1. A biphase medium elastic wave reverse time migration imaging method based on longitudinal and transverse wave decoupling is characterized in that a first-order velocity-stress equation of a biphase isotropic medium is deduced; deriving a longitudinal and transverse wave decoupling equation of the biphase medium, and separating a mixed wave field in the biphase medium into a pure longitudinal wave (P wave) wave field and a pure transverse wave (S wave) wave field; obtaining an imaging result by using the seismic source normalized imaging condition; correcting a polarity inversion phenomenon caused by wave field separation; the method specifically comprises the following steps:

the method comprises the following steps: inputting a longitudinal and transverse wave velocity field, a multi-component actual observation gun record and an observation system file;

step two: calculating an elastic wave source mixed wave field extended forward by a two-phase medium;

step three: separating the mixed wave field obtained in the second step into a pure longitudinal wave field and a pure transverse wave field by utilizing a Helmholtz decoupling method;

step four: calculating an elastic wave source mixed wave field of the reverse continuation of the two-phase medium;

step five: separating the mixed wave field obtained in the fourth step into a pure longitudinal wave field and a pure transverse wave field by utilizing a Helmholtz decoupling method;

step six: obtaining a multi-component imaging result of the elastic wave of the two-phase medium under a Cartesian coordinate system;

step seven: carrying out polarity correction on the imaging result with the polarity inversion;

step eight: and outputting longitudinal and transverse wave decoupled elastic wave biphase medium reverse time migration imaging.

2. The method for biphasic medium elastic wave reverse time migration imaging based on longitudinal and transverse wave decoupling as claimed in claim 1, wherein the second specific process is as follows:

based on the Biot theory, the vector form of the two-dimensional two-phase isotropic medium elastic wave equation is (1):

wherein in the formulaAndrespectively representing the displacement components of the solid and liquid phases; q represents the coupling between the volume of the liquid phase and the change in volume of the solid phase; mu and lambda are solid phase elasticity parameters corresponding to Lame constants in elasticity theory; r is the elasticity parameter of the fluid phase, p₁₁，ρ₁₂，ρ₂₂As a mass coefficient, p₁₁Representing the total equivalent mass, p, of the solid phase fraction of the volume element of the medium₁₂Represents the solid-liquid phase coupling mass coefficient, rho₂₂Representing the total equivalent mass of the flow phase portion in the media volume element; b is dissipation coefficient, and when the relative motion between the fluid and the solid is not considered, the elastic wave equation without dissipation can be obtained by making B equal to 0;

the elastic wave equation vector form is derived with time, and a first-order velocity-stress equation (2) of the two-phase isotropic medium can be obtained through sorting:

wherein v is_x，v_zRepresents the velocity of the solid phase; v_x，V_zRepresents the velocity of the flow phase; sigma_xx，σ_zz，τ_xzRepresents the solid phase stress and S represents the flow phase stress.

3. The method for biphasic medium elastic wave reverse time migration imaging based on longitudinal and transverse wave decoupling as claimed in claim 1, wherein the third specific process is as follows: according to Helmholtz decoupling theory, the longitudinal wave field is a non-rotating field, the transverse wave field is a non-scattered field, and the velocities of the longitudinal wave and the transverse wave of the liquid phase are defined as phi and phi respectivelyThe velocities of the longitudinal and transverse solid waves are phi and phi respectivelySatisfies the relationship of formula (3):

then there are:

wherein, the rotation field of the flow phaseAnd solid phase rotation fieldOnly with shear wave velocity and not with longitudinal wave velocity; the solid phase divergence field theta and the flow phase divergence field theta are only related to longitudinal wave velocity and are not related to transverse wave velocity, so that the solid phase displacement vector and the flow phase displacement vector can respectively obtain a pure transverse wave (S) wave field and a pure longitudinal wave (P) wave field through rotation and divergence field operation, and the wave field separation is the basis of the wave field separation in the two-phase medium.

