CN113031063A - Reverse time migration imaging method based on imaging gather correlation weighting - Google Patents
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Abstract
The invention relates to a reverse time migration imaging method based on imaging gather correlation weighting, which belongs to the field of seismic exploration migration imaging and specifically comprises the following steps: firstly, an imaging section of each cannon is obtained by adopting a reverse time migration method, imaging channels at the same position in the imaging section of each cannon are respectively extracted and arranged from small to large according to the distance between the channels and the cannon, so as to obtain imaging channel sets at the positions, then, in each imaging channel set, the noise part of a far channel is preliminarily cut off, all the imaging channels in the imaging channel set are correlated with a reference channel one by one, N channels with larger correlation values are selected for superposition, so that a new imaging channel is formed, each imaging channel set is processed, and each new imaging channel is placed at a corresponding position, so that the final reverse time migration imaging section is obtained. The result shows that the method can effectively suppress noise, improve the continuity and the balance of the in-phase axis and further obviously improve the imaging precision of reverse time migration.
Description
Technical Field
The invention belongs to the field of seismic exploration migration imaging, and particularly relates to a reverse time migration imaging method based on correlated weighting of an imaging gather.
Background
The reverse time migration is a currently accepted seismic imaging algorithm with the highest precision, a seismic source wavelet is used as a timing wave field disturbance, a timing wave field is obtained based on a two-way wave equation finite difference numerical simulation technology, meanwhile, an actual seismic record is used as the reverse time disturbance, a reverse time wave field is obtained based on the two-way wave equation finite difference numerical simulation technology, and finally a cross-correlation imaging condition is applied to obtain a final migration profile. According to the method, the two-way wave is used for imaging, the strong speed change condition can be processed, and the complex area which cannot be accurately imaged by the conventional imaging method can be accurately imaged, so that the current reverse time migration is considered as an important method capable of further improving the oil-gas exploration capability of seismic exploration, and is one of the research hotspots in the current geophysical field.
In the conventional reverse time migration imaging, the imaging section is formed by directly adding the imaging results of each shot, and actually, when the distance between an imaging point and a shot point is far, the current shot hardly contributes to the imaging of the imaging point, on the contrary, irregular noise is brought, so that the imaging quality is influenced, and no method for well selecting an effective imaging shot so as to improve the imaging quality exists at present.
Disclosure of Invention
The invention aims to provide a reverse time migration imaging method based on correlated weighting of an imaging gather. Firstly, an imaging section of each cannon is obtained by adopting a reverse time migration method, imaging channels at the same position in the imaging section of each cannon are respectively extracted, and the images are arranged according to the distance between the channels and the cannons from small to large so as to obtain imaging channel sets at various positions, then in each imaging channel set, primarily cutting off the noise part of the far path, correlating all the imaging paths in the imaging path set with the reference path (namely the result of stacking all the paths in each processed imaging path set) one by one, selecting N paths with larger correlation values (N is the given effective stacking path number, generally 1/2 of the full coverage times of the seismic survey line) for stacking, thereby forming a new imaging path, and carrying out the processing on each imaging gather, and placing each new imaging gather at a corresponding position to obtain a final reverse time migration imaging section. Model experiment results show that the method can effectively suppress noise and improve the continuity and the balance of the in-phase axis, thereby obviously improving the imaging precision of reverse time migration.
