Microseismic reverse time positioning method based on improved imaging condition

Abstract

The invention belongs to the field of seismic exploration, and particularly relates to a microseism reverse time positioning method based on an improved imaging condition, which comprises the following steps: constructing a speed model, and carrying out gridding processing; loading the wave field data to corresponding grid points in a speed model according to the position of a detector to form a detection point; wave detection points are divided into N groups, the number of the wave detection points in each group is consistent, reverse time continuation is carried out on the wave field data in each group at the same time, in the reverse time continuation process, the squares of wave field amplitude values obtained through reverse time continuation are superposed along a time axis, and an initial positioning section is obtained; performing grouping cross-correlation operation on the wave field of each group of initial positioning profiles; and carrying out self-correlation processing on the wave field after grouping cross-correlation operation to obtain an accurate positioning profile for seismic source positioning. The problems of low positioning accuracy, poor imaging quality and high calculation cost existing in conventional imaging conditions in existing seismic source reverse time positioning are solved, more accurate seismic source positioning is realized, and meanwhile, the risk of errors and illusions is reduced.

Description

Technical Field

[0001] The invention belongs to the field of seismic exploration, and in particular relates to a microseismic time-reverse positioning method based on improved imaging conditions. Background Art

[0002] The microseismic time-reversal positioning method is widely used in oil and gas exploration, mine monitoring, seismological research, etc. Its advantage is that it can provide high-resolution source imaging, which helps to better understand underground structures and activities.

[0003] Microseismic time reversal positioning is a seismic signal processing technology that uses the time reversal invariance principle of the wave equation to estimate the location of the earthquake source. That is, by reversely analyzing the seismic wave records and using a variety of imaging technologies to improve the imaging effect, the coherence of the wave field at the earthquake source is enhanced, and the wave field is re-aggregated at the excitation point to achieve earthquake source positioning.

[0004] Among them, choosing appropriate imaging conditions is crucial for reverse time location of earthquake sources. Common imaging conditions for reverse time location of earthquake sources face some challenges and limitations, including: The purpose of autocorrelation is to accurately determine the source location by fully utilizing the data at each moment of the reverse-time wave field propagation. However, during the reverse-time wave field propagation process, the interference waves outside the source will also undergo autocorrelation, which may cause certain interference to the final imaging results and affect the accuracy of positioning.

[0005] Cross-correlation locates the earthquake source by comparing the similarity of the reverse time wave fields received by different detectors, which may lead to false focusing of the earthquake source near the sensor. It is also sensitive to individual or a small number of abnormal records, which may significantly affect the imaging quality. It also requires processing a large amount of data, so the computational cost is high. Summary of the invention

[0006] One of the technical problems to be solved by the present invention is to provide a microseismic reverse time positioning method based on improved imaging conditions, so as to solve the problems of low positioning accuracy, poor imaging quality and high calculation cost existing in conventional imaging conditions in the existing source reverse time positioning.

[0007] The technical solution adopted by the present invention is as follows: A microseismic reverse time positioning method based on improved imaging conditions includes: constructing a velocity model and performing gridding processing; Load the wave field data to the corresponding grid points in the velocity model according to the positions of the detectors to form detection points; The detection points are divided into N groups, each group has the same number of detection points, and the wave field data in each group are simultaneously reverse-time extended. The squares of the wave field amplitude values ​​obtained by reverse-time extension are superimposed along the time axis to obtain the initial positioning profile; Perform group cross-correlation operation on each group of wave fields after reverse time extension; The wave field after the group cross-correlation operation is subjected to autocorrelation processing to obtain the precise positioning profile for earthquake source location.

[0008] Furthermore, the reverse time extension refers to adding the wave field of the detector to the corresponding detection point position in time starting from the record of the maximum time, and performing reverse propagation of the wave field according to the numerical calculation method of the wave equation.

[0009] Furthermore, the wave field data in each group are simultaneously subjected to reverse time extension, including: , In the formula, For the The detection points are located in space and propagation time The reverse time-delay extension wave field; is the total number of detection points used, For each group of wave field data, the imaging value in the imaging section is simultaneously reverse-time extended. The maximum time for recording wave field data.

[0010] Furthermore, a group cross-correlation operation is performed on each group of wave fields after inverse time delay, which is expressed as: , in, is the serial number of the detector group, M represents the total number of detector groups, For the The reverse time-delay extension wave field of the group, is the imaging value in the imaging section after group cross-correlation operation, The maximum time for recording wave field data.

[0011] Furthermore, the wave field after the group cross-correlation operation is subjected to autocorrelation operation, which is expressed as: , is the imaging value in the imaging section after autocorrelation operation.

