Moving medium seismic wave forward modeling method

Abstract

The invention belongs to the technical field of geophysical exploration for petroleum, and particularly relates to a forward modeling method for moving medium seismic waves, which constructs a moving medium acoustic wave fluctuation equation by introducing seawater flow attributes into the acoustic wave fluctuation equation and can more accurately simulate the propagation of seismic waves in flowing seawater and static solid rock strata.

Description

Technical Field

[0001] This invention belongs to the field of petroleum geophysical exploration technology, specifically relating to a forward modeling method for seismic waves in moving media, which is particularly suitable for marine seismic exploration. Background Technology

[0002] Seismic exploration comprises three basic processes: data acquisition, data processing, and post-imaging interpretation. In the data processing stage, velocity modeling and migration imaging are crucial steps. Forward modeling of seismic waves based on full-wavelength information is the foundation and core component of full-wavelength inversion modeling and reverse-time migration imaging; its simulation accuracy directly determines the accuracy of subsequent modeling and imaging.

[0003] Compared to onshore seismic exploration, offshore seismic exploration requires special attention to the impact of seawater properties and characteristics on seismic data acquisition and imaging processing. For example, offshore data often contains interfering wavefield information such as multiples and ghost waves. Fully considering the propagation mechanisms of such waves during forward modeling helps improve simulation accuracy, thereby providing a more reliable theoretical basis for subsequent full-waveform inversion modeling and reverse-time migration imaging.

[0004] In onshore seismic exploration, the motion properties of the background medium are usually not considered, and conventional acoustic or elastic wave equations are sufficient. In marine exploration, marine seismic data obtained through air gun excitation and towed cable reception typically only contains P-wave components. Therefore, the industry usually uses conventional acoustic wave equations to simulate the propagation of seismic waves in seawater. However, seawater has an often overlooked characteristic: it is not static like solid rock layers, but rather fluid. Therefore, considering and incorporating the fluid properties of seawater into the acoustic wave equations is of great significance for improving the accuracy of forward modeling and thus serving the modeling and imaging processing processes. Summary of the Invention

[0005] To address the challenge of high-precision seismic wave propagation simulation in seismic data processing during offshore oil and gas seismic exploration in my country, the inventors have developed a forward modeling method for seismic waves in moving media through long-term practical research. Based on the moving background medium (mainly seawater), the method incorporates the seawater flow properties into the acoustic wave equation to construct the acoustic wave equation for moving media. This equation can more accurately simulate the propagation of seismic waves in flowing seawater and stationary solid rock strata.

[0006] The forward modeling method for seismic waves in a moving medium according to the present invention is as follows:

[0007] Step 1: Input the P-wave velocity model of the background medium, the flow velocity model of the background medium, the density of the background medium, and the seismic source wavelet used;

[0008] Step 2: Input the seismic acquisition system parameters and receiver point parameters, and input the forward modeling parameters;

[0009] Step 3: Simulate the propagation process of sound waves in a moving medium using the motion medium sound wave wave equation;

[0010] Step 4: Record the simulated acoustic wave field and simulated earthquake records.

[0011] Furthermore, in step one, the P-wave velocity model of the background medium is v p (x, z), the velocity model of the background medium is v m (x, z)=(v x , v z The density of the background medium is ρ, and the wavelet of the earthquake source used is f(t).

[0012] Where x and z are spatial coordinate variables, t is a time variable, and v x and v z These are the velocity model vectors v. m Components in the x and z directions.

[0013] Further, in step two, the parameters of the seismic acquisition system include: number of shots Ns, shot spacing Ds, and shot point coordinate arrangement; the parameters of the receiving points include: number of receiving points Nr, receiving point spacing Dr, and receiving point coordinate arrangement; the parameters of the forward modeling include: horizontal and vertical grid spacing dx and dz, time sampling interval dt, total seismic recording duration nt, and source dominant frequency f. m .

