Depth-keeping design method for shallow cable

Abstract

The invention belongs to the technical field of ocean seismic data acquisition, and particularly relates to a depth-keeping design method for a shallow-section cable, which comprises the following steps: firstly, determining the resistance, tension and lift of the cable in seawater, and respectively calculating the required maximum tension under the conditions of no ocean current, countercurrent and downstream; the weight of the depth setting equipment is designed through the obtained maximum tension; two optimization design schemes of auxiliary cable sinking equipment and depth-keeping equipment and single depth-keeping equipment are obtained, a final design scheme is determined by combining a specific sea test experiment, according to the scheme, the profile during collection can be kept consistent, is not influenced by downstream and countercurrent, and is stabilized at a specific water depth position, and the design efficiency is improved. The optimal acquisition result profile can be obtained, and the method has high practical application value and application effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of marine seismic data acquisition, and particularly relates to a depth determination design method for a shallow profile cable. Background Art

[0002] For shallow profile acquisition, in order to obtain the best acquisition effect, the cable needs to be stabilized at a specific depth below the sea surface. Currently, the isofloating cable is mainly sunk to the designated position by adding weights such as lead blocks. This method is relatively simple, but it is difficult to determine the sinking depth of the cable in actual application.

[0003] In addition, the sinking depth of the cable will rise and fall respectively under the influence of ocean currents flowing with and against the current. When the current flows against the current or with the current, the sinking depth of the cable cannot be automatically adjusted, and the lifting and lowering amplitudes are relatively large, which will result in inconsistent appearances of the final acquisition profiles and the acquisition quality not meeting the requirements of later processing and interpretation. Therefore, a depth determination method for shallow profiles is needed to keep the cable at a specific depth during the acquisition process. Summary of the Invention

[0004] In order to solve the defects of the traditional method of sinking the cable by adding weights such as lead blocks, the present invention proposes a depth determination design method for a shallow profile cable. By designing a depth determination device combining a floating ball and a weight, two optimized design schemes are proposed to achieve precise control of the cable depth.

[0005] The present invention is implemented by the following technical solutions: A depth determination design method for a shallow profile cable includes the following steps:

[0006] Step 1: Determine the resistance, tension, and lift force of the cable in seawater;

[0007] Step 2: Determine the relevant parameters of the shallow profile device, and calculate the maximum upward tension in the cases of no ocean current, against the current, and with the current respectively; the relevant parameters include the cable diameter, seawater density, cable length, height between the ship and the sea surface, release length of the towed cable from the stern of the ship, ship speed during acquisition, and ocean current speed;

[0008] Step 3: Design the weight of the depth determination device according to the maximum tension calculated in Step 2, and the maximum tension is equivalent to the vertical component of the tension;

[0009] Step 4: On the basis of Step 3, optimize the depth determination scheme design, including:

[0010] Scheme 1: Auxiliary cable sinking device + depth determination device: Set a guiding steel frame at the stern of the ship to sink the cable to a specific depth below the sea surface at the stern of the ship, with the cable inclination angle being 0, and place the depth determination device behind the geophone section;

[0011] Solution 2: Single depth-fixing device: Place the depth-fixing device directly in front of the geophone section, and achieve a cable tilt angle θ of 0 by balancing the pulling force of the ship and the resistance of the geophone;

[0012] Step 5: Further optimize the depth-fixing design in combination with actual sea trials. Select a sea area with large tidal current changes. According to the tidal current prediction, conduct comparative tests on the seismic profiles collected when there is no tidal current and when the tidal current is large, and determine the final solution. Specifically:

[0013] When there is no tidal current, first collect the standard shallow profile by using the conventional lead-sheath weight method, and then remove the lead-sheath weight and reconfigure according to the two solutions in Step 4 respectively, and conduct collection experiments on the same survey line; when collecting, first ensure the stability of the equipment. If the noise is large, appropriately reduce the weight of the depth-fixing device and keep it away from the geophone section, and continuously adjust the design solution until the quality of the collected profile is comparable to that of the conventional method;

[0014] Then select to collect on the same survey line when the tidal current is large, and compare with the collection effect when there is no sea current in the conventional collection method. If the quality of the two is comparable, it is considered that the design solution meets the actual collection requirements; otherwise, judge whether to increase or decrease the weight of the depth-fixing device according to the profile effect until the collected profile meets the requirements.

[0015] Furthermore, in Step 3, regarding the design of the weight of the depth-fixing device, among them, the float is designed to be 2 - 5 times the maximum pulling force, and the counterweight is designed to be 1 - 3 times the maximum pulling force.