4. The method for imaging elastic waves of the isotropic two-phase medium based on longitudinal and transverse wave decoupling by reverse time migration of the longitudinal and transverse waves as claimed in claim 1, wherein the sixth specific process comprises the following steps:

using the normalized imaging conditions of the seismic source as shown in formula (5) for the separated longitudinal and transverse wave components in the seismic source wave field and the wave field of the detector,

wherein, I_pp，I_ps，I_spAnd I_ssImaging of the PP, PS, SP and SS fractions representing the solid phases, respectively; i is_PP，I_PS，I_SPAnd I_SSImaging of the PP, PS, SP and SS components of the flow phase, respectively;

SF_pand SF_sAnd respectively representing the separated solid-phase longitudinal wave field and the solid-phase transverse wave field in the seismic source wave field propagated from the forward direction; RF (radio frequency)_pAnd RF_sRespectively representing solid-phase longitudinal wave fields and solid-phase transverse wave fields separated from the wave field of the detection point which reversely propagates; SF_PAnd SF_SRespectively representing liquid-phase longitudinal wave fields and transverse wave fields separated from a forward-propagated seismic source wave field; RF (radio frequency)_PAnd RF_SRespectively representing liquid phase longitudinal and transverse wave fields separated from a backward propagation wave detection point wave field, (x, y) representing space coordinates in a Cartesian coordinate system, and nt representing the total number of samples in the calculation time.

5. The method for imaging elastic waves of the isotropic two-phase medium based on longitudinal and transverse wave decoupling by reverse time migration of the longitudinal and transverse waves as claimed in claim 1, wherein the seven concrete processes of the step are as follows:

polarity inversion corrects the polarity by calculating the incident angle of the longitudinal wave at each imaging point and then taking the negative value of the incident angle or the opposite number of positive value imaging, so the normalized cross-correlation imaging conditions of the seismic source are rewritten as (taking solid phase PS imaging as an example)

In the formula, theta_pRepresents the incident angle of the longitudinal wave, and

Background

The underground rock is actually composed of a solid phase and a liquid phase, and the solid phase porous framework is a uniform isotropic elastic solid; the liquid phase is a compressible fluid with viscosity that fills the void space. Production practices show that the stratum has extensive development of fracture pores, and the pores are filled with fluid. Therefore, the imaging research on the two-phase medium is beneficial to improving the oil-gas exploration precision, and has important significance.

At present, Reverse Time Migration (RTM) in a two-phase medium is imaging by using a mixed wave field, and only coupling information is obtained, which is easy to generate longitudinal and transverse wave crosstalk and is not beneficial to identification and analysis of the wave field. Whereas the traditional elastic wave inverse time migration (ERTM) based on the decoupling of the longitudinal and transverse waves is imaged based on the pure solid model assumption. Compared to a pure solid model, a liquid or gas filled biphasic medium is more realistic, but traditional ERTM is ineffective in biphasic media. Furthermore, fluid or gas reservoirs are difficult to identify in single phase imaging.

Disclosure of Invention

Aiming at the defects, the invention provides a dual-phase medium elastic wave reverse time migration imaging method based on longitudinal and transverse wave decoupling, which is characterized in that a first-order velocity-stress equation of a dual-phase isotropic medium is deduced; deriving a longitudinal and transverse wave decoupling equation of the biphase medium, and separating a mixed wave field in the biphase medium into a pure longitudinal wave (P wave) wave field and a pure transverse wave (S wave) wave field; and solving an imaging result by using the seismic source normalized imaging condition.

The invention specifically adopts the following technical scheme:

a method for imaging elastic wave reverse time migration of a two-phase medium based on longitudinal and transverse wave decoupling is used for deducing a first-order velocity-stress equation of the two-phase isotropic medium; deriving a longitudinal and transverse wave decoupling equation of the biphase medium, and separating a mixed wave field in the biphase medium into a pure longitudinal wave (P wave) wave field and a pure transverse wave (S wave) wave field; obtaining an imaging result by using the seismic source normalized imaging condition; correcting a polarity inversion phenomenon caused by wave field separation; the method specifically comprises the following steps:

the method comprises the following steps: inputting a longitudinal and transverse wave velocity field, a multi-component actual observation gun record and an observation system file;

step two: calculating an elastic wave source mixed wave field extended forward by a two-phase medium;

step three: separating the mixed wave field obtained in the second step into a pure longitudinal wave field and a pure transverse wave field by utilizing a Helmholtz decoupling method;

step four: calculating an elastic wave source mixed wave field of the reverse continuation of the two-phase medium;

step five: separating the mixed wave field obtained in the fourth step into a pure longitudinal wave field and a pure transverse wave field by utilizing a Helmholtz decoupling method;

step six: obtaining a multi-component imaging result of the elastic wave of the two-phase medium under a Cartesian coordinate system;

step seven: carrying out polarity correction on the imaging result with the polarity inversion;

step eight: and outputting longitudinal and transverse wave decoupled elastic wave biphase medium reverse time migration imaging.

