Attenuation-considered scattered wave reverse time imaging seismic positioning method and system for complex structure

Abstract

The invention discloses an attenuation-considered scattered wave reverse time imaging seismic positioning method and system for a complex structure, and the method comprises the steps: building a frequency domain viscous sound wave equation, and representing a stratum absorption attenuation effect through a complex velocity model; introducing a contrast source based on a scattering theory, and separating a background wave field from a scattering wave field; inverse time continuation considering attenuation compensation is carried out, and energy focusing is realized through reverse wave field propagation and filtering processing; determining a focus position by applying imaging conditions; the invention specifically relates to an earthquake event positioning method based on a scattering theory, in particular to an inverse time continuation earthquake source positioning method under the condition of considering stratum heterogeneity and energy attenuation compensation, which can be applied to micro-earthquake monitoring related to industrial activities, hydrofracture effect evaluation and earthquake event positioning in complex geological structures. The technical problems of a traditional earthquake positioning method in a complex structure and an attenuation medium are effectively solved, and a new technical support is provided for underground space fine detection and engineering safety monitoring.

Description

Technical Field

[0001] This application relates to the fields of geophysical exploration and earthquake monitoring technology, and in particular to a method and system for earthquake location based on reverse-time imaging of scattered waves considering attenuation in complex structures. Background Technology

[0002] Microseismic monitoring technology has significant application value in fields such as oil and gas field development, geothermal resource exploration, and engineering safety monitoring. Traditional source location methods are mainly divided into two categories: travel-time-based methods and waveform-based imaging methods. Travel-time-based methods, such as the Geiger method and the double-difference method, rely on the accurate acquisition of the first arrival time of seismic waves. However, the acquisition of the first arrival time is difficult under low signal-to-noise ratio conditions, leading to a significant decrease in location accuracy. Waveform-based methods, such as reverse-time imaging technology, can make full use of the entire waveform information, but they face two key technical bottlenecks in practical applications: First, the real underground medium has significant viscoelastic characteristics. Seismic waves undergo energy attenuation and dispersion effects during propagation. Traditional methods, based on the assumption of a perfectly elastic medium, ignore the attenuation factor, resulting in amplitude distortion and phase error. Second, non-homogeneous bodies such as faults and fractures in complex geological structures generate strong scattered wave interference, affecting the wavefield focusing effect.

[0003] In recent years, researchers have attempted to improve positioning accuracy by modifying imaging conditions and stacking methods, but the results in applications under strongly attenuating media and complex structural conditions remain unsatisfactory. While frequency-domain reverse-time imaging methods can effectively handle attenuation compensation, existing methods have shortcomings in the separation of scattered wavefields and numerical stability. Especially when the target area differs significantly from the surrounding rock properties, the contribution of the scattered wavefield to the total wavefield is significant, making it difficult for traditional methods to accurately separate the background wavefield from the scattered wavefield, leading to reduced imaging resolution. Furthermore, the high-frequency noise amplification problem during the reverse-time extrapolation process also limits the practical application of the method. Therefore, there is an urgent need to develop a novel seismic positioning method that can simultaneously consider the attenuation characteristics of strata and the scattering effects of complex structures. Summary of the Invention

[0004] To address the aforementioned issues, this application provides a method and system for seismic location based on attenuation-considered reverse-time imaging of scattered waves in complex structures. This effectively solves the key technical challenges faced by traditional seismic location methods in complex structures and attenuating media, providing new technical support for fine-grained underground space exploration and engineering safety monitoring. The technical solution is as follows: The first aspect of this application provides a seismic location method for inverse-time imaging of scattered waves considering attenuation in complex structures, comprising the following steps: establishing a frequency-domain viscous acoustic wave equation and characterizing the formation absorption attenuation effect through a complex velocity model; introducing a contrast source based on scattering theory to separate the background wave field and the scattered wave field; performing inverse-time extension considering attenuation compensation and achieving energy focusing through reverse wave field propagation and filtering; and determining the source location by applying imaging conditions.

