A direct reverse-time migration imaging method based on DAS collected strain rate data

Abstract

The application discloses a kind of direct reverse-time migration imaging methods based on DAS acquisition strain rate data, it is related to the technical field of exploration geophysics, including s1, based on velocity-pressure wave equation and strain rate-velocity relationship establishes pressure-strain rate equation;S2, based on pressure-strain rate equation, obtains positive transmission wave field and reverse transmission wave field;S3, using cross-correlation imaging condition to positive transmission wave field and reverse transmission wave field are imaged, and imaging result I (x) is obtained;S4, the comprehensive illumination matrix of source and wave field of geophone is calculated;S5, the comprehensive illumination compensation of imaging result I (x) is obtained final imaging result I final (x).The application directly uses strain rate data for imaging, not only avoids the error caused by data conversion, significantly improves the fidelity of data and the accuracy of imaging, but also simplifies the data processing procedure.In addition, the method improves the illumination effect of deep underground area through comprehensive illumination strategy, so that the imaging result is clearer and more detailed.

Description

Technical Field

[0001] This invention relates to the field of exploration geophysics, and in particular to a direct reverse time migration imaging method based on strain rate data acquired by DAS. Background Technology

[0002] Distributed Acoustic Sensing (DAS) has demonstrated significant advantages over traditional seismic detectors in seismic data acquisition, attracting widespread attention in the industry. These advantages are mainly reflected in higher spatial acquisition density, lower cost, and ease of long-term deployment. DAS systems utilize optical fibers as the sensing medium, transmitting light pulses of different frequencies into the fiber and accurately measuring the phase change caused by Rayleigh backscattering, thereby capturing the strain or strain rate information along the fiber axial direction with high sensitivity and high resolution. With the continuous advancement of DAS technology, its application in oil and gas exploration is becoming increasingly widespread, making it a current research hotspot. Significant progress has been made in areas such as vertical seismic profiling, surface seismic exploration, and reservoir dynamic monitoring. In seismic imaging technology, reverse time migration (RTM) exhibits unique advantages compared to Kirchhoff migration and one-way wave equation migration, which are widely used in practical exploration. RTM is simple in principle, has high imaging accuracy, and is not limited by formation dip angle, effectively handling complex geological structures. However, traditional reverse time offset technology has certain limitations when processing DAS acquired data due to data type restrictions.

[0003] Traditional seismic detectors record particle velocity information, while DAS records axial strain rate information. These two types of data differ significantly in amplitude and phase. Directly applying traditional seismic imaging methods to DAS-acquired data often leads to unsatisfactory imaging results. This is because traditional seismic imaging algorithms are designed based on particle velocity data, while strain rate data differs fundamentally from particle velocity data in both physical meaning and mathematical expression. Therefore, to utilize DAS data for seismic imaging, current research generally converts the strain rate data recorded by DAS into particle velocity data before applying conventional imaging procedures. However, this data conversion inevitably introduces conversion errors, reducing data fidelity and thus affecting imaging accuracy. Furthermore, the conversion process increases the complexity and computational load of data processing and may also lead to the loss of useful information, thereby limiting the application potential of DAS acquisition technology in seismic imaging.

[0004] Therefore, there is an urgent need to develop a stable, efficient and practical method for implementing reverse time migration in order to address the challenges faced in DAS data imaging. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention discloses a direct reverse time migration imaging method based on strain rate data acquired via DAS. This method first accurately simulates the forward propagation wavefield generated by the seismic source based on the pressure-strain rate equation. Then, it uses the actually observed strain rate data as boundary conditions for reverse propagation, thereby constructing the reverse propagation wavefield. High-quality imaging is achieved by applying cross-correlation imaging conditions to the forward and reverse propagation wavefields, and illumination compensation is performed on the imaging results using a source and receiver integrated illumination strategy, further improving the depth resolution of the imaging results.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

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

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

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

[0010] s3. Using cross-correlation imaging conditions, the forward propagation wave field and the reverse propagation wave field are imaged to obtain the imaging result I(x);

[0011] s4. Calculate the combined illumination matrix of the source and receiver wavefields;

[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 is expressed as follows:

[0015]

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

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

[0018]

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

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

[0021]

[0022] Then, substituting equation (2) into equation (3), we finally obtain the pressure-strain rate equation, which takes the following form:

[0023]

[0024] Optionally, in step s2, for the shot point located at position x s =(x s ,y s ,z s =0), the fiber is located at position x r =(x r ,y r ,z r =0) Received strain rate record Using the source wavelet function f(t) and P-wave velocity model v(x) from the observed data, and then updating and solving based on the pressure-strain rate equation, the propagating wave field p is obtained. F , means as follows:

[0025]

[0026] Also based on the pressure-strain rate equation, the observed strain rate is recorded Using this as a boundary condition, reverse time propagation is performed to obtain the propagated wavefield p. B , means as follows:

[0027]

[0028] Where x = (x, y, z) represents the spatial location of the model. Let δ be the Laplace operator, and δ be the Dirac impulse function.