Preferably, the specific process of the step two is as follows:

based on the Biot theory, the vector form of the two-dimensional two-phase isotropic medium elastic wave equation is (1):

wherein in the formulaAndrespectively representing the displacement components of the solid and liquid phases; q represents the coupling between the volume of the liquid phase and the change in volume of the solid phase; mu and lambda are solid phase elasticity parameters corresponding to Lame constants in elasticity theory; r is the elasticity parameter of the fluid phase, p₁₁，ρ₁₂，ρ₂₂As a mass coefficient, p₁₁Representing the total equivalent mass, p, of the solid phase fraction of the volume element of the medium₁₂Represents the solid-liquid phase coupling mass coefficient, rho₂₂Representing the total equivalent mass of the flow phase portion in the media volume element; b is dissipation coefficient, and when the relative motion between the fluid and the solid is not considered, the elastic wave equation without dissipation can be obtained by making B equal to 0;

the elastic wave equation vector form is derived with time, and a first-order velocity-stress equation (2) of the two-phase isotropic medium can be obtained through sorting:

wherein v is_x，v_zRepresents the velocity of the solid phase; v_x，V_zRepresents the velocity of the flow phase; sigma_xx，σ_zz，τ_xzRepresents the solid phase stress and S represents the flow phase stress.

Preferably, the step three comprises the following specific processes: according to Helmholtz decoupling theory, the longitudinal wave field is a non-rotating field, the transverse wave field is a non-scattered field, and the velocities of the longitudinal wave and the transverse wave of the liquid phase are defined as phi and phi respectivelyThe velocities of the longitudinal and transverse solid waves are phi and phi respectivelySatisfies the relationship of formula (3):

then there are:

in the formula, the rotation field of the flow phaseAnd solid phase rotation fieldOnly with shear wave velocity and not with longitudinal wave velocity; the solid phase divergence field theta and the flow phase divergence field theta are only related to longitudinal wave velocity and are not related to transverse wave velocity, so that the solid phase displacement vector and the flow phase displacement vector can respectively obtain a pure transverse wave (S) wave field and a pure longitudinal wave (P) wave field through rotation and divergence field operation, and the wave field separation is the basis of the wave field separation in the two-phase medium.

Preferably, the step six comprises the following specific processes:

using the normalized imaging conditions of the seismic source as shown in formula (5) for the separated longitudinal and transverse wave components in the seismic source wave field and the wave field of the detector,

wherein, I_pp，I_ps，I_spAnd I_ssImaging of the PP, PS, SP and SS fractions representing the solid phases, respectively; i is_PP，I_PS，I_SPAnd I_SSImaging of the PP, PS, SP and SS components of the flow phase, respectively;

SF_pand SF_sAnd respectively representing the separated solid-phase longitudinal wave field and the solid-phase transverse wave field in the seismic source wave field propagated from the forward direction; RF (radio frequency)_pAnd RF_sRespectively representing solid-phase longitudinal wave fields and solid-phase transverse wave fields separated from the wave field of the detection point which reversely propagates; SF_PAnd SF_SRespectively representing liquid-phase longitudinal wave fields and transverse wave fields separated from a forward-propagated seismic source wave field; RF (radio frequency)_PAnd RF_SRespectively, representing the liquid phase longitudinal and transverse wave wavefields separated from the counter-propagating demodulator probe wavefields. (x, y) represents spatial coordinates in a Cartesian coordinate system, and nt represents the total number of samples in computation time.