[0016] Furthermore, in Step 1, the specific calculation method is as follows:

[0017] (1) When the cable moves in seawater, the resistance it receives is: Among them, C d is the resistance coefficient, ρ is the seawater density, A is the projected area of the cable perpendicular to the direction of motion, is the relative motion speed of the cable with respect to the fluid;

[0018] (2) The lift force is: Among them, C is the lift coefficient;

[0019] (3) The pulling force is: Among them, θ is the cable tilt angle, h is the height between the ship and the sea surface, and x is the length of the cable released from the stern of the ship; and then the vertical component of the pulling force is obtained as: F 拉垂直分量 = F 阻 tanθ.

[0020] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0021] This solution proposes two optimized design schemes, namely the auxiliary cable sinking equipment + depth fixing equipment and the single depth fixing equipment, by designing a depth fixing device combining a floating ball and a counterweight, and combining the counterweight design of the depth fixing device. The final design scheme is determined through specific sea trial experiments. This solution can keep the profile consistent during acquisition, being unaffected by downstream and upstream currents, and stabilizing at a specific water depth position, enabling the best acquisition result profile. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of the force condition when the shallow profile cable is stably acquired in the embodiment of the present invention;

[0023] Figure 2 It is a schematic diagram of the optimized design scheme in the embodiment of the present invention. (a) is Scheme 1, and (b) is Scheme 2;

[0024] Among them, 1. guiding steel frame; 2. depth fixing device; 3. geophone. Specific Embodiments

[0025] In order to more clearly understand the above-mentioned objects, features, and advantages of the present invention, the present invention will be further described below with reference to the drawings and embodiments. Many specific details are set forth in the following description in order to fully understand 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.

[0026] Embodiment. First of all, it should be noted that in the field data acquisition activity, the force condition of the cable in seawater is as Figure 1 shown. Among them, the upward buoyancy comes from the natural lifting effect after the cable is immersed in seawater, while the gravity comes from the mass of the cable itself. The conventionally used cable is designed as a neutrally buoyant cable, which means that in seawater, the gravity and buoyancy of the cable cancel each other out, making it in a suspended state. The pulling force comes from the traction effect when the working ship drags the cable, and this force usually has a certain inclination angle due to the relative position difference between the ship and the cable in the water. The resistance is the reaction force generated by the cable to resist the pulling force, pointing in the opposite direction of the drag. During the acquisition process, a lifting force will also be generated when the cable is inclined. The key factor for the cable to float up and down is the vertical force. Among them, the upward vertical force includes the lifting force, buoyancy, and vertical component of the pulling force, and the downward vertical force includes the cable gravity and the counterweight gravity. If the upward vertical force > the downward vertical force, the cable will move upward, and vice versa.

[0027] However, in the actual marine environment, it is inevitable to encounter the influence of ocean currents. Ocean currents can increase the relative speed of the cable (against the current) or decrease it (along the current). Since the resistance in the countercurrent state is significantly greater than that in the downstream, the pulling force during countercurrent increases significantly, causing the cable to be almost close to the sea surface. On the contrary, when operating along the current, the cable may sink below the sea surface, with a depth of up to 2 meters or even deeper. This directly affects the quality of the acquisition profile and the difficulty and uniformity of subsequent data processing, and the processing effect is difficult to meet the analysis requirements.

[0028] In current field acquisition practices, it is difficult to precisely control the depth of the cable by directly adding weights to the cable. Analysis shows that the root cause of the problem is that when only the cable is in water, it is extremely vulnerable to external forces and lacks an effective mechanism to balance the up and down fluctuations caused by different changes in pulling forces. Through research, it is found that the ratio between buoyancy and gravity can be increased, that is, by designing a depth-fixing device combining floating balls and weights, the influence of external force fluctuations on the cable depth can be weakened to achieve precise control of the cable depth. Therefore, only by clarifying the influence of each force on the cable can the design scheme of the depth-fixing device be more scientific.

[0029] In addition, since the gravity and buoyancy of the isoneustic cable in seawater are basically equal, the main forces related to the lifting and lowering of the cable are the pulling force and the lifting force. If the angle θ is 0, the vertical component of the pulling force and the lifting force are both zero, and the cable will be in a stable operating state, which is also conducive to achieving depth control of the cable.