[0005] For example, in the complex structure-considered attenuation-based reverse-time imaging seismic location method provided in one embodiment, the step of establishing the frequency domain viscous acoustic wave equation includes: calculating the frequency-phase velocity using the Kolsky-Futterman attenuation model; constructing a complex velocity model including the quality factor Q; and discretizing and solving the frequency domain viscous acoustic wave equation using a 9-point difference scheme.

[0006] For example, in the complex structure-considered attenuated scattered wave reverse-time imaging seismic location method provided in one embodiment, the complex velocity model is expressed as: in, It is a complex velocity. It is the frequency-dependent phase velocity, and Q is the quality factor. ω is the angular frequency, and i is the imaginary unit.

[0007] For example, in the complex structure-considered attenuation-based reverse-time imaging seismic location method provided in one embodiment, the step of introducing a contrast source includes: decomposing the total wave field into a background wave field and a scattered wave field based on perturbation theory; characterizing the difference in physical parameters between the target area and the background medium through a contrast function; and constructing a contrast source term to describe the generation mechanism of the scattered wave field.

[0008] For example, in the complex structure-considered attenuation-based reverse-time imaging seismic location method provided in one embodiment, the attenuation compensation step includes: employing a compensation velocity model during the reverse-time extension process; applying a low-pass filter to suppress high-frequency noise amplification; and achieving frequency domain wavefield stabilization through a Butterworth filter.

[0009] For example, in the complex structure-considered attenuated scattered wave reverse-time imaging seismic location method provided in one embodiment, the compensated velocity model is expressed as: in, This is the reference angular frequency.

[0010] For example, in the complex structure-considered attenuation-based reverse-time imaging seismic location method provided in one embodiment, the reverse-time extension step includes: using the seismic record as a boundary condition for wavefield back propagation; implementing attenuation compensation and filtering during the time-backpropagation process; and determining the energy focusing location through the maximum amplitude imaging condition.

[0011] For example, in one embodiment of the complex structure-considered attenuated scattering wave reverse-time imaging seismic location method, the method further includes: using a sparse storage strategy to process a large impedance matrix; applying the LU decomposition method to solve the discretized wave equation; and converting the frequency domain wave field to the time domain through inverse Fourier transform.

[0012] A second aspect of this application provides an earthquake monitoring system, comprising: a wavefield simulation module configured to perform the above-described method; a data acquisition module for receiving seismic record signals; a processing and control module for realizing reverse-time imaging and source location; and a result display module for outputting location results and imaging data.

[0013] For example, in one embodiment of the earthquake monitoring system, the system is applied to at least one of the following scenarios: shale gas hydraulic fracturing microseismic monitoring, underground fluid migration trajectory tracking, earthquake event location in complex geological structures, and spatiotemporal evolution analysis of earthquakes induced by engineering activities.

[0014] The seismic location method and system based on reverse-time imaging of scattered waves with attenuation in complex structures, provided by some embodiments of this application, have the following significant advantages: (1) Improved positioning accuracy under complex geological conditions: By establishing a frequency domain viscous acoustic wave equation and introducing a complex velocity model, the physical mechanism of formation absorption attenuation was accurately characterized, overcoming the limitations of the traditional elastic medium assumption. This method characterizes energy dissipation through the quantization of the imaginary part of the complex velocity model, fully considering the attenuation effect in the forward modeling stage, laying the foundation for subsequent accurate compensation, and reducing the positioning error in strongly attenuated formations.

[0015] (2) Enhanced imaging capability in non-homogeneous media: Based on scattering theory, the concept of contrast source is introduced, which effectively separates the background wave field and the scattered wave field, significantly improving the imaging quality under complex structural conditions. The contrast function sensitively identifies the differences in physical properties between the target area and the surrounding rock, and the contrast source term completely describes the process of scattered wave generation. This method is particularly suitable for high-precision imaging in fault-developed areas and fracture zones, improving lateral resolution.

