A method for correcting high-resolution seismic uneven cables

Abstract

The present invention discloses a method for correcting high-resolution seismic uneven cables. Based on the actual seabed collected, a seabed velocity model is constructed, and then the seabed reflection arrival time when the cable floats on the sea surface is obtained through forward simulation. Then, the actual collected seabed reflection time is corrected to the theoretically calculated seabed time, so as to achieve the purpose of uneven cable correction. This method can effectively eliminate ghost waves caused by uneven receiving cables, overcome the defect that the phase axis is difficult to accurately align during superposition, achieve more accurate velocity pickup, and obtain better superposition imaging effect, which meets the theoretical requirements of seismic data processing.

Description

Technical Field

[0001] The invention belongs to the technical field of marine geological exploration, and in particular relates to a method for correcting a high-resolution seismic uneven cable. Background Art

[0002] In marine high-resolution seismic exploration, the acquisition cable is generally 100-300m long, and the acquisition cable is relatively short. It is generally assumed that the acquisition cable floats on the sea surface. However, when the acquisition cable is used for a long time, the cable weight is often unbalanced, resulting in excessive cable density. At this time, it is difficult for the cable to float on the sea surface during acquisition, and it often sinks below the sea surface. In addition, due to the short length of the cable, it is often difficult to install water birds to control the cable sinking depth. In this case, affected by factors such as ocean currents, the cable sinking depth is often difficult to control, the cable posture is relatively complex, and the cable sinking depth in the seawater varies.

[0003] Due to the inconsistent cable laying depth, ghost waves appear in the receiving cable, the phase axis is difficult to align accurately during superposition, and the velocity spectrum energy group cannot be focused, resulting in difficulty in velocity picking. Therefore, it is very important to eliminate the imbalance of high-resolution seismic cable posture. Summary of the invention

[0004] In order to solve the defects caused by the imbalance of cable posture in the prior art, the present invention proposes a method for correcting high-resolution seismic uneven cables to achieve more accurate velocity picking and obtain better stacking imaging effects, which meets the theoretical requirements of seismic data processing.

[0005] The present invention is implemented by adopting the following technical scheme: a method for correcting a high-resolution seismic uneven cable, comprising the following steps:

[0006] Step 1: Construct a one-dimensional coordinate system; unify the seabed position coordinates and the shot point and detection point position coordinates into the one-dimensional coordinate system;

[0007] Step 2: Construct seabed velocity model:

[0008] First, select the first seismic data, perform post-stack time migration on the first seismic data, and obtain the true position of the seafloor after homing; then convert the time domain profile into a depth domain profile; finally, pick the seafloor and construct a seafloor velocity model based on its depth;

[0009] Step 3: Determine the input parameters of the forward modeling, and calculate the seabed reflection time based on the forward modeling;

[0010] Step 4: Uneven cable correction: Pick up the seabed reflection time and correct it to the seabed reflection time calculated by forward modeling to complete the correction of the uneven cable.

[0011] Furthermore, in step 3, forward modeling is performed based on a ray tracing method, and its input parameters include the one-dimensional position coordinates of the shot points and the detection points converted in step 1.

[0012] Furthermore, in step 3, forward simulation is performed based on the wave equation method, and its input parameters include reconstruction of the seabed velocity model, one-dimensional position coordinates of the shot point and the detection point, grid size, wavelet main frequency and calculation step size, wherein the reconstruction of the seabed velocity model refers to intercepting the seabed velocity model according to the shot point position of each shot and the detection point position.

[0013] Furthermore, in step 1, the end point of the survey line close to the initial blasting point is used as the starting point of the one-dimensional coordinate system, and the distances of the blasting point and the detection point of each shot relative to the starting point, as well as the distance of the seabed position relative to the starting point, are calculated to construct a unified one-dimensional coordinate system.

[0014] Furthermore, after the uneven cable correction is completed in step 4, the step of quality control after the uneven cable correction is also included, which is specifically:

[0015] The effect of uneven cable correction is verified by comparing CMP gathers with stacked profiles. After correction, the CMP gathers are flattened at flat seabed locations, the signal-to-noise ratio of the stacked profiles is improved, and the continuity and clarity of the formations are improved.

[0016] Furthermore, in step 4, assuming that the picked seabed reflection time is T, and the seabed reflection time calculated by forward simulation is T1, the correction amount is T2=T1-T, and the correction of the uneven cable can be achieved by moving each shot data by T2.

