A method for analyzing sea wave noise characteristics in a conductivity stratified sea water

Abstract

The application discloses a method for analyzing characteristics of sea wave noise in layered seawater with conductivity, and belongs to the technical field of sea wave noise analysis. The method is used for solving the problem that the existing calculation of sea wave induced electromagnetic field often assumes that the seawater conductivity is uniformly distributed, and the marine environmental electromagnetic noise cannot be accurately analyzed. The method comprises the following steps: S1, a calculation model of sea wave induced electromagnetic field based on layered seawater with limited depth and conductivity is established; S2, the sea wave induced electromagnetic field is solved by using Maxwell differential equation and seawater wave equation conditions; S3, noise generated by sea wave motion cutting geomagnetic field is analyzed according to the solved sea wave induced electromagnetic field, and the spatial distribution of unit amplitude sea wave magnetic noise and electric noise is derived; and S4, sea wave electromagnetic noise power spectrum under different sea conditions is analyzed in combination with sea wave spectrum. The application considers the characteristic of uneven seawater conductivity, establishes a four-layer medium sea wave induced electromagnetic field calculation model, and analyzes the sea wave electromagnetic noise.

Description

Technical Field

[0001] This invention relates to the field of ocean wave noise analysis technology, and more specifically, to a method for analyzing the characteristics of ocean wave noise in conductive stratified seawater. Background Technology

[0002] The electromagnetic fields generated by ocean waves cutting through the Earth's magnetic field are unavoidable noise sources in magnetic anomaly detection by spacecraft. JT. Weaver provides a theoretical model for the electromagnetic fields generated by ocean waves in infinitely deep, uniform seawater, and this model has been validated. The geometric diagram used is shown below. Figure 1 As shown in the figure, the vertical downward direction along the z-axis is positive, meaning the seawater is in the z>0 region, the air is in the z<0 region, and the waves move along the x-direction.

[0003] In reality, seawater depth is finite. Compared to the Weaver model, solving for ocean wave magnetic noise requires incorporating a layer of seabed / land surface media, such as... Figure 2 As shown. Let the depth of the uniform seawater be d, with other conditions remaining unchanged. In this case, the boundary z=d needs to be considered in the solution of the seawater velocity potential and the magnetic noise of the waves.

[0004] In reality, due to the uneven distribution of salinity and temperature, the conductivity of seawater varies with depth. Existing wave models that assume a uniform distribution of seawater conductivity cannot accurately obtain the electromagnetic field induced by the waves. The electromagnetic field generated by the movement of waves in the Earth's magnetic field is an important component of electromagnetic noise in the marine environment. Therefore, the obtained electromagnetic field is difficult to accurately analyze the electromagnetic noise in the marine environment. Summary of the Invention

[0005] The technical problem to be solved by this invention is:

[0006] Existing calculations of wave-induced electromagnetic fields often assume a uniform distribution of seawater conductivity, which fails to accurately analyze electromagnetic noise in the marine environment.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0008] This invention provides a method for analyzing the wave noise characteristics in conductive stratified seawater, comprising the following steps:

[0009] S1. Establish a calculation model for wave-induced electromagnetic field based on stratified seawater with finite conductivity at a certain depth;

[0010] S2. Solve for the electromagnetic field induced by ocean waves using the conditions of Maxwell's differential equation and the ocean wave equation.

[0011] S3. Based on the obtained wave-induced electromagnetic field, analyze the noise generated by the wave motion cutting the geomagnetic field and derive the spatial distribution of wave magnetic noise and electrical noise per unit amplitude.

[0012] S4. Combine wave spectrum analysis to analyze the electromagnetic noise power spectrum of waves under different sea conditions.

[0013] Furthermore, the wave-induced electromagnetic field calculation model for finite-depth conductive stratified seawater described in S1 includes four media layers: air, two layers of seawater, and the seabed / land. The specific modeling process is as follows:

[0014] Assuming seawater is an incompressible, nonviscous liquid, meaning its velocity vector is irrotational and divergent, a velocity potential satisfying Laplace's equations is introduced to calculate the velocity vector. ,satisfy Ocean waves propagate along the x-axis; assuming the waves are sine waves with a wave height of a, the wave amplitude is very small relative to the wavelength, the sea surface is approximately a plane, and the velocity potential is as shown in (1):

[0015] (1)

[0016] in, The wave angular frequency, For time, For transmission distance, For depth distance, The imaginary unit;

[0017] At this point, the wave number m satisfies the dispersion equation (2):

[0018] (2)

[0019] Where, g = 9.8 m / For gravitational acceleration, ω = 2π / T. For the wave cycle, This represents the total depth of the seawater.

