A method for forward modeling of seismic wave in a moving medium

Abstract

The application belongs to the technical field of petroleum geophysical exploration, and particularly relates to a moving medium seismic wave forward simulation method. The flowing property of seawater is introduced into an acoustic wave equation to construct a moving medium acoustic wave equation, and the equation can more accurately simulate the propagation of seismic waves in flowing seawater and stationary solid rock layers.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of petroleum geophysical exploration, and particularly relates to a method for simulating seismic wave forward modeling of moving medium, which is especially suitable for offshore seismic exploration. BACKGROUND

[0002] Seismic exploration includes three basic processes of data acquisition, data processing and imaging interpretation. In the data processing link, velocity modeling and migration imaging are key steps. Seismic wave forward modeling based on full wave field information is the basis and core component of full waveform inversion modeling and reverse-time migration imaging, and the simulation accuracy directly determines the accuracy of subsequent modeling and imaging.

[0003] Compared with land seismic exploration, offshore seismic exploration needs to pay special attention to the influence of the nature and characteristics of seawater on seismic data acquisition and imaging processing. For example, offshore acquisition data often contains multiple wave, ghost wave and other interference wave field information. Fully considering the propagation mechanism of such waves in the forward modeling process helps to improve the simulation accuracy, thereby providing a more reliable theoretical basis for subsequent full waveform inversion modeling and reverse-time migration imaging.

[0004] In land seismic exploration, the motion attribute of the background medium is usually not considered, and the conventional acoustic or elastic wave equation can meet the demand. In offshore exploration, offshore seismic data based on air gun excitation and streamer reception usually only contains P-wave components, therefore, the industry usually uses the conventional acoustic wave equation to simulate the propagation of seismic waves in seawater. However, seawater has a characteristic that is often overlooked: it is not static like solid rock, but has fluidity, so it is important to consider and introduce the flow attribute of seawater in the acoustic wave equation to improve the forward simulation accuracy and serve the modeling and imaging processing process. SUMMARY

[0005] In view of the high-precision seismic wave propagation simulation problem faced by seismic data processing in China's offshore oil and gas seismic exploration, the inventors have designed a method for simulating seismic wave forward modeling of moving medium based on long-term practice, which introduces the flow attribute of seawater into the acoustic wave equation based on moving background medium (mainly seawater), and constructs a moving medium acoustic wave equation, which can more accurately simulate the propagation of seismic waves in flowing seawater and static solid rock.

[0006] The method for simulating seismic wave forward modeling of moving medium of the present application is as follows:

[0007] Step one, 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 two, input the seismic acquisition system parameters and the receiver point parameters, and input the forward simulation parameters;

[0009] Step three, simulating the propagation of acoustic wave in moving medium by using acoustic wave equation of moving medium;

[0010] Step four, recording the simulated acoustic wave field and simulated seismic record.

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

[0012] wherein x and z are spatial coordinate variables, t is a time variable, v x and v z are respectively the components of the flow velocity model vector v m in the x and z directions.

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

[0014] Further, to ensure the accuracy of the forward simulation and avoid numerical dispersion, the lateral grid interval, the vertical grid interval, the source main frequency, and the minimum P-wave velocity v pmin of the model satisfy the following relationship:

[0015] (1).

[0016] To ensure the stability of the acoustic wave propagation, the lateral grid interval, the vertical grid interval, the time sampling interval, and the maximum P-wave velocity v pmax of the model satisfy the following relationship:

[0017] (2).

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

[0019] (3).

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

[0021] Further, in step three, the propagation process of the acoustic wave in the moving medium is simulated, and the specific steps are as follows:

[0022] a. Source loading: the seismic source wavelet f(t) is loaded on the pressure component p of the acoustic wave;

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

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

[0025] d. Wave field continuation: from 0 time, the moving medium acoustic wave equation (3) is continued in time until nt time.

[0026] Further, the seismic source wavelet f(t) is loaded on the pressure component p of the acoustic wave, specifically, the Ricker wavelet is used as the excitation source and is loaded on the pressure component p, and the source function is:

[0027] (4) ;

[0028] Wherein, A represents the amplitude, and the value is 1000, and e represents the base of natural logarithm.

[0029] The beneficial effects of the present application are:

[0030] By introducing the flow properties of the background medium in offshore seismic exploration into the traditional acoustic wave equation, the accuracy of simulating the propagation of the acoustic wave in seawater is improved, and the acoustic wave field simulation results and the simulated seismic record results are closer to the actual situation. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is the flow chart of the method of the present application.

[0032] Figure 2 is a schematic diagram of the P wave velocity model of the background medium of the present application.

[0033] Figure 3 is a schematic diagram of the simulated acoustic wave field slice at 1 s obtained by different methods.

[0034] Figure 4 is a schematic diagram of the seismic record at 4 s obtained by different methods. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort fall within the scope of protection of the present application.

