Microseismic event S-wave first arrival pickup method

Abstract

The invention provides a microseismic event S-wave first arrival pickup method, which belongs to the technical field of seismic waveform data processing, and comprises the following steps: carrying out detrending and band-pass filtering preprocessing on three-component original data; calculating a covariance matrix, solving a characteristic value and a characteristic vector, and carrying out polarization correction; a Bezier curve is adopted to accurately position waveform cross points; and determining the S-wave first arrival by combining the waveform cross point and the amplitude characteristics. According to the method, the dielectric anisotropy influence is eliminated through polarization correction, accurate intersection point detection is achieved through the Bezier curve, the amplitude judgment criterion is combined, the accuracy and reliability of S-wave first arrival recognition are remarkably improved, and environmental noise interference is effectively restrained. The method realizes full-process automatic processing, and is suitable for the fields of micro-seismic monitoring, seismic exploration and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of seismic waveform data processing, and in particular to a method for picking the first arrival of S waves of microseismic events. Background Art

[0002] In the field of microseismic monitoring and seismic exploration, accurately identifying and picking up the first arrival time of S waves is of great significance for earthquake source location and the study of underground medium properties. Currently, the identification of the first arrival of S waves is still mainly done manually, which is not only time-consuming but also prone to errors. Although automatic picking algorithms have been developed, such as the STA / LTA method (short time window / long time window ratio method), this method calculates the ratio of the signal energy or amplitude in the short time window to the long time window, and determines the first arrival of S waves when the ratio exceeds a preset threshold. However, this type of method has obvious limitations: the selection of window length and threshold is too dependent on experience, inaccurate threshold judgment will affect the accuracy of event picking, and the recognition accuracy is significantly reduced under low signal-to-noise ratio conditions. Summary of the Invention

[0003] In view of this, the present invention aims to propose a method for picking the first arrival of S waves of microseismic events. By eliminating the influence of medium anisotropy through polarization correction, and realizing precise intersection detection using Bezier curves, combined with the amplitude judgment criterion, it can not only significantly improve the accuracy and reliability of S-wave first arrival identification, but also effectively suppress the interference of environmental noise.

[0004] To achieve the above object, the technical solution of the present invention is implemented as follows: A method for picking the first arrival of an S wave of a microseismic event comprises the following steps:

[0005] Data preprocessing: preprocess the three-component raw data obtained by three-component geophone monitoring;

[0006] Polarization correction is performed to eliminate the influence of medium anisotropy: the covariance matrix C of the three-component waveform data is constructed based on the preprocessed waveform data, the eigenvalues ​​and eigenvectors are calculated, and the waveform data of the P-wave component after polarization correction is obtained;

[0007] Use Bezier curves to achieve accurate intersection detection: Based on Bezier curve intersection detection, a linear equation system of two line segments is established, the intersection coordinates are solved, and it is determined whether the intersection point is inside the two line segments. If the intersection point is inside the two line segments, it is considered a valid waveform intersection point;

[0008] Determine the first arrival time of the S wave: Calculate the amplitude of each valid waveform intersection point and determine the first arrival time of the S wave.

[0009] Furthermore, the data preprocessing step includes: performing detrending processing on the three-component raw data obtained by the three-component detector monitoring to eliminate the baseline drift and the overall trend item in the data;

[0010] A bandpass filter is used to filter the detrended data, and an appropriate cutoff frequency range is selected to filter out high-frequency and low-frequency noise and enhance the signal-to-noise ratio.

[0011] Furthermore, the steps of polarization correction include:

[0012] First, construct a three-component waveform data matrix

[0013]

[0014] Where C is the 3×3 covariance matrix, N, E, and Z represent the north-south, east-west, and vertical components of the detector, respectively;

[0015] Then, the covariance matrix is ​​calculated, and the eigenvalues ​​and eigenvectors are solved. The eigenvector corresponding to the maximum eigenvalue is taken as the main polarization direction;

[0016] Finally, the original waveform is projected onto the main polarization direction to obtain the polarization-corrected waveform.