[0020] The equation for the free water surface is shown in equation (3):

[0021] (3)

[0022] The velocities satisfy the boundary conditions of the sea surface wave and the seabed, respectively, as given by equations (4) and (5):

[0023] (4)

[0024] (5)

[0025] in, The z-axis component of the seawater velocity field;

[0026] Therefore, the obtained velocity potential is given by equation (6):

[0027] (6)

[0028] The velocity vector is further obtained as shown in equation (7):

[0029] (7)

[0030] Assuming the geomagnetic field in the study area remains constant, the geomagnetic vector is given by equation (8):

[0031] (8)

[0032] Where F is the total magnitude of the geomagnetic field, I is the magnetic inclination, and θ is the geomagnetic declination, i.e., the angle between magnetic north and the direction of ocean wave propagation. Unit vectors along the X, Y, and Z axes. It is the geomagnetic tilt angle. It is the magnetic declination;

[0033] Compared to the Earth's magnetic field, the amplitude of the changing magnetic field caused by the induced current is very small and can be ignored. The corrected Ampere circuital law in seawater is shown in equation (9):

[0034] (9)

[0035] in, To induce a magnetic field in ocean waves within stratified seawater. These represent the upper and lower layers of seawater, respectively. The electric field induced by ocean waves in stratified seawater;

[0036] Combining Faraday's law of electromagnetic induction, the wave equation satisfied by the magnetic field in seawater is shown in equation (10):

[0037] (10)

[0038] The magnetic permeability is the same in air and seawater, which is μ=4π×10-7 H / m.

[0039] Furthermore, the solution to the wave-induced electromagnetic field described in S2 using the conditions of Maxwell's differential equation and the wave equation is as follows:

[0040] Since the magnetic field has a resonant form similar to the velocity in formula (6), the magnetic field vector generated by seawater is as shown in formula (11):

[0041] (11)

[0042] Where hx and hz are the magnetic field components of the X-axis and Z-axis, respectively;

[0043] Due to the divergence-free nature of the magnetic field Equation (12) is obtained:

[0044] (12)

[0045] At the same time h y =0;

[0046] The electrical conductivity of air is 0, therefore the amount of air containing h z The scalar partial differential equation satisfied by the components is shown in equation (13):

[0047] (13)

[0048] The electrical conductivity of seawater and seabed is used to obtain h in seawater and seabed. z The scalar partial differential equations satisfied by the components are shown in equations (14) and (15), respectively:

[0049] (14)

[0050] (15)

[0051] In the formula, , ;

[0052] The boundary conditions of the electromagnetic field are given by equation (16) for the x and z components of the magnetic field on both sides of the interface at z=0, z=d1 and z=d.

[0053] (16)

[0054] The magnetic field components h in each medium are obtained from the wave equation of the magnetic field. z The general solutions for each component are shown in equations (17)-(21):

[0055] (17)

[0056] (18)

[0057] (19)

[0058] (20)

[0059] The undetermined coefficients C1-C6 are obtained from the boundary condition (16), and the magnetic field component h is further obtained from equation (11). x The expression for this is used to obtain the magnetic field vector of each region;

[0060] Further, the induced electric field vectors in each part of the medium are obtained from (21). Since the conductivity of air is set to 0, the electric field of air is 0.

[0061] (twenty one).

[0062] Furthermore, the spatial distribution of magnetic and electrical noise of unit amplitude ocean waves described in S3 is derived, and the magnetic noise generated by unit amplitude ocean waves with wave periods of T=5 s, 10 s, 20 s and 40 s is analyzed, as well as the electrical noise generated by unit amplitude ocean waves with wave periods of T=5 s, 6 s, 7 s, 8 s, 9 s and 10 s is analyzed.

[0063] Furthermore, in S4, the analysis of the electromagnetic noise power spectrum of ocean waves under different sea conditions, using the wave spectrum, is represented by the PM spectrum, i.e.:

[0064] (twenty two)

[0065] The calculation method for wind speed at different altitudes is as follows:

[0066] (twenty three)

[0067] The sea state is calculated using wind speed, and the wind speed at 10 m above sea level is calculated according to formula (24):

[0068] (twenty four)

[0069] B represents the Beaufort scale wind level.