[0016] (3) The numerical stability problem of inverse time-delay topology was solved: the high-frequency divergence phenomenon in the wave field back propagation process was effectively suppressed by the combined application of attenuation compensation mechanism and Butterworth filter. The compensation velocity model adopts the mathematical design of complex velocity conjugate to realize the accurate inverse transformation of the attenuation process, while the fourth-order Butterworth filter cuts off the noise frequency band while maintaining the effective signal, thus improving the numerical stability of inverse time-delay topology.

[0017] (4) Improved computational efficiency and practicality: The adoption of a 9-point difference scheme and sparse matrix storage strategy significantly improves the efficiency of numerical simulation while ensuring computational accuracy. The 9-point difference scheme reduces the sampling point requirement within one wavelength, sparse storage reduces memory usage, and LU decomposition provides a stable and efficient solution, enabling the method to handle large-scale data in actual exploration.

[0018] (5) Expanded the scope of engineering applications: Through complete system integration and clear scenario definition, the system has successfully transformed from theoretical methods to engineering applications. The modular design of the system supports various application needs such as shale gas fracturing monitoring and fluid migration tracking, providing reliable technical means for unconventional oil and gas resource development and engineering safety monitoring. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this specification or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 Flowchart of the implementation of the attenuated scattered wave reverse-time imaging seismic location method considering the complex structure of this application; Figure 2a For the original wavefield, the ground receives the seismic record; Figure 2b The attenuated wavefield is used to receive seismic records on the ground. Figure 2c This is an attenuated seismic record with a signal-to-noise ratio of 0.1; Figure 3a The source location results are noise-free from the original earthquake record; Figure 3b To compensate for the noise-free seismic source location results in the post-earthquake record; Figure 4a Source location results after adding noise to the original earthquake record; Figure 4b The source location results after adding noise to the earthquake record for compensation. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] Unless otherwise defined, the technical or scientific terms used in this disclosure shall have the ordinary meaning understood by one of ordinary skill in the art to which this disclosure pertains. The terms “first,” “second,” and similar terms used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as “comprising” or “including” mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as “connected” or “linked” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as “upper,” “lower,” “left,” and “right” are used only to indicate relative positional relationships, and these relative positional relationships may change accordingly when the absolute position of the described objects changes.

[0023] This application provides a seismic location method for complex structures using reverse-time imaging of scattered waves, considering attenuation. Figure 1 As shown, the specific implementation includes the following three core steps: Step 1: Establishing and solving the frequency domain viscous acoustic wave equation Seismic waves suffer energy loss during propagation due to absorption and attenuation by strata and fluids. Numerical simulations considering viscosity can be broadly categorized into two types: methods using quality factors in the time domain and methods using complex velocities in the frequency domain. In the time domain, a series of viscous parameters are used to describe the viscosity of the medium. These methods are based on various approximate constant Q models, but they struggle to describe frequency-domain attenuation coefficients and dispersion effects. In the frequency domain, however, the viscosity of seismic wave propagation can be characterized simply and quickly by introducing complex velocities.