[0030] Specifically, this embodiment proposes a depth-fixing design method for a shallow profile cable, including the following steps:

[0031] Step 1: Determine the resistance, pulling force, and lifting force of the cable in seawater;

[0032] It should be noted that according to different model assumption conditions (such as the influence of vertical ocean currents, etc.), the calculation formulas for these forces will become extremely complex, greatly increasing the calculation cost in actual applications. Moreover, the focus of this invention is on the design of the depth-fixing device rather than obtaining precise results for each force. Therefore, in this embodiment, only based on the actual situation, an optimized model is made, and the relevant calculation equations are simplified. Specifically:

[0033] (1) When the cable moves in seawater, the calculation method of the resistance is as follows:

[0034]

[0035] Among them, C d is the resistance coefficient (the towing cable is in the shape of a cylinder, C d ≈0.1), ρ is the seawater density (ρ = 1025 kg / m 3 or so), A is the projected area of the cable perpendicular to the direction of motion, is the relative speed of the cable with respect to the fluid;

[0036] Among them, for a cable in the shape of a cylinder, the projected area A perpendicular to the direction of motion of the cable is expressed as follows:

[0037] A = dL (2)

[0038] where d is the cable diameter and L is the cable length.

[0039] (2) When the cable is completely submerged in seawater during acquisition, according to Archimedes' principle, the buoyant force on the cable at this time is:

[0040] F 浮 = ρVg (3)

[0041] where ρ is the seawater density, V is the volume of seawater displaced, and g is the acceleration due to gravity. Since marine seismic exploration cables are generally designed as neutrally buoyant cables, that is, the gravity and buoyant force are basically equal in seawater. At this time, G 重 = F 浮 ;

[0042] (3) The lift force calculation formula is:

[0043]

[0044] where 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 also 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 linearly increase from 0 to about 0.1 - 0.2; when the angle of attack increases from 5° to 15°, within this range of the angle of attack, the lift coefficient may increase from about 0.1 to about 0.5. Therefore, the lift coefficient is generally between 0 and 0.5 (the actual situation may vary due to factors such as the surface characteristics of the cable), A is the projected area of the cable perpendicular to the direction of motion, ρ is the seawater density, is the relative velocity of the cable with respect to the fluid.

[0045] (4) The tension calculation formula is

[0046]

[0047] where θ is the cable inclination angle. The cable is roughly parabolic in shape from the stern to the end. For simplicity in calculation, it is approximated as a triangle. Then there is

[0048]

[0049] where h is the height between the ship and the sea surface, and x is the length of the cable released from the stern of the ship.

[0050] The calculation formula for the vertical component of the tension is:

[0051] F 拉垂直分量 = F 阻 tanθ (7)

[0052] Since, F 重 = F 浮 and when the cable is in a horizontal state during normal shallow profile acquisition, the lift force is zero at this time. Therefore, the main factor affecting the cable's lifting and lowering lies in F 拉垂直分量 and G 配重 Then it is necessary to balance the upward pulling force so as to maintain the stability of the cable.

[0053] Step 2: Identify the relevant parameters of the specific shallow profile equipment and calculate the maximum required maximum pulling force under the conditions of no ocean current, countercurrent, and downstream;

[0054] Among them, the relevant parameters of the specific shallow profile equipment include cable diameter, seawater density, cable length, height from the ship to the sea surface, release length of the tow cable starting from the stern of the ship, ship speed during acquisition, ocean current speed, etc. Based on these parameters, calculate the maximum upward pulling force under the conditions of no ocean current, countercurrent, and downstream according to Formulas 1 - 7 respectively.

[0055] Step 3: Design the counterweight of the cable depth fixing equipment according to the maximum pulling force calculated in Step 2;

[0056] Generally, it is preferred that the counterweight is designed as 2 times the calculated maximum pulling force, and the floating ball is designed as 4 times the maximum pulling force. Of course, the specific situation can be fine-tuned. A larger counterweight and floating ball of the depth fixing equipment are more conducive to maintaining the stability of the cable equipment, but an overly heavy equipment is not conducive to field construction. In addition, an overly large equipment will cause additional noise, resulting in a larger interference noise for the field acquisition signal.

[0057] Step 4: Optimize the design of the depth fixing scheme based on Step 3;

[0058] According to Formulas 4 and 7, when the inclination angle θ is 0, the cable will be in a stable running state, and at this time the cable will maintain balance, which is conducive to the depth fixing control of the cable. However, during actual acquisition, affected by factors such as instantaneous turbulence and sudden changes in the ship's pulling force, the cable depth will still change. Therefore, the floating ball and counterweight, as a technical means to balance the vertical pulling force, are still essential.

