CN103675911B - A kind of method based on compressional wave and converted shear wave joint inversion intercept and gradient - Google Patents

Abstract

The invention relates to a method for joint inversion of intercept and gradient based on longitudinal wave and converted shear wave. The method includes the following steps: obtaining longitudinal wave and converted shear wave angle domain gathers, obtaining the longitudinal wave and shear wave velocity and logging interpretation results at the same time; extracting wavelets from the longitudinal wave and converted shear wave angle domain gathers to obtain the angle domain reflection coefficient profile, and using the obtained Calculate the S/P wave velocity ratio profile of the P-wave and S-wave velocities; use the deduced converted S-wave reflection coefficient formula and the P-wave reflection coefficient approximate formula with the intercept and gradient as parameters, and use the singular value decomposition method for joint inversion to obtain the intercept and gradient; use the logging interpretation results to calibrate the inverted intercept and gradient to establish a fluid identification model to detect fluids in undrilled areas. The invention makes full use of the received multi-component seismic wave field information, reduces the multiple solutions in the inversion process, improves the stability of the inversion process, and provides a reliable method for oil and gas prediction.

Description

A method for joint inversion of intercept and gradient based on P-wave and converted S-wave

technical field

The invention belongs to the field of geophysical exploration, and in particular relates to a method for joint inversion of intercept and gradient based on longitudinal waves and converted shear waves.

Background technique

Amplitude Versus Offset (Amplitude Versus Offset) is a technique for predicting oil and gas reservoirs by studying the relationship between reflection amplitude and offset to obtain lithological parameters such as velocity and density. Using the Shuey (1985) formula to extract the corresponding intercept (P) and gradient (G) or their combination parameters can effectively carry out oil and gas identification and fluid prediction. Among them, the intercept (P) profile, that is, the vertical incidence reflection coefficient profile, has a high signal-to-noise ratio and resolution. It is vertically incident and reflected at approximately zero offset, and no converted shear wave will be generated. It can be regarded as a longitudinal wave profile . The gradient (G) profile is related to Poisson's ratio, and contains information about the lithology changes above and below the reflection interface. The intercept (P) and gradient (G) are good parameters for oil and gas indication and fluid prediction.

However, the traditional method of extracting intercept (P) and gradient (G) only relies on P-wave data, and it is difficult to overcome the instability in the extraction process and the multi-solution of the extraction results; on the other hand, multi-component seismic data records a wealth of Seismic wave field information can make up for the shortcomings of the method of simply using longitudinal waves, reduce the multi-solution nature of the inversion process, increase the stability of the inversion results, and provide a reliable method for oil and gas prediction. The invention comprehensively utilizes the longitudinal wave and the converted shear wave to jointly extract the intercept (P) and the gradient (G), fully utilizes the received multi-component seismic wave field information, reduces the multiple solutions of the inversion process, and improves the stability of the inversion process , providing a reliable method for oil and gas prediction.

Contents of the invention

In order to overcome the deficiencies of independently using longitudinal wave data to invert intercept (P) and gradient (G) parameters, the present invention provides a method for jointly inverting intercept (P) and gradient (G) based on longitudinal wave and converted shear wave; fully Utilizing the wave field information of P-wave and converted S-wave, based on the approximate expression of P-wave reflection coefficient (Shuey, 1985) with intercept (P) and gradient (G) as parameters, the intercept (P) and gradient (G) are further deduced G) is the approximate formula of the converted shear wave reflection coefficient, using the obtained longitudinal wave and converted shear wave angle gathers, and using the singular value decomposition method for joint inversion to obtain the intercept (P) and gradient (G).

To achieve the above object, the technical scheme of the present invention is as follows:

The method of joint inversion of intercept and gradient based on longitudinal wave and converted shear wave includes the following steps:

Step 1: Obtain P-wave and converted shear-wave angle domain gathers, and simultaneously obtain P-wave and shear-wave velocities and logging interpretation results

Step 2: Extract wavelets from the P-wave and converted S-wave angle-domain gathers to obtain the angle-domain reflection coefficient profile, and use the obtained P-wave and S-wave velocities to obtain the S-wave velocity ratio profile

Step 3: Using the deduced transformed shear wave reflection coefficient formula and the approximate expression of longitudinal wave reflection coefficient with the intercept (P) and gradient (G) as parameters, the singular value decomposition method is used for joint inversion to obtain the intercept (P) and Gradient (G)

Step 4: Use the logging interpretation results to calibrate the inverted intercept (P) and gradient (G) to establish a fluid identification model, and perform fluid detection in undrilled areas

Compared with the prior art, the present invention has the following beneficial effects: the present invention comprehensively utilizes the longitudinal wave and the converted shear wave to jointly extract the intercept (P) and the gradient (G), fully utilizes the received multi-component seismic wavefield information, and reduces the inversion The multi-solution of the process improves the stability of the inversion process and provides a reliable method for oil and gas prediction.

