by the longitudinal wave geophones; the rearranging of the longitudinal wave geophones and the seismic source comprises selecting soil near the top of the pipeline, arranging a survey line in a direction roughly perpendicular to the distribution of the pipeline, placing longitudinal wave geophones at equal intervals on the survey line, and placing the seismic source between any two geophones; S3, performing collective empirical mode decomposition combined with wavelet threshold denoising on the acoustic signal collected in step S2 to obtain a denoised received signal; S4. Extracting the reflected longitudinal wave from the received signal after denoising in step S3 based on the energy intensity of various sound waves in the received signal and the time sequence of arrival at the longitudinal wave detector described in step S2; by establishing a no-pipeline model as a reference model, subtracting the signal received under the no-pipeline model from the signal received under the pipeline model to obtain the reflected longitudinal wave signal of the buried non-metallic pipeline; S5. Establish a rectangular coordinate system, divide the measurement surface, and calculate the longitudinal wave propagation time of each reflection unit. Dividing the measurement surface refers to dividing the measurement surface located in the third and fourth quadrants into units. The measurement surface is a rectangular area symmetrical about the y-axis, whose height is approximately three times the buried depth of the pipeline and whose width is five times the rectangular area of ​​the longitudinal wave detector array length. The unit division is generally performed with a side length less than the diameter of the pipeline as the length, and the measurement surface is discretely divided into a plurality of square units. S6, calculating the mutual information function between the source signal and each reflected longitudinal wave signal; further calculating the frequency domain mutual information superposition value of the signal, for each reflection unit, based on the longitudinal wave propagation time calculated in step S5, obtaining a superposition value matrix; S7. Use the superposition value matrix obtained in step S6 to perform two-dimensional color imaging to obtain the buried depth information of the pipeline. In the imaging map of the detection area, the larger the superposition value, the brighter the color of the corresponding reflection unit. The brightest area in the map is the location of the pipeline.

2. The method for locating the depth of buried non-metallic pipelines based on acoustic mutual information according to claim 1, characterized in that: Step S3 performs collective empirical mode decomposition combined with wavelet threshold denoising on the sound wave signal collected in step S2 to obtain a denoised received signal. The specific denoising process is as follows: First, the signal to be processed is decomposed by collective empirical mode decomposition to obtain a set of natural frequency components of intrinsic mode functions, which are arranged from high frequency to low frequency, wherein the high-frequency intrinsic mode functions mainly contain noise, while the low-frequency intrinsic mode functions mainly contain the main components of the signal; then, by analyzing the frequency characteristics of each intrinsic mode function component, some high-frequency intrinsic mode function components that need to be denoised are selected; then, wavelet transform is applied to the selected high-frequency intrinsic mode function components to decompose the signal into approximate coefficients and detail coefficients of different scales, and thresholding is performed on the detail coefficients to remove the noise component; the thresholding process can be performed using a soft threshold; Then, the denoised intrinsic mode function components are reconstructed through inverse wavelet transform; finally, the intrinsic mode function components processed by wavelet threshold are recombined with the unprocessed low-frequency intrinsic mode function components and residual terms to obtain the denoised signal.

3. The method for depth positioning of buried non-metallic pipelines based on acoustic mutual information according to claim 1, characterized in that: The rectangular coordinate system established in step S5 is based on the earthquake source as the coordinate origin O, the horizontal rightward direction is the positive direction of the x-axis, and the vertical upward direction is the positive direction of the y-axis, and the measurement surface is divided into the third and fourth quadrants; The longitudinal wave propagation time refers to the propagation time for the sound wave to propagate from the earthquake source through the soil to the surface of the pipe, then be reflected by the pipe surface, reach the ground surface and be received by the longitudinal wave detector. Each reflection unit is assumed to be the reflection point at the center point of each unit, that is, each unit has a pipe. The specific method for calculating the propagation time is: The height of the measuring surface is H, the width is W, and the side length of the division unit is dx. Then the measuring surface has a total of H / dx rows and W / dx columns. The four longitudinal wave geophones are arranged symmetrically about the source, with an adjacent spacing of l. Their coordinates can be recorded as (x k ,0), where x k = -1.5·l+l·(k-1), k=1,...,4, the coordinates of the earthquake source are (0,0), that is, located between longitudinal wave detectors 2 and 3. Assuming that the buried depth of the pipeline is h, that is, the distance from the upper surface of the pipeline to the ground, and the horizontal distance between the center of the pipeline and the earthquake source is d, the coordinates of the pipeline are (d,-h); For each reflection unit (i, j), i = 1, ..., W / dx, j = 1, ..., H / dx, the geometric center coordinates of the unit need to be calculated, where cx is the horizontal coordinate of the reflection unit center and cy is the vertical coordinate of the reflection unit center. The calculation formulas are: The distance that the sound wave travels from the earthquake source to the surface of the buried pipeline and then reflected by the pipeline surface back to the kth longitudinal wave detector is: Let the longitudinal wave velocity be c, then the propagation time of the sound wave from the source to the pipe surface and then reflected by the pipe surface back to the kth longitudinal wave detector is: t k =S k / c Based on this, the longitudinal wave propagation time of each reflection unit can be obtained.

4. The method for locating the depth of buried non-metallic pipelines based on acoustic mutual information according to claim 1, characterized in that: Step S6 is to calculate the mutual information function between the source signal and each reflected longitudinal wave signal, and the calculation process is as follows: The source signal is represented by x(t), and the signal received by the longitudinal wave geophone is represented by y(t). First, the continuous-time signals x(t) and y(t) are sampled to obtain discrete signals x(n) and y(n). Then, x(n) and y(n) are regarded as random variables, and the joint probability distribution P(x, y) and marginal probability distributions P(x) and P(y) between the x(n) and y(n) signals under different time delays are estimated by statistical histogram data. The mutual information function between the x(t) and y(t) signals is calculated based on the two obtained distribution results. The formula is as follows: The maximum value of the mutual information function corresponds to the best time delay estimate of the signal y(t) relative to x(t); The frequency domain mutual information superposition value of the reflection unit is calculated based on the determination of the maximum mutual information value, and the maximum mutual information value is used as the dividing point to intercept the mutual information function with a period length backward; the one period length is the signal length of one period of the source signal; then for each reflection unit, according to the longitudinal wave propagation time obtained in step S5, the intercepted mutual information functions are shifted to the left by the corresponding longitudinal wave propagation time; finally, the intercepted mutual information functions are added together to obtain the frequency domain mutual information superposition value of a reflection unit; the remaining reflection units are calculated according to this method to obtain a superposition value matrix.

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