[0024] If we disregard the absorption and attenuation effects of the formation, the underground medium can be considered as an elastic medium. In this case, the isotropic acoustic wave equation in the frequency domain can be expressed as: (Equation 1) In the formula For spatial wave fields, For the longitudinal wave velocity, As the epicenter, For the Laplace operator, Let be the angular frequency. The complex velocity model can be expressed as: (Equation 2) in, It is a complex velocity. This is the frequency-dependent phase velocity, and Q is the quality factor. The phase velocity is calculated using the Kolsky-Futterman decay rate model: (Equation 3) in, For the real velocity of the formation, The reference angular frequency is used. The final complex velocity model is as follows: (Equation 4) Substituting Equation 4 into Equation 1, we obtain the Helmholtz equation considering the absorption and attenuation effect of the formation: (Equation 5) In the formula, Let be the complex wave number. The matrix form of this expression is: (Equation 6) After obtaining the frequency-domain viscous acoustic wave equation, a 9-point difference scheme is used for discretization calculation. This operator only requires four sampling points within one wavelength during forward modeling, improving computational efficiency while maintaining the accuracy of the forward modeling. The 9-point difference scheme can be expressed as a weighted sum of the 0° difference operator and the 45° rotation operator: (Equation 7) The difference schemes for the 0° and 45° rotation Laplace operators are expressed as follows: (Equation 8) Substituting Equation 8 into Equation 5, we obtain Equation 5 after the finite difference: (Equation 9) The expressions for each coefficient are as follows: (Equation 10) Rearrange Equation 9 into matrix form: (Equation 11) in This is the forward impedance coefficient matrix. In actual calculations, to reduce computer memory consumption, for the matrix... A sparse storage strategy and LU decomposition method are employed for solving the problem. After solving, the simulated wavefield in the frequency domain is subjected to an inverse Fourier transform to obtain the forward-modeled seismic record from the time-domain source method. (Equation 12).

[0025] Step 2: Introduction of a contrast source and wavefield separation based on scattering theory The complex underground structure can lead to the generation of scattered waves. In order to obtain more comprehensive wavefield information including the scattered wavefield, this application uses the finite difference contrast source forward modeling method to simulate the wavefield.

[0026] Based on perturbation theory, the total wave field From background wave field and scattered wave field constitute: (Equation 13) In the frequency-wavenumber domain, the relationship for the above wave field can be obtained as follows: (Equation 14) in, Let the wavenumber be the sound wave propagating in the background wave field. Defined as contrast, it represents the degree of difference in physical parameters between a target area and its background. Contrast is not equal to 0 when there is a difference between the background and the target area; when their parameters are the same, contrast is zero, meaning no scattered waves are generated. The expression is as follows: (Equation 15) Here we introduce the concept of a comparison source, which we define as: (Equation 16) With the help of comparison sources The matrix forms of equations 13 and 14 can be written as: (17) in, It is by The diagonal matrix formed The background impedance coefficient matrix can be obtained from Equation 11. Integrating the above formulas, we can obtain: (18) in Since this is a forward modeling operator for a finite-difference contrast source, the solution for the scattered wavefield requires the background wavefield. The final expression for the total wavefield is: (Equation 19).

[0027] Step 3: Reverse-time imaging localization considering attenuation compensation The seismic record is reverse-time transformed and used as a boundary condition for the original receiver location. The wavefield is then backpropagated and filtered, with attenuation compensation performed during the process. After obtaining the backpropagated wavefield, appropriate imaging conditions are applied to the seismic record, and the location of the source is obtained by searching for the maximum energy.

[0028] Attenuation compensation can be viewed as the inverse operation of the attenuation process. When compensating in the frequency domain, the relationship between velocity and complex velocity can be expressed as: (Equation 20) During forward propagation of the wavefield, energy is gradually dissipated, and this process is relatively stable. However, during reverse propagation, the high-frequency components of the signal are greatly amplified, and noise is often a high-frequency component. Therefore, the influence of noise is amplified, leading to instability in the wave equation. In this case, low-pass filtering of the wavefield is required, and the cutoff frequency needs to be determined based on the frequency distribution of the effective signal and noise in the seismic record. The filtering operator used in this application is a fourth-order Butterworth filter.

[0029] Reverse-time imaging localization can be viewed as the process of backward expansion of the wave field along time -t, ultimately achieving energy focusing. The reverse-time-delayed topological wave field can be obtained by solving the following acoustic wave equation using the finite difference method: (Equation 21) Where R is the inverse time-delayed topology, This represents the seismic record received by the receiver. T is the total duration of the microseismic record, and t is the inverse wavefield propagation time. During the reverse time-delay topology process, a maximum amplitude imaging condition is applied, and the final energy focal point is the location of the seismic source. (Equation 22).