[0059] When solving the problem of cable depth fixing, this scheme designs the scheme based on these two principles, mainly including two schemes: a single depth fixing equipment and an auxiliary cable sinking equipment + depth fixing equipment, as Figure 2 shown:

[0060] Solution 1: By setting a guiding steel frame 1 at the stern of the ship, the cable is sunk to a specific depth below the sea surface at the stern, preferably at a depth of 50 cm. Subsequently, the inclination angle θ is made 0. In this case, the depth-setting device 2 is placed behind the geophone 3, which can reduce the wake noise impact brought by the depth-setting device 2, and this solution can be designed as a lighter counterweight.

[0061] Solution 2: The depth-setting device 2 is directly placed in front of the geophone 3. This solution achieves an inclination angle θ of 0 for the cable by balancing the pulling force of the ship and the resistance of the geophone. The depth-setting device of this solution is simpler, but the counterweight of the depth-setting device in this solution needs to be larger than that of Solution 1, and it may cause relatively large water flow noise at the same time.

[0062] Step 5: Further optimize the depth-setting design in combination with actual sea trials. Compare the seismic profiles actually collected without sea current and with sea current to determine the final solution. Specifically, a sea area with large tidal current changes can be selected. According to the tidal current prediction, comparative tests are carried out respectively when there is no tidal current and when the tidal current is large. The length of the test survey line is generally selected as 2 km, or it is sufficient to compare the acquisition effects. When there is no tidal current, first collect the standard shallow profile by using the conventional lead sheet counterweight method, and then remove the lead sheet counterweight and reconfigure it according to the two sets of solutions in Step 4 respectively, and conduct acquisition experiments on the same survey line respectively. When collecting, it is necessary to ensure the stability of the equipment first, that is, mainly observe whether the floating body is stable during collection. If it is not stable, the stability problem of the depth-setting device needs to be solved first. After the collection is relatively stable, compare the seismic profiles collected by the two sets of solutions with the standard profile. If the noise is too large, the size of the equipment can be appropriately reduced and the section far from the geophone can be adjusted, and the design solution is continuously adjusted until the quality of the collected profile is comparable to that of the conventional method. Then select to collect on the same survey line when the tidal current is large and compare it with the acquisition effect of the conventional acquisition method when there is no sea current. If the quality of the two is comparable, it is considered that the design solution meets the actual acquisition requirements; otherwise, increase or decrease the counterweight according to the profile effect until the collected profile meets the requirements.

[0063] Specific case:

[0064] One of the most commonly used acquisition parameters for shallow profile acquisition. The length of the shallow profile cable is generally 3 m, the cable diameter is 5 cm, the standard conventional acquisition speed is 5 knots (about 2.5 m / s), the seawater density ρ = 1025 kg / m 3 , the height between the ship and the sea surface is 3 - 5 m, and the towed cable is generally at a position 20 - 30 m behind the ship. Calculate the vertical forces in the cases of no sea current, countercurrent, and downstream respectively.

[0065] (1) Acquisition without sea current

[0066] During conventional acquisition, the influence of countercurrent and downstream is not considered. Then, according to Formulas 1 and 2, F can be calculated 阻 = 48 N, and according to Formula 3, F is calculated 浮= 59 N. According to Formula 6, the range of its tilt angle can be obtained as 8.53 - 9.46°. The larger the tilt angle, according to Formulas 5 and 7, F 拉垂直分量 is larger. We assume the maximum tilt angle is 10° and approximately calculate F 拉垂直分量max = 8.5 N. Compared with the cable gravity of 59 N, its order of magnitude is significantly one level smaller. In this case, a lead sheath counterweight of approximately 0.85 kg (the buoyancy of the lead sheath is negligible due to its high density) can keep the cable balanced, which is why only a small amount of lead sheath is generally configured in conventional acquisition.

[0067] (2) Upstream acquisition

[0068] With other conditions unchanged, considering the maximum flow velocity of 2 knots upstream, the relative velocity of the cable increases to 3.5 m / s. Then F 阻 = 93 N, F 浮 = 59 N, F 拉垂直分量max = 16.6 N. In this case, if the traditional method is used, that is, using a lead sheath counterweight of approximately 0.85 kg cannot keep the cable balanced, and the cable will float upward until it floats on the sea surface, which will cause the seismic data collected to be affected by excessive wind and wave noise and affect the penetration depth.