[0017] Furthermore, the steps of performing intersection detection using the Bezier curve include:

[0018] First, for the line segment determined by two points P0(m0,n0) and P1(m1,n1), its linear Bezier expression is:

[0019] B(t)=P0+t(P0-P1)=(1-t)P0+tP1 (2)

[0020] Among them, t is the linear Bezier parameter, which represents the distance from the starting point, 0≤t≤1, which is the linear Bezier curve;

[0021] Then, for the waveform data obtained by the two different detectors, construct corresponding line segments L1 and L2, where the endpoints of L1 are (x1, y1) and (x2, y2), and the endpoints of L2 are (x3, y3) and (x4, y4). The intersection of the two line segments L1 and L2 is (x0, y0). The formula is organized and written in matrix form. By solving the matrix equation A×T=B, the intersection coordinates (x0, y0) and parameter values ​​(t1, t2) are obtained.

[0022]

[0023] The parameter t1 represents the ratio of the distance from the starting point of L1 to the intersection point to the length of L1; the parameter t2 represents the ratio of the distance from the starting point of L2 to the intersection point to the length of L2;

[0024] Finally, determine whether the intersection point is inside the two line segments. If the intersection point is inside the two line segments, it is considered a valid waveform intersection point.

[0025] Furthermore, the steps of determining the first arrival of the S wave include:

[0026] Traverse all detected intersections;

[0027] For each intersection point, calculate the maximum amplitude value within a certain time window thereafter;

[0028] If the maximum amplitude value in the time window is greater than or equal to 1.5 times the amplitude at the intersection point, the time corresponding to the intersection point is determined to be the first arrival time of the S wave;

[0029] If the maximum amplitude is less than 1.5 times the amplitude at the intersection point, continue to compare with the next intersection point until the condition is met.

[0030] Compared to existing technologies, the method for picking the first arrival of microseismic S waves described in this paper offers the following advantages: It eliminates the effects of medium anisotropy through polarization correction, uses Bezier curves for precise intersection detection, and, combined with amplitude determination criteria, significantly improves the accuracy and reliability of S-wave first arrival identification, effectively suppressing environmental noise interference. This method achieves fully automated processing and is suitable for applications in microseismic monitoring, seismic exploration, and other fields.

[0031] (1) Polarization correction eliminates the influence of dielectric anisotropy

[0032] By calculating the covariance matrix and solving for eigenvalues ​​and eigenvectors, the original waveform is projected onto the main polarization direction, correcting for waveform deflection caused by formation anisotropy. This ensures accurate S-wave first arrival detection even in situations with large variations in formation dip angles or medium velocity.

[0033] (2) The linear Bezier curve is introduced to locate the intersection point. By solving the parametric equation group to determine whether the intersection point is inside the line segment, the picking accuracy can be further improved.

[0034] (3) By comparing the ratio of the amplitude at the intersection point to the maximum amplitude in the next fixed time window, false intersection points caused by environmental noise, instrument errors or other random factors can be effectively eliminated.

[0035] (4) The multiple verification method based on polarization and intersection detection reduces the dependence on empirical parameters and improves the adaptability of the picking results to different geological conditions and different noise levels.

[0036] (5) The method proposed in the present invention can realize the automatic picking of massive three-component data, accelerate the efficiency of identifying the first arrival of S waves, reduce human errors, and has significant practical value in application scenarios such as microseismic monitoring and seismic exploration that have high requirements for real-time or batch processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0038] Figure 1 It is the technical roadmap of the present invention;

[0039] Figure 2 Schematic diagram of the intersection points picked up in the embodiment. DETAILED DESCRIPTION

[0040] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0041] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0042] The present invention proposes a method for picking the first arrival of S waves of microseismic events, which combines three key technologies: polarization correction technology to eliminate the influence of medium anisotropy, Bezier curve to achieve accurate intersection detection, and amplitude-based judgment criteria. The method includes the following steps: detrending and bandpass filtering preprocessing of the three-component raw data; polarization correction by calculating the covariance matrix and solving the eigenvalues ​​and eigenvectors; using Bezier curves to accurately locate the waveform intersection points; combining the waveform intersection points and amplitude characteristics to determine the first arrival of the S wave. Specifically, if Figure 1 As shown, the following steps are included:

[0043] Starting from the three-component raw data of microseismic monitoring, the three-component raw data obtained by three-component geophone monitoring is preprocessed; first, it is detrended and then band-pass filtered for noise reduction. The details are as follows:

[0044] Detrending is performed on the three-component raw data obtained by the three-component geophone monitoring to eliminate the baseline drift and overall trend items in the data;

[0045] A bandpass filter is used to filter the detrended data, and an appropriate cutoff frequency range is selected to filter out high-frequency and low-frequency noise and enhance the signal-to-noise ratio. The cutoff frequency range is 5 Hz to 45 Hz for surface microseismic data and 50 Hz to 301 Hz for underground microseismic data.

