Cable depth control system and method for high-resolution small multi-channel seismic acquisition

Abstract

The invention belongs to the technical field of marine seismic data acquisition, and particularly relates to a cable depth control system and method for high-resolution small multi-channel seismic acquisition. The design of a depth-keeping control system combining buoys in a head buoy system, a middle buoy system and a tail buoy system and a lead sheath roll counter weight is combined, and the influence of external force fluctuation on the depth of the cable is weakened through a corresponding depth-keeping control method, so that the purpose of accurately controlling the depth of the cable is achieved. During specific implementation, construction and operation are convenient, the cost is low, the small multi-channel seismic acquisition cable can be stabilized at the specific water depth position, the optimal acquisition result profile is obtained, and high practical application value and application effect are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of marine seismic data acquisition equipment, and in particular relates to a cable depth control system and method for high-resolution small multi-channel seismic acquisition. Background Art

[0002] In high-resolution small multi-channel seismic acquisition, the cable length is generally within 300 meters. In the existing technology, small multi-channel acquisition is basically to place it directly in the seawater and let the construction ship tow it, or to use a buoy to float the cable directly on the sea surface ( Figure 3 (b)). In this case, since the cable is affected by factors such as ocean currents during the data collection process, it is difficult to maintain a stable posture in the seawater, and generally there will be tilted cables, irregular twisted cables, etc. ( Figure 3 (b), (c)); Cable tilt or twist seriously affects the quality of seismic data. For example, when the cable is tilted, the cable ghost reflection increases with the increase of offset distance, causing the ghost reflection to be completely separated from the effective signal ( Figure 4 ), this separated wavelet will bring great trouble to the subsequent seismic data processing and seriously affect the data processing effect.

[0003] Although there are related system designs for cable depth control in the prior art, such as the water bird system, the water bird system is expensive, with a set of water bird systems starting at at least 700,000 yuan. Expensive water bird equipment is mainly used in multi-channel seismic acquisition for oil and gas exploration. For small multi-channel acquisition, the overall cost is too high, and the maintenance cost of the water bird system after damage is also very high, which is difficult to match the benefits of the small multi-channel acquisition system. In addition, small multi-channel seismic has higher requirements for depth setting during acquisition. Even if the water bird system is used, it is difficult for the water bird to solve the cable depth control requirements of small multi-channel seismic because the cable is too short, especially the front half of the cable is greatly affected by the tension of the ship. At this time, it is difficult for the water bird system to control the sinking depth of the cable and to keep the cable stable.

[0004] In summary, for small multi-channel seismic acquisition, it is urgent to design a matching cable depth control system and method to solve the cable posture problem during seismic acquisition so that the cable can remain stable at a certain depth below the sea surface. Summary of the invention

[0005] In order to solve the problems of unstable cable depth control and serious impact on seismic data quality in traditional small multi-channel seismic acquisition, the present invention proposes a cable depth control system and method for high-resolution small multi-channel seismic acquisition. By designing a head float system, a middle float system and a tail float system and a combination of corresponding floats and lead coil weights in each system, precise control of the depth of the small multi-channel seismic acquisition cable can be achieved, which can effectively ensure the quality of small multi-channel seismic data acquisition.

[0006] The present invention is implemented by adopting the following technical scheme: a cable depth control system for high-resolution small multi-channel seismic acquisition, comprising a head float system, a tail float system and a plurality of intermediate float systems arranged therebetween;

[0007] The head float system includes a head float and a head lead reel, the head lead reel is arranged at the front of the cable and far away from the detector, the head float and the head lead reel are connected by an auxiliary rope, and the buoyancy of the head float is at least twice the weight of the head lead reel; the tail float system includes a tail float and a tail lead reel, the tail lead reel is arranged at the end of the cable, the tail float and the tail lead reel are connected by an auxiliary rope, and the tail float system has the same parameter configuration as the head float system;

[0008] The intermediate float system includes an intermediate float and an intermediate lead roll. The intermediate float and the intermediate lead roll are connected by an auxiliary rope. The head float, the middle float and the tail float are connected by a Kevlar rope. The closer to the front of the cable, the heavier the weight of the intermediate lead roll. The buoyancy of the intermediate float is at least twice the weight of the corresponding intermediate lead roll.

