A depth-domain inversion method based on the variable density acoustic wave equation

Abstract

The present invention belongs to the field of inversion, and particularly relates to a depth-domain inversion method based on the variable-density acoustic wave equation. It includes obtaining a depth-domain wavelet by equivalent constant-velocity substitution, obtaining a depth-domain amplitude-preserved angle gather by wavelet convolution with the reflection coefficient, and linear inversion of the depth-domain angle gather. The specific process is as follows: The seismic wavelet in the time domain is converted into a seismic wavelet in the depth domain by the equivalent constant-velocity method, and the depth-domain amplitude-preserved angle gather is obtained by convolving with the reflection coefficient after the corresponding coordinate transformation; then, taking the depth-domain amplitude-preserved angle gather as the input data set, using linear inversion with prior knowledge, the relationship between the variation of the reflectivity with the angle and the density and velocity perturbations is obtained. This method fully considers the influence of the density parameter on the reflection coefficient, and through precise wavefield direction decomposition, an amplitude-preserved reflection coefficient is obtained, which has an important guiding role in the actual reservoir resource location and exploration.

Description

Technical Field

[0001] The present invention belongs to the field of inversion, and particularly relates to a depth-domain inversion method based on a variable-density acoustic wave equation. Background Art

[0002] In an increasingly tense environment of great power confrontation, being able to achieve self-sufficiency in energy can gain a certain advantage in the fierce competition. Currently, in the imaging of underground media, the angle gather is the most reliable imaging gather because it is not affected by the wavefield multipath and has a clear physical meaning. And the inversion of underground elastic parameters through the angle gather has always been the focus of current research. The traditional AVA (Amplitude Versus Angle) inversion technology focuses on the time domain. By transforming the data in the depth domain to the time domain through depth-time conversion, the relatively mature time-domain AVA inversion technology is used to obtain the elastic parameter estimation in the time domain, and then the elastic parameter field in the depth domain is obtained through time-depth conversion. In this processing flow, especially in the process of time-depth conversion, the overall average velocity or layer average velocity is used, which is equivalent to smoothing the elastic parameters and causing the loss of high wavenumber components of the elastic parameters. And directly performing AVA inversion in the depth domain can obtain more accurate elastic parameter information while saving the additional conversion calculation amount. In actual seismic exploration, density is a weak parameter and is difficult to invert. However, if the influence of the density parameter on the amplitude is not considered in places where the interface density difference is large, then the obtained depth-domain angle gather is likely to be incorrect and the AVA inversion cannot be correctly performed. Therefore, the process of using the depth-domain angle gather AVA inversion based on the variable-density acoustic wave equation to invert the underground elastic properties and elastic parameters is a more reasonable and more in line with the actual underground physical meaning choice. Summary of the Invention

[0003] The present invention studies an inversion process of amplitude versus angle in the depth domain, converts the inversion of underground elastic coefficients in the time domain into the inversion of elastic coefficients in the depth domain, and finally obtains a high-precision inversion result of underground elastic coefficients through a reasonable selection of linear inversion equations. It not only demonstrates and strengthens the guiding significance of extracting variable-density angle gathers for actual seismic exploration from the perspective of the inversion of amplitude versus angle in the depth domain, but also further realizes the construction of an accurate underground elastic parameter field.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A depth-domain inversion method based on a variable-density acoustic wave equation, comprising the following steps:

[0006] Step 1, convert the seismic wavelet in the time domain to the seismic wavelet in the depth domain by the equivalent constant velocity method, and convolve the reflection coefficient after the corresponding coordinate transformation to obtain the depth-domain true-amplitude angle gather;

[0007] Step 2: Take the depth-domain true-amplitude angle gather as the input data set, and use linear inversion with prior knowledge to obtain the relationship between the variation of reflectivity with angle and the density and velocity perturbations.

[0008] Among them, Step 1 includes:

[0009] Step 1-1: In the depth domain, construct reflective layers, and the depth and velocity of each layer are:

[0010] ;

[0011] Replace the velocity of each layer with a constant velocity , then the corresponding layer thickness is replaced with :

[0012] ;

[0013] Then the corresponding normal-incidence reflection coefficient R undergoes a coordinate transformation with the numerical value unchanged;

[0014] Step 1-2: Resample the transformed layer thickness according to the minimum layer thickness:

[0015] ;

[0016] In the formula, is the resampled layer thickness;

[0017] Step 1-3: Re-establish the mapping relationship R - R’ between the reflection coefficients in the two coordinate systems before and after sampling, R’ is the reflection coefficient after coordinate transformation; and use the reflection coefficient R’ in the depth domain with the transformed coordinates and the depth-domain wavelet to convolve to obtain the corresponding true-amplitude depth-domain angle gather S :

[0018] .

[0019] Among them, Step 2 includes:

[0020] Step 2-1: Determine the approximation of the reflection and transmission of the longitudinal wave in the fluid medium, and then there are the following relational expressions:

[0021] ;

[0022] In the formula, the reflection coefficient varying with the angle is obtained from the true-amplitude depth-domain angle gather S , is the change in velocity, is the longitudinal wave velocity, is the change in density, is the density of the medium, is the tangent of the reflection angle;

[0023] The relational expression is re - expressed using two attributes: intercept and gradient:

[0024] ;

[0025] In the formula, represents the intercept, represents the gradient;

[0026] Step 2 - 2, the solution of the equation re - expressed for the two attributes of intercept and gradient is:

[0027] ;

[0028] where is:

[0029] ;

[0030] Substitute the formula of into the formula of to obtain the solutions of the elastic properties and as:

[0031] ;

[0032] Thus, the relationship between the variation of the reflectivity with the angle and the density and velocity perturbations can be obtained.