Description of drawings

Fig. 1 is the flow chart of the method for extracting intercept (P) and gradient (G) jointly by longitudinal wave and converted shear wave;

Fig. 2 is the layered model data for the joint extraction of intercept (P) and gradient (G) by longitudinal wave and converted shear wave;

Figure 3a is the P-wave angle domain gather (0-38°) directly synthesized by the Zoeppritz equation;

Figure 3b is the converted shear wave angle domain gather (0-38°) directly synthesized by the Zoeppritz equation;

Figure 4a is the reflection coefficient in the longitudinal wave angle domain obtained by removing the wavelets of the longitudinal wave angle domain gather in Figure 3a by using the principle of least squares pulse deconvolution;

Figure 4b is the converted shear wave angle domain reflection coefficient obtained by removing the wavelets of the converted shear wave angle domain gather in Figure 3b by using the principle of least squares pulse deconvolution;

Fig. 5a is the comparison between the intercept (P) extracted jointly by P-wave and converted S-wave, and the intercept (P) extracted only by P-wave and the true value of intercept (P);

Figure 5b is the comparison between the gradient (G) extracted jointly by the longitudinal wave and the converted shear wave and the gradient (G) extracted by only the longitudinal wave, and the true value of the gradient (G);

Figure 6a is a longitudinal wave angle domain gather (0-38°) directly synthesized by the Zoeppritz equation with 25dB background noise added;

Figure 6b is the converted shear wave angle domain gather (0-38°) directly synthesized by the Zoeppritz equation with 25dB background noise added;

Figure 7a is the reflection coefficient in the longitudinal wave angle domain obtained by removing the wavelets of the longitudinal wave angle domain gather in Figure 6a by using the principle of least squares pulse deconvolution;

Fig. 7b is the converted shear wave angle domain reflection coefficient obtained by removing the wavelet of the converted shear wave angle domain gather in Fig. 6b by using the principle of least squares pulse deconvolution;

Fig. 8a The comparison between the intercept (P) extracted jointly by P-wave and converted S-wave and the intercept (P) extracted only by P-wave and the true value of intercept (P) when random noise is added to the input angular gather;

Fig. 8b compares the gradient (G) extracted jointly by P-wave and converted S-wave and the gradient (G) extracted by only P-wave with the true value of gradient (G) when random noise is added to the input angular gather.

detailed description

As shown in Figure 1, the method of joint inversion of intercept (P) and gradient (G) based on P-wave and converted S-wave includes the following steps:

(1) Obtain the longitudinal wave and converted shear wave angle domain gathers, and obtain the longitudinal wave and shear wave velocity and logging interpretation results at the same time; the specific method is as follows:

Based on the seismic data collected in the field, the seismic imaging method is used to extract the pre-stack seismic angle gather data;

Obtain compressional and shear wave velocities and logging interpretation results based on field logging data;

(2) Extract wavelets from the angle domain gathers of longitudinal waves and converted shear waves to obtain the angle domain reflection coefficient profile, and use the obtained longitudinal wave and shear wave velocities to obtain the shear and longitudinal wave velocity ratio profile; the specific method is as follows:

Using the pre-stack seismic geological calibration method (Shen Guoqiang. Research on pre-stack seismic inversion method and influencing factors [J]. 2013) to obtain the wavelets in the angle gathers of P-wave and converted S-wave, and then use the principle of least square pulse deconvolution to remove the P-wave , converting the wavelet of the shear wave angle domain gather, obtaining the longitudinal wave, and converting the shear wave angle domain reflection coefficient.

Through the Zoeppritz equation (see "Seismic Wave Theory and Method", Sun Chengyu, p. 65, formula 3-2-7), using the logging P-wave velocity, S-wave velocity and density parameters, the forward modeling P-wave and converted S-wave side-hole angle domains can be obtained Seismic traces, the comparison between the P-wave and converted shear-wave angle gathers and the actually obtained pre-stack P-wave and converted shear-wave angle gathers can estimate the wavelets in the actually obtained angle gathers. Then, when the wavelets are known, the least square pulse deconvolution method (Li Zhenchun's "Seismic Data Processing Method" p. 67) is used to remove the wavelets of the angle-domain gathers of the longitudinal waves and converted shear waves, and obtain the angle-domain reflections of the longitudinal waves and converted shear waves coefficient.