[0046] Based on the pre-processed data, polarization correction is performed on it. This step can eliminate the waveform polarization deflection caused by factors such as formation dip and medium anisotropy. The details are as follows:

[0047] The covariance matrix C of the three-component waveform data is constructed based on the preprocessed waveform data, the eigenvalues ​​and eigenvectors are calculated, and the waveform data of the P-wave component after polarization correction is obtained;

[0048] First, construct the three-component waveform data matrix as shown below:

[0049]

[0050] Where C is the 3×3 covariance matrix, N, E, and Z represent the north-south, east-west, and vertical components of the detector, respectively;

[0051] Then, the covariance matrix is ​​calculated, and the matrix eigenvalues ​​and eigenvectors are solved. The eigenvector corresponding to the maximum eigenvalue is taken as the main polarization direction;

[0052] Finally, the original waveform is projected onto the main polarization direction to obtain the polarization-corrected waveform.

[0053] The S-wave first arrival time is determined by superimposing seismic data monitored by two geophones, primarily identifying the starting point of the "butterfly plot" generated by the 180-degree phase difference between the two seismic traces. This starting point marks the first arrival time of the shear wave signal, graphically represented by the intersection of the waveform. After polarization processing, the method of the present invention improves recognition accuracy by simultaneously considering waveform intersections and amplitude characteristics: when the maximum amplitude value within a certain time window after a certain intersection is greater than or equal to 1.5 times the amplitude at the intersection, the time corresponding to the intersection is determined to be the first arrival time of the S wave.

[0054] Bezier curves are used to accurately locate waveform intersections. Based on Bezier curve intersection detection, a linear equation system is established for the two line segments, the intersection coordinates are solved, and it is determined whether the intersection is inside the two line segments. If the intersection is inside the two line segments, it is considered a valid waveform intersection.

[0055] First, for the line segment determined by two points P0(m0,n0) and P1(m1,n1), its linear Bezier expression is:

[0056] B(t)=P0+t(P0-P1)=(1-t)P0+tP1 (2)

[0057] Among them, t is the linear Bezier parameter, which represents the distance from the starting point, 0≤t≤1, which is the linear Bezier curve; this form is equivalent to linear interpolation.

[0058] Then, for the waveform data obtained by two different detectors, construct the corresponding line segments L1 and L2 respectively, and set the endpoints of the two given line segments L1 and L2:

[0059] The endpoints of L1 are: (x1, y1); (x2, y2)

[0060] The endpoints of L2 are: (x3, y3); (x4, y4)

[0061] The intersection of two line segments L1 and L2 is (x0, y0); the parameter t1 represents the ratio of the distance from the starting point of L1 to the intersection point to the length of L1; the parameter t2 represents the ratio of the distance from the starting point of L2 to the intersection point to the length of L2; therefore, four equations can be written:

[0062] x0=x1+t1(x2-x1) (3)

[0063] x0=x3+t2(x4-x3) (4)

[0064] y0=y1+t1(y2-y1) (5)

[0065] y0=y3+t2(y4-y3) (6)

[0066] Rearranging and expressing it in matrix form, the given expression (2) can be rewritten as formula (7) (matrix form).

[0067]

[0068] The given linear equation system (7) can be expressed as A×T=B. Therefore, in order to solve for the variable T, it can be expressed as T=A -1 B, where A -1 Represents the inverse matrix of matrix A. By solving this matrix equation, we can simultaneously obtain the intersection coordinates (x0, y0) and parameter values ​​(t1, t2).

[0069]

[0070] To determine the S-wave first arrival, the signal data is first loaded and polarized. Since the waveform data is discretely sampled, the algorithm describes the relationship between adjacent sampling points using a linear Bezier curve. By systematically solving the linear equations corresponding to adjacent data points, Formula (7), the Bezier parameters defining the intersection point are obtained.