[0017] Compared with the prior art, the advantages and positive effects of the present invention are:

[0018] The seismic uneven cable correction method proposed in this scheme is based on the actual seabed acquisition, constructs a seabed velocity model, and then obtains the seabed reflection arrival time when the cable floats on the sea surface through forward simulation, and then corrects the actual acquired seabed reflection time to the theoretically calculated seabed time to achieve the purpose of uneven cable correction. This method can effectively overcome the defect that the phase axis is difficult to accurately align when the effective wave is superimposed, achieve more accurate velocity pickup, and obtain better superposition imaging effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the generation principle of cable ghost reflection and its performance on the shot gather according to the embodiment of the present invention;

[0020] Figure 2 This is a schematic diagram of the uneven cable correction process according to an embodiment of the present invention;

[0021] Figure 3This is a schematic diagram of constructing a velocity model using the picked seabed according to an embodiment of the present invention;

[0022] Figure 4 A schematic diagram of seabed reflection time obtained by forward simulation according to an embodiment of the present invention;

[0023] Figure 5 A schematic diagram showing the comparison of CMP gathers before and after cable unevenness correction according to an embodiment of the present invention;

[0024] Figure 6 It is a schematic diagram for comparing superimposed cross sections before and after cable unevenness correction according to an embodiment of the present invention. DETAILED DESCRIPTION

[0025] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein, and therefore, the present invention is not limited to the specific embodiments disclosed below.

[0026] First, the principle of cable virtual reflection is explained:

[0027] The most obvious manifestation of cable unevenness in seismic gun gathering is the presence of cable ghost reflections: Figure 1 As shown in the figure, the right figure shows the seismic shot gathering when the cable is uneven, where ① is the seabed reflection, corresponding to Figure 1 The earthquake propagation path ① in the left figure. ② is the cable virtual reflection reflected by the seabed, corresponding to Figure 1 The seismic propagation path ② in the left figure. Obviously, the time taken by path ① is less than that of path ②, so the seabed reflection arrives first in the seismic shot set, and the cable virtual reflection of the seabed reflection arrives later. Since the far end is often buried deeper when the cable is uneven, the difference between the seabed reflection and the cable virtual reflection of the seabed reflection is greater, which is manifested as the difference in time between the cable virtual reflection of the seabed reflection and the seabed reflection will increase with the increase of the cable distance. When the cable is uneven, the CMP gather after dynamic correction no longer conforms to the law of in-phase superposition, so it is necessary to correct the cable to the sea surface when the cable is uneven.

[0028] The present invention proposes a method for correcting a high-resolution seismic uneven cable, and the correction idea is as follows:

[0029] Since the actual seabed for collecting seismic data is relatively clear and the results are relatively reliable, the present invention constructs a seabed velocity model based on this, and then obtains the seabed reflection arrival time when the cable floats on the sea surface through forward simulation. The actual collected seabed reflection time is then corrected to the theoretically calculated seabed time to achieve the purpose of uneven cable correction. The fundamental purpose of the above idea is to correct the uneven cable to the sea level so that it meets the assumptions of high-resolution cable collection. The process is as follows Figure 2 As shown, the specific steps include:

[0030] Step 1: Construct a one-dimensional coordinate system and unify the velocity model and the shot point detection point positions into the one-dimensional coordinate system;

[0031] After the observation system is established, the coordinates of the shot point, the receiver point and the seabed position can be determined, but at this time they are two-dimensional coordinates. The model construction will be more complicated during the forward simulation calculation, so the two-dimensional coordinates are converted to one-dimensional coordinates. The key to converting two-dimensional coordinates to one-dimensional coordinates is to determine the starting point of the one-dimensional coordinates. The starting point of the one-dimensional coordinates is generally the end point of the survey line close to the initial shot point. The distances of the shot point and the receiver point of each shot in the observation system relative to the starting point, as well as the distance of the seabed position relative to the starting point, can be calculated to construct a unified one-dimensional coordinate system.

[0032] Step 2: construct a seabed velocity model;

[0033] According to practical experience, the front cable often floats on the sea surface, and the seabed reflection is clear, and this embodiment constructs the velocity model based on this.

[0034] First, select the first seismic data and perform post-stack time migration on the first data to obtain the true position of the seafloor after homing. Post-stack time migration can choose Kirchhoff time migration, FX domain time migration or STOLT time migration. The key to migration is to give a velocity model. Since only the seafloor is migrated, it is only necessary to establish a velocity model according to the seawater velocity of 1500m / s.

[0035] Then the time domain profile is converted into a depth domain profile, and the calculation formula is D = T*1.5 / 2, where T is the time domain profile and D is the depth domain profile;

[0036] Finally, pick up the seabed and build a seabed velocity model based on its depth. For example, a velocity of 1500m / s is given above the seabed, and a velocity of 2000m / s can be given below the seabed.

[0037] Step 3, determine the input parameters of the forward modeling, and calculate the seabed reflection time based on the forward modeling;

[0038] (1) The most critical step in forward modeling is to determine the input parameters of the forward modeling. Forward modeling can be done by wave equation forward modeling and ray tracing methods.

[0039] If wave equation forward modeling is used, the most critical input parameter is the reconstruction of its velocity model. Because wave equation forward modeling has higher accuracy, but the calculation efficiency is too low, it is difficult to use the entire velocity model for calculation in actual work. Generally, it is necessary to intercept the seabed velocity model according to the position of each shot point and the position of the detection point to improve the calculation efficiency. The remaining parameters such as the positions of the shot point and the detection point use the one-dimensional coordinate position converted in step 1, the grid size is set to the spacing of the CMP, the main frequency of the wavelet should be consistent with the main frequency of the actual source wavelet collected, and the calculation step size is the time interval of the actual data collection. If the ray tracing method is used, due to its relatively high calculation efficiency, the entire seabed velocity model is generally used. It is only necessary to determine the coordinates of the shot point and the detection point position for calculation. Its position is the one-dimensional coordinate position converted in step 1.