Preferably, the step seven comprises the following specific processes:

polarity inversion corrects the polarity by calculating the incident angle of the longitudinal wave at each imaging point and then taking the negative value of the incident angle or the opposite number of positive value imaging, so the normalized cross-correlation imaging conditions of the seismic source are rewritten as (taking solid phase PS imaging as an example)

In the formula, theta_pRepresents the incident angle of the longitudinal wave, and

the invention has the following beneficial effects:

the longitudinal wave field and the transverse wave field in a solid phase medium and a liquid phase medium can be effectively separated from a complex mixed wave field by the aid of the longitudinal wave and transverse wave decoupling-based dual-phase medium elastic wave reverse time migration imaging method, so that high-precision pure longitudinal wave and pure transverse wave fields are respectively obtained in the solid phase and the liquid phase, and multi-component imaging is carried out on the solid phase and the liquid phase in the dual-phase medium.

The invention can accurately image the liquid medium, suppress the crosstalk noise of longitudinal and transverse waves and improve the recognition capability of fluid or gas reservoir, which has important significance for oil-gas exploration.

Drawings

FIG. 1 is a bi-phase isotropic Hess model and parameters in a Cartesian coordinate system in an embodiment of the present invention;

(a) representing a longitudinal wave velocity parameter; (b) representing a shear wave velocity parameter; (c) expressing the parameters and density of the dual-phase medium;

FIG. 2 is a graphical representation of results of a two-phase isotropic Hess model ERTM-B imaging in Cartesian coordinates in accordance with an embodiment of the present invention;

(a) representing solid phase PP imaging; (b) representing solid phase PS imaging; (c) represents solid phase SP imaging; (d) represents solid phase SS imaging; (e) representing liquid phase PP imaging; (f) representing liquid phase PS imaging; (g) indicating liquid phase SP imaging; (h) indicating liquid phase SS imaging.

Detailed Description

The following description of the embodiments of the present invention will be made with reference to the accompanying drawings:

a method for imaging elastic wave reverse time migration of a two-phase medium based on longitudinal and transverse wave decoupling is used for deducing a first-order velocity-stress equation of the two-phase isotropic medium; deriving a longitudinal and transverse wave decoupling equation of the biphase medium, and separating a mixed wave field in the biphase medium into a pure longitudinal wave (P wave) wave field and a pure transverse wave (S wave) wave field; obtaining an imaging result by using the seismic source normalized imaging condition; correcting a polarity inversion phenomenon caused by wave field separation; the method specifically comprises the following steps:

the method comprises the following steps: inputting a longitudinal and transverse wave velocity field, a multi-component actual observation gun record and an observation system file;

step two: calculating an elastic wave source mixed wave field extended forward by a two-phase medium;

step three: separating the mixed wave field obtained in the second step into a pure longitudinal wave field and a pure transverse wave field by utilizing a Helmholtz decoupling method;

step four: calculating an elastic wave source mixed wave field of the reverse continuation of the two-phase medium;

step five: separating the mixed wave field obtained in the fourth step into a pure longitudinal wave field and a pure transverse wave field by utilizing a Helmholtz decoupling method;

step six: obtaining a multi-component imaging result of the elastic wave of the two-phase medium under a Cartesian coordinate system;

step seven: carrying out polarity correction on the imaging result with the polarity inversion;

step eight: and outputting longitudinal and transverse wave decoupled elastic wave biphase medium reverse time migration imaging.

Preferably, the specific process of the step two is as follows:

based on the Biot theory, the vector form of the two-dimensional two-phase isotropic medium elastic wave equation is (1):

wherein in the formulaAndrespectively representing the displacement components of the solid and liquid phases; q represents the coupling between the volume of the liquid phase and the change in volume of the solid phase; mu and lambda are solid phase elasticity parameters corresponding to Lame constants in elasticity theory; r is the elasticity parameter of the fluid phase, p₁₁，ρ₁₂，ρ₂₂As a mass coefficient, p₁₁Representing the total equivalent mass, p, of the solid phase fraction of the volume element of the medium₁₂Represents the solid-liquid phase coupling mass coefficient, rho₂₂Representing the total equivalent mass of the flow phase portion in the media volume element; b is the dissipation coefficient, if not consideredWhen the relative motion between the fluid and the solid is considered, B is 0 to obtain an elastic wave equation without dissipation;

the elastic wave equation vector form is derived with time, and a first-order velocity-stress equation (2) of the two-phase isotropic medium can be obtained through sorting:

wherein v is_x，v_zRepresents the velocity of the solid phase; v_x，V_zRepresents the velocity of the flow phase; sigma_xx，σ_zz，τ_xzRepresents the solid phase stress and S represents the flow phase stress.

