Secondary positioning method of dynamic drift of marine vertical cable nodes

Abstract

The present invention discloses a method for dynamic drift secondary positioning of marine vertical cable nodes, which comprises the following steps: defining initial coordinates for the vertical cable nodes, picking the first arrival time of the direct wave, dividing the picked first arrival time into different intervals according to the acquisition time and the spatial position of the shot point, inverting the first arrival time of each interval using the equivalent velocity secondary positioning method, obtaining the coordinates of the different intervals, verifying the rationality of the interval division through the inversion results, and finally obtaining high-precision node coordinates and depths. The present invention takes into account the dynamic changes of the vertical cable in the water, divides the construction process into several time periods, rationally selects shot points of different spatial positions during inversion, performs dynamic secondary positioning using the equivalent velocity secondary positioning method, calculates the coordinates and depth of the vertical cable node in each time period, thereby improving the node positioning accuracy. The present invention is suitable for the precise positioning of vertical cables.

Description

Technical Field

[0001] The present invention belongs to the field of ocean exploration and relates to an ocean positioning method, in particular to a secondary positioning method for dynamic drift of ocean vertical cable nodes. Background Art

[0002] In the oil and gas industry, during vertical cable data acquisition, the actual position of the vertical cable on the seabed often differs significantly from the designed position due to factors such as waves and currents. This is because, unlike land-based VSPs, where the receivers are arranged in a straight line perpendicular to the ground, the vertical cable's position often changes with the seabed currents, causing the coordinates and depth of the receivers to change in real time, resulting in vertical cable node drift. This drift in the vertical cable node can lead to errors in the receiver position, thus affecting the imaging accuracy of the vertical cable data.

[0003] To ensure that the results of vertical cable inspections are closer to the actual location, secondary positioning of the data collected by the vertical cable is required. The secondary positioning methods currently used for vertical cable data are primarily developed from OBS positioning. OBS positioning methods are divided into two categories: acoustic wave positioning and first-arrival wave secondary positioning. Acoustic wave positioning requires a specialized acoustic system and is performed at the collection site, making it costly. Secondary first-arrival wave positioning, on the other hand, utilizes the time of the earthquake's first arrival for positioning, which is low-cost and widely used for secondary positioning of vertical cables.

[0004] The first arrival wave secondary positioning method includes:

[0005] 1. Circle intersection method: Draw a circle with the shot point as the center and the distance from the shot point to the detection point as the radius. The intersection of the circles obtained by three or more shot points is the location of the detection point.

[0006] 2. Nearly regular tetrahedron method: Three shot points need to be selected to form a nearly regular tetrahedron with the detection points to improve positioning accuracy.

[0007] Both of the above methods require careful screening of the calculated data, which is a very large workload and greatly reduces production efficiency.

[0008] Search method: Divide the grid around the initial coordinate point and perform linear dynamic correction (LMN) on each grid point, assuming it represents the actual position. The point with the best initial flattening effect is the desired result. This method is limited by the grid area and cannot locate points outside the grid. If the grid area is too large, the calculation efficiency will be seriously affected.

[0009] Fourth, the surface fitting method: The area around the receiver is divided into a grid. On the common receiver gather, the sum of the squares of the differences between the theoretical traveltimes of each trace from different shots and the actual first arrival traveltimes of the primary positioning point should be minimized. Secondary positioning of the receiver coordinates is achieved through grid node search, least squares surface fitting, and finding the surface extrema. However, this method cannot directly calculate the receiver depth.

[0010] 5. Equivalent velocity secondary positioning method: When solving the coordinates of the detection point, the seawater velocity is added as a variable to the inversion equation group. An equivalent velocity is also obtained in the process of solving the coordinates. This method avoids the uncertainty brought by the artificial setting of the seawater velocity, can obtain three-dimensional coordinates, has high positioning accuracy and fast calculation speed.

[0011] However, the traditional equivalent velocity secondary positioning method for vertical cables currently primarily refers to OBS, assuming that the node remains in the same position after being lowered to the seabed. The vertical cable's posture constantly changes during construction, and if only a single coordinate is located during the entire acquisition process, significant errors will be incurred.