The invention adopts the following technical scheme:
a reverse time migration imaging method based on imaging gather correlation weighting specifically comprises the following steps:
(1) giving a two-dimensional velocity model V (x, z), where x, z represent spatial position coordinates, x is 1,2,3, …, Nx, z is 1,2,3, …, Nz, Nx, Nz represent the total number of model lateral and longitudinal grid points, respectively; given seismic wavelet Wi(t), i represents the gun number, i is 1,2,3, …, S; s represents the total number of guns, t represents time, and based on a speed model, forward modeling is carried out by applying an acoustic wave equation through finite difference to obtain a timing wave field U of each guni(x,z,t)；
(2) Seismic records P based on V (x, z) at given shotsi(xrT) performing a reverse time continuation as a reverse time perturbation to obtain a reverse time wave field R of each guni(x, z, t), wherein xrRepresenting the position of the wave detection point;
(3) carrying out shot point cross-correlation normalized imaging by using a formula (1) to obtain an imaging section Image of each shoti(x, z) such that a total of S imaging profiles are obtained; the formula (1) is as follows:
(4) for the S imaging sections, the X position of each section is determinedExtracting the channels to form Nx imaging channel sets, and recording as IGx(i, z) wherein x is 1,2,3, …, Nx; i is 1,2,3, …, S, each imaging trace is concentrated and arranged from small to large according to the distance between the shot position and the current x;
(5) in each imaging gather IGx(i, z) the noise part of the far track is primarily cut off, and then all the tracks are superposed by using the formula (2) to form a reference track Gx(z); the formula (2) is as follows:
(6) using formula (3) to integrate IG of each imaging channelx(i, z) each track is associated with a reference track G of the imaging gatherx(z) correlating to obtain a correlation value C of each track with respect to the reference trackx(i) (ii) a The formula (3) is as follows:
(7) in each imaging gather IGxAnd (i, z) superposing the front N tracks with the maximum correlation value by using a formula (4) to form a new imaging section Imagenew(x, z), wherein N is a given number of valid superposed tracks; the formula (4) is:
further, N in the step (7) is 1/2 of the number of times of full coverage of the seismic survey line.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a reverse time migration imaging method based on imaging gather correlation weighting, firstly adopting a reverse time migration method to obtain an imaging cross section of each shot, respectively extracting imaging channels at the same position in the imaging cross section of each shot, arranging the imaging channels according to the distance between the channels and the shot from small to large so as to obtain imaging gathers at each position, then preliminarily cutting off the noise part of a far channel in each imaging gather, correlating all the imaging channels in the imaging gathers with a reference channel (namely the result of overlapping all the channels of each processed imaging gather) one by one, selecting N channels with larger correlation values (N is the given number of effective overlapping channels, generally 1/2 of the number of full coverage times of the seismic survey line) to overlap so as to form a new imaging channel, carrying out the above processing on each imaging gather, and placing each new imaging channel at the corresponding position, and obtaining a final reverse time migration imaging section. Model experiment results show that the method can effectively suppress noise and improve the continuity and the balance of the in-phase axis, thereby obviously improving the imaging precision of reverse time migration.
Drawings
FIG. 1 is a conventional reverse time migration flow diagram;
FIG. 2 is a reverse time migration flow chart based on imaging gather dependent weighting;
FIG. 3 velocity model;
FIG. 4 is a trace set diagram of trace 100;
FIG. 5 is a photograph of a 100 th imaging gather after ablation;
FIG. 6 shows the reference trace of the 100 th imaging gather;
FIG. 7 is a conventional reverse time migration imaging diagram;
FIG. 8 is a reverse time migration imaging plot based on imaging gather dependent weighting.
Detailed Description
The detailed description is made using a 200 x 200 mesh model. The model is a layered model with three layers from top to bottom, and the speeds are 1500m/s, 2000m/s and 2500m/s respectively. The grid step length of the model in the x direction and the z direction is 5m, the model is 1000m long in the transverse direction and 1000m deep in the longitudinal direction (the speed model is shown in figure 3).
Based on the model, a full-array observation mode is adopted, blasting is carried out for 40 times in total, the shot set records that each shot has 200 receiving channels, the shot interval is 25m, the channel interval is 5m, and the depth of a shot point and the depth of a receiving point are both 0 m.
