A method for jointly detecting the burial depth of submarine optical cables using a magnetic antenna and inertial navigation

Abstract

The present invention discloses a method for jointly detecting the burial depth of submarine optical cables by using a magnetic antenna and inertial navigation, that is, an underwater three-dimensional probe device and inertial navigation are installed together and then dropped to the seabed as an underwater fish. The underwater fish is connected to the receiving device on the operation ship through an underwater cable. The operation ship pulls the underwater fish to move forward in an "S" shape along the axial direction of the submarine optical cable. The burial depth of the submarine optical cable is calculated by measuring the three-component alternating magnetic field signals at two different positions around the energized submarine optical cable, including the following steps: Step 1, use the phase characteristics of the field components to determine whether the underwater fish crosses the submarine optical cable when moving from position 1 to position 2 along a straight line; Step 2, calculate the burial depth of the submarine optical cable by using the amplitude value of the field components. The method disclosed by the present invention has a high burial depth calculation accuracy regardless of the position or angle between the underwater fish and the submarine optical cable under the premise of ensuring sufficient signal-to-noise ratio. In particular, the burial depth with guaranteed accuracy can reach more than one hundred meters under the same conditions.

Description

Technical Field

[0001] The present invention belongs to the field of application research on the propagation of low-frequency electromagnetic waves across water-air media, and particularly relates to a method for jointly detecting the burial depth of submarine optical cables using a magnetic antenna and inertial navigation in this field. Background Art

[0002] The currently widely used and relatively mature method for detecting the burial depth of submarine cables is the active electromagnetic detection method. Its basic principle is to pass an ultra-low frequency (30 Hz - 300 Hz) alternating current through the submarine cable, and judge the burial depth of the submarine cable by detecting the electromagnetic field signal generated around the submarine cable. Zhou Xuejun and Wang Hongxia of the Naval University of Engineering proposed a method for detecting the burial depth of submarine cables using a dual three-dimensional array probe based on this principle. This method is to fix two probes at both ends of a rigid rod with a length of d, and three mutually orthogonal magnetic rod antennas are respectively installed on the probes to form two arrays to detect the magnetic field signals in the x, y, and z directions. As Figure 1 shown, a rectangular coordinate system is established with the centers o1o2 of the two arrays as the origin respectively, o1z1 and o2z2 are the directions perpendicular to the seabed, and o1x1 and o2x2 are the directions of the rod. The angles α between the projection of the sum of the magnetic field vectors of each probe in the oxy plane and the x-axis, and the angles β between the projection and o1z1, o2z2 i (i = 1, 2) are obtained by measuring the magnetic field signals. Then, the relative position relationship between the array and the submarine cable is divided into three types: the array straddles the submarine cable, the array is in the upper left of the submarine cable, and the array is in the upper right of the submarine cable. The relational expressions between the burial depth h of the submarine cable and d, α and β i (i = 1, 2) in these three cases are respectively deduced, and then the burial depth value is obtained.

[0003] The advantages of this method are that the burial depth of the submarine cable can be measured regardless of whether the array is above the submarine cable or whether the submarine cable route is perpendicular to the array. However, this method also has three deficiencies: First, when the array is far from the submarine cable or the angle between the connecting rod of the array and the axial direction of the submarine cable is small, the calculation accuracy is poor. Because the connecting rod of the array can only be about 2 meters in engineering, this limitation results in a maximum burial depth of about 6 meters to ensure accuracy; Second, this method is not applicable to the case where any array is directly above the submarine cable; Third, two sets of three-dimensional orthogonal magnetic rod antennas need to be used, and these two sets of antennas are fixedly connected by a rigid rod, so the maneuverability is not strong. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for measuring the burial depth of submarine optical cables using a measuring device composed of a set of three-dimensional orthogonal magnetic rod antennas and inertial navigation, which can accurately measure the burial depth of submarine optical cables without considering the relative distance and azimuth between the measuring device and the submarine optical cable.

[0005] The present invention adopts the following technical solutions:

[0006] A method for jointly detecting the burial depth of submarine optical cables using a magnetic antenna and inertial navigation. The improvement lies in that an underwater three-dimensional probe device and inertial navigation are installed together and then dropped to the seabed as an underwater towed fish. The underwater towed fish is connected to the receiving device on the operation ship through an underwater cable. The operation ship pulls the underwater towed fish to move forward in an S shape along the axial direction of the submarine optical cable. The burial depth of the submarine optical cable is calculated by measuring the three-component alternating magnetic field signals at two different positions 1 and 2 around the energized submarine optical cable, including the following steps:

[0007] Step 1: Use the phase characteristics of the field components to determine whether the underwater towed fish crosses the submarine optical cable when moving linearly from position 1 to position 2:

[0008] Define the submarine optical cable coordinate system: The direction perpendicular to the submarine optical cable is the x direction, and the direction along the submarine optical cable is the y direction; Define the underwater towed fish coordinate system: The direction from position 2 to position 1 is the x' direction, and the direction perpendicular to the moving trajectory of the underwater towed fish is the y' direction; The z and z' directions are the same, both perpendicular to the sea surface direction. The angle between the x' direction and the x direction is θ, θ ∈ [0, π / 2];

[0009] Then the direction of the underwater towed fish coordinate system in the submarine optical cable coordinate system is:

[0010]

[0011] When a low-frequency alternating current is passed through the submarine optical cable, the magnetic field generated by the current on the submarine optical cable is approximately a concentric circle around the submarine optical cable, that is:

[0012]

[0013] H x 、H y and H z are the magnetic field intensities generated by the current on the submarine optical cable in the x, y, and z axis directions respectively, I is the value of the low-frequency alternating current,

[0014] If the phase difference of the magnetic field z component between the two measured positions on the underwater three-dimensional probe device is 0 degrees, the underwater towed fish has not crossed the submarine optical cable; if the phase difference of the magnetic field z component between the two measured positions on the underwater three-dimensional probe device is 180 degrees, the underwater towed fish has crossed the submarine optical cable;

[0015] Step 2: Calculate the burial depth of the submarine optical cable using the field component amplitude value:

[0016] If the horizontal distance from position 1 to the submarine optical cable is x1, the horizontal distance from position 2 to the submarine optical cable is x2, and the burial depth of the submarine optical cable is z, where x1, x2, and z are all non-negative numbers. According to the angle θ between the x' direction and the x direction, it can be known that:

[0017] x2 = d cosθ ± x1 (4)

[0018] In the above formula, the positive sign is taken when the underwater towed fish does not cross the submarine optical cable, and the negative sign is taken when the underwater towed fish crosses the submarine optical cable;

[0019] According to the coordinate axis conversion relationship, the amplitude values of the antenna magnetic fields at positions 1 and 2 can be obtained as follows:

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026] Among them, equations (5), (6), and (7) represent the amplitude value of the antenna magnetic field at position 1, and equations (8), (9), and (10) represent the amplitude value of the antenna magnetic field at position 2;

[0027] According to equations (5) and (6), θ can be obtained:

[0028]

[0029] Let Judge whether the total field strength H1 at position 1 is less than the total field strength H2 at position 2. If so, swap the signal values at positions 1 and 2 to ensure that position 1 is closer to the submarine optical cable than position 2;

[0030] Let According to equations (5), (6), and (7), we can get:

[0031]

[0032] When the underwater towed fish does not cross the submarine optical cable:

[0033] a) If Use to calculate, and the equation is:

[0034] A(B - 1)(A 2 + 1)z 2 + d cosθ[(B - 2)A 2 + B]z - Ad 2 cos 2 θ = 0 (13)

[0035] d is the straight-line distance from position 1 to position 2;

[0036] b) If using to calculate, the equation is:

[0037] (1 - B)(A 2 + 1)z 2 + 2Ad cosθz + d 2 cos 2 θ = 0 (14)

[0038] When towing a fish underwater across a submarine optical cable on the seabed:

[0039] a) If using to calculate, the equation is:

[0040] A(B + 1)(A 2 + 1)z 2 - d cosθ[(B + 2)A 2 + B]z + Ad 2 cos 2 θ = 0 (15)

[0041] b) If using to calculate, the equation is:

[0042] (B - 1)(A 2 + 1)z 2 + 2Adcosθz - d 2 cos 2 θ = 0 (16)

[0043] According to equations (13), (14), (15) or (16), the positive root z1 of the equation can be obtained, which is the burial depth of the submarine optical cable.

[0044] The beneficial effects of the present invention are as follows:

[0045] For the method disclosed by the present invention, on the premise of ensuring sufficient signal-to-noise ratio, regardless of the position or angle between the underwater fish and the submarine optical cable, the burial depth calculation accuracy is very high. In particular, the burial depth with high accuracy can reach more than one hundred meters under the same conditions; second, this method is also applicable to the case where the underwater fish is directly above the submarine optical cable; third, a method for judging whether the underwater fish crosses the submarine optical cable is given. Therefore, it is not necessary to measure the burial depth of the submarine optical cable when it is known whether the underwater fish crosses the submarine optical cable; fourth, only a set of three-dimensional orthogonal magnetic rod antennas is required, and the controllability is strong. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a schematic diagram of an existing dual three-dimensional array probe;