[0012] Based on the defects of the above methods, there is an urgent need for a new method to perform secondary positioning of the information collected by the vertical cable to obtain more accurate data. Summary of the Invention

[0013] The purpose of the present invention is to provide a method for dynamic drift secondary positioning of marine vertical cable nodes, which uses the "equivalent velocity secondary positioning method" for dynamic secondary positioning and calculates the coordinates and depth of the vertical cable nodes in each time period, thereby improving the node positioning accuracy.

[0014] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0015] A dynamic drift secondary positioning method for marine vertical cable nodes is disclosed. The method comprises the following steps: defining initial coordinates for the vertical cable nodes, picking the first arrival time of the direct wave, dividing the picked first arrival time into different intervals according to the acquisition time and the spatial position of the shot point, inverting the first arrival time of each interval using the equivalent velocity secondary positioning method to obtain the coordinates of the different intervals, verifying the rationality of the interval division through the inversion results, and ultimately obtaining high-precision node coordinates and depth.

[0016] As a limitation of the invention, the method comprises the following steps performed in sequence:

[0017] 1. Place the initial coordinates of the vertical cable provided by the acquisition into the seismic data trace header to define the observation system;

[0018] 2. Modify the offset trace header and perform linear time-of-motion correction (LTK).

[0019] 3. Pick the first arrival travel time on the receiver point gather after linear time difference correction;

[0020] Fourth, the first arrival information is divided into different intervals according to the construction time, and each interval is input into the computer program. Given the initial water velocity, the equivalent velocity secondary positioning method is used for inversion. Each detection point obtains multiple position information in different time periods;

[0021] 5. Place the node location information into the seismic data header corresponding to the construction period to complete the secondary positioning processing of the single node;

[0022] 6. Modify the shot offset header according to step 2, perform linear time difference correction, and check the leveling of the direct wave first arrival. If the direct wave first arrival can be leveled, the secondary positioning result is more accurate. Then, perform steps 1 to 6 on all nodes on the vertical cable to complete the drift correction of the vertical cable.

[0023] As a limitation of the present invention: the formula for linear time difference correction of seismic traces with different shot offsets in step 2 is:

[0024]

[0025] Where, T x is the travel time of the reflected seismic wave, X is the offset, and V is the linear time difference correction speed. As a limitation of step 2 of the present invention: before the linear time difference correction in step 2, the offset is modified to

[0026]

[0027] Where S is the shot point, R is the receiver point, SR' is the horizontal distance from the shot point to the receiver point, and RR' is the depth of the receiver point minus the depth of the shot point.

[0028] As a limitation of step 4 of the present invention: the specific method of secondary positioning by the equivalent velocity secondary positioning method in step 4 is: the picked first arrival is divided into intervals according to the acquisition time, and a set of coordinates and water depths of the nodes are obtained in each time interval. The first arrival travel time, shot point coordinates and shot point water depth of the same time interval are substituted into the following equation group

[0029]

[0030] In the formula, (x, y, z) are the coordinates of the node to be taken, (x n 、y n 、z n ) is the coordinate of the shot point, v is the velocity of seawater, t n The first arrival time.

[0031] As a further limitation of step 4 of the present invention: the node coordinates and seawater velocity are obtained in the following manner: given the initial node coordinates and initial seawater velocity, the overdetermined equations are solved using the Newton iteration method.

[0032] Due to the adoption of the above technical solution, the present invention has achieved the following technical advancements compared with the prior art:

[0033] The present invention uses the vertical cable deployment coordinates provided by the collection to define the observation system and perform linear time difference correction on the common detection point channel set. Through the dynamic secondary positioning processing of the vertical cable by the present invention, it can be timely discovered whether the coordinates and water depths of the same node in different construction time periods have changed significantly. If there is a significant change, it means that the posture of the vertical cable is indeed changing dynamically during the construction process; the present invention can detect whether the distance of all secondary positioning coordinates has moved compared with the initial deployment coordinates. If there has been a movement, it means that the vertical cable has been affected by the ocean current or the ship speed during the process of being deployed and sunk to the seabed, and the position has changed significantly. The secondary positioning of the present invention can correct the above-mentioned posture changes and position changes to obtain a more accurate position of the node. Therefore, the present invention can better solve the problem of node coordinate error caused by the dynamic drift of the vertical cable.