[0069] (3) Downstream acquisition

[0070] With other conditions unchanged, considering the maximum flow velocity of 2 knots downstream, the relative velocity of the cable decreases to 1.5 m / s. Then F 阻 = 17 N, F 浮 = 59 N, F 拉垂直分量max = 3.1 N. In this case, if the traditional method is used, that is, using a lead sheath counterweight of approximately 0.85 kg will cause the cable to sink downward due to excessive counterweight, making the tilt angle further increase, and F 拉垂直分量max further increases to balance the influence of the counterweight, making the cable reach a balanced state again. The cable sinking too deep will cause the appearance of virtual reflection of the cable, thus affecting the seismic acquisition resolution.

[0071] The above calculations can lead to two key conclusions: 1) θ is the key factor causing the cable to rise and fall. If θ = 0, there will be no cable rise and fall; 2) The maximum vertical tension of the cable is 16.6 N, and a counterweight of approximately 1.6 kg can keep the cable stable. According to Step 3, set the maximum counterweight to approximately 3.2 kg, and the buoyancy of the floating ball is 6.4 kg. Then, according to Step 4, design two sets of schemes and manufacture related equipment. Finally, according to Step 5, conduct sea trials to determine the final scheme.

[0072] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention in any other form. Any person skilled in the art may use the technical content disclosed above to make changes or modifications into equivalent embodiments with equivalent changes and apply them to other fields. However, as long as it does not depart from the technical solution content of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A depth design method for shallow-section cables, characterized in that: The depth-setting device combining a floating ball and a counterweight is designed to achieve precise control of the cable depth, which specifically includes the following steps: Step 1: Determine the resistance, tension and lift of the cable in seawater; Step 2, determine the relevant parameters of the shallow profiling equipment, and calculate the maximum upward pulling force in the absence of current, upstream and downstream conditions respectively; the relevant parameters include cable diameter, seawater density, cable length, height between the ship and the sea surface, length of the towline released from the stern, ship speed during collection, and ocean current speed; Step 3: Design the weight of the depth-setting equipment according to the maximum pulling force calculated in step 2; Step 4: Based on step 3, optimize the depth setting scheme design, including: Solution 1: Auxiliary cable laying equipment + depth-setting equipment: A guide steel frame is set at the stern to allow the cable to be laid at a specific depth below the sea surface at the stern. The cable inclination angle is 0, and the depth-setting equipment is placed behind the detector section. Solution 2: Single depth-fixing device: Place the depth-fixing device directly in front of the detector section to achieve a cable inclination angle θ of 0 by balancing the ship's pulling force and the detector's resistance.

2. The depth design method for shallow-section cables according to claim 1 is characterized in that: After step 4, step 5 of optimizing the depth design scheme in combination with the sea trial is also included, namely: Combined with the actual sea trial, the depth setting design was further optimized. A sea area with large tidal changes was selected. According to the tidal forecast, the seismic profiles collected when there was no tidal current and when there was a large tidal current were compared to determine the final solution. Specifically: When there is no tidal current, first use the conventional lead weight method to collect the standard shallow profile, then remove the lead weight and reconfigure according to the two schemes in step 4, and conduct collection experiments on the same survey line respectively; when collecting, first ensure the stability of the equipment. If the noise is loud, appropriately reduce the weight of the depth-setting equipment and keep it away from the detector section, and constantly adjust the design plan until the quality of the collected profile is equivalent to that of the conventional method; Then select the same survey line to collect data when the tide is strong, and compare the collection effect with the conventional collection method when there is no current. If the quality of the two is equivalent, it is considered that the design scheme meets the actual collection requirements. Otherwise, increase or decrease the weight of the depth-fixing equipment based on the profile effect until the collected profile meets the requirements.

3. The depth design method for shallow-section cables according to claim 1 is characterized in that: In step 3, regarding the design of the weight of the depth-fixing equipment, the float is designed to be 2-5 times the maximum tension, and the counterweight is designed to be 1-3 times the maximum tension.

4. The depth design method for shallow-section cables according to claim 1 is characterized in that: In step 1, the specific calculation method is as follows: (1) When the cable moves in seawater, the resistance it encounters is: Among them, C d is the drag coefficient, ρ is the seawater density, A is the projected area of ​​the cable perpendicular to the direction of motion, is the speed of the cable relative to the fluid; (2) The lift force is: Where C is the lift coefficient; (3) The pulling force is: Where θ is the cable tilt angle, h is the height from the ship deck to the sea surface, x is the length of the cable released from the stern; the vertical component of the tension is: F 拉垂直分量 =F 阻 tanθ.

5. The depth design method for shallow-section cables according to claim 1 is characterized in that: In step 2, the maximum tension is equivalent to the vertical component of the tension.

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