[0070] Furthermore, as described in S4, the electromagnetic noise power spectrum of ocean waves under different sea conditions is analyzed by combining ocean wave spectrum analysis. The air height is set to three points: h=0 m, h=50 m, and h=100 m, and the depth of the upper seawater is set to 100 m. The magnetic noise power spectrum of ocean waves at depths of h=0 m, h=50 m, and h=100 m in the upper seawater and at depths of h=150 m, h=200 m, and h=250 m in the lower seawater is analyzed. The electromagnetic noise power spectrum of ocean waves at depths of h=0 m, h=20 m, h=50 m, and h=100 m in the seawater is also analyzed.

[0071] A system for analyzing the wave noise characteristics in stratified seawater with conductivity, the system having a program module corresponding to the steps of any of the above-mentioned technical solutions, and executing the steps in the above-mentioned method for analyzing the wave noise characteristics in stratified seawater with conductivity when running.

[0072] A computer-readable storage medium storing a computer program configured to, when invoked by a processor, implement the steps of the method for analyzing wave noise characteristics in conductive stratified seawater as described in any of the above technical solutions.

[0073] Compared with the prior art, the beneficial effects of the present invention are:

[0074] This invention provides a method for analyzing wave noise characteristics in stratified seawater with conductivity. Based on existing derivations of wave electromagnetic noise in infinitely deep and finitely deep uniform seawater, this method further considers the non-uniform conductivity of seawater. According to the characteristic that the conductivity of seawater is approximately stratified in the vertical direction, a wave-induced electromagnetic field calculation model for stratified seawater with conductivity of finite depth, including four media: air, two layers of seawater, and seabed / land, is established. The method derives and analyzes the wave electromagnetic noise when the seawater is approximated as two conductive media. Attached Figure Description

[0075] Figure 1 This refers to the infinitely deep uniform seawater model in the background technology of this invention;

[0076] Figure 2 This refers to the finite-depth model of uniform seawater in the background art of this invention;

[0077] Figure 3 This is a flowchart of the wave noise characteristic analysis method in conductive stratified seawater according to an embodiment of the present invention;

[0078] Figure 4 This is a schematic diagram of a wave-induced electromagnetic field calculation model based on stratified seawater with finite-depth conductivity in an embodiment of the present invention.

[0079] Figure 5 This is a comparison chart showing the variation of ocean wave magnetic noise with height or depth obtained from two models in this embodiment of the invention.

[0080] Figure 6 This is a trend diagram showing the relative difference in ocean wave magnetic noise obtained from the two models in this embodiment of the invention.

[0081] Figure 7 This is a graph showing the variation of unit amplitude wave electrical noise with seawater depth in different regions according to an embodiment of the present invention;

[0082] Figure 8 The power spectrum is shown in the embodiment of the invention, where the wind speed is 15 m / s at a distance of 10 m above the sea surface.

[0083] Figure 9 This is a magnetic noise spectrum of sea waves under sea states 5-7 in an embodiment of the present invention;

[0084] Figure 10 This is a wave magnetic noise spectrum at different depths under sea state 7 in this embodiment of the invention;

[0085] Figure 11 This is a wave electrical noise spectrum diagram under sea states 5-7 in the embodiments of the present invention;

[0086] Figure 12 This is a comparison chart of the power spectrum of ocean wave electrical noise at four points in the seawater at depths of h=0 m, h=20 m, h=50 m and h=100 m in an embodiment of the present invention. Detailed Implementation

[0087] In the description of this invention, it should be noted that the terms "first," "second," and "third" mentioned in the embodiments of this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," and "third" may explicitly or implicitly include one or more of that feature.

[0088] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0089] Combination Figure 3 and Figure 4 As shown, this invention provides a method for analyzing the wave noise characteristics in conductive stratified seawater, comprising the following steps:

[0090] S1. Establish a calculation model for wave-induced electromagnetic field based on stratified seawater with finite conductivity at a certain depth;

[0091] S2. Solve for the electromagnetic field induced by ocean waves using the conditions of Maxwell's differential equation and the ocean wave equation.

[0092] S3. Based on the obtained wave-induced electromagnetic field, analyze the noise generated by the wave motion cutting the geomagnetic field and derive the spatial distribution of wave magnetic noise and electrical noise per unit amplitude.

[0093] S4. Combine wave spectrum analysis to analyze the electromagnetic noise power spectrum of waves under different sea conditions.