[0034] In summary, this invention takes into account the dynamic changes of vertical cables in water, divides the construction process into several time periods, rationally selects shot points at different spatial locations during inversion, and uses the equivalent velocity secondary positioning method for dynamic secondary positioning. The coordinates and depths of the vertical cable nodes in each time period are calculated, thereby improving the node positioning accuracy. This invention is suitable for the precise positioning of vertical cables. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a line graph of each node after being processed in step 2 in an embodiment of the present invention;

[0036] Figure 2 This is a line graph of each node after being processed in step 3 in an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of the vertical cable construction route according to an embodiment of the present invention;

[0038] Figure 4a Linear time difference correction gathers before secondary positioning of all nodes in the embodiment of the present invention;

[0039] Figure 4b This is the linear time difference correction gather after secondary positioning of all nodes in the embodiment of the present invention. DETAILED DESCRIPTION

[0040] In order to better explain the present invention and facilitate understanding, the present invention is described in detail below through specific implementation methods in conjunction with the accompanying drawings.

[0041] Example 1: Dynamic drift secondary positioning method for marine vertical cable nodes

[0042] This embodiment provides a method for secondary positioning of marine vertical cable nodes by dynamic drift. In this embodiment, secondary positioning is performed on 12 nodes measured on a vertical cable, including the following steps performed in sequence:

[0043] 1. Place the initial coordinates of the vertical cable provided by the acquisition into the seismic data header to define the observation system.

[0044] 2. Modify the shot offset trace header and perform linear time-of-motion correction.

[0045] The formula for linear time difference correction of seismic traces with different offsets in this step is:

[0046]

[0047] Where, T x is the travel time of the reflected seismic wave, X is the offset, and V is the linear time difference correction speed. As a limitation of step 2 of the present invention: before the linear time difference correction in step 2, the offset is modified to

[0048]

[0049] Where S is the shot point, R is the receiver point, SR' is the horizontal distance from the shot point to the receiver point, and RR' is the depth of the receiver point minus the depth of the shot point.

[0050] Using the calculation formula in step 2, in this embodiment, after performing linear time differential correction on all 12 nodes, the following is obtained: Figure 1 As shown, through Figure 1 It can be seen that the direct wave arrivals of all nodes are not flat after linear time differential correction, and the distortion is very serious, indicating that the true coordinates have changed significantly compared with the initial delivery coordinates.

[0051] 3. Pick the first arrival travel time on the detection point gather after linear time difference correction.

[0052] At the beginning of this step, pick up Figure 2 As shown, the picked first arrival must be a direct wave first arrival. First arrivals can be picked through human-computer interaction or automatically. Before picking, they must be identified based on their velocity. Direct wave velocity is around 1500 m / s, while refracted wave velocity is generally greater than 1500 m / s. In deep water, first arrivals at medium and near offsets are generally direct waves.

[0053] Fourth, the initial arrival information is divided into different intervals according to the construction time, and input into the computer program respectively. Given the initial water velocity, the equivalent velocity secondary positioning method is used for inversion, and each detection point obtains multiple position information in different time periods.

[0054] The specific method of secondary positioning by the equivalent velocity secondary positioning method in this step is as follows: the picked first arrival is divided into intervals according to the acquisition time, and a set of coordinates and water depths of the nodes are obtained in each time interval. The first arrival travel time, shot point coordinates and shot point water depth of the same time interval are substituted into the following equation group:

[0055]

[0056] In the formula, (x, y, z) are the coordinates of the node to be taken, (x n 、y n 、z n ) is the coordinate of the shot point, v is the velocity of seawater, t n The first arrival time.

[0057] In this step, the node coordinates and seawater velocity are obtained in the following way: given the initial node coordinates and initial seawater velocity, the overdetermined equations are solved using the Newton iteration method.

