阅读说明：本技术 一种基于伴随状态法初至波走时层析的海水速度建模方法 (Seawater speed modeling method based on first-arrival wave travel-time chromatography of adjoint state method ) 是由 刘玉柱 刘雯 董良国 杨积忠 李华 于 2021-08-06 设计创作，主要内容包括：本发明涉及一种基于伴随状态法初至波走时层析的海水速度建模方法,包括以下步骤：1)提取OBS资料中海水透射波到达时,进行预处理得到炮点检波点和对应走时数据,即观测走时；2)根据一般情况下海水速度分布设计初始模型；3)进行基于伴随状态法初至波走时层析方法的反演迭代,得到最终成像结果。与现有技术相比,本发明具有受季节洋流因素影响小、计算效率高、适用于海洋地震勘探等优点。(The invention relates to a seawater speed modeling method based on first-arrival wave travel time chromatography of an adjoint state method, which comprises the following steps: 1) extracting seawater transmitted waves from OBS data, and preprocessing the seawater transmitted waves to obtain shot point wave detection points and corresponding travel time data, namely observation travel time; 2) designing an initial model according to the seawater speed distribution under the general condition; 3) and performing inversion iteration based on the first-motion wave travel time tomography method by using the adjoint state method to obtain a final imaging result. Compared with the prior art, the method has the advantages of small influence of seasonal ocean current factors, high calculation efficiency, suitability for marine seismic exploration and the like.)

1. A seawater speed modeling method based on first-arrival wave travel-time chromatography of an adjoint state method is characterized by comprising the following steps:

1) extracting seawater transmitted waves from OBS data, and preprocessing the seawater transmitted waves to obtain shot point wave detection points and corresponding travel time data, namely observation travel time;

2) designing an initial model according to the seawater speed distribution under the general condition;

3) and performing inversion iteration based on the first-motion wave travel time tomography method by using the adjoint state method to obtain a final imaging result.

2. The seawater velocity modeling method based on adjoint state method first-arrival travel-time chromatography according to claim 1, wherein the step 3) specifically comprises the following steps:

31) solving a path function equation by using a direct scanning type shortest path method, and calculating a travel time field of the current model;

32) calculating the travel time difference between the travel time field and the observed travel time of the current model, judging whether the target function meets a termination condition, if so, stopping iteration and outputting an inversion result, otherwise, continuing inversion, and performing step 33);

33) calculating a adjoint field lambda according to the travel time difference and the adjoint equation;

34) calculating a gradient from the adjoint field λ and the current model;

35) determining a step length, modifying the model, and outputting a current inversion result;

36) repeating steps 31-35) until the final inversion result is obtained.

3. The seawater velocity modeling method based on adjoint state method first-arrival travel-time chromatography according to claim 2, wherein the step 31) specifically comprises the following steps:

311) determining the position of a seismic source and determining the minimum travel time of adjacent nodes of the seismic source;

312) and directly carrying out full-field scanning in all directions to obtain the travel time field of the current model.

4. The seawater velocity modeling method based on adjoint state method first-arrival travel-time chromatography according to claim 2, wherein the step 33) specifically comprises the following steps:

331) making a difference value according to the travel time field of the current model and the sea water transmitted wave travel time extracted from the actual data;

332) calculating an accompanying variable at a detection point according to an accompanying equation;

333) scanning according to the adjoint equation to obtain the adjoint field of the full space.

5. The method as claimed in claim 4, wherein in step 332), the expression of the adjoint equation:

wherein λ (x) is an accompanying variable at the current position x, x is the current position, Ω is a calculation region, T (x) is a travel time field of the current model, and T (x)^obs(x) For observing the resulting travel time field, δ is a function of δ, ani denotes the number of the detection point, r_iIs the position of the ith detection point, N is the number of detection points,is a gradient operator.

6. The seawater velocity modeling method based on adjoint state method first-arrival travel-time chromatography according to claim 5, wherein in the step 34), adjoint variables and gradients satisfy the relation:

wherein J (x) is an objective function, and c is a model velocity.

7. The method as claimed in claim 6, wherein the objective function is expressed as:

8. the method as claimed in claim 1, wherein in step 2), during the marine seismic data processing, the initial model is optimized on the basis of the seawater velocity constant assumed to be constant when more detailed seawater velocity distribution information exists in the survey area.

9. The method of claim 8, wherein the seawater velocity is 1500 m/s.

10. The seawater velocity modeling method based on adjoint state method first-arrival travel-time chromatography according to claim 1, wherein in the step 1), the OBS data is derived from OBS travel-time data acquired by a marine active earthquake at a short time, so as to reduce the influence of seasonal and ocean current factors.

Technical Field

The invention relates to the field of seismic imaging in exploration seismology, in particular to a seawater velocity modeling method based on first-arrival wave travel time stratification and using OBS data of an adjoint state method.