[0029] Optionally, in step s3, the cross-correlation imaging condition is used to image the forward propagating wave field p. F With the reverse propagation wave field p B The imaging is performed, and the imaging result I(x) is represented 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 integrated lighting matrix H(x) is represented 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 effects of this invention are as follows: Its core innovation lies in constructing a pressure-strain rate equation specifically adapted for strain rate wavefield simulation and imaging, and proposing a method for direct reverse time migration imaging of strain rate data based on this equation. This method first accurately simulates the forward propagation wavefield generated by the seismic source based on the pressure-strain rate equation, and then uses the actually observed strain rate data as boundary conditions for reverse propagation, thereby constructing the reverse propagation wavefield. High-quality imaging is achieved by applying cross-correlation imaging conditions to the forward and reverse propagation wavefields, and illumination compensation is performed on the imaging results using a comprehensive illumination strategy for the seismic source and receiver points, further improving the deep resolution of the imaging results. Compared to traditional strain rate data imaging methods, this invention, by directly utilizing strain rate data for imaging, not only avoids errors caused by data conversion, significantly improving data fidelity and imaging accuracy, but also simplifies the data processing workflow. Furthermore, this method improves the illumination effect in deep subsurface areas through a comprehensive illumination strategy, making the imaging results clearer and richer in detail. The pressure-strain rate equation disclosed in this invention and its direct reverse time migration imaging technology adapted to strain rate data provide a stable and efficient framework for processing DAS acquired data, and are expected to promote the technological innovation and application development of DAS in the field of exploration geophysics. Attached Figure Description

[0036] Figure 1 This represents the actual velocity field of the Dongqiu model;

[0037] Figure 2 The smoothed offset velocity field of the Dongqiu model;

[0038] Figure 3 For observing strain rate records;

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

[0040] Figure 5 Illuminate the source of the earthquake;

[0041] Figure 6 Energy illumination for the detector point;

[0042] Figure 7Integrated lighting for the seismic source and detector points;

[0043] Figure 8 The results are direct imaging results from strain rate data;

[0044] Figure 9 The results are obtained by direct imaging of strain rate data using source illumination compensation.

[0045] Figure 10 The results are direct imaging results using strain rate data with integrated illumination compensation. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort 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 accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

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

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

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

[0050]

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

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

[0053]

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

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

[0056]

[0057] Then, substituting equation (2) into equation (3), we finally obtain the pressure-strain rate equation, which takes the following form:

[0058]

[0059] s2. Based on the pressure-strain rate equation, obtain the forward and reverse propagation wave fields; specifically including: for the shot point located at position x s =(x s ,y s ,z s =0), the fiber is located at position x r =(x r ,y r ,z r =0) Received strain rate record Using the source wavelet function f(t) and P-wave velocity model v(x) from the observed data, and then updating and solving based on the pressure-strain rate equation, the propagating wave field p is obtained. F , means as follows:

[0060]

[0061] Also based on the pressure-strain rate equation, the observed strain rate is recorded Using this as a boundary condition, reverse time propagation is performed to obtain the propagated wavefield p. B , means as follows:

[0062]

[0063] Where x = (x, y, z) represents the spatial location of the model. Let δ be the Laplace operator, and δ be the Dirac impulse function.

[0064] s3. Using cross-correlation imaging conditions for the propagating wave field p F With the reverse propagation wave field p B The imaging is performed, and the imaging result I(x) is represented as:

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

[0066] s4. Calculate the combined illumination matrix H(x) of the source and receiver wavefields, 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 in this invention was applied to reverse-time migration imaging of the Eastern Autumn model in western China, achieving good imaging results. The actual Eastern Autumn velocity model is as follows: Figure 1 As shown, the offset model of the imaging input is as follows: Figure 2 As shown. The source function of the observed strain rate data is a Ricker wavelet with a main frequency of 15 Hz. The observation dataset consists of 169 shot gathers, uniformly distributed on the surface, with a maximum fiber offset of 2.5 km. Figure 3 A shot set of observed strain rate data is presented. Figure 4 This is a shot set of synthetic strain rate data obtained from the offset velocity model. Figure 5 The source energy illumination matrix is ​​displayed. Figure 6 For the detector point energy illumination matrix, Figure 7 It is a combined illumination matrix for the seismic source and receiver. Figure 8 The results are based on direct reverse time migration imaging using the pressure-strain rate equation. Although this method can directly use strain rate data for imaging, the directionality of the DAS data acquisition results in very low resolution for deep imaging. Figure 9 Showing the Figure 8 The effect of source illumination compensation on the imaging results shows that conventional source illumination compensation cannot effectively enhance the quality of deep imaging. Figure 10 That is to Figure 8 The imaging results shown are after comprehensive illumination compensation of the source and receiver points. It can be seen that after comprehensive illumination compensation, the depth resolution of the imaging results is significantly improved, and high-quality imaging of strain rate data is achieved.

[0071] This invention proposes a direct reverse time migration imaging method for strain rate data acquired by distributed acoustic sensing (DAS), aiming to provide a stable, efficient, and practical reverse time migration implementation scheme to address the challenges in DAS data imaging. Traditional seismic detectors record particle velocity information, while DAS records axial strain rate information, which differ significantly in amplitude and phase. Directly applying traditional seismic imaging methods to DAS-acquired data often leads to unsatisfactory imaging results. Furthermore, converting DAS data to particle velocity data before applying conventional imaging procedures not only introduces conversion errors, reducing imaging accuracy, but also increases the complexity and workload of data processing. To address this issue, this invention constructs a pressure-strain rate equation specifically adapted for strain rate wavefield simulation and imaging, and proposes a direct reverse time migration imaging method for strain rate data based on this equation. This method first uses the pressure-strain rate equation to accurately simulate the forward propagation wavefield generated by the seismic source, and then uses the actually observed strain rate data as boundary conditions for reverse propagation, thereby constructing the reverse propagation wavefield. High-quality imaging is achieved by applying cross-correlation imaging conditions to the forward and reverse propagation wavefields. Finally, an integrated illumination strategy using both source and receiver points is employed to compensate for the illumination, further enhancing the deep resolution of the imaging results. This method not only avoids errors caused by data conversion, significantly improving data fidelity and imaging accuracy, but also simplifies the data processing workflow. Furthermore, the integrated illumination strategy improves the lighting effect in deep subsurface regions, resulting in clearer and more detailed imaging results.

[0072] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the 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 acquisition of strain rate data, characterized in that, The method comprises the following steps: s1, establishing a pressure-strain rate equation based on a velocity-pressure fluctuation equation and a strain rate-velocity relationship; s2, obtaining a forward wave field and a reverse wave field based on the pressure-strain rate equation; s3, imaging the forward-propagating wavefield and the backward-propagating wavefield using the cross-correlation imaging condition to obtain an imaging result ; s4, calculating a comprehensive illumination matrix of a wave field of a seismic source and a geophone; s5, the imaging result is obtained s5, the imaging result is obtained ; The step s1 specifically comprises: An expression form of the first-order velocity-pressure fluctuation equation is as follows: （1） wherein is the pressure, is the medium density, is the particle velocity, is the time, , , is the spatial direction; The relationship between the strain rate and the particle velocity is derived and expressed as: （2） wherein, denotes the strain rate, denotes the strain, is the displacement; The spatial derivatives of the second, third and fourth equations in formula (1) are solved respectively, the first equation is kept unchanged, and the following is obtained: （3） Then, formula (2) is brought into formula (3), and finally, the pressure-strain rate equation is obtained, and the form is as follows: （4）； In step s2, for the shot located at position , the fiber located at position , the received strain rate record , the source wavelet function of the observation data , and the P-wave velocity model , the forward wavefield is then obtained by solving the update equation based on the pressure-strain rate equation, and is represented as follows: （5） Also based on the pressure-strain rate equation, the observed strain rate record Inverse time-reversal as a boundary condition, obtaining the time-reversed wavefield is expressed as follows: （6） wherein representing a model space position, is a Laplacian operator, is a Dirac delta function.

2. The direct reverse-time migration imaging method based on DAS collected strain rate data of claim 1, wherein, In step s3, the forward propagating wavefield is imaged using the cross-correlation imaging condition The imaging result is represented as: （7）。 3. The direct reverse-time migration imaging method based on DAS for collecting strain rate data of claim 2, wherein, In step s4, the synthesis illumination matrix is denoted as: （8）。 4. The direct reverse-time migration imaging method based on DAS for collecting strain rate data of claim 3, wherein, In step s5, the final imaging result is: （9）。

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