[0009] Furthermore, the weight of the first lead coil is determined by:

[0010] (1) When a small multi-channel seismic cable moves in seawater, it is subject to resistance, tension and lift:

[0011] The resistance is: The lift force is: The pulling force is: The vertical component of the tension is: F 拉垂直分量 =F 拉 sinθ;

[0012] Among them, C d is the drag coefficient, ρ is the seawater density, D is the cable diameter, is the speed of the cable relative to the fluid, L is the cable length, C is the lift coefficient, θ is the cable inclination angle, h is the height between the ship deck and the sea surface, and x is the length of the cable released from the stern;

[0013] (2 Considering that the small multi-channel cable should be in a horizontal state when working, the lift force is zero and the upward pulling force is F 拉垂直分量 , then the weight of the first lead roll is at least F 拉垂直分量 twice as much.

[0014] Furthermore, the weight of the middle lead roll is determined in the same principle as the first lead roll, with the difference being that the cable length becomes smaller and the inclination angle becomes smaller, but the inclination angle of the middle part of the cable is required to be no greater than 5°.

[0015] The present invention further provides a control method for a cable depth control system for high-resolution small multi-channel seismic acquisition, comprising the following steps:

[0016] Step A: Cable release:

[0017] Fully unfold the cable and Kevlar rope, align their heads and tails, and mark the positions of the first lead roll, each middle lead roll, and the tail lead roll on the cable, and mark the corresponding positions of the first float, each middle float, and the tail float on the Kevlar rope;

[0018] Synchronously release the cable and Kevlar rope outwards, install the corresponding float and lead roll respectively when encountering colored marks, and connect the corresponding float and lead roll with auxiliary ropes; after the Kevlar rope and cable are released, tighten them at the same time, and the cable release work is completed after they are fixed;

[0019] Step B: Seismic data acquisition:

[0020] Conduct acquisition experiments to determine whether the depth control system design is reasonable. If the seabed reflection of the acquired shot gather is abnormal or the event axis is tilted or twisted on the seabed of the CMP gather, it indicates that the cable is not level during acquisition. Then adjust the spacing of the intermediate float system based on the actual sea trial to ensure that the acquired seismic data meets the engineering quality standards.

[0021] Step C: Cable recycling:

[0022] After the seismic data collection is completed, the Kevlar rope and cable are recovered simultaneously, and the corresponding float and lead reel are removed until the cable is recovered.

[0023] Compared with the prior art, the advantages and positive effects of the present invention are:

[0024] This scheme designs a cable depth control system composed of a float and a counterweight and its corresponding control method, which increases the ratio between buoyancy and gravity, thereby weakening the influence of external force fluctuations on the cable depth and realizing precise control of the cable depth. The cable depth control effect is better than that of the water bird system, and the cost is much lower than that of the water bird system, the subsequent maintenance cost is low, and during the collection process, there is no need to set and adjust a series of related parameters like the water bird system, which reduces the difficulty of use and has wide promotion and application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of the structure of a small multi-channel cable depth control system according to an embodiment of the present invention;

[0026] Figure 2 It is a schematic diagram of the force when a small multi-channel cable is stably collecting data in an embodiment of the present invention;

[0027] Figure 3Several cable postures during small multi-channel seismic acquisition, where (a) is the ideal state, (b) is the tilted state, (c) is the twisted state, and (d) is the sea surface state;

[0028] Figure 4 This is a schematic diagram of virtual reflections caused by cable tilt during small multi-channel seismic acquisition according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] 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. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0030] Example 1: Cable depth control system for high-resolution small multi-channel seismic acquisition. For small multi-channel seismic acquisition, in order to ensure that the cable reaches an ideal state during the acquisition process ( Figure 3 (a)) requires consideration of many factors and comprehensive consideration.