[0058] Figure 3 This is a schematic diagram of the vertical cable construction route of this embodiment. The two routes at adjacent times are located on both sides of the vertical cable. If the conditions of the shot points in a time interval are not in a straight line, the two routes can be divided into one time interval. This embodiment divides the construction time of the same node into 15 intervals, and locates the coordinates once in each interval. The same initial coordinates and node water depths are given to different time intervals, and the picked initial arrivals are used for inversion to obtain the coordinates and node depths of different time intervals. The specific data are shown in Table 1

[0059] Table 1

[0060]

[0061] As shown in Table 1, the inverted coordinates and node water depths reflect the dynamic drift of the nodes during the acquisition process; the secondary positioning coordinates are more than 100 meters away from the initial coordinates, indicating that the vertical cable changed position due to the influence of ship speed and ocean currents during the process of being deployed and sunk to the seabed.

[0062] 5. Place the node location information into the seismic data header corresponding to the construction period to complete the secondary positioning processing of a single node.

[0063] 6. Modify the shot offset header according to step 2, perform linear time difference correction, and check the leveling of the direct wave first arrival. If the direct wave first arrival can be leveled, the secondary positioning result is more accurate. Then, perform steps 1 to 6 on all nodes on the vertical cable to complete the drift correction of the vertical cable.

[0064] In this embodiment, the 12 nodes are positioned twice before Figure 4a As shown, after the second positioning is performed through steps one to six, Figure 4b As shown, through Figure 4a and Figure 4b The comparison shows that all nodes are basically leveled at the initial arrival, indicating that the positioning coordinates are highly accurate.

[0065] The above description is only the best description of this embodiment and does not constitute a limitation on the scope of protection of the present invention.

Claims

1. A method for secondary positioning of marine vertical cable nodes by dynamic drift, characterized in that: The dynamic drift secondary positioning method for marine vertical cable nodes is as follows: initial coordinates are defined for the vertical cable nodes, the first arrival time of the direct wave is picked, the picked first arrival is divided into different intervals according to the acquisition time and the spatial position of the shot point, the first arrival time of each interval is inverted using the equivalent velocity secondary positioning method to obtain the coordinates of the different intervals, the rationality of the interval division is verified by the inversion results, and finally high-precision node coordinates and depth are obtained; The method comprises the following steps performed in sequence:

1. Place the initial coordinates of the vertical cable provided by the acquisition into the seismic data trace header to define the observation system; 2. Modify the offset trace header and perform linear time-of-motion correction (LTK).

3. Pick the first arrival travel time on the receiver point gather after linear time difference correction; Fourth, the first arrival information is divided into different intervals according to the construction time, and each interval is input into the computer program. Given the initial water velocity, the equivalent velocity secondary positioning method is used for inversion. Each detection point obtains multiple position information in different time periods; 5. Place the node location information into the seismic data header corresponding to the construction period to complete the secondary positioning processing of the single node; 6. Modify the shot offset trace according to step 2, perform linear time difference correction, and check the leveling of the direct wave first arrival. If the direct wave first arrival can be leveled, the secondary positioning result is more accurate. Then, perform steps 1 to 6 on all nodes on the vertical cable to complete the drift correction of the vertical cable.

2. The method for dynamic drift secondary positioning of marine vertical cable nodes according to claim 1, characterized in that: The formula for linear time difference correction of seismic traces with different offsets in step 2 is: , Where, is the travel time of the reflected seismic wave, is the offset, is the linear time difference correction speed.

3. The method for dynamic drift secondary positioning of marine vertical cable nodes according to claim 2, characterized in that: Before the linear time differential correction in step 2, the offset Modified to , Where, For the gun point, is the detection point, is the horizontal distance from the shot point to the receiver point, It is the depth of the detection point minus the depth of the shot point.

4. The method for dynamic drift secondary positioning of marine vertical cable nodes according to any one of claims 1 to 3, characterized in that: The specific method of the secondary positioning method of equivalent velocity secondary positioning in step 4 is as follows: the picked first arrival is divided into intervals according to the acquisition time, and a set of coordinates and water depths of the node are obtained in each time interval. The first arrival travel time, shot point coordinates and shot point water depth of the same time interval are substituted into the equation group. , Where, is the coordinate of the node to be taken, is the shot point coordinate, is the seawater velocity, The first arrival time.

5. The method for dynamic drift secondary positioning of marine vertical cable nodes according to claim 4, characterized in that: The node coordinates and seawater velocity are obtained in the following manner: given the initial node coordinates and initial seawater velocity, the overdetermined equations are solved using the Newton iteration method.

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