Background

The first-arrival waves are seismic information which arrives first in seismic data and comprise refracted waves and direct waves, have the characteristics of strong energy and clear seismic facies, and can be observed and picked up clearly. At present, the industry has developed a mature method for automatically picking up first-arrival wave information in seismic data by using a computer, because the first-arrival wave information does not include deep reflected wave information and the like, the first-arrival wave tomography technology is generally applied to building a near-surface velocity model at present, the first-arrival wave information includes travel time and waveform, and the currently most mature tomography is a first-arrival wave travel time tomography method, and compared with wave equation tomography, the method has better stability and higher computational efficiency.

In modern seismic exploration, three-dimensional seismic exploration modes are more and more widely applied, the data volume of seismic information is more and more large, and the traditional travel time tomography method based on ray tracing still needs huge calculation amount. An adjoint state method first-arrival wave travel time tomography technology is introduced in 2012 of Shechun and the like, finite frequency adjoint state method tomography based on an acoustic wave equation is deduced, the method and a traditional ray tomography method are respectively applied to theoretical model experiments and actual data processing, and results show that the adjoint state method first-arrival wave travel time tomography method can obtain an inversion result similar to the traditional ray tomography, but the calculation efficiency is greatly improved. In 2017, lie Yongde and the like propose accompanying state method first-arrival wave travel time tomography using an approximate Heissian matrix for preconditions, the method obtains a matrix similar to ray density by adding one-time calculation, the inverse of the matrix is used as a precondition, the inversion accuracy under the condition that an initial model is not ideal is improved, and theoretical model and actual data processing result experiments show that the accompanying state method first-arrival wave travel time tomography of the preconditions not only retains the advantages of the accompanying state method first-arrival wave travel time tomography, but also can overcome the limitation of a first-order direction, the inversion accuracy and efficiency are improved, 2021, a Dong nations and the like propose an improved accompanying state method travel time tomography method independent of a surface normal vector, the defect that a field in the traditional method depends on the surface normal vector is overcome, and a residual error can be correctly returned when a detection point travels out, so that the inversion accuracy is improved.

In recent years, with the continuous perfection of marine seismic exploration technology, people have higher and higher requirements on seismic data processing precision, and each parameter is required to be as accurate as possible. At present, two methods are mainly used for measuring the speed in seawater: direct measurement and indirect measurement, wherein an instrument used in the direct measurement method is generally called as a sound velocity meter, and generally utilizes a transmitting-receiving transducer to measure sound velocity in a fixed distance; the indirect measurement method is to measure the temperature, salinity and depth of seawater by using a hydrological instrument, then to obtain a sound velocity profile by calculation by using an empirical formula of environmental measurement and sound velocity. However, the two common methods are greatly influenced by temperature, salinity and pressure in the measuring process and continuously change along with the spatial position and time, so that the measuring result is not accurate enough. Therefore, if the actual marine seismic survey data can be used to determine the velocity of sound in the sea, the measurement accuracy is greatly improved, thereby improving the accuracy and efficiency of marine seismic data processing.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a seawater speed modeling method based on the first-arrival travel-time chromatography of the adjoint state method.

The purpose of the invention can be realized by the following technical scheme:

a seawater speed modeling method based on first-arrival wave travel-time chromatography of an adjoint state method comprises the following steps:

1) extracting seawater transmitted waves from OBS data, and preprocessing the seawater transmitted waves to obtain shot point wave detection points and corresponding travel time data, namely observation travel time;

2) designing an initial model according to the seawater speed distribution under the general condition;

3) and performing inversion iteration based on the first-motion wave travel time tomography method by using the adjoint state method to obtain a final imaging result.

The step 3) specifically comprises the following steps:

31) solving a path function equation by using a direct scanning type shortest path method, and calculating a travel time field of the current model;

32) calculating the travel time difference between the travel time field and the observed travel time of the current model, judging whether the target function meets a termination condition, if so, stopping iteration and outputting an inversion result, otherwise, continuing inversion, and performing step 33);

33) calculating a adjoint field lambda according to the travel time difference and the adjoint equation;

34) calculating a gradient from the adjoint field λ and the current model;

35) determining a step length, modifying the model, and outputting a current inversion result;

36) repeating steps 31-35) until the final inversion result is obtained.

The step 31) specifically comprises the following steps:

311) determining the position of a seismic source and determining the minimum travel time of adjacent nodes of the seismic source;

312) and directly carrying out full-field scanning in all directions to obtain the travel time field of the current model.

The step 33) specifically comprises the following steps:

331) making a difference value according to the travel time field of the current model and the sea water transmitted wave travel time extracted from the actual data;

332) calculating an accompanying variable at a detection point according to an accompanying equation;

333) scanning according to the adjoint equation to obtain the adjoint field of the full space.