[0047] Figure 2 It is a schematic diagram of the operation mode for detecting the burial depth of submarine optical cables;

[0048] Figure 3 It is a schematic flow diagram of the method of the present invention;

[0049] Figure 4 It is a schematic diagram when the underwater fish sled does not cross the submarine optical cable;

[0050] Figure 5 It is a schematic diagram when the underwater fish sled crosses the submarine optical cable. Detailed implementation manners

[0051] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0052] Embodiment 1. This embodiment discloses a method for jointly detecting the burial depth of submarine optical cables using a magnetic antenna and an inertial navigation system. As Figure 2 shown, an underwater three-dimensional probe device (detector) and an inertial navigation system are installed together and then dropped to the seabed as an underwater fish sled. The underwater fish sled is connected to the receiving device on the operation ship through an underwater cable. The operation ship pulls the underwater fish sled to move forward in an S shape along the axial direction of the submarine optical cable. The burial depth of the submarine optical cable is calculated by measuring the three-component alternating magnetic field signals at two different positions 1 and 2 around the energized submarine optical cable. As Figure 3 shown, it includes the following steps:

[0053] Step 1, use the phase characteristics of the field components to determine whether the underwater fish sled crosses the submarine optical cable when moving from position 1 to position 2 along a straight line:

[0054] Define the submarine optical cable coordinate system: the direction perpendicular to the submarine optical cable is the x direction, and the direction along the submarine optical cable is the y direction; define the underwater fish sled coordinate system: the direction from position 2 to position 1 is the x' direction, and the direction perpendicular to the moving trajectory of the underwater fish sled is the y' direction; the z and z' directions are the same, both perpendicular to the sea surface direction, and the included angle between the x' direction and the x direction is θ, θ ∈ [0, π / 2];

[0055] Then the direction of the underwater fish sled coordinate system in the submarine optical cable coordinate system is:

[0056]

[0057] When a low-frequency alternating current is applied to the submarine optical cable, the magnetic field generated by the current on the submarine optical cable is approximately a concentric circle around the submarine optical cable, that is:

[0058]

[0059] Before calculating the burial depth of the submarine optical cable, it is first necessary to determine whether the underwater towed fish crosses the state of the submarine optical cable when moving in a straight line from position 1 to position 2. H x 、H y and H z are the magnetic field intensities generated by the current on the submarine optical cable in the x, y, and z-axis directions respectively, I is the value of the low-frequency alternating current,

[0060] If the phase difference of the magnetic field z-component between two measured positions on the underwater three-dimensional probe device is 0 degrees, then as Figure 4 shown, the underwater towed fish does not cross the submarine optical cable; if the phase difference of the magnetic field z-component between two measured positions on the underwater three-dimensional probe device is 180 degrees, then as Figure 5 shown, the underwater towed fish crosses the submarine optical cable;

[0061] Based on this, it is possible to judge whether the underwater towed fish has crossed the submarine optical cable according to the phase difference of the magnetic field z-component between the two measured positions.

[0062] Step 2, calculate the burial depth of the submarine optical cable using the field component amplitude value:

[0063] If the horizontal distance from position 1 to the submarine optical cable is x1, the horizontal distance from position 2 to the submarine optical cable is x2, and the burial depth of the submarine optical cable is z, and x1, x2, and z are all non-negative numbers, according to the angle θ between the x' direction and the x direction, it can be known that:

[0064] x2 = d cosθ ± x1 (4)

[0065] In the above formula, the plus sign is taken when the underwater towed fish does not cross the submarine optical cable, and the minus sign is taken when the underwater towed fish crosses the submarine optical cable;

[0066] According to the coordinate axis conversion relationship, the amplitude values of the antenna magnetic fields at positions 1 and 2 can be obtained as follows:

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073] Among them, equations (5), (6), and (7) represent the amplitude values of the antenna magnetic field at position 1, and equations (8), (9), and (10) represent the amplitude values of the antenna magnetic field at position 2;

[0074] θ can be obtained according to equations (5) and (6):

[0075]

[0076] Let Judge whether the total field strength H1 at position 1 is less than the total field strength H2 at position 2. If so, swap the signal values at position 1 and position 2 to ensure that position 1 is closer to the submarine optical cable than position 2;

[0077] Let According to equations (5), (6) and (7), we can get:

[0078]

[0079] The following is a specific analysis of two cases: when the underwater towed fish does not cross the submarine optical cable and when the underwater towed fish crosses the submarine optical cable:

[0080] Case 1: When the underwater towed fish does not cross the submarine optical cable:

[0081] a) If Use to calculate, and the equation is:

[0082] A(B - 1)(A 2 + 1)z 2 + d cosθ[(B - 2)A 2 + B]z - Ad 2 cos 2 θ = 0 (13)

[0083] d is the straight-line distance from position 1 to position 2, which can be obtained according to the information output by the inertial navigation;

[0084] b) If Use to calculate, and the equation is:

[0085] (1 - B)(A 2 + 1)z 2 + 2Ad cosθz + d 2 cos 2 θ = 0 (14)

[0086] Case 2: When the underwater towed fish crosses the submarine optical cable:

[0087] a) If Use to calculate, and the equation is:

[0088] A(B + 1)(A 2 + 1)z 2 - dcosθ[(B + 2)A 2 + B]z + Ad2 cos 2 θ = 0 (15)

[0089] b) If using to calculate, the equation is:

[0090] (B - 1)(A 2 + 1)z 2 + 2Ad cosθz - d 2 cos 2 θ = 0 (16) According to Equation (13), (14), (15) or (16), the positive root z1 of the equation can be obtained, which is the burial depth of the submarine optical cable.

Claims

1. A method for jointly detecting the burial depth of submarine optical cables using a magnetic antenna and inertial navigation, characterized in that, After an underwater three-dimensional probe device and an inertial navigation system are installed together and then deployed to the seabed as an underwater towed fish, the underwater towed fish is connected to the receiving device on the operation ship through an underwater cable. The operation ship towes the underwater towed fish to move forward in an S shape along the axial direction of the undersea optical cable. The burial depth of the undersea optical cable is calculated by measuring the three-component alternating magnetic field signals at two different positions 1 and 2 around the energized undersea optical cable, including the following steps: Step 1, use the phase characteristics of the field components to determine whether the underwater towed fish crosses the undersea optical cable when moving linearly from position 1 to position 2: Define the coordinate system of the undersea optical cable: the direction perpendicular to the undersea optical cable is the x direction, and the direction along the undersea optical cable is the y direction; define the coordinate system of the underwater towed fish: the direction from position 2 to position 1 is the x' direction, and the direction perpendicular to the moving trajectory of the underwater towed fish is the y' direction; the z and z' directions are the same, both perpendicular to the sea surface, and the included angle between the x' direction and the x direction is θ, θ ∈ [0, π / 2]; Then the direction of the underwater towed fish coordinate system in the undersea optical cable coordinate system is: When a low-frequency alternating current is applied to the undersea optical cable, the magnetic field generated by the current on the undersea optical cable is approximately a concentric circle around the undersea optical cable, that is: H x 、H y and H z are the magnetic field intensities generated by the current on the submarine optical cable in the x, y, and z-axis directions respectively, and I is the value of the low-frequency alternating current. If the phase difference of the magnetic field z component between the two measured positions on the underwater three-dimensional probe device is 0 degrees, the underwater towed fish does not cross the undersea optical cable; If the phase difference of the magnetic field z component between the two measured positions on the underwater three-dimensional probe device is 180 degrees, the underwater towed fish crosses the undersea optical cable; Step 2, calculate the burial depth of the undersea optical cable using the amplitude value of the field component: If the horizontal distance from position 1 to the undersea optical cable is x1, the horizontal distance from position 2 to the undersea optical cable is x2, and the burial depth of the undersea optical cable is z1, and x1, x2, and z1 are all non-negative numbers, according to the included angle θ between the x' direction and the x direction, it can be known that: x2 = dcosθ ± x1 (4) In the above formula, the plus sign is taken when the underwater towed fish does not cross the undersea optical cable, and the minus sign is taken when the underwater towed fish crosses the undersea optical cable; According to the coordinate axis conversion relationship, the amplitude values of the antenna magnetic fields at positions 1 and 2 can be obtained respectively as: Among them, equations (5), (6), and (7) represent the amplitude value of the antenna magnetic field at position 1, and equations (8), (9), and (10) represent the amplitude value of the antenna magnetic field at position 2; θ can be obtained according to equations (5) and (6): Let Determine whether the total field strength H1 at position 1 is less than the total field strength H2 at position 2. If so, swap the signal values at position 1 and position 2 to ensure that position 1 is closer to the submarine optical cable than position 2; Let According to equations (5), (6), and (7), we can obtain: When the underwater towed fish does not cross the undersea optical cable: a) If using for calculation, the equation is: d is the straight-line distance from position 1 to position 2; b) If is used for calculation, the equation is: When the underwater towed fish crosses the undersea optical cable: a) If using for calculation, the equation is: b) If using for calculation, the equation is: According to equations (13), (14), (15), or (16), the positive root z1 of the equation can be obtained, which is the burial depth of the undersea optical cable.

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