[0031] The basic principle is as follows: During field data collection activities, the stress conditions of the cable in seawater are as follows: Figure 2 As shown, the upward buoyancy comes from the natural lifting effect of the cable after it is immersed in seawater, while the gravity comes from the mass of the cable itself and is in a downward direction. The cable commonly used is designed as a neutral buoyancy cable, which means that in seawater, the gravity and buoyancy of the cable offset each other, making it suspended. The tension comes from the traction of the operating vessel when dragging the cable. This force usually has a certain inclination angle due to the relative position difference between the vessel and the cable in the water. The resistance is the reaction force generated by the cable to resist the tension, pointing in the opposite direction of the drag. During the acquisition process, the cable will also generate lift when it is tilted. The key factor that makes the cable float up and down is the vertical force, of which the upward vertical force includes the vertical components of lift, buoyancy, and tension, and the downward vertical force includes the cable weight and the counterweight weight. If the upward vertical force > the downward vertical force, the cable will move upward, and vice versa.

[0032] When a small multi-channel seismic cable moves in seawater, the resistance it encounters is calculated using formula (1):

[0033]

[0034] Among them, C d is the resistance coefficient. Under ideal conditions, when the cable is dragged in the seawater, the seawater is approximately in a state of advection, and the resistance coefficient is small (the actual value needs to be measured based on the actual cable used). Let C d =0.01, ρ is the fluid density (seawater density ρ = 1025 kg / m3 About), D is the cable diameter, is the speed of the object relative to the fluid, and L is the length of the cable.

[0035] The lift calculation formula is:

[0036]

[0037] Among them, C is the lift coefficient, which is related to the angle of attack. When the angle of attack is close to 0, the lift coefficient is usually close to 0. As the angle of attack gradually increases, the lift coefficient begins to rise slowly. When the angle of attack increases from 0 to 5°, the lift coefficient may increase linearly from 0 to about 0.1-0.2. When the angle of attack increases from 5° to 15°, the lift coefficient may increase from about 0.1 to about 0.5 within this angle of attack range. Therefore, the lift coefficient is generally between 0-0.5 (the actual situation may vary due to factors such as cable surface characteristics). D is the cable diameter, is the speed of the object relative to the fluid, L is the cable length, and ρ is the density of seawater.

[0038] The formula for calculating the pulling force is:

[0039]

[0040] Where θ is the tilt angle ( Figure 2 ). The cable is roughly in the shape of a parabola from the stern to the end. To simplify the calculation, this embodiment approximates it to a triangle, so there is

[0041]

[0042] Where h is the height between the ship and the sea surface, and x is the length of the towline released from the stern.

[0043] The vertical component of tension is calculated as:

[0044] F 拉垂直分量 =F 拉 sinθ (5)

[0045] Because F 重 =F 浮 , F 升 It is zero when the cable is horizontal. It will only occur when the cable is tilted by external forces (such as waves). The deformation of the cable is only a small change, and its force is much smaller than the influence of cable tension, that is, the influence of cable buoyancy is not large. Therefore, the main factor affecting the lifting of the cable is F 拉垂直分量 . G 配重 The upward pulling force needs to be balanced to keep the cable stable.