The S-wave velocity ratio profile is obtained by dividing the S-wave and P-wave velocities obtained from logging data.

(3), using the deduced conversion shear wave reflection coefficient formula (formula 1.5) and Shuey (1985) longitudinal wave reflection coefficient approximate formula (formula 1.1) with intercept (P) and gradient (G) as parameters, using singular value decomposition method to perform joint inversion to obtain the intercept (P) and gradient (G); the specific method is as follows:

Shuey (1985) rewrote the approximate formula of longitudinal wave reflection coefficient derived by Aki and Richard (1980) into a formula with intercept (P) and gradient (G) as parameters:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; pp</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; G</span>}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; tan</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1.1</span>}<span\; class="notranslate">\; )</span>$

in, $P=\frac{1}{2}(\frac{\mathrm{\&Delta;}{v}_p}{v_p}+\frac{\mathrm{\&Delta;\&rho;}}{\mathrm{\&rho;}}),$ $G=\frac{1}{2}\frac{\mathrm{\&Delta;}{v}_p}{v_p}-2(2\frac{\mathrm{\&Delta;}{v}_p}{v_p}+\frac{\mathrm{\&Delta;\&rho;}}{\mathrm{\&rho;}})+\frac{\mathrm{\&Delta;\&sigma;}}{{(1-\mathrm{\&sigma;})}^2};$ V _p and ΔV _p are the average velocity of the longitudinal waves in the upper and lower layers and the difference of the longitudinal waves in the upper and lower layers, respectively, and θ is the average of the incident angle and transmission angle of the longitudinal waves.

Using the intercept (P) and gradient (G) as parameters, an approximate formula for the converted shear wave reflection coefficient (Equation 1.5) was derived. Aki and Richard et al. (1980) gave an approximate formula for the converted shear wave reflection coefficient:

where θ, are the average values of the incident angle and transmission angle of the longitudinal wave, the average value of the reflection angle and the transmission angle of the converted wave, V _p , V _s , and ρ are the average values of the longitudinal wave velocity, shear wave velocity, and density of the upper and lower layers, respectively, and ΔV _p , ΔV _s , Δρ is the difference in P-wave velocity, S-wave velocity, and density between the upper and lower layers.

In an isotropic medium, the relationship between Poisson's ratio and the ratio of compressional-to-short-wave velocity:

$\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&sigma;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1.3</span>}<span\; class="notranslate">\; )</span>$

Among them, V _p , V _s , and σ are the average values of P-wave velocity, S-wave velocity, and Poisson's ratio in the upper and lower layers, respectively, and σ is a function of V _p , V _s . Differentiate and arrange both sides of formula 1.2 to get:

$\frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\frac{\mathrm{<span\; class="notranslate">\; \&Delta;\&sigma;</span>}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&sigma;</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1.4</span>}<span\; class="notranslate">\; )</span>$

Substitute Equation 1.4 into Equation 1.2 and arrange the following equation:

In order to overcome the shortcomings of using the P-wave data alone to invert the intercept (P) and gradient (G) parameters, and make full use of the P-wave and converted S-wave field information, formula 1.1 and formula 1.5 are combined, and the SVD (singular value decomposition) method is used Perform a joint inversion of intercept and gradient:

$\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; pp</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>\\ {\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; pp</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; ps</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>\\ {\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; ps</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>\end{array}\right]\left[\begin{array}{ccc}\mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>\\ \mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>\\ \mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>\\ <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>& <span\; class="notranslate">\; .</span>\\ \mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>& \mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>\end{array}\right]\left[\begin{array}{c}\\ \mathrm{<span\; class="notranslate">\; P</span>}\\ \mathrm{<span\; class="notranslate">\; G</span>}\\ \frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}\end{array}\right]$

Among them, B(θ _i ), C(θ _i ) (i=1, 2, 3...n) are different angles θ _i (i=1, 2, 3...n ) in the Rpp equation, the coefficients before G, ΔV _p /V _p , D(θ _i ), E(θ _i ), F(θ _i ) (i=1, 2, 3...n) are different angles θ _i (i=1, 2, 3...n) Coefficients before P, G, ΔV _p /V _p in the Rps equation, Rpp(θ _i ), Rps(θ _i ) (i=1, 2 , 3.....n) are the reflection coefficients of longitudinal wave and converted shear wave at different angles θ _i (i=1, 2, 3...n).