[0014] Furthermore, to ensure the accuracy of the forward modeling and avoid numerical dispersion, the grid spacing, the dominant source frequency, and the minimum P-wave velocity v of the model are... pmin The following relationship must be satisfied:

[0015] (1);

[0016] To ensure the stability of sound wave propagation, the grid spacing, time sampling interval, and the model's maximum P-wave velocity v are... pmax The following relationship must be satisfied:

[0017] (2).

[0018] Furthermore, in step three, the acoustic wave equation of the moving medium is expressed in stress-velocity form, as follows:

[0019] (3);

[0020] In the formula, p represents the pressure component of the sound wave, and u and w represent the velocity components of the sound wave in the x and z directions, respectively.

[0021] Furthermore, in step three, the propagation process of sound waves in a moving medium is simulated, and the specific steps are as follows:

[0022] a. Source loading: The earthquake source wavelet f(t) is loaded onto the pressure component p of the sound wave;

[0023] b. Velocity field decomposition: Based on the principle of vector decomposition, the velocity model v is decomposed. m (x, z) is decomposed into horizontal and vertical components v. x and v z ;

[0024] c. Determine the numerical solution format: The partial derivative terms in equation (3) are numerically solved using the classic staggered grid finite difference method;

[0025] d. Wave field extension: Starting from time 0, the wave equation (3) of the moving medium is extended in a clockwise direction until time nt.

[0026] Furthermore, the earthquake source wavelet f(t) is loaded onto the pressure component p of the sound wave. Specifically, a Ricker wavelet is used as the excitation source and loaded onto the sound pressure component p, with the source function being:

[0027] (4);

[0028] In the formula, A represents the amplitude, which takes the value of 1000, and e represents the base of the natural logarithm.

[0029] The beneficial effects of this invention are:

[0030] By incorporating the flow properties of the background medium in marine seismic exploration into the traditional acoustic wave equation, the accuracy of simulating the propagation of sound waves in seawater is improved, and the resulting acoustic wave field simulation results are closer to the actual situation than the simulated seismic records. Attached Figure Description

[0031] Figure 1 This is a flowchart of the method of the present invention.

[0032] Figure 2 This is a schematic diagram of the P-wave velocity model of the background medium of this invention.

[0033] Figure 3 These are schematic diagrams of simulated acoustic wave field slices obtained by different methods at 1 s.

[0034] Figure 4 These are schematic diagrams of earthquake records at 4 s obtained by different methods. Detailed Implementation

[0035] 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 a part of the embodiments of this application, and not all of the 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.

[0036] Seismic wave propagation simulation is highly sensitive to the accuracy of the subsurface geological model. Inaccurate models can lead to significant deviations between the simulated wavefield and the actual propagation process, thereby reducing the accuracy of subsequent seismic migration imaging. In marine seismic exploration, fully considering the fluidity of seawater can significantly improve the accuracy of the simulated wavefield. Therefore, this invention innovatively introduces the motion properties of the background medium into the conventional acoustic wave equation, aiming to accurately simulate the propagation process of seismic waves in flowing seawater and provide more precise acoustic wavefield information for subsequent seismic migration imaging.

[0037] To address the challenge of high-precision seismic wave propagation simulation in seismic data processing during offshore oil and gas seismic exploration in my country, this invention proposes a forward modeling method for seismic waves in moving media. (See [link to relevant documentation]). Figure 1 The specific steps are as follows:

[0038] Step 1: Input the P-wave velocity model of the background medium, the flow velocity model of the background medium, the density of the background medium, and the seismic source wavelet used.

[0039] Specifically, the P-wave velocity model of the background medium is v p (x, z), the velocity model of the background medium is v m (x, z)=(v x , v z The background medium has a density of ρ, and the earthquake source wavelet used is f(t). Here, x and z are spatial coordinate variables, t is a time variable, and v... x and v z These are the velocity model vectors v. m Components in the x and z directions.

[0040] Step 2: Input the parameters of the seismic acquisition system and the receiver point, and input the forward modeling parameters.