[0094] Example 1

[0095] like Figure 4 As shown, the calculation model for the wave-induced electromagnetic field of finite-depth conductive stratified seawater includes four media layers: air, two layers of seawater, and the seabed / land. The specific modeling process is as follows:

[0096] Assuming seawater is an incompressible, nonviscous liquid, meaning its velocity vector is irrotational and divergent, a velocity potential satisfying Laplace's equations is introduced to calculate the velocity vector. ,satisfy Ocean waves propagate along the x-axis; assuming the waves are sine waves with a wave height of a, the wave amplitude is very small relative to the wavelength, the sea surface is approximately a plane, and the velocity potential is as shown in (1):

[0097] (1)

[0098] in, The wave angular frequency, For time, For transmission distance, For depth distance, The imaginary unit;

[0099] At this point, the wave number m satisfies the dispersion equation (2):

[0100] (2)

[0101] Where g = 9.8 m / For gravitational acceleration, ω = 2π / T. For the wave cycle, The total depth of the seawater; the variation of wave period and wave number with ocean depth;

[0102] The equation for the free water surface is shown in equation (3):

[0103] (3)

[0104] The velocities satisfy the boundary conditions of the sea surface wave and the seabed, respectively, as given by equations (4) and (5):

[0105] (4)

[0106] (5)

[0107] in, The z-axis component of the seawater velocity field;

[0108] Therefore, the obtained velocity potential is given by equation (6):

[0109] (6)

[0110] The velocity vector is further obtained as shown in equation (7):

[0111] (7)

[0112] Assuming the geomagnetic field in the study area remains constant, the geomagnetic vector is given by equation (8):

[0113] (8)

[0114] Where F is the total magnitude of the geomagnetic field, I is the magnetic inclination, and θ is the geomagnetic declination, i.e., the angle between magnetic north and the direction of ocean wave propagation. Unit vectors along the X, Y, and Z axes. It is the geomagnetic tilt angle. It is the magnetic declination;

[0115] Compared to the Earth's magnetic field, the amplitude of the changing magnetic field caused by the induced current is very small and can be ignored. The corrected Ampere circuital law in seawater is shown in equation (9):

[0116] (9)

[0117] in, To induce a magnetic field in ocean waves within stratified seawater. These represent the upper and lower layers of seawater, respectively. The electric field induced by ocean waves in stratified seawater;

[0118] Combining Faraday's law of electromagnetic induction, the wave equation satisfied by the magnetic field in seawater is shown in equation (10):

[0119] (10)

[0120] The magnetic permeability is the same in air and seawater, which is μ=4π×10-7 H / m.

[0121] Example 2

[0122] The solution to the wave-induced electromagnetic field described in S2, using the conditions of Maxwell's differential equation and the wave equation, is as follows:

[0123] Since the magnetic field has a resonant form similar to the velocity in formula (6), the magnetic field vector generated by seawater is as shown in formula (11):

[0124] (11)

[0125] Where hx and hz are the magnetic field components of the X-axis and Z-axis, respectively;

[0126] Due to the divergence-free nature of the magnetic field Equation (12) is obtained:

[0127] (12)

[0128] At the same time h y =0;

[0129] The electrical conductivity of air is 0, therefore the amount of air containing h z The scalar partial differential equation satisfied by the components is shown in equation (13):

[0130] (13)

[0131] The electrical conductivity of seawater and seabed is used to obtain h in seawater and seabed. z The scalar partial differential equations satisfied by the components are shown in equations (14) and (15), respectively:

[0132] (14)

[0133] (15)

[0134] In the formula, , ;

[0135] The boundary conditions of the electromagnetic field are given by equation (16) for the x and z components of the magnetic field on both sides of the interface at z=0, z=d1 and z=d.

[0136] (16)

[0137] The magnetic field components h in each medium are obtained from the wave equation of the magnetic field. z The general solutions for each component are shown in equations (17)-(21):

[0138] (17)

[0139] (18)

[0140] (19)

[0141] (20)

[0142] The undetermined coefficients C1-C6 are obtained from the boundary condition (16), and the magnetic field component h is further obtained from equation (11). x The expression for this is used to obtain the magnetic field vector of each region;

[0143] Further, the induced electric field vectors in each part of the medium are obtained from (21). Since the conductivity of air is set to 0, the electric field of air is 0.

[0144] (twenty one)

[0145] Example 3

[0146] Analysis of the spatial distribution of unit amplitude ocean wave magnetic and electrical noise:

[0147] Three locations were selected, one in the South China Sea, one in the Bohai Sea, and one in the Yellow Sea. The locations of the three locations are shown in the figure below. Figure 4As shown in Table 1, the total magnetic field, geomagnetic inclination, and magnetic declination of each coordinate point were obtained from GEOPROBE queries, with the elevation set to 0 m. Based on the actual seawater depth and conductivity of the three sea areas, the seawater was divided into upper and lower layers. In the South China Sea, the water depth is 410 m; therefore, 0-100 m is designated as the upper layer with an average conductivity of 5.099 S / m, and 100-410 m as the lower layer with an average conductivity of 3.43 S / m. In the Bohai Sea, the water depth is 20 m; 0-5 m is designated as the upper layer with an average conductivity of 3.65 S / m, and 5-20 m as the lower layer with an average conductivity of 3.591 S / m. In the Yellow Sea, the water depth is 55 m; 0-15 m is designated as the upper layer with an average conductivity of 3.918 S / m, and 15-55 m as the lower layer with an average conductivity of 3.480 S / m.