[0012] Compared with the prior art, the present invention has the following beneficial effects: the present invention first performs group cross-correlation imaging, and then performs autocorrelation imaging. The accuracy of earthquake source location is improved. The possible illusion phenomenon in the location process is reduced. More accurate earthquake source location is achieved, while the risk of error and illusion is reduced. The method of the present invention can give an accurate earthquake source location; has a high spatial resolution; can suppress background noise to a certain extent; can maximize the coherent wave field at the earthquake source; and overcome the problem of earthquake source "illusion". BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings are used to provide a further understanding of the technical solution of the present application or the prior art, and constitute a part of the specification. Among them, the accompanying drawings expressing the embodiments of the present application are used together with the embodiments of the present application to explain the technical solution of the present application, but do not constitute a limitation on the technical solution of the present application.

[0014] Figure 1 It is a schematic diagram of the flow of the method for reverse time location of earthquake sources provided by the present invention; Figure 2 is a schematic diagram of a velocity model in Example 1 of the present invention; Figure 3 This is a schematic diagram of receiving and recording by a surface geophone in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of positioning in Example 1 of the present invention; Figure 5 This is a schematic diagram of receiving and recording by a surface geophone in Embodiment 2 of the present invention; Figure 6 This is a schematic diagram of positioning in Example 2 of the present invention. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0016] See also Figure 1 A microseismic reverse time positioning method based on improved imaging conditions is shown, comprising: constructing a velocity model, in this embodiment, a Marmousi model is used, and performing grid processing; Load the wave field data to the corresponding grid points in the velocity model according to the positions of the detectors to form detection points; The detection points are divided into N groups, each group has the same number of detection points, and the wave field data in each group are simultaneously reverse-time extended. During the reverse-time extension process, the square of the wave field amplitude value obtained by reverse-time extension is superimposed along the time axis to obtain the initial positioning section; Performing group cross-correlation operation on the wave fields of each group of initial positioning sections; The wave field after the group cross-correlation operation is subjected to autocorrelation processing to obtain the precise positioning profile for earthquake source location.

[0017] Reverse time extension refers to adding the wave field of the detector to the corresponding detection point position in time starting from the record of the maximum time, and performing reverse propagation of the wave field according to the numerical calculation method of the wave equation.

[0018] The equation used in the numerical calculation method of the wave equation is: , represents the time after discreteness, represents the discretized spatial coordinates, Represents the particle stress wave field, where the particle stress wave field corresponds to different times and different spatial coordinates.

[0019] The derivation process is: Using the second-order acoustic wave equation, it can be expressed as: , Choose traditional meshing and finite difference method to realize the numerical discretization of second-order acoustic wave equation and define the spatial sampling step size and , corresponding to and Discretization interval in direction, introducing time sampling step , is the velocity of the medium space; In order to make the finite difference method have second-order time accuracy, the particle stress wave field At the moment Use Taylor series expansion at: , , , , Both are particle stress wave fields Representations at different moments.

[0020] Adding the two equations together, we can get: , This is the time second-order accuracy difference format of the finite difference algorithm, and the time truncation error is . It is expressed as: , Substituting into the second-order acoustic wave equation, we get the time second-order difference format: , The derivation of the spatial 2N-order difference format is similar to the derivation of the temporal 2-order difference format. For any integer ,for: , is the spatial truncation error, , , is the particle stress wave field Representation of coordinates in different spaces.

[0021] use , represents the coefficients of the difference format, ignoring the truncation error, we get Difference format in the axis direction: , Similarly, Difference format in the axis direction: , Will The difference format in the axis direction is the same as Substituting the difference format in the axial direction into the second-order time difference format, we get: , The reverse time extension in the present invention is to perform reverse propagation of the wave field according to the numerical calculation method of the wave equation mentioned above.

[0022] The square of the wave field amplitude value obtained by inverse time extension is superimposed along the time axis to obtain the initial positioning profile, including: , In the formula, For the The detection points are located in space and propagation time The reverse time-delay extension wave field; is the total number of detection points used, For each group of wave field data, the imaging value in the imaging section is simultaneously reverse-time extended. The maximum time for recording wave field data.

[0023] The group cross-correlation operation is performed on the wave field of each group of initial positioning sections, which can be expressed as: , in, is the sequence number of the detector group, Represents the total number of detector groups, For the The reverse time-delay extension wave field of the group, is the imaging value in the imaging section after group cross-correlation operation, The maximum time for recording wave field data.

[0024] The wave field after the group cross-correlation operation is subjected to autocorrelation operation, which is expressed as: , is the imaging value in the imaging section after autocorrelation operation.