The third concrete process is as follows: according to Helmholtz decoupling theory, the longitudinal wave field is a non-rotating field, the transverse wave field is a non-scattered field, and the velocities of the longitudinal wave and the transverse wave of the liquid phase are defined as phi and phi respectivelyThe velocities of the longitudinal and transverse solid waves are phi and phi respectivelySatisfies the relationship of formula (3):

then there are:

in the formula, the rotation field of the flow phaseAnd solid phase rotation fieldOnly with shear wave velocity and not with longitudinal wave velocity; while the solid phase powderThe degree field theta and the flow phase divergence field theta are only related to longitudinal wave velocity and are unrelated to transverse wave velocity, which shows that a pure transverse wave (S) wave field and a pure longitudinal wave (P) wave field can be respectively obtained by performing rotation and divergence field operations on solid phase and flow phase displacement vectors, and the method is the basis of wave field separation in a two-phase medium.

The concrete process of the step six is as follows:

using the normalized imaging conditions of the seismic source as shown in formula (5) for the separated longitudinal and transverse wave components in the seismic source wave field and the wave field of the detector,

wherein, I_pp，I_ps，I_spAnd I_ssImaging of the PP, PS, SP and SS fractions representing the solid phases, respectively; i is_PP，I_PS，I_SPAnd I_SSImaging of the PP, PS, SP and SS components of the flow phase, respectively;

SF_pand SF_sAnd respectively representing the separated solid-phase longitudinal wave field and the solid-phase transverse wave field in the seismic source wave field propagated from the forward direction; RF (radio frequency)_pAnd RF_sRespectively representing solid-phase longitudinal wave fields and solid-phase transverse wave fields separated from the wave field of the detection point which reversely propagates; SF_PAnd SF_SRespectively representing liquid-phase longitudinal wave fields and transverse wave fields separated from a forward-propagated seismic source wave field; RF (radio frequency)_PAnd RF_sRespectively, representing the liquid phase longitudinal and transverse wave wavefields separated from the counter-propagating demodulator probe wavefields. (x, y) represents spatial coordinates in a Cartesian coordinate system, and nt represents the total number of samples in computation time.

The concrete process of the step seven is as follows:

polarity inversion corrects the polarity by calculating the incident angle of the longitudinal wave at each imaging point and then taking the negative value of the incident angle or the opposite number of positive value imaging, so the normalized cross-correlation imaging conditions of the seismic source are rewritten as (taking solid phase PS imaging as an example)

In the formula, theta_pRepresents the incident angle of the longitudinal wave, and

the above-described techniques are applied to the Hess model data, and an ideal calculation effect is obtained. A typical set of Hess model parameters was selected as shown in fig. 1. In fig. 1, (a) is a longitudinal wave velocity parameter, (b) is a transverse wave velocity parameter, and (c) is a two-phase medium parameter and density. The target fluid or gas reservoir in the two-phase medium region can be seen from the graph, the range of the model is 3616 m × 3152 m, the global interval is 8 m × 8 m, the time sampling interval is 0.5ms, in the region where the detectors are fixedly distributed, 56 Ricker explosion sources with the main frequency of 25Hz are uniformly arranged on the ground surface, the grid interval is 8 m, a total of 452 double-component detection points are uniformly distributed on the model, the distance between the detection points is 8 m, and the input offset model is a real model after 10 times of smoothing.

In fig. 2, (a), (b), (c) and (d) show RTM imaging results of solid phases PP, PS, SP and SS, respectively, and (e), (f), (g) and (h) show RTM imaging results of flow phases PP, PS, SP and SS, respectively. As can be seen from the results, ERTM-B proposed by the present invention produces good imaging results in both solid and liquid phase portions, and since the wavelength of the transverse wave is shorter than that of the longitudinal wave, the image of SS component has higher resolution than that of PP component, and the polarity inversion of the transverse wave in the images of solid phase PS, solid phase SP, liquid phase PS and liquid phase SP is accurately corrected. In an example where a fluid or gas reservoir is typically the target imaging region, the proposed ERTM-B of the present invention is able to accurately image the target reservoir in liquid phase imaging, and has a high signal-to-noise ratio and high accuracy, as compared to conventional RTM methods, which helps identify liquid or gas reservoirs.

It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.