Direct reverse time migration imaging method for acquiring strain rate data based on DAS

Abstract

The invention discloses a direct reverse time migration imaging method for acquiring strain rate data based on a DAS, and relates to the technical field of exploration geophysics, and the method comprises the steps: s1, building a pressure-strain rate equation based on a speed-pressure wave equation and a strain rate-speed relation; s2, obtaining a forward propagation wave field and a backward propagation wave field based on the pressure intensity-strain rate equation; s3, imaging the forward propagation wave field and the backward propagation wave field by using a cross-correlation imaging condition to obtain an imaging result I (x); s4, calculating a comprehensive illumination matrix of the seismic source and the detection point wave field; and s5, performing comprehensive illumination compensation on the imaging result I (x) to obtain a final imaging result Ifinal (x). According to the method, the strain rate data is directly utilized for imaging, so that errors caused by data conversion are avoided, the fidelity of the data and the imaging precision are remarkably improved, and the data processing flow is simplified. In addition, according to the method, the illumination effect of the deep underground area is improved through a comprehensive illumination strategy, the imaging result is clearer, and details are richer.

Description

Technical Field

[0001] The invention relates to the field of exploration geophysical technology, and in particular to a direct reverse time migration imaging method based on DAS acquisition of strain rate data. Background Art

[0002] Distributed Acoustic Sensing (DAS) has shown significant advantages over traditional seismic detectors in the field of seismic data acquisition and has attracted widespread attention in the industry. These advantages are mainly reflected in higher spatial acquisition density, lower cost and easy long-term deployment. The DAS system uses optical fiber as a sensing medium, emits light pulses of different frequencies into the optical fiber, and accurately measures the phase change caused by Rayleigh backscattering, thereby capturing the strain or strain rate information of the optical fiber axis with high sensitivity and high resolution. With the continuous advancement of DAS technology, its application in the field of oil and gas exploration has become more and more extensive, and has become a hot topic in current research. DAS technology has made significant progress in the fields of vertical seismic profiling, surface seismic exploration and reservoir dynamic monitoring. In seismic imaging technology, the reverse time migration method shows unique advantages over the Kirchhoff migration and one-way wave equation migration widely used in actual exploration. The principle of reverse time migration is simple, the imaging accuracy is high, and it is not limited by the formation dip angle, and it can effectively handle complex geological structures. However, traditional reverse time migration technology has certain limitations when processing DAS collected data due to data type restrictions.

[0003] Traditional seismic geophones record particle velocity information, while DAS records axial strain rate information. There are significant differences in amplitude and phase between these two types of data. Directly applying traditional seismic imaging methods to DAS acquisition data usually leads to unsatisfactory imaging results. This is because traditional seismic imaging algorithms are designed based on particle velocity data, while strain rate data is essentially different from particle velocity data in terms of physical meaning and mathematical expression. Therefore, in order to use DAS data for seismic imaging, current research generally converts the strain rate data recorded by DAS into particle velocity data, and then applies conventional imaging processes. However, this data conversion inevitably introduces conversion errors, reduces the fidelity of the data, and thus affects the accuracy of imaging. In addition, the conversion process increases the complexity and computational complexity of data processing, and may also lead to the loss of effective information, thereby limiting the application potential of DAS acquisition technology in seismic imaging.

[0004] Therefore, it is urgent to develop a stable, efficient and practical reverse time migration implementation method to meet the challenges faced in DAS data imaging. Summary of the invention

[0005] In order to solve the above technical problems, the present invention discloses a direct reverse time migration imaging method based on DAS acquisition of strain rate data, which first accurately simulates the forward wave field generated by the earthquake source based on the pressure-strain rate equation, and then uses the actually observed strain rate data as the boundary condition for reverse propagation, thereby constructing the reverse wave field. By applying the cross-correlation imaging condition to the forward wave field and the reverse wave field for high-quality imaging, and using the comprehensive illumination strategy of the earthquake source and the detector point to perform illumination compensation on the imaging results, the deep resolution of the imaging results is further improved.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A direct reverse time migration imaging method based on DAS to collect strain rate data includes the following steps:

[0008] s1. Establish the pressure-strain rate equation based on the velocity-pressure wave equation and the strain rate-velocity relationship;

[0009] s2. Based on the pressure-strain rate equation, the forward wave field and the reverse wave field are obtained;

[0010] s3. Use the cross-correlation imaging condition to image the forward wave field and the reverse wave field to obtain the imaging result I(x);

[0011] s4. Calculate the comprehensive illumination matrix of the wave field of the source and the detection point;

[0012] s5. Perform comprehensive illumination compensation on the imaging result I(x) to obtain the final imaging result I final (x).