[0025] In order to quantitatively evaluate the focusing effect of seismic imaging, the kurtosis index in statistics is used. Kurtosis is a statistic that measures the distribution morphology of random variables, reflecting the sharp or flat characteristics of the distribution. The calculation of kurtosis is based on the fourth-order center distance of the data, which is obtained by calculating the average of the fourth variance of the data points relative to the mean and performing normalization. By comparing the characteristics of kurtosis with those of the normal distribution, the steepness of the data distribution can be described. The level of kurtosis is related to the concentration of imaging. High kurtosis means that the imaging results are more concentrated and the imaging quality is better. On the contrary, low kurtosis indicates that the imaging results are dispersed, which may lead to reduced imaging resolution. The formula for calculating kurtosis is: , Where K is the kurtosis; is the total number of positioning result imaging values ​​I; For the The imaging value at ; is the average of all imaging values.

[0026] Embodiment 1, as Figure 2 The Marmousi velocity model established in this embodiment is shown in FIG. 1. When the velocity model is processed in a network, the grid size is 1000m*300m, the grid spacing is 1m, the sampling spacing is 1ms, the recording length is 1.6s, and a total of 1000 geophones are arranged along the surface of the target work area with a receiving spacing of 1m. The source uses Ricker wavelet with a main frequency of 20Hz. The source is placed at the (600,200) grid for excitation. The surface geophone receives the record as shown in FIG. Figure 3 shown.

[0027] Then the earthquake source is located by forward modeling and reverse time extension of the earthquake source wave field. Figure 4 As shown, the kurtosis value is 13571.6586. It can be seen from the positioning image that the positioning accuracy is high and the energy concentration is high.

[0028] Example 2, also using Figure 2 The Marmousi velocity model shown in the figure is different in that white noise with a signal-to-noise ratio of 0.1 is added to the received record. The surface geophone received record is as follows: Figure 5 shown.

[0029] Follow the steps in Example 1 to locate the earthquake source. Figure 6 As shown in the figure, the kurtosis value is 13138.7969. After adding noise, the positioning effect is still very accurate, almost unaffected by noise, and has strong noise resistance. The method of the present invention can highlight the effective information in the wave field, suppress the background noise to a certain extent, and suppress the irrelevant noise between the imaging back channels, and at the same time maximize the coherent wave field at the source.

[0030] The method of the present invention can display the effective information at the source in the positioning section with high quality under the premise of ensuring the number of detectors, and further overcomes the shortcomings of the prior art that individual or a small number of abnormal records of cross-correlation imaging conditions will have a serious impact on the imaging results and the large amount of calculation. It has strong adaptability to low signal-to-noise ratio data. By grouping, while ensuring the calculation efficiency, the applicability of microseismic reverse time imaging to the number of detection points, low signal-to-noise ratio microseismic records and the spatial resolution of imaging are improved within a reasonable calculation time. It has high computational efficiency. Compared with cross-correlation imaging, its calculation time is greatly reduced, and it is more suitable for microseismic monitoring with high real-time requirements. In addition, by grouping the detection points, the spatial resolution and signal-to-noise ratio of source imaging are effectively improved, and the interference problem in autocorrelation imaging is avoided.

[0031] 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 and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A microseismic reverse time positioning method based on improved imaging conditions, characterized in that: include: Construct velocity model and perform meshing; Load the wave field data to the corresponding grid points in the velocity model according to the positions of the detectors to form detection points; The detection points are divided into N groups, each group has the same number of detection points, and the wave field data in each group are simultaneously reverse-time extended. During the reverse-time extension process, the square of the wave field amplitude value obtained by reverse-time extension is superimposed along the time axis to obtain the initial positioning section; Performing group cross-correlation operation on the wave fields of each group of initial positioning sections; The wave field after the group cross-correlation operation is subjected to autocorrelation processing to obtain the precise positioning profile for earthquake source location.

2. A microseismic reverse time positioning method based on improved imaging conditions according to claim 1, characterized in that: The reverse time extension refers to adding the wave field of the detector to the corresponding detection point position in time starting from the record of the maximum time, and performing reverse propagation of the wave field according to the numerical calculation method of the wave equation.

3. The microseismic reverse time positioning method based on improved imaging conditions according to claim 1, characterized in that: The square of the wave field amplitude value obtained by inverse time extension is superimposed along the time axis to obtain the initial positioning profile, including: ， In the formula, For the The detection points are located in space and propagation time The reverse time-delay extension wave field; is the total number of detection points used, For each group of wave field data, the imaging value in the imaging section is simultaneously reverse-time extended. The maximum time for recording wave field data.

4. The microseismic reverse time positioning method based on improved imaging conditions according to claim 1, characterized in that: The group cross-correlation operation is performed on the wave field of each group of initial positioning sections, which can be expressed as: ， in, is the sequence number of the detector group, M represents the total number of detector groups, For the The reverse time-delay extension wave field of the group, is the imaging value in the imaging section after group cross-correlation operation, The maximum time for recording wave field data.

5. The method for reverse time positioning of microseismic earthquakes based on improved imaging conditions according to claim 4, characterized in that: The wave field after the group cross-correlation operation is subjected to autocorrelation operation, which is expressed as: , is the imaging value in the imaging section after autocorrelation operation.

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