The following describes the specific implementation process of the present invention in detail, and the flow is shown in fig. 2:
(1) a two-dimensional velocity model V (x, z) with a mesh size of 200 × 200 is given, where x and z represent spatial position coordinates (x ═ 1,2,3, …,200, z ═ 1,2,3, …, 200); given placeSeismic wave Wi(t), 40 shots are shot, i represents the serial number of the shot (i is 1,2,3, … and 40), t represents time, forward modeling is carried out by applying a sound wave equation through finite difference based on a speed model, and a timing wave field U of each shot is obtainedi(x,z,t)；
(2) Seismic records P based on V (x, z) at given shotsi(xrT) performing a reverse time continuation as a reverse time perturbation to obtain a reverse time wave field R of each guni(x, z, t), wherein xrRepresenting the position of the wave detection point;
(3) carrying out shot point cross-correlation normalized imaging by using a formula (1) to obtain an imaging section Image of each shoti(x, z) such that a total of 40 imaging profiles are obtained; the formula (1) is
(4) For these 40 imaged sections, the traces at the same x-position in each section are extracted to form 200 imaged gather denoted as IGx(i, z) wherein x is 1,2,3, …,200, i is 1,2,3, …,40, and the trace set map for the 100 th trace is shown in fig. 4; each imaging channel is concentrated and arranged from small to large according to the distance between the shot point position and the current x;
(5) in each imaging gather IGx(i, z) the noise part of the far track is primarily cut off, and then all the tracks are superposed by using the formula (2) to form a reference track Gx(z); the formula (2) is as follows:
the picture after the 100 th imaging gather is cut is shown in fig. 5;
(6) using formula (3) to integrate IG of each imaging channelx(i, z) each track is associated with a reference track G of the imaging gatherx(z) correlating to obtain a correlation value C of each track with respect to the reference trackx(i) (ii) a The formula (3) is as follows:
the reference trace for the 100 th imaging gather is shown in FIG. 6.
(7) In each imaging gather IGxThe first 20 paths with the maximum correlation value in (i, z) are overlapped by using a formula (4) to form a new imaging section Imagenew(x, z); the formula (4) is:
to illustrate the effectiveness of the method of the present invention, a comparison is made here with conventional reverse time migration imaging plots. The conventional reverse time migration imaging process is shown in fig. 1, fig. 7 is a conventional reverse time migration imaging chart, and fig. 8 is a reverse time migration imaging chart based on correlated weighting of the imaged gather. Comparing the two graphs, it can be found that the low-frequency noise at the top of fig. 8 is eliminated, the continuity of the in-phase axes at the two sides of the low-frequency noise is enhanced, and the equalization of the in-phase axes becomes better, which shows that the imaging precision of the reverse time migration based on the correlated weighting of the imaging gather is obviously improved.
Claims (2)
1. A reverse time migration imaging method based on imaging gather correlation weighting is characterized by comprising the following steps:
(1) giving a two-dimensional velocity model V (x, z), where x, z represent spatial position coordinates, x is 1,2,3, …, Nx, z is 1,2,3, …, Nz, Nx, Nz represent the total number of model lateral and longitudinal grid points, respectively; given seismic wavelet Wi(t), i represents the gun number, i is 1,2,3, …, S; s represents the total number of guns, t represents time, and based on a speed model, forward modeling is carried out by applying an acoustic wave equation through finite difference to obtain a timing wave field U of each guni(x,z,t)；
(2) Seismic records P based on V (x, z) at given shotsi(xrT) performing a reverse time continuation as a reverse time perturbation to obtain a reverse time wave field R of each guni(x, z, t), wherein xrRepresenting the position of the wave detection point;
(3) carrying out shot point cross-correlation normalized imaging by using a formula (1) to obtain an imaging section Image of each shoti(x, z) such that a total of S imaging profiles are obtained; the formula (1) is as follows:
(4) for the S imaging sections, extracting the trace at the same x position in each section to form Nx imaging trace sets, denoted as IGx(i, z) wherein x is 1,2,3, …, Nx; i is 1,2,3, …, S, each imaging trace is concentrated and arranged from small to large according to the distance between the shot position and the current x;
(5) in each imaging gather IGx(i, z) the noise part of the far track is primarily cut off, and then all the tracks are superposed by using the formula (2) to form a reference track Gx(z); the formula (2) is as follows:
(6) using formula (3) to integrate IG of each imaging channelx(i, z) each track is associated with a reference track G of the imaging gatherx(z) correlating to obtain a correlation value C of each track with respect to the reference trackx(i) (ii) a Said formula
(7) In each imaging gather IGxAnd (i, z) superposing the front N tracks with the maximum correlation value by using a formula (4) to form a new imaging section Imagenew(x, z), wherein N is a given number of valid superposed tracks; the formula (4) is:
2. the method of claim 1, wherein N in step (7) is 1/2 of the number of times the seismic line is fully covered.
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