[0033] This method fully considers the influence of the density parameter on the reflection coefficient. Through the accurate decomposition of the wave - field direction, an amplitude - preserved reflection coefficient is obtained, which has an important guiding role in the actual reservoir resource positioning and exploration. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is the overall system flow chart of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] To enable those skilled in the art to better understand the solution of the present invention, the following will clearly and completely describe the technical solution in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments, and are not intended to limit the scope of the present invention disclosed. In addition, in the following description, the description of common technologies is omitted to avoid unnecessarily confusing the concept of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0036] The present invention will be further elaborated below in conjunction with the accompanying drawings:

[0037] A depth domain inversion method based on the variable density acoustic wave equation, as Figure 1 shown, includes the following steps:

[0038] Step 1, convert the seismic wavelet in the time domain to the seismic wavelet in the depth domain by the equivalent constant velocity method, and convolve the reflection coefficient after the corresponding coordinate transformation to obtain the depth domain true amplitude angle gather;

[0039] The specific steps of Step 1 include:

[0040] Step 1-1, in the depth domain, build reflection layers, and the depth and velocity of each layer are:

[0041] ;

[0042] Replace the velocity of each layer with the constant velocity , then the corresponding layer thickness is replaced with :

[0043] ;

[0044] Then the corresponding normal incidence reflection coefficient R is subjected to coordinate transformation with the numerical value unchanged;

[0045] Step 1-2, resample the converted layer thickness according to the minimum layer thickness:

[0046] ;

[0047] In the formula, is the layer thickness after sampling;

[0048] Step 1-3, re-establish the mapping relationship R - R’ of the reflection coefficients in the two coordinate systems before and after sampling R’is the reflection coefficient after coordinate transformation; and the reflection coefficient in the depth domain with transformed coordinates R’ is convolved with the wavelet in the depth domain to obtain the corresponding angle gather in the depth domain with preserved fidelity S :

[0049] .

[0050] Step 2: Using the angle gather in the depth domain with preserved fidelity as the input data set, the relationship between the variation of reflectivity with angle and the density and velocity perturbations is obtained by linear inversion with prior knowledge

[0051] The specific steps of Step 2 include

[0052] Step 2-1: Determine the approximation of the reflection and transmission of the longitudinal wave in the fluid medium, and then there are the following relational expressions

[0053] ;

[0054] In the formula, the reflection coefficient varying with the angle is obtained from the angle gather in the depth domain with preserved fidelity S , is the change in velocity is the longitudinal wave velocity is the change in density is the density of the medium is the tangent of the reflection angle

[0055] The relational expression is re-expressed in terms of two attributes: intercept and gradient

[0056] ;

[0057] In the formula represents the intercept represents the gradient

[0058] Step 2-2: The solutions of the equation re-expressed for the two attributes of intercept and gradient are

[0059] ;

[0060] where is

[0061] ;

[0062] Substitute the formula of into the formula of , and the solutions of the elastic properties and are

[0063] ;

[0064] The relationship between the variation of reflectivity with angle and density and velocity perturbations can be obtained.

Claims

1. A depth domain inversion method based on the variable density acoustic wave equation, characterized in that, Including the following steps: Step 1: Convert the seismic wavelet in the time domain to the seismic wavelet in the depth domain using the equivalent constant velocity method, and convolve the reflection coefficient after the corresponding coordinate transformation to obtain the depth-domain true-azimuth angle gather; Step 2: Use the depth-domain true-azimuth angle gather as the input data set, and utilize linear inversion with prior knowledge to obtain the relationship between the variation of reflectivity with angle and the density and velocity perturbations; Among them, Step 1 includes: Step 1-1: In the depth domain, construct n reflection layers, and the depth and velocity of each layer are: {(d1,v1)(d2,v2)……(d i ,v i )……(d n ,v n )} (1) Replace the velocity v of each layer i with a constant velocity v c , then the corresponding layer thickness is replaced by d′ i : Then, perform coordinate transformation on the corresponding vertically incident reflection coefficient R, and the value remains unchanged; Step 1-2: Resample the converted layer thickness according to the minimum layer thickness d c ≤ min[d′ i , i = 1, 2, …, n, (3) where d c is the layer thickness after sampling; Steps 1-3, re-establish the mapping relationship R-R' between the reflection coefficients of the two coordinate systems before and after sampling, where R' is the reflection coefficient after coordinate transformation; and use the reflection coefficient R' in the depth domain with transformed coordinates and the wavelet W in the depth domain v to convolve to obtain the corresponding true-amplitude depth-domain common-angle gather S: S = n' * W v (4).

2. The angle-domain imaging method based on the acoustic wave equation according to claim 1, wherein Step 2 includes: Step 2-1: Determine the approximation of the reflection and transmission of the longitudinal wave in the fluid medium, and then there are the following relational expressions: In the formula, the reflection coefficient Rpp(θ) varying with the angle is obtained from the angle gather S in the true depth domain, ΔV p is the change in velocity, V p is the P-wave velocity, Δρ is the change in density, ρ is the density of the medium, and tanθ is the tangent of the reflection angle; Re-express Equation (5) using two attributes, intercept and gradient: Rpp(θ) = P + Gtan 2 θ, (6) In the formula, P represents the intercept, and G represents the gradient; Step 2-2: The solution directly listed for Equation (6) is: P = (M T M) -1 M T Rpp(θ), (7) Among them, M is: Substitute Equation (8) into Equation (7) to obtain the solutions of the elastic attributes P and G as: The relationship between the variation of reflectivity with angle and the density and velocity perturbations can be obtained.

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