[0013] Optionally, step s1 specifically includes:

[0014] The first-order velocity-pressure wave equation can be expressed as follows:

[0015]

[0016] Among them, p is pressure, ρ is medium density, v is particle velocity, t is time, x, y, z are spatial directions;

[0017] The relationship between strain rate and particle velocity is derived as follows:

[0018]

[0019] in, represents strain rate, ε represents strain, and u is displacement;

[0020] Taking the spatial derivatives of the second, third, and fourth equations in equation (1) respectively, keeping the first equation unchanged, we obtain:

[0021]

[0022] Then, by substituting equation (2) into equation (3), we finally obtain the pressure-strain rate equation, which is as follows:

[0023]

[0024] Optionally, in step s2, for a shot point at position x s =(x s ,y s ,z s =0), the optical fiber is located at position x r =(x r ,y r ,z r =0) received strain rate records Using the source wavelet function f(t) and the P-wave velocity model v(x) of the observed data, the forward wave field p is obtained by updating and solving the pressure-strain rate equation. F , which is expressed as follows:

[0025]

[0026] Also based on the pressure-strain rate equation, the observed strain rate is recorded As the boundary condition, reverse time propagation is performed to obtain the reverse propagation wave field p B , which is expressed as follows:

[0027]

[0028] Among them, x = (x, y, z) represents the model space position, is the Laplace operator, and δ is the Dirac impulse function.

[0029] Optionally, in step s3, the forward wave field p is imaged using the cross-correlation imaging condition. F With the back propagation field p B Imaging is performed, and the imaging result I(x) is expressed as:

[0030] I(x)=∫∫p F (x; t; x s )·p B (x; t; x s )dtdx s (7).

[0031] Optionally, in step s4, the comprehensive lighting matrix H(x) is expressed as:

[0032] H(x)=∫∫p F (x; t; x s ) 2 dtdxs ·∫∫p B (x; t; x s ) 2 dtdx s (8).

[0033] Optionally, in step s5, the final imaging result I final (x) is:

[0034]

[0035] The beneficial effect of the present invention is that the core innovation of the present invention lies in constructing a pressure-strain rate equation specifically adapted for strain rate wave field simulation and imaging, and based on the equation, a method for direct reverse time migration imaging of strain rate data is proposed. The method first accurately simulates the forward wave field generated by the source based on the pressure-strain rate equation, and then reversely propagates the actual observed strain rate data as the boundary condition, thereby constructing the reverse wave field. By applying the forward wave field and the reverse wave field to high-quality imaging under the cross-correlation imaging condition, and using the comprehensive illumination strategy of the source and the detection point to perform illumination compensation on the imaging results, the deep resolution of the imaging results is further improved. Compared with the traditional strain rate data imaging method, the present invention not only avoids the errors caused by data conversion, significantly improves the fidelity of the data and the accuracy of imaging, but also simplifies the data processing process by directly using the strain rate data for imaging. In addition, the method improves the lighting effect of the deep underground area through a comprehensive illumination strategy, making the imaging results clearer and richer in details. The pressure-strain rate equation disclosed in the present invention and the direct reverse time migration imaging technology adapted to strain rate data provide a stable and efficient framework for processing DAS acquisition data, and are expected to promote the technological innovation and application development of DAS in the field of exploration geophysics. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is the true velocity field of the Dongqiu model;

[0037] Figure 2 is the migration velocity field after smoothing of the Dongqiu model;

[0038] Figure 3 To observe the strain rate record;

[0039] Figure 4 is the resultant strain rate record obtained from the offset velocity;

[0040] Figure 5 Illuminating the source energy;

[0041] Figure 6 Provide energy illumination for detection point;

[0042] Figure 7Comprehensive lighting for the source and receiver points;