Said step 332), the expression of the accompanying equation:

wherein λ (x) is an accompanying variable at the current position x, x is the current position, Ω is a calculation region, T (x) is a travel time field of the current model, and T (x)^obs(x) For observing the resulting travel time field, δ is a function of δ, anDenotes the number of the detection point, r_iIs the position of the ith detection point, N is the number of detection points,is a gradient operator.

In the step 34), the accompanying variable and the gradient satisfy the relation:

wherein J (x) is an objective function, and c is a model velocity.

The expression of the objective function is as follows:

in the step 2), in the marine seismic data processing process, the seawater velocity is generally assumed to be constant, and when more detailed seawater velocity distribution information exists in the survey area, the initial model is optimized on the basis.

Assuming a seawater velocity of 1500 m/s.

In the step 1), the OBS data are derived from OBS travel time data acquired by one marine active earthquake in a short time, so that the influence of seasons and ocean current factors is reduced.

Compared with the prior art, the invention has the following advantages:

firstly, the travel time data obtained according to a large amount of OBS travel time data acquired by one-time marine active earthquake in a short time is less influenced by factors such as seasons, ocean currents and the like.

Secondly, due to the introduction of the adjoint state method, the gradient calculation process in the invention avoids the calculation of the Frechet derivative, and the gradient is obtained by indirect calculation, so that the calculation efficiency is higher, and the parallel calculation is easy to realize.

And thirdly, the seawater velocity inversion process is similar to the seismic exploration inversion process, and the method is easy to realize in the actual marine seismic exploration process.

Drawings

FIG. 1 is a flow chart of the method of the present invention.

Fig. 2 is a theoretical seawater velocity model in example 1.

Fig. 3 is an inverted initial model in example 1.

Fig. 4 shows the results of transmission wave time-lapse tomography in example 1.

Fig. 5 is a velocity profile of the inversion result in the vertical direction in example 1, where fig. 5a is a velocity profile at X500 m and fig. 5b is a velocity profile at X750 m.

Fig. 6 is a horizontal velocity profile of the inversion result in example 1, where fig. 6a is a velocity profile at Y of 200 m and fig. 6b is a velocity profile at Y of 700 m.

Fig. 7 is a theoretical seawater velocity model in example 2.

Fig. 8 is an inverted initial model in example 2.

Fig. 9 shows the results of transmission wave time-lapse tomography in example 2.

Fig. 10 is a velocity profile of the inversion result in example 2, where X is 600 meters, Y is a velocity profile in the vertical direction at 600 meters, and fig. 10b is a velocity profile in the horizontal direction at Z is 250 meters, and Y is 600 meters.

Fig. 11 is an inverted initial model in example 3.

Fig. 12 shows the results of transmission wave time-lapse tomography in example 3.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments.

Example 1:

according to the distribution characteristics of the seawater speed, the following two-dimensional theoretical model is set in the example: the model size is 2000 meters by 1000 meters, the grid is 200 meters by 100, the spacing is 10 meters, the background speed variation trend is 'high speed-low speed-high speed', the maximum speed is 1500 meters/second, the speed reaches the minimum at the depth of about 500m, a high-speed abnormal body is arranged at the position (1000,600), the speed is 1480 meters/second, the seabed is below 950 meters, and the speed is 3000 meters/second, as shown in fig. 2. Setting shot points to the top of the model, namely the sea surface, setting OBS on the seabed, wherein the number of the shot points is 201, the distance between the shots is 10 meters, the number of the OBS is 201, and the distance is 10 meters. The inversion was performed using a constant velocity model with a seawater velocity of 1500m/s as the initial model, which is shown in fig. 3. The initial step size is 50, decreasing with the objective function. The inversion is iterated 603 generations, the initial objective function is 39.9, the final objective function is 0.0066, the objective function drops to 0.016% in the case of the initial model, and the total time is 1 hour and 31 minutes.

The specific implementation mode is as follows:

1) performing seismic wave ray propagation forward modeling on the theoretical model by a direct scanning type minimum path method to obtain theoretical travel time;

2) performing seismic wave ray propagation forward modeling on the initial model by a direct scanning type minimum path method to obtain calculated travel time;

3) calculating the difference between the theoretical travel time and the calculated travel time at the receiving point, and calculating by an adjoint equation to obtain an adjoint variable field;

4) calculating to obtain a gradient field according to the relation between the accompanying variable and the gradient;

5) determining a model modifier according to a steepest descent method to obtain a new speed model;

6) taking the new speed model as the initial model to continue iteration, repeating the steps 2-5) until the new speed model meets the convergence condition, ending the iteration, and outputting the final result

The inversion results are shown in fig. 4. The inversion result is observed, and the improved accompanying state method travel time chromatography can accurately invert the speed distribution condition of the seawater, the position and the size of the high-speed abnormal body are well expressed, the travel time is basically fitted, and the error at the boundary is obvious. The horizontal resolution is obviously higher than the vertical resolution, the shape of the abnormal body in the vertical direction is stretched, and the background speed tends to be average.