[0046] In the actual marine environment, it is inevitable to encounter the influence of ocean currents, which will increase the relative speed of the cable (upstream) or decrease (downstream). According to Formula 1, if the upstream velocity is greater than the downstream velocity, the upstream resistance is significantly greater than the downstream velocity, resulting in a significant increase in the tension during the upstream operation, making the cable almost close to the sea surface; on the contrary, when operating downstream, the cable may sink below the sea surface, with a depth of up to 2 meters or even deeper, which directly affects the quality of the acquisition profile and the difficulty and uniformity of subsequent data processing, and cannot meet the needs of actual engineering geology. In the current field acquisition practice, when only the cable is in the water, it is very susceptible to external interference, and there is a lack of effective mechanisms to balance the ups and downs caused by different tension changes. Therefore, the core strategy of the innovation of this embodiment is the design of the float and the lead reel weight, which increases the ratio between buoyancy and gravity, that is, by designing the head float system, the middle float system and the tail float system, and combining the head float system, the middle float system and the lead reel weight in the tail float system to design a fixed depth control system, so as to weaken the influence of external force fluctuations on the cable depth, thereby achieving the purpose of precise control of the cable depth.

[0047] from Figure 2 From the stress conditions and formula 1-5, it can be seen that the main factor affecting the balance of the cable is the inclination angle. When the inclination angle is zero, F 拉垂直分量 and F 升 are all zero, at which point the cable will be stably sunk to a certain depth. Considering the actual construction conditions, the multi-channel cable depth control system designed in this embodiment according to this principle is as follows: Figure 1 As shown, specifically:

[0048] The cable depth control system includes a bow float system, a tail float system and multiple intermediate float systems arranged therebetween, the bow float system includes a bow float and a bow lead reel, the bow lead reel is arranged at the front of the cable and away from the detector, the bow float is connected to the bow lead reel by an auxiliary rope, the tail float system includes a tail float and a tail lead reel, the tail lead reel is arranged at the end of the cable, the tail float is connected to the tail lead reel by an auxiliary rope, the intermediate float system includes an intermediate float and an intermediate lead reel, the intermediate floats are connected to the intermediate lead reels by an auxiliary rope, the bow float is connected to the ship by a Kevlar rope, and the bow float, the intermediate float and the tail float are connected by a Kevlar rope. Among them, each float mainly plays the role of providing buoyancy. The buoyancy provided by the float on the water surface needs to be much greater than the gravity of the entire device. The float is shuttle-shaped to minimize the resistance of seawater. The float needs to meet the minimum buoyancy requirements. The main function of the lead reel is to provide counterweight. The heavier head float system is designed to balance the inclination angle, while the middle float system at the back is relatively light, which mainly plays the role of balancing the instantaneous force exerted on the cable, so that the cable can always be stable at a certain depth during the collection process. The tail float system is equivalent to the head float system.

[0049] Specifically, for each floating system, among them:

[0050] (1) First buoy system: Since the ultimate goal of the scheme is to basically maintain the horizontal state of the cable, the lift force is zero at this time, and the upward pulling force is mainly F 拉垂直分量 This component is calculated by formula 1,3-5, from which the minimum weight of the lead roll can be obtained, that is, the minimum weight of the first lead roll is equal to the vertical component F of the upward pulling force 拉垂直分量 Since the first float system is located at the front of the cable, far away from the detector, it has little effect on the actual acquisition. In order to better balance the impact of instantaneous external force, the actual lead roll weight is at least twice the calculated weight, and the buoyancy of the first float is at least twice the weight of the first lead roll.

[0051] (2) Intermediate buoy system:

[0052] Intermediate float spacing: The smaller the intermediate float spacing, the better the cable depth control effect. However, too much equipment not only increases the acquisition cost, but also has a certain impact on the acquisition because the intermediate lead coil is close to the detector. Therefore, the spacing determination method is as follows: the float system is arranged at intervals of 50m, 100m and 150m respectively, and the same survey line is collected. The intermediate float spacing is determined by comparing the seismic data after collection. Based on the comprehensive efficiency and acquisition effect, 100m or 150m spacing is preferably used.