[0071] When constructing a matrix to solve a linear system of equations: Matrix A contains the coefficients derived from the coordinates of two adjacent points, Matrix T is a column vector containing the unknown Bessel parameters and the coordinates of the intersection point, and Matrix B contains the constant terms on the right side of the equation. Specifically, the matrix construction process is as follows:

[0072] A matrix: 4×4 coefficient matrix, reflecting the direction information of the line segment;

[0073] T matrix: 4×1 column vector, containing [t1, t2, x0, y0];

[0074] B matrix: 4×1 column vector containing known endpoint information;

[0075] The matrix T contains all the parameters needed to describe the intersection of the two line segments L1 and L2, namely the intersection coordinates (x0, y0) and the parameters t1 and t2. When the value range of t1 and t2 is limited to the interval [0, 1], the intersection point is located inside the line segments L1 and L2. By constructing matrices A and B with the coordinates of the endpoints of the two line segments L1 and L2, and then solving the equation T = A -1 B, we can determine the parameters describing the intersection of the two line segments, namely (x0, y0).

[0076] Finally, the first arrival time of the S wave is determined. For each intersection point, the maximum amplitude value within a certain time window thereafter is calculated in sequence. If the maximum amplitude value is greater than or equal to 1.5 times the amplitude at the intersection point, the time corresponding to the intersection point is determined to be the first arrival time of the S wave. If this condition is not met, the next intersection point is compared until the condition is met, and the process of the present invention is finally completed. The details are as follows:

[0077] Calculate the amplitude of each valid waveform intersection and determine the first arrival time of the S wave. Including:

[0078] Traverse all detected intersections;

[0079] For each intersection point, the maximum amplitude value within a certain time window thereafter is calculated in turn; wherein, the certain time window refers to a time window that can effectively capture the S-wave amplitude characteristics; it should be noted that in actual application, the time window is selected according to the sampling rate and the signal main frequency range. The present invention does not limit the specific value range of the time window. The value requirement of the time window is that it can effectively capture the S-wave amplitude characteristics. Therefore, the specific value is closely related to the application scenario.

[0080] If the maximum amplitude value in the time window is greater than or equal to 1.5 times the amplitude at the intersection point, the time corresponding to the intersection point is determined to be the first arrival time of the S wave;

[0081] If the maximum amplitude value is less than 1.5 times the amplitude at the intersection point, the comparison continues with the next intersection point until the condition is met, and the process of the present invention is finally completed.

[0082] Example 1

[0083] The present invention proposes a method for picking the first arrival of S waves of microseismic events. The complete processing flow is as follows:

[0084] 1. Data preprocessing

[0085] 1.1, load three-component original data, including north-south (N), east-west (E), and vertical (Z) components;

[0086] 1.2, detrending each component to eliminate baseline drift;

[0087] 1.3, using a bandpass filter for noise reduction.

[0088] 2. Polarization correction implementation

[0089] 2.1, construct the covariance matrix C:

[0090] For a given three-component waveform data [N(t), E(t), Z(t)], calculate the 3×3 covariance matrix:

[0091]

[0092] 2.2, Eigenvalue decomposition:

[0093] Perform eigenvalue decomposition on the covariance matrix C to obtain three eigenvalues ​​λ1, λ2, λ3 (λ1≥λ2≥λ3) and their corresponding eigenvectors.

[0094] 2.3, Polarization direction determination:

[0095] The eigenvector corresponding to the maximum eigenvalue λ1 is the main polarization direction.

[0096] 2.4, Waveform Projection:

[0097] The original waveform is projected onto the main polarization direction to obtain the polarization-corrected waveform.

[0098] 3. Bezier curve intersection detection

[0099] 3.1, construct a linear Bezier curve for adjacent sampling points:

[0100] For a line segment determined by two points P0(m0,n0) and P1(m1,n1), its linear Bezier expression is:

[0101] B(t)=P0+t(P0-P1)=(1-t)P0+tP1 (2)

[0102] Among them, t is the linear Bezier parameter, which represents the distance from the starting point, 0≤t≤1. This form is called a linear Bezier curve; this form is equivalent to linear interpolation.

[0103] 3.2, construct the intersection to solve the matrix equation:

[0104] For two line segments L1 and L2, let their endpoints be:

[0105] Endpoints of L1: (x1, y1); (x2, y2)

[0106] Endpoints of L2: (x3, y3); (x4, y4)

[0107] The intersection of two line segments L1 and L2 is at (x0, y0). The parameter t1 represents the ratio of the distance from the start of L1 to the intersection point to the length of L1. Similarly, the parameter t2 represents the ratio of the distance from the start of L2 to the intersection point to the length of L2.

[0108] Construct the matrix equation AT=B:

[0109]

[0110] 3.3, solve the intersection coordinates:

[0111] To solve for the variable T, we can express it as T = A -1 B, where A -1 Represents the inverse matrix of matrix A. By solving this matrix equation, we can simultaneously obtain the intersection coordinates (x0, y0) and parameter values ​​(t1, t2).