[0043] Figure 8 It is the direct imaging result of strain rate data;

[0044] Fig. 9 Direct imaging results for strain rate data using source illumination compensation;

[0045] Fig.10 Results for direct imaging of strain rate data using integrated illumination compensation. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0047] A direct reverse time migration imaging method based on DAS to collect strain rate data includes the following steps:

[0048] s1. Establish the pressure-strain rate equation based on the velocity-pressure wave equation and the strain rate-velocity relationship; specifically including:

[0049] The first-order velocity-pressure wave equation can be expressed as follows:

[0050]

[0051] Among them, p is pressure, ρ is medium density, v is particle velocity, t is time, x, y, z are spatial directions;

[0052] The relationship between strain rate and particle velocity is derived as follows:

[0053]

[0054] in, represents strain rate, ε represents strain, and u is displacement;

[0055] Taking the spatial derivatives of the second, third, and fourth equations in equation (1) respectively, keeping the first equation unchanged, we obtain:

[0056]

[0057] Then, by substituting equation (2) into equation (3), we finally obtain the pressure-strain rate equation, which is as follows:

[0058]

[0059] s2. Based on the pressure-strain rate equation, the forward wave field and the reverse wave field are obtained; specifically, for the shot point at position x s =(x s ,y s ,z s =0), the optical fiber is located at position x r =(x r ,y r ,z r =0) received strain rate records Using the source wavelet function f(t) and the P-wave velocity model v(x) of the observed data, the forward wave field p is obtained by updating and solving the pressure-strain rate equation. F , which is expressed as follows:

[0060]

[0061] Also based on the pressure-strain rate equation, the observed strain rate is recorded As the boundary condition, reverse time propagation is performed to obtain the reverse propagation wave field p B , which is expressed as follows:

[0062]

[0063] Among them, x = (x, y, z) represents the model space position, is the Laplace operator, and δ is the Dirac impulse function.

[0064] s3. Use the cross-correlation imaging condition to analyze the forward wave field p F With the back propagation field p B Imaging is performed, and the imaging result I(x) is expressed as:

[0065] I(x)=∫∫p F (x; t; x s )·p B (x; t; x s )dtdx s (7).

[0066] s4. Calculate the comprehensive illumination matrix H(x) of the source and receiver wave fields, expressed as:

[0067] H(x)=∫∫p F (x; t; xs ) 2 dtdx s ·∫∫p B (x; t; x s ) 2 dtdx s (8).

[0068] s5. Perform comprehensive illumination compensation on the imaging result I(x) to obtain the final imaging result I final (x) is:

[0069]

[0070] The method provided by the present invention is applied to the reverse time migration imaging of the east autumn model in western China, and good imaging results are obtained. Figure 1 As shown, the migration model of the imaging input is as follows Figure 2 The source function of the observed strain rate data is a Ricker wavelet with a main frequency of 15 Hz. The observed data set consists of 169 shot collections, which are evenly distributed on the surface. The maximum offset distance of the optical fiber is 2.5 km. Figure 3 shows an observed strain rate data shot, Figure 4 is the synthetic strain rate data shot gather obtained by the migration velocity model. Figure 5 The source energy illumination matrix is ​​shown. Figure 6 is the detection point energy illumination matrix, Figure 7 It is a comprehensive lighting matrix of seismic sources and receiver points. Figure 8 This is the result of direct reverse time migration imaging based on the pressure-strain rate equation. Although this method can directly use strain rate data for imaging, the deep imaging resolution is very low due to the directional nature of DAS acquisition data. Fig. 9 Shows the Figure 8 The imaging results are shown after source illumination compensation. The results show that conventional source illumination compensation cannot effectively enhance the quality of deep imaging. Fig.10 is right Figure 8 The imaging results shown are the results of comprehensive illumination compensation for the source and the receiver points. It can be seen that after comprehensive illumination compensation, the deep resolution of the imaging results is significantly improved, achieving high-quality imaging of the strain rate data.