[0030] Example verification Example 1: Verification of positioning accuracy under noise-free conditions use Figure 2a The original wavefield record shown was tested. Figure (a) shows that the localization results of the traditional method have significant deviations, while... Figure 3b The location results obtained using the method presented in this application show a high degree of agreement with the actual seismic source location. Quantitative analysis indicates that the method presented in this application reduces the location error from 35m using traditional methods to 8m.

[0031] Example 2: Stability verification under noisy environments The test was conducted under attenuated seismic recording conditions with a signal-to-noise ratio of 0.1, as shown in Figure 2(c). Figure 4(a) shows that the traditional method is severely affected by noise, and multiple false energy clusters appear in the imaging results, while the method of this application shown in Figure 4(b) can still accurately identify the source location, demonstrating the strong noise resistance of the method.

[0032] This application presents a seismic location method for complex geological structures using inverse-time imaging of scattered waves. This method establishes a complete system comprising four key steps: establishing frequency-domain viscous acoustic wave equations, introducing a contrast source, applying inverse-time extension with attenuation compensation, and applying imaging conditions. This represents a fundamental shift from traditional elastic medium models to viscoelastic medium models. The method systematically considers both formation absorption attenuation and complex geological scattering effects. By combining wavefield separation and compensation techniques, it effectively overcomes amplitude distortion and phase errors caused by neglecting attenuation in traditional methods, significantly improving the accuracy and reliability of source location under complex geological conditions. By employing the Kolsky-Futterman attenuation model and a 9-point difference scheme for discretization, a wavefield simulation foundation more consistent with actual formation characteristics is established. The specific attenuation model accurately describes the frequency-dependent absorption attenuation characteristics, while the 9-point difference scheme improves numerical stability while maintaining computational accuracy. This combination provides a high-quality wavefield data foundation for subsequent inverse-time extension, reducing simulation errors at the source. By defining the specific mathematical expression of the complex velocity model, the influence mechanism of the formation quality factor on wave propagation is quantified. The imaginary part introduced in the model directly characterizes the energy dissipation characteristics of seismic waves during propagation in the strata, enabling numerical simulation to realistically reproduce the amplitude attenuation and waveform changes observed in actual operation, providing a theoretical basis for accurate attenuation compensation; by introducing a contrast source based on perturbation theory, the accurate separation of effective wavefield information under complex tectonic conditions is achieved.The contrast function sensitively identifies the differences in physical properties between the target region and the background medium, while the contrast source term fully describes the generation process of the scattered wave field. This separation process enables the method to maintain stable positioning performance even in highly heterogeneous formations. By implementing a compensated velocity model and Butterworth filtering in the reverse time-delay topology, the numerical instability problem in the reverse time propagation process is solved. The compensated model effectively recovers the high-frequency components lost due to attenuation, while the application of a specific filter suppresses the exponential growth of noise. This dual control ensures the convergence and practicality of the reverse time-delay topology process. By using complex velocity conjugate as the compensated velocity model, an accurate mathematical inverse transformation of the attenuation process is achieved. The conjugate operation is equivalent to attenuation compensation under time reversal in the frequency domain. This design ensures both... Maintaining mathematical rigor while facilitating numerical implementation, this method provides core algorithmic support for high-quality reverse-time imaging. By using seismic records as boundary conditions and combining them with maximum amplitude imaging conditions, a complete reverse-time imaging localization process is constructed. The reasonable setting of boundary conditions ensures the accuracy of wavefield backpropagation, while the maximum amplitude imaging condition fully utilizes the energy focusing characteristics of the wavefield. This combination gives the determination of the seismic source location clear physical meaning and mathematical basis. By adopting a sparse storage strategy and the LU decomposition method, the feasibility of large-scale numerical computation is significantly improved. Sparse storage effectively reduces memory requirements, while LU decomposition provides a stable and efficient equation solution scheme. This optimization enables the method to handle large-scale computational problems in actual exploration and has engineering application value.