[0053] Middle float weight: Different from the first float system, the middle float mainly responds to the cable tilt caused by instantaneous tension. Assuming that the tension causes the cable to tilt and produces an instantaneous inclination angle, the maximum weight of the lead roll can be calculated based on this. The specific design is similar to the weight design of the first lead roll, and it is calculated using formulas 1, 3-5. The difference is that as the cable length L becomes smaller, the inclination angle becomes smaller, and it is generally assumed that the maximum inclination is 5°. Therefore, the closer to the front of the cable, the greater its weight, and the closer to the back, the smaller its weight. The weight of the middle lead roll is also at least twice the calculated weight, and the buoyancy of the middle float is at least twice that of the corresponding middle lead roll.

[0054] (3) Tail float system: The tail of the cable is easily affected by turbulence and thus sways. The tail float needs to keep the cable straight, so it needs a larger weight. In this embodiment, the same parameters as the head buoy system are used.

[0055] Embodiment 2: A method for controlling the depth of a small multi-channel seismic acquisition cable based on the solution described in Embodiment 1, specifically comprising the following steps:

[0056] Step A: Cable release:

[0057] Fully unfold the cable and Kevlar rope, and align their heads and tails, and use colored tape to mark the positions of the first lead roll, the middle lead rolls and the tail lead roll on the cable, and mark the corresponding positions of the first float, the middle float and the tail float on the Kevlar rope; simultaneously release the cable and Kevlar rope outwards, and install the corresponding floats and lead rolls when encountering colored marking tapes, and connect the corresponding floats and lead rolls with auxiliary ropes; after the Kevlar rope and cable are released, tighten them at the same time, and the cable release work is completed after they are fixed.

[0058] Step B: Seismic data acquisition:

[0059] First, conduct an acquisition experiment to determine whether the design of the depth control system is reasonable. During acquisition, first observe the acquired shot gathers to see whether the reflection of the seabed is smooth and continuous. If there are abnormal phenomena such as distortion, it means that the cable is still not flat during acquisition. Secondly, after the experimental survey line is completed, perform routine data processing on the survey line to observe whether the seabed of the CMP gathers after dynamic correction is flat. If there are phenomena such as inclination or distortion of the phase axis, it means that the cable is not flat during acquisition. Whether there are abnormal phenomena in the shot gathers or CMP gathers, it is necessary to locate the position of the abnormal section and eliminate reasons such as operational errors. Specifically, the spacing of the intermediate float system can be adjusted in combination with the actual sea trial. Generally, measures such as increasing the spacing of the depth control system can be taken to ensure that the acquired seismic data meets the engineering quality standards.

[0060] Step C: Cable recycling:

[0061] After the seismic data collection is completed, the Kevlar rope and cable are recovered simultaneously. The connector is untied at the lead roll location, the winding tape is loosened, and the lead roll is separated until the cable is recovered.

[0062] Specific examples:

[0063] In order to understand this scheme more clearly, we take the case where the cable length is 300m, the distance between the middle floats is 150m, and the cable diameter is 5cm as an example. Assume that the maximum seawater velocity during the reverse flow is 7 knots (about 3.5m / s), and the seawater density ρ = 1025kg / m 3 The height between the ship and the sea surface is 3-5m, and the towline is generally 20-30m behind the ship.

[0064] According to formula 4, the inclination angle range is 8.53-9.46°. The larger the inclination angle, the larger the F 拉垂直分量 The larger the angle, assuming the maximum inclination angle is 10°, the resistance of the first float is approximately calculated according to formula 1:

[0065] F 阻 =0.5*1025*0.01*3.5 2*300*0.05=941N

[0066] According to formulas 3 and 5, the vertical component of the tension is calculated as

[0067] F 拉垂直分量 =F 阻 *tan(10°)=165N

[0068] At this time, the weight of the first lead coil is approximately 165 / 10*2=33KG, and the buoyancy of the first float is designed to be 66KG.