Respectively carrying out velocity extraction analysis on the inversion result in the horizontal direction and the vertical direction at the positions where X is 500 meters, X is 750 meters, Y is 200 meters and Y is 700 meters, so that the high inversion precision and high resolution in the horizontal direction can be obviously seen, the true velocity can be well approached, and the velocity transverse change can be well expressed; the inversion resolution ratio in the vertical direction is low, the inversion accuracy is low, the inversion speed can approximately approach to the real speed, and the reason that the resolution ratio difference between the horizontal direction and the vertical direction is large is that due to the adoption of the observation system for single-side excitation and single-side reception and the travel time inversion, under the condition, the inversion is insensitive to the speed change along the ray direction and sensitive to the speed change among rays.

Example 2:

according to the two-dimensional theoretical model, a three-dimensional theoretical model is designed, the size of the model is 1000 meters by 500 meters, the grid is 100 by 50, the grid interval is 10 meters, the seawater speed part is designed along with a two-dimensional idea, the seawater background speed is in a trend of changing from high to low to high, the maximum speed is 1500 meters per second, the depth speed reaches the minimum value when reaching 200 meters, a high-speed abnormal body exists in the center position of the model, and the speed of the abnormal body is 1480 meters per second, as shown in fig. 6. In order to simulate an OBS observation system, shot points are arranged on the top of the model, namely the sea surface, the number of shots is 51 x 51, 2601 shots are shared, the intervals between the shots are 10 meters, and the sea surface is uniformly covered; the OBS are positioned at the bottom of the model, namely the seabed, and are 10201 in number of 101 × 101, spaced by 10 meters and uniformly distributed on the seabed. The inversion was performed using a homogeneous model with a seawater velocity of 1500m/s as an initial model, which is shown in fig. 7. And performing inversion iteration for 21 rounds, wherein the initial objective function is 35.5, the final objective function is 0.87, the objective function is reduced to 2.4% of the initial model, the specific implementation flow is similar to that of example 1, and the difference is that the calculation of the two-dimensional situation is expanded to three-dimensional.

The inversion result is observed, and the improved time-lapse chromatography with the state method under the three-dimensional condition can be seen to accurately invert the speed distribution condition of the seawater, the position and the size of the high-speed abnormal body are well expressed, the time-lapse basic fitting is realized, the horizontal direction resolution is obviously higher than the vertical direction, the shape of the abnormal body in the vertical direction is stretched, and the background speed tends to be average.

The speed vertical line drawing analysis is carried out at the position where X is 600 meters, Y is 600 meters, the speed horizontal line drawing analysis is carried out at the position where Y is 600 meters and Z is 250 meters, so that the inversion accuracy in the horizontal direction is high, the resolution is high, the real speed can be well approached, and the speed transverse change can be well expressed; the inversion resolution ratio in the vertical direction is low, the inversion accuracy is low, and the inversion speed can approximately approach to the real speed.

Example 3:

in the embodiment, the seawater speed modeling method based on the OBS provided by the invention is applied to real OBS data of the east China sea, an initial model used for inversion is shown in FIG. 11, the horizontal length of the initial model is 11000 m, and the depth is 90 m; the size of the grid is 11000 x 9, the spacing is 1 m, and the speed of the seawater is uniform 1500 m/s. A total of 91 OBSs with a depth of 81 meters and a distribution from 638 meters to 10715 meters; the depth of shot points is 9 meters, and the shot points are distributed from 0 meter to 10999 meters for 283 shots. The average depth of the sea bottom is 85 meters. The inversion was stopped after 14 iterations, the initial objective function was 788.2, the final objective function was 226.3, and the objective function dropped to 28.7% of when the initial model was used.

Analyzing the inversion result, wherein the improved first-arrival wave travel time tomography method with the adjoint state method can roughly show the real speed distribution of the seawater when being applied and actual data, and the inversion result conforms to the theoretical speed distribution of the seawater; meanwhile, the change of the seawater speed in the horizontal direction is better shown, the change of the seawater speed in the vertical direction is smaller, and one reason is presumed that the change of the actual seawater in the vertical direction is smaller because the seawater in the actual data measuring area is shallower; secondly, because of the limitation of the observation system, the vertical resolution is lower than the transverse resolution. In addition, the obvious inversion accuracy on two sides of the calculation area is lower, and the reason is presumed that the ray density of the area is sparse and the illumination is insufficient.