[0033] A second aspect of this application provides an earthquake monitoring system, comprising: The wave field simulation module is configured to execute the above method, using a multi-core parallel computing architecture to achieve fast simulation of frequency domain wave fields; The data acquisition module is used to receive seismic recording signals; The processing and control module integrates an LU decomposition solver and a Butterworth filter to achieve reverse time imaging and seismic source localization; The results display module outputs positioning results and imaging data, providing three-dimensional energy focusing display and source parameter output.

[0034] By constructing a complete system encompassing wavefield simulation, data acquisition, processing and control, and result display, the transformation from theoretical methodology to practical tool has been achieved. The collaborative work of each module provides an end-to-end solution, enabling advanced positioning methods to be directly applied to actual production environments, greatly enhancing the practicality and promotional value of the technology.

[0035] Although the embodiments of this application have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for this application. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, this application is not limited to the specific details and the illustrations shown and described herein.

Claims

1. A seismic location method using reverse-time imaging of scattered waves considering attenuation in complex structures, characterized in that, Includes the following steps: A frequency-domain viscous acoustic wave equation was established, and the formation absorption attenuation effect was characterized by a complex velocity model. Based on scattering theory, a contrast source is introduced to separate the background wave field from the scattered wave field; Inverse time delay topology considering attenuation compensation is performed, and energy focusing is achieved through reverse wave field propagation and filtering. The location of the earthquake source is determined by applying imaging conditions.

2. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves according to claim 1, characterized in that, The steps for establishing the frequency-domain viscous acoustic wave equation include: The Kolsky-Futterman attenuation model was used to calculate the frequency-conversion phase velocity; Construct a complex velocity model that includes the quality factor Q; The frequency domain viscous acoustic wave equation is discretized and solved using a 9-point difference scheme.

3. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves, as described in claim 2, is characterized in that... The complex velocity model is expressed as: in, It is a complex velocity. It is the frequency-dependent phase velocity, and Q is the quality factor. ω is the angular frequency, and i is the imaginary unit.

4. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves, as described in claim 1, is characterized in that... The step of introducing a comparison source includes: Based on perturbation theory, the total wave field is decomposed into a background wave field and a scattered wave field; The difference in physical parameters between the target area and the background medium is characterized by a contrast function; A contrast source term is constructed to describe the generation mechanism of the scattered wave field.

5. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves according to claim 1, characterized in that, The attenuation compensation step includes: A compensated velocity model is used in the reverse time delay topology process; Low-pass filters are used to suppress high-frequency noise amplification; Frequency domain wave field stabilization is achieved using a Butterworth filter.

6. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves according to claim 5, characterized in that, The compensation velocity model is expressed as follows: in, This is the reference angular frequency.

7. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves, as described in claim 1, is characterized in that... The inverse time delay step includes: The seismic record was used as a boundary condition for backpropagation of the wave field. Attenuation compensation and filtering are performed during the time-reversal process; The energy focusing location is determined by the maximum amplitude imaging condition.

8. The seismic location method for complex structures considering attenuation through reverse-time imaging of scattered waves according to claim 1, characterized in that, Also includes: A sparse storage strategy is used to process large impedance matrices; The discretized wave equation is solved using the LU decomposition method. The frequency domain wave field is converted to the time domain by inverse Fourier transform.

9. An earthquake monitoring system, characterized in that, include: The wave field simulation module is configured to perform the method described in any one of claims 1-8; The data acquisition module is used to receive seismic recording signals; The processing and control module enables reverse-time imaging and seismic source localization; The results display module outputs the positioning results and imaging data.

10. The earthquake monitoring system according to claim 9, characterized in that, The system is applied to at least one of the following scenarios: microseismic monitoring of hydraulic fracturing in shale gas, tracking of underground fluid migration trajectories, location of seismic events in complex geological structures, and spatiotemporal evolution analysis of earthquakes induced by engineering activities.

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