[0069] The middle float is located exactly in the middle of the cable (if the distance between the middle floats is 100m, two middle float systems are required), then:

[0070] F 阻 =0.5*1025*0.01*3.5 2 *150*0.05=470N

[0071] F 拉垂直分量 =F 阻 *tan(5°)=41N

[0072] The weight of the middle lead coil is 41 / 10*2=8.2kg, and the middle float is designed to be 16.4kg; the tail float is designed with the same parameters as the head float. Through specific implementation on site, this solution method can achieve better cable depth control effect and effectively improve the quality of small multi-channel seismic data acquisition.

[0073] 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 cable depth control system for high-resolution small multi-channel seismic acquisition, characterized in that: It includes a front float system, a rear float system and a plurality of intermediate float systems arranged therebetween; The bow float system includes a bow float and a bow lead coil, the bow lead coil is arranged at the front of the cable and away from the detector, the bow float and the bow lead coil are connected by an auxiliary rope, and the buoyancy of the bow float is at least twice the weight of the bow lead coil; the tail float system includes a tail float and a tail lead coil, the tail lead coil is arranged at the end of the cable, the tail float and the tail lead coil are connected by an auxiliary rope, and the parameter configuration of the tail float system is the same as that of the bow float system; The intermediate float system includes an intermediate float and an intermediate lead roll. The intermediate float and the intermediate lead roll are connected by an auxiliary rope. The head float, the intermediate float and the tail float are connected by a Kevlar rope. The closer to the front of the cable, the heavier the weight of the intermediate lead roll. The buoyancy of the intermediate float is at least twice the weight of the corresponding intermediate lead roll.

2. The cable depth control system for high-resolution small multi-channel seismic acquisition according to claim 1, characterized in that: The weight of the first lead coil is determined in the following manner: (1) When a small multi-channel seismic cable moves in seawater, it is subject to resistance, tension, and lift: The resistance is: The lift force is: The pulling force is: The vertical component of the tension is: F 拉垂直分量 =F 拉 sinθ; Among them, C d is the drag coefficient, ρ is the seawater density, D is the cable diameter, is the speed of the cable relative to the fluid, L is the cable length, C is the lift coefficient, θ is the cable inclination angle, h is the height between the ship deck and the sea surface, and x is the length of the cable released from the stern; (2 Considering that the small multi-channel cable should be in a horizontal state when working, the lift force is zero and the upward pulling force is F 拉垂直分量 , then the weight of the first lead roll is at least F 拉垂直分量 twice as much.

3. The cable depth control system for high-resolution small multi-channel seismic acquisition according to claim 1, characterized in that: The spacing between the intermediate floats is 50m-150m.

4. The cable depth control system for high-resolution small multi-channel seismic acquisition according to claim 1, characterized in that: The weight of the middle lead roll is determined using the same principle as the first lead roll. The difference is that the cable length becomes smaller and the inclination angle becomes smaller, but the inclination angle of the middle part of the cable is required to be no more than 5°.

5. The control method of the cable depth control system for high-resolution small multi-channel seismic acquisition according to claim 1, characterized in that: The following steps are involved: Step A: Cable release: Fully unfold the cable and Kevlar rope, align their heads and tails, and mark the positions of the first lead coil, each middle lead coil, and the tail lead coil on the cable, and mark the corresponding positions of the first float, each middle float, and the tail float on the Kevlar rope; Synchronously release the cable and Kevlar rope outwards, install the corresponding float and lead roll when encountering a colored mark, and connect the corresponding float and lead roll with an auxiliary rope; after the Kevlar rope and cable are released, tighten them at the same time, and the cable release work is completed after they are fixed; Step B: Seismic data acquisition: Conduct acquisition tests to determine whether the depth control system design is reasonable. If the seabed reflections of the acquired shot gathers are abnormal or the CMP gathers have tilted or twisted events on the seabed, it indicates that the cable was not level during acquisition. Based on the actual sea trials, adjust the spacing of the intermediate float system to ensure that the acquired seismic data meets engineering quality standards. Step C: Cable recycling: After the seismic data collection is completed, the Kevlar rope and cable are recovered simultaneously, and the corresponding float and lead reel are removed until the cable is recovered.

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