[0041] Specifically, the parameters of the seismic acquisition system include: number of shots Ns, shot spacing Ds, and shot point coordinate arrangement; the parameters of the receiver points include: number of receiver points Nr, receiver point spacing Dr, and receiver point coordinate arrangement. The parameters of the forward modeling include: horizontal and vertical grid spacing dx and dz, time sampling interval dt, total seismic recording duration nt, and source dominant frequency f. m .

[0042] To ensure the accuracy of forward modeling and avoid numerical dispersion, the grid spacing, source dominant frequency, and minimum P-wave velocity v of the model are adjusted. pmin The following relationship should be satisfied:

[0043] (1).

[0044] To ensure the stability of sound wave propagation, the grid spacing, time sampling interval, and the model's maximum P-wave velocity v are adjusted. pmax The following relationship should be satisfied:

[0045] (2).

[0046] Step 3: Simulate the propagation process of sound waves in a moving medium using the motion medium sound wave equation. The motion medium sound wave equation expressed in stress-velocity form is shown below:

[0047] (3);

[0048] In the formula, p represents the pressure component of the sound wave, and u and w represent the velocity components of the sound wave in the x and z directions, respectively.

[0049] The specific steps of simulating the propagation of sound waves in a moving medium are as follows:

[0050] a. Source loading: The earthquake source wavelet f(t) is loaded onto the pressure component p of the sound wave; specifically, the Ricker wavelet is used as the excitation source and loaded onto the sound pressure component p, and its source function is:

[0051] (4);

[0052] In the formula, A represents the amplitude, which takes the value of 1000, and e represents the base of the natural logarithm.

[0053] b. Velocity field decomposition: Based on the principle of vector decomposition, the velocity model v is decomposed. m (x, z) is decomposed into horizontal and vertical components v. x and v z ;

[0054] c. Determine the numerical solution format: The partial derivative terms in equation (3) are numerically solved using the classic staggered grid finite difference method;

[0055] d. Wave field extension: Starting from time 0, the wave equation (3) of the moving medium is extended in a clockwise direction until time nt.

[0056] Step 4: Record the simulated acoustic wave field and simulated earthquake records.

[0057] The following specific implementation example further illustrates the forward modeling method for seismic waves in moving media of the present invention.

[0058] Step 1: Input the P-wave velocity model of the background medium, the flow velocity model of the background medium, the density of the background medium, and the seismic source wavelet used. Figure 2 The model shows the P-wave velocity, flow velocity, and density of the background medium. The model is divided into two layers: the upper layer represents the seawater layer flowing uniformly to the right at a speed of 30 m / s, and the lower layer represents the stationary solid layer.

[0059] Step 2: Input the seismic acquisition system parameters. In this embodiment, the number of shots Ns is 1, the shot spacing Ds = 0, and the shot coordinates are (2.5 km, 2 km); the number of receivers Nr is 501, the spacing Dr is 10 m, the receivers are horizontally uniformly distributed, and the depth of each receiver is 10 m. The input forward modeling parameters are: horizontal and vertical grid spacing dx and dz are both 10 m, the time sampling interval dt = 0.5 ms, the total seismic recording duration nt = 4 s, and the source dominant frequency f. m =10 Hz. After verification, the forward modeling parameters satisfy the dispersion condition formula (1) and the stability condition formula (2).

[0060] Step 3: The propagation process of sound waves in moving seawater and stationary solid rock layers is simulated using the motion medium sound wave equation (3) of this invention. The detailed steps are as follows:

[0061] a. Source loading: In this embodiment, the Ricker wavelet is used as the excitation source and loaded onto the sound pressure component p. Its source function is formula (4).

[0062] b. Velocity field decomposition: Decomposing the horizontal component v of the velocity model x The velocity is set to 30 m / s. Since seawater mainly flows horizontally, the vertical component v of the velocity model is used in this embodiment. z Set to 0;

[0063] c. Determine the numerical solution format: The partial derivative terms in equation (3) are numerically solved using the classic staggered grid finite difference method;

[0064] d. Wave field extension: Starting from time 0, the wave equation (3) of the moving medium is extended in a clockwise direction until time 4 s.