[0071] The present invention proposes a direct reverse time migration imaging method for distributed acoustic sensing DAS to collect strain rate data, aiming to provide a stable, efficient and practical reverse time migration implementation scheme to meet the challenges in DAS data imaging. Traditional seismic detectors record particle velocity information, while DAS records axial strain rate information, and there are significant differences between the two in amplitude and phase. If the traditional seismic imaging method is directly applied to DAS acquisition data, it often leads to unsatisfactory imaging results. In addition, if the DAS data is converted into particle velocity data and then the conventional imaging process is applied, it will not only introduce conversion errors and reduce the accuracy of imaging, but also increase the complexity and workload of data processing. To solve this problem, the present invention constructs a pressure-strain rate equation specifically adapted to strain rate wave field simulation and imaging, and proposes a direct reverse time migration imaging method for strain rate data based on the equation. The method first uses the pressure-strain rate equation to accurately simulate the forward wave field generated by the earthquake source, and then uses the actually observed strain rate data as the boundary condition for reverse propagation, thereby constructing the reverse wave field. By applying cross-correlation imaging conditions to the forward wavefield and the reverse wavefield for high-quality imaging, and finally using a comprehensive illumination strategy for the source and the receiver point to compensate for the illumination of the imaging results, the deep resolution of the imaging results can be further improved. This method not only avoids the errors caused by data conversion, significantly improves the fidelity of the data and the accuracy of imaging, but also simplifies the data processing process. In addition, the comprehensive illumination strategy improves the illumination effect of the deep underground area, making the imaging results clearer and more detailed.

[0072] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A direct reverse time migration imaging method based on DAS to collect strain rate data, characterized in that: The steps include: s1. Establish the pressure-strain rate equation based on the velocity-pressure wave equation and the strain rate-velocity relationship; s2. Based on the pressure-strain rate equation, the forward wave field and the reverse wave field are obtained; s3. Use the cross-correlation imaging condition to image the forward wave field and the reverse wave field to obtain the imaging result I(x); s4. Calculate the comprehensive illumination matrix of the wave field of the source and the receiver points; s5. Perform comprehensive illumination compensation on the imaging result I(x) to obtain the final imaging result I final (x).

2. A direct reverse time migration imaging method based on DAS acquisition of strain rate data as claimed in claim 1, characterized in that: Step s1 specifically includes: The first-order velocity-pressure wave equation can be expressed as follows: Among them, p is pressure, ρ is medium density, v is particle velocity, t is time, x, y, z are spatial directions; The relationship between strain rate and particle velocity is derived as follows: in, represents strain rate, ε represents strain, and u is displacement; Taking the spatial derivatives of the second, third, and fourth equations in equation (1) respectively, keeping the first equation unchanged, we obtain: Then, by substituting equation (2) into equation (3), we finally obtain the pressure-strain rate equation, which is as follows:

3. A direct reverse time migration imaging method based on DAS acquisition of strain rate data as claimed in claim 2, characterized in that: In step s2, for the shot point at position x s =(x s ,y s ,z s =0), the optical fiber is located at position x r =(x r ,y r ,z r =0) received strain rate records Using the source wavelet function f(t) and the P-wave velocity model v(x) of the observed data, the forward wave field p is obtained by updating and solving the pressure-strain rate equation. F , which is expressed as follows: Also based on the pressure-strain rate equation, the observed strain rate is recorded As the boundary condition, reverse time propagation is performed to obtain the reverse propagation wave field p B , which is expressed as follows: Among them, x = (x, y, z) represents the model space position, is the Laplace operator, and δ is the Dirac impulse function.

4. A direct reverse time migration imaging method based on DAS acquisition of strain rate data as claimed in claim 3, characterized in that: In step s3, the forward wave field p is imaged using the cross-correlation imaging condition. F With the back propagation field p B Imaging is performed, and the imaging result I(x) is expressed as: I(x)=∫∫p F (x;t;x s )·p B (x;t;x s )dtdx s (7)。 5. A direct reverse time migration imaging method based on DAS acquisition of strain rate data as claimed in claim 4, characterized in that: In step s4, the comprehensive lighting matrix H(x) is expressed as: H(x)=∫∫p F (x;t;x s ) 2 dtdx s ·∫∫p B (x;t;x s ) 2 dtdx s (8)。 6. A direct reverse time migration imaging method based on DAS acquisition of strain rate data as claimed in claim 5, characterized in that: In step s5, the final imaging result I final (x) is:

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