[0112]

[0113] 4. Determination of the first arrival of the S wave

[0114] 4.1, traverse all detected intersections;

[0115] 4.2. For each intersection point, calculate the maximum amplitude value within a certain time window (e.g., 20ms) thereafter;

[0116] 4.3. If the maximum amplitude exceeds 1.5 times the amplitude at the intersection point, the point is determined to be the first arrival of the S wave;

[0117] 4.4, if the condition is not met, continue to check the next intersection until the condition is met.

[0118] Based on the above polarization processing and intersection detection algorithm, the intersection of the waveform can be identified and the first arrival time of the S wave can be determined. Figure 2 The figure shows a schematic diagram of the picked intersections, where the square symbols represent the detected intersection positions.

[0119] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for picking the first arrival of S waves of microseismic events, characterized in that: The following steps are involved: Data preprocessing: preprocess the three-component raw data obtained by three-component geophone monitoring; Polarization correction is performed to eliminate the influence of medium anisotropy: the covariance matrix C of the three-component waveform data is constructed based on the preprocessed waveform data, the eigenvalues ​​and eigenvectors are calculated, and the waveform data of the P-wave component after polarization correction is obtained; Use Bezier curves to achieve accurate intersection detection: Based on Bezier curve intersection detection, a linear equation system of two line segments is established, the intersection coordinates are solved, and it is determined whether the intersection point is inside the two line segments. If the intersection point is inside the two line segments, it is considered a valid waveform intersection point; Determine the first arrival time of the S wave: Calculate the amplitude of each valid waveform intersection point and determine the first arrival time of the S wave.

2. A method for picking the first arrival of microseismic events S waves according to claim 1, characterized in that: The steps of data preprocessing include: detrending the three-component raw data obtained by the three-component detector monitoring to eliminate the baseline drift and overall trend items in the data; A bandpass filter is used to filter the detrended data, and an appropriate cutoff frequency range is selected to filter out high-frequency and low-frequency noise and enhance the signal-to-noise ratio.

3. The method for picking the first arrival of microseismic events S waves according to claim 1, characterized in that: The steps of polarization correction include: First, construct a three-component waveform data matrix Where C is the 3×3 covariance matrix, N, E, and Z represent the north-south, east-west, and vertical components of the detector, respectively; Then, the covariance matrix is ​​calculated, and the eigenvalues ​​and eigenvectors are solved. The eigenvector corresponding to the maximum eigenvalue is taken as the main polarization direction; Finally, the original waveform is projected onto the main polarization direction to obtain the polarization-corrected waveform.

4. The method for picking the first arrival of microseismic events S waves according to claim 1, characterized in that: The steps for intersection detection using Bezier curves include: First, for the line segment determined by two points P0(m0,n0) and P1(m1,n1), its linear Bezier expression is: B(t)=P0+t(P0-P1)=(1-t)P0+tP1 (2) Among them, t is the linear Bezier parameter, which represents the distance from the starting point, 0≤t≤1, which is the linear Bezier curve; Then, for the waveform data obtained by the two different detectors, construct corresponding line segments L1 and L2, where the endpoints of L1 are (x1, y1) and (x2, y2), and the endpoints of L2 are (x3, y3) and (x4, y4). The intersection of the two line segments L1 and L2 is (x0, y0). The formula is organized and written in matrix form. By solving the matrix equation A×T=B, the intersection coordinates (x0, y0) and parameter values ​​(t1, t2) are obtained. The parameter t1 represents the ratio of the distance from the starting point of L1 to the intersection point to the length of L1; the parameter t2 represents the ratio of the distance from the starting point of L2 to the intersection point to the length of L2; Finally, determine whether the intersection point is inside the two line segments. If the intersection point is inside the two line segments, it is considered a valid waveform intersection point.

5. A method for picking the first arrival of microseismic events S waves according to claim 4, characterized in that: The steps for determining the first arrival of an S wave include: Traverse all detected intersections; For each intersection point, calculate the maximum amplitude value within a certain time window thereafter; If the maximum amplitude value in the time window is greater than or equal to 1.5 times the amplitude at the intersection point, the time corresponding to the intersection point is determined to be the first arrival time of the S wave; If the maximum amplitude is less than 1.5 times the amplitude at the intersection point, continue to compare with the next intersection point until the condition is met.

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