[0065] Step 4: Record the simulated acoustic wave field and seismic record, and compare them with the acoustic wave field and seismic record obtained using the traditional acoustic wave equation (ignoring seawater movement). The traditional acoustic wave equation is shown below:

[0066] (5);

[0067] Figure 3 -a is a slice of the acoustic wave field at 1 s obtained using equation (3) of this invention. Figure 3 -b is a slice of the acoustic wave field obtained using the traditional acoustic wave equation (5); Figure 4 -a is the seismic record at 4 s obtained using equation (3) of this invention. Figure 4 -b is the seismic record obtained using the traditional acoustic wave equation (5), and the red dashed line is the auxiliary line.

[0068] By comparison, it can be seen that the equation (3) of this invention can accurately simulate the propagation characteristics and propagation law of sound waves in the moving medium, highlighting the influence of seawater flow, while the traditional sound wave equation (5) ignores the motion properties of the background medium and cannot be used for high-precision forward modeling of marine seismic waves.

[0069] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A forward modeling method for seismic waves in a moving medium, characterized in that, The specific method is as follows: Step 1: Input the P-wave velocity model of the background medium, the flow velocity model of the background medium, the density of the background medium, and the seismic source wavelet used; Step 2: Input the seismic acquisition system parameters and receiver point parameters, and input the forward modeling parameters; Step 3: Simulate the propagation process of sound waves in a moving medium using the motion medium sound wave wave equation; Step 4: Record the simulated acoustic wave field and simulated earthquake records.

2. The method according to claim 1, characterized in that, In step one, the P-wave velocity model of the background medium is v p (x, z), the velocity model of the background medium is v m (x, z)=(v x , v z The density of the background medium is ρ, and the wavelet of the earthquake source used is f(t); Where x and z are spatial coordinate variables, t is a time variable, and v x and v z These are the velocity model vectors v. m Components in the x and z directions.

3. The method according to claim 1, characterized in that, In step two, the parameters of the seismic acquisition system include: number of shots Ns, shot spacing Ds, and shot point coordinate arrangement; the parameters of the receiving points include: number of receiving points Nr, receiving point spacing Dr, and receiving point coordinate arrangement; the parameters of the forward modeling include: horizontal and vertical grid spacing dx and dz, time sampling interval dt, total seismic recording duration nt, and source dominant frequency f. m .

4. The method according to claim 3, characterized in that, To ensure the accuracy of forward modeling and avoid numerical dispersion, the grid spacing, source dominant frequency, and minimum P-wave velocity v of the model are specified. pmin The following relationship must be satisfied: （1）； To ensure the stability of sound wave propagation, the grid spacing, time sampling interval, and the model's maximum P-wave velocity v are... pmax The following relationship must be satisfied: （2）。 5. The method according to claim 1, characterized in that, In step three, the acoustic wave equation of the moving medium is expressed in stress-velocity form, as follows: （3）； In the formula, p represents the pressure component of the sound wave, and u and w represent the velocity components of the sound wave in the x and z directions, respectively.

6. The method according to claim 5, characterized in that, Step three simulates the propagation process of sound waves in a moving medium. The specific steps are as follows: a. Source loading: The earthquake source wavelet f(t) is loaded onto the pressure component p of the sound wave; b. Velocity field decomposition: Based on the principle of vector decomposition, the velocity model v is decomposed. m (x, z) is decomposed into horizontal and vertical components v. x and v z ; c. Determine the numerical solution format: The partial derivative terms in equation (3) are numerically solved using the classic staggered grid finite difference method; d. Wave field extension: Starting from time 0, the wave equation (3) of the moving medium is extended in a clockwise direction until time nt.

7. The method according to claim 6, characterized in that, The earthquake source wavelet f(t) is loaded onto the pressure component p of the sound wave. Specifically, a Ricker wavelet is used as the excitation source and loaded onto the sound pressure component p. The source function is: （4）； In the formula, A represents the amplitude, which takes the value of 1000, and e represents the base of the natural logarithm.

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