The expressions of the above inversion intercept and gradient can be summarized as:

y=Ax

Among them, y is the angle-domain quasi-reflection coefficient matrix, x is the attribute parameter matrix to be extracted, and A is the attribute parameter x front coefficient matrix.

Using the singular value method to perform AVO inversion can obtain the attribute parameter x matrix, namely

x＝UΛ ^-1 V ^T y

Among them, U and V are the eigenvalues of ^AAT obtained by performing singular value decomposition on matrix A, and Λ is the singular value matrix of ^AAT obtained by performing singular value decomposition on matrix A.

(4) Use the logging interpretation results to calibrate the inverted intercept (P) and gradient (G) to establish a fluid identification model, and perform fluid detection in undrilled areas; the specific method is as follows:

Firstly, calibrate the inverted intercept (P) and gradient (G) profiles and the logging interpretation results of wells passing through the profile, that is, according to the logging interpretation results, the intercept (P) and gradient (G) profiles on the Oil and gas layers and water layers are correspondingly calibrated on the intercept (P) and gradient (G) sections of the inversion, and the unknown Predict the oil layer and gas layer on the intercept and gradient profile of the drilling area, and obtain the favorable drilling area.

By testing the four-layer model proposed by Mahmoudian and Margrave (2004), Fig. 5 shows the intercept (P) and gradient (G) attributes extracted by joint longitudinal wave and converted shear wave and the intercept (P) and gradient (G) attribute extracted by single longitudinal wave Compared with the real value of the intercept (P) and gradient (G), it is found that the intercept (P) and gradient (G) of the joint inversion have higher accuracy, especially the gradient (G) attribute; Figure 8 is In the case that the input angle gather data is noisy, the intercept (P) and gradient (G) attributes are extracted by combining the longitudinal wave and the converted shear wave, and the intercept (P) and gradient (G) attributes are extracted by using a single longitudinal wave and the intercept (P ) and the true value of the gradient (G), the comparison shows that the intercept (P) and gradient (G) of the joint inversion have higher accuracy, especially the gradient (G) attribute, and the joint has a certain effect on the background noise suppress.

Claims (4)

1. A method for jointly inverting intercept and gradient based on compressional waves and converted shear waves is characterized by comprising the following steps:

step 1: acquiring a longitudinal wave angle domain gather, a converted transverse wave angle domain gather, and simultaneously acquiring longitudinal wave and transverse wave speeds and well logging interpretation results;

step 2: extracting sub-waves from the longitudinal wave and converted shear wave angle domain gathers to obtain an angle domain reflection coefficient profile, and solving a shear-longitudinal wave velocity ratio profile by using the obtained longitudinal wave and shear wave velocities;

and step 3: performing joint inversion by using a converted shear wave reflection coefficient formula and a longitudinal wave reflection coefficient approximate formula which take intercept P and gradient G as parameters and adopting a singular value decomposition method to obtain the intercept P and the gradient G; the specific method comprises the following steps:

approximate formula of longitudinal wave reflection coefficient with intercept P and gradient G as parameters:

$R_{pp}\left(\mathrm{\&theta;}\right)=P+G{\sin }^2\mathrm{\&theta;}+\frac{1}{2}\frac{{\mathrm{\&Delta;V}}_p}{V_p}({\tan }^2\mathrm{\&theta;}-{\sin }^2\mathrm{\&theta;})---\left(1.1\right)$

wherein, $P=\frac{1}{2}(\frac{{\mathrm{\&Delta;V}}_p}{V_p}+\frac{\mathrm{\&Delta;}\mathrm{\&rho;}}{\mathrm{\&rho;}}),G=\frac{1}{2}\frac{{\mathrm{\&Delta;V}}_p}{V_p}-2(2\frac{{\mathrm{\&Delta;V}}_p}{V_p}+\frac{\mathrm{\&Delta;}\mathrm{\&rho;}}{\mathrm{\&rho;}})+\frac{\mathrm{\&Delta;}\mathrm{\&sigma;}}{{(1-\mathrm{\&sigma;})}^2};$

V_prho and sigma are respectively the average values of the longitudinal wave speed, the density and the Poisson ratio of the upper layer and the lower layer; Δ V_pAnd delta rho and delta sigma are respectively the longitudinal wave velocity difference, the density difference and the Poisson's ratio difference of the upper layer and the lower layer; theta is the average of the incident angle and the transmission angle of the longitudinal wave;

approximate formula of reflection coefficient of converted shear wave with intercept P and gradient G as parameters:

wherein theta,Respectively, the average value of the incident angle and the transmission angle of the longitudinal wave, the average value of the reflection angle and the transmission angle of the converted wave, V_p、V_sRho is the average value of the longitudinal wave velocity, the transverse wave velocity and the density of the upper layer and the lower layer respectively,ΔV_p、ΔV_sand the delta rho is the longitudinal wave velocity difference, the transverse wave velocity difference and the density difference of the upper layer and the lower layer respectively;

by utilizing a longitudinal wave and converted shear wave angle domain reflection coefficient profile obtained after the extraction of the wavelet and a calculated shear-longitudinal wave velocity ratio profile, the following system equation is solved by a Singular Value Decomposition (SVD) method to carry out joint inversion, and intercept P and gradient G are obtained:

$\left[\begin{array}{c}{R}_{pp}\left({\mathrm{\&theta;}}_1\right)\\ \begin{array}{c}.\\ .\\ .\end{array}\\ R_{pp}\left({\mathrm{\&theta;}}_n\right)\\ R_{ps}\left({\mathrm{\&theta;}}_1\right)\\ \begin{array}{c}.\\ .\\ .\end{array}\\ R_{ps}\left({\mathrm{\&theta;}}_n\right)\end{array}\right]=\left[\begin{array}{ccc}1& B\left({\mathrm{\&theta;}}_1\right)& C\left({\mathrm{\&theta;}}_1\right)\\ \begin{array}{c}.\\ .\\ .\end{array}& \begin{array}{c}.\\ .\\ .\end{array}& \begin{array}{c}.\\ .\\ .\end{array}\\ 1& B\left({\mathrm{\&theta;}}_n\right)& C\left({\mathrm{\&theta;}}_n\right)\\ D\left({\mathrm{\&theta;}}_1\right)& E\left({\mathrm{\&theta;}}_1\right)& F\left({\mathrm{\&theta;}}_1\right)\\ \begin{array}{c}.\\ .\\ .\end{array}& \begin{array}{c}.\\ .\\ .\end{array}& \begin{array}{c}.\\ .\\ .\end{array}\\ D\left({\mathrm{\&theta;}}_n\right)& E\left({\mathrm{\&theta;}}_n\right)& F\left({\mathrm{\&theta;}}_n\right)\end{array}\right]\left[\begin{array}{c}P\\ G\\ \frac{{\mathrm{\&Delta;V}}_p}{V_p}\end{array}\right]$

wherein, B (theta)_i)，C(θ_i) N, each of which is a different angle θ_i，i＝1，2，3......n，R_ppG in the equation,A coefficient of front; d (theta)_i)、E(θ_i)、F(θ_i) N, i being different angles θ, 1, 2, 3_i，i＝1，2，3......n，R_psP, G in the equation,A coefficient of front; r_pp(θ_i)、R_ps(θ_i) N, i being different angles θ, 1, 2, 3_iN, i 1, 2, 3.. n, and converted shear wave reflection coefficients;

and 4, step 4: and calibrating the inverted intercept P and the gradient G by using the well logging interpretation result to establish a fluid identification mode, and carrying out fluid detection on the non-drilled area.

2. The method for jointly inverting the intercept and the gradient based on the compressional wave and the converted shear wave according to claim 1, wherein the specific method in the step 1 is as follows:

extracting pre-stack seismic angle gather data by using a seismic imaging method based on seismic data acquired in the field;

and obtaining longitudinal wave and transverse wave speeds and well logging interpretation results based on the well logging data of the field well logging.

3. The method for jointly inverting the intercept and the gradient based on the compressional wave and the converted shear wave according to claim 2, wherein the specific method in the step 2 is as follows:

obtaining wavelets in the longitudinal wave and converted transverse wave angle domain channel sets by using a prestack seismic geological calibration method, and then removing the wavelets in the longitudinal wave and converted transverse wave angle domain channel sets by using a least square pulse deconvolution method to obtain reflection coefficients in the longitudinal wave and converted transverse wave angle domains;

and dividing the transverse wave velocity and the longitudinal wave velocity obtained by the logging data to obtain a transverse-longitudinal wave velocity ratio profile.

4. The method for jointly inverting the intercept and the gradient based on the compressional wave and the converted shear wave according to claim 3, wherein the specific method in the step 4 is as follows:

firstly, calibrating inverted intercept P and gradient G sections and well logging interpretation results of wells passing through the sections, namely correspondingly calibrating oil-gas layers and water layers on the intercept P and gradient G sections on the inverted intercept P and gradient G sections according to the well logging interpretation results, and predicting oil layers and gas layers on the intercept and gradient sections of an undrilled well area according to display phenomena of the oil-gas layers and the water layers on the intercept P and gradient G sections to obtain a favorable well area.