[0040] (2) After the forward modeling input parameters are determined, the seabed reflection time can be calculated by forward modeling according to the adopted ray tracing method or wave equation forward modeling method. These methods are specific applications of mature technologies in the present invention and the details will not be described in detail.

[0041] Step 4: Uneven cable correction;

[0042] Pick up the seabed reflection time and correct it to the seabed reflection time calculated by forward simulation to complete the correction of uneven cables. Assume that the picked seabed reflection time is T, and the seabed reflection time calculated by forward simulation is T1, then the correction amount is T2 = T1-T, and the uneven cable correction can be completed by moving each shot data by T2.

[0043] Step 5: Quality control after uneven cable correction;

[0044] The effect of uneven cable correction is mainly verified by comparing CMP gathers and stacked profiles. The CMP gathers after uneven cable correction should be flattened at flat seabed locations. The stacked profile after uneven cable correction has a higher signal-to-noise ratio and the strata are more continuous and clear.

[0045] Specific examples

[0046] The high-resolution small multi-channel seismic data collected in a deep sea area has obvious cable unevenness. According to the solution of the patent of this invention, the cable is corrected to the sea level. In this case, the ray tracing method is used to calculate the seabed reflection time. After constructing a one-dimensional coordinate system, the constructed velocity model is as follows Figure 3 As shown in the figure, the circle is the picked seafloor, and the velocity is set to 1500m / s. Since only the seafloor reflection needs to be simulated during forward modeling, the velocity model is relatively simple. After constructing the velocity model, the positions of the shot points and receiver points are input, and the seafloor reflection time is calculated using the ray tracing method, and then superimposed on the actual collected seismic shot set, as shown in the figure. Figure 4As shown, it can be seen that the travel time obtained by the forward simulation is approximately half of the seabed reflection time and the cable virtual reflection time of the seabed reflection. Then the seabed reflection is picked up and corrected to the theoretical calculation time to complete the correction of the uneven cable. Figure 5 This is a comparison of the CMP gathers before and after the cable unevenness correction. It can be seen that the seabed of the CMP gathers without cable unevenness correction is obviously uneven, while the CMP gathers are flat after the cable unevenness correction. Figure 6 It is a comparison of the stacked sections before and after the cable unevenness correction. After the cable unevenness correction, the stacked sections have a higher signal-to-noise ratio and the strata are more continuous and clear.

[0047] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for correcting a high-resolution seismic unevenness cable, characterized in that: The following steps are involved: Step 1: Construct a one-dimensional coordinate system, take the end point of the survey line close to the initial blast point as the starting point of the one-dimensional coordinate system, calculate the distances of the blast point and the detection point of each blast relative to the starting point, and the distance of the seabed relative to the starting point; unify the coordinates of the seabed position and the blast point and detection point positions into the one-dimensional coordinate system; Step 2: Construct seabed velocity model: First, select the first seismic data, perform post-stack time migration on the first seismic data, and obtain the true position of the seafloor after homing; then convert the time domain profile into a depth domain profile; finally, pick the seafloor and construct a seafloor velocity model based on its depth; Step 3, determining the input parameters of the forward modeling, wherein the input parameters include the one-dimensional position coordinates of the shot points and the detection points converted in step 1, and calculating the seabed reflection time based on the forward modeling; Step 4: Uneven cable correction: Pick up the seabed reflection time and correct it to the seabed reflection time calculated by forward modeling. Suppose the picked seabed reflection time is T, and the seabed reflection time calculated by forward modeling is T1. The correction amount is T2=T1-T. The correction of the uneven cable can be completed by moving each shot data by T2.

2. The method for correcting a high-resolution seismic unevenness cable according to claim 1, characterized in that: In step 3, forward modeling is performed based on the ray tracing method, and its input parameters include the one-dimensional position coordinates of the shot points and the detection points converted in step 1.

3. The method for correcting a high-resolution seismic unevenness cable according to claim 1, characterized in that: In step 3, forward simulation is performed based on the wave equation method, and its input parameters include reconstruction of the seabed velocity model, one-dimensional position coordinates of the shot point and the detection point, grid size, wavelet main frequency and calculation step size, wherein the reconstruction of the seabed velocity model refers to intercepting the seabed velocity model according to the position of each shot point and the detection point.

4. The method for correcting a high-resolution seismic unevenness cable according to claim 1, characterized in that: After the uneven cable correction is completed in step 4, the uneven cable correction quality control step is also included, which is specifically: The effect of uneven cable correction is verified by comparing CMP gathers with stacked profiles. After correction, the CMP gathers are flattened at flat seabed locations, the signal-to-noise ratio of the stacked profiles is improved, and the continuity and clarity of the formations are improved.

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