[0036] Seismic wave propagation simulation is highly sensitive to the accuracy of the underground geological model. Inaccurate model can cause large deviation between the simulated wave field and the real propagation process, and thus reduce the accuracy of subsequent seismic migration imaging. In offshore seismic exploration, fully considering the flow characteristics of seawater can significantly improve the accuracy of the simulated wave field. Therefore, the motion properties of the background medium are innovatively introduced into the conventional acoustic wave equation, aiming to accurately simulate the propagation process of seismic waves in flowing seawater, and to provide more accurate acoustic wave field information for subsequent seismic migration imaging

[0037] In view of the problem of high-precision seismic wave propagation simulation faced by seismic data processing in China's offshore oil and gas seismic exploration, the present application provides a method for simulating seismic wave propagation in moving medium, as shown in Figure 1 , the specific steps are as follows:

[0038] Step one, 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 flow velocity model of the background medium is v m (x, z)=(v x , v z ), the density of the background medium is p, and the seismic source wavelet used is f(t). Wherein, x and z are spatial coordinate variables, t is a time variable, v x and v z are the components of the flow velocity model vector v m in the x and z directions.

[0040] Step two, input the seismic acquisition system parameters and the receiver point parameters, and input the forward simulation parameters.

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

[0042] To ensure the accuracy of the forward simulation and avoid numerical dispersion, the horizontal grid spacing, the vertical grid spacing, the source dominant frequency and the minimum P-wave velocity v pmin The following relationship should be met:

[0043] (1).

[0044] To ensure the stability of the sound wave propagation, the horizontal grid spacing, the vertical grid spacing, the time sampling interval and the maximum P-wave velocity v pmax The following relationship should be met:

[0045] (2).

[0046] Step three, the motion medium acoustic wave equation is used to simulate the propagation process of the sound wave in the motion medium, and the motion medium acoustic wave equation in the form of stress-velocity is as follows:

[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 process of the sound wave in the motion medium are as follows:

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

[0051] (4);

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

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

[0054] c. Determine the numerical solution format: the classic staggered grid finite difference method is used to numerically solve the partial derivative term in equation (3);

[0055] d. Wave field continuation: starting from 0 time, the motion medium acoustic wave equation (3) is continued in time until nt time.

[0056] Step four, record the simulated sound wave field and the simulated seismic record.

[0057] The motion medium seismic wave forward modeling method of the present application is further illustrated by a specific implementation case.

[0058] Step one: 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 P-wave velocity, flow velocity and density models of the background medium are shown, and the model is divided into two layers, the upper layer represents a seawater layer flowing uniformly to the right at a speed of 30 m / s, and the lower layer represents a stationary solid layer.

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

[0060] Step three: use the motion medium acoustic wave equation (3) of the present application to simulate the propagation process of acoustic waves in the moving seawater and the stationary solid rock layer, and the detailed steps are as follows:

[0061] a. source loading: a Ricker wavelet is used as the excitation source and loaded on the acoustic pressure component p in this embodiment, and the source function is formula (4);

[0062] b. flow velocity field decomposition: the horizontal component v x of the flow velocity model is set to 30 m / s, and since the seawater mainly flows in the horizontal direction, the vertical component v z of the flow velocity model is set to 0 in this embodiment;

[0063] c. determine the numerical solution format: use the classic staggered grid finite difference method to numerically solve the partial derivative terms in equation (3);

[0064] d. wave field continuation: start from 0 time, and continue the motion medium acoustic wave equation (3) forward in time until 4 s.

[0065] Step four: record the simulated acoustic wave field and seismic record, and compare them with the acoustic wave field and seismic record obtained by using the traditional acoustic wave equation (without considering the motion of seawater). The traditional acoustic wave equation is as follows:

[0066] (5).

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

[0068] It can be seen by comparison that the equation (3) of the application can accurately simulate the propagation characteristics and propagation rules of acoustic waves in a moving medium, highlighting the influence of seawater flow, while the traditional acoustic wave equation (5) ignores the motion attribute of the background medium and cannot be used for high-precision seismic wave forward modeling at sea.

[0069] The above only describes the embodiments of the application, and does not limit the patent scope of the application, and any equivalent structure or equivalent process transformation using the content of the application specification and drawings, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the 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; 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; Step 2: Input the seismic acquisition system parameters and receiver point parameters, and input the forward modeling parameters; 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 grid spacing dx, vertical grid spacing dz, time sampling interval dt, total seismic recording duration nt, and source dominant frequency f. m ; Step 3: Simulate the propagation process of sound waves in a moving medium using the sound wave equation of 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 the acoustic wave equation of the moving medium are numerically solved using the classic staggered grid finite difference method; d. Wave field extension: Starting from time 0, the wave equation of the sound wave in the moving medium is extended in a clockwise direction until time nt; Step 4: Record the simulated acoustic wave field and simulated earthquake records.

2. The method according to claim 1, characterized in that, To ensure the accuracy of forward modeling and avoid numerical dispersion, the horizontal grid spacing, vertical 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 horizontal grid spacing, vertical grid spacing, time sampling interval, and the model's maximum P-wave velocity v are specified. pmax The following relationship must be satisfied: （2）。 3. 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.

4. The method according to claim 1, 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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