[0148] Table 1

[0149]

[0150] Magnetic noise analysis: Spatial distribution of wave magnetic noise per unit wave amplitude estimated based on the four-layer medium model of air-layered seawater-seabed and land in Example 1, and compared with the existing three-layer medium model based on the assumption of uniform seawater conductivity ( Figure 2 The estimated wave magnetic noise was compared. Taking the point in the South China Sea in Table 1 as an example, the distribution of wave magnetic noise above and below the sea surface obtained by the two models was compared. The conductivity distribution of the uniform seawater medium model was 4 S / m. The results are as follows. Figure 5 As shown in the figure, (a) and (b) represent the variations of ocean wave magnetic noise in the air above the sea surface and in the seawater below the sea surface, respectively. The four sets of curves with different line types (dotted line, dashed line, dotted line and solid line) from the inside out represent the magnetic noise generated by ocean waves with a unit amplitude and a period of T = 5 s, 10 s, 20 s and 40 s, respectively.

[0151] according to Figure 5As can be seen, the overall trend of each set of curves is the same: the magnetic noise amplitude reaches its maximum near the sea level, and decreases with increasing altitude in the air and depth in the seawater. This verifies the accuracy of the non-uniform conductivity seawater model of this invention. Meanwhile, the magnitudes of the wave magnetic noise obtained from the two models differ significantly: under the analyzed wave period, the conductivity distribution of the upper layer of seawater selected based on the non-uniform seawater medium model is greater than that of the uniform seawater medium model, while the conductivity distribution of the lower layer of seawater is smaller than that of the uniform seawater medium model. Therefore, the amplitude values ​​of wave magnetic noise obtained within the upper 100 m of the seawater are generally larger than those assumed under uniform seawater. Since the conductivity of actual seawater is non-uniform, preliminary stratification of seawater according to conductivity makes the estimated wave magnetic noise closer to the actual value. Furthermore, from... Figure 5 (b) It can be seen that the difference in wave magnetic noise between the two models is the largest in the area near the sea surface. As the seawater depth increases, the difference in wave magnetic noise amplitude between the two models decreases. When approaching the seabed and land, the wave magnetic noise obtained by the two models tends to be consistent.

[0152] Furthermore, the difference between the wave magnetic noise amplitudes calculated by the two models is used to obtain the percentage difference relative to the estimated value from the stratified seawater model, as shown below. Figure 6 As shown.

[0153] Depend on Figure 6 As can be seen, the relative difference between the two is mainly distributed between 5% and 20%. With the increase of seawater depth, the relative difference gradually decreases to a negative value, mainly because the conductivity of the lower layer of seawater in the four-layer medium model is lower than that of the uniform seawater medium model. Wave noise is mainly distributed at the sea-air interface. The relative difference is smaller underwater and larger in the air, and the relative difference decreases as the wave motion period increases.

[0154] Electrical noise analysis:

[0155] The electrical noise of ocean waves with different periods and unit amplitudes at three points in three sea areas, as shown in Table 1, was analyzed. Since the electrical noise in the air is zero, the variation of ocean wave electrical noise with depth underwater was analyzed as follows: Figure 7 As shown, six wave periods T=5 s, 6 s, 7 s, 8 s, 9 s and 10 s were analyzed.

[0156] The results show that the wave noise variation pattern with the wave cycle is the same at each location in each sea area; however, for waves of the same cycle, the wave noise amplitude varies significantly at the three locations, especially at the deeper points in the South China Sea where the noise amplitude is significantly lower than at the other two locations. Spatially, the maximum wave noise value is still near the sea surface, approximately at a depth of 5 m or more, near the sea-air interface. With increasing depth, the wave noise initially decreases rapidly and then changes gradually. The larger the wave cycle, the more gradual the change in wave noise amplitude with seawater depth. In terms of amplitude, the magnitude of wave noise per unit amplitude at sea surface is 1–8 μV / m, with the specific value depending on the wave cycle and location.

[0157] Example 4

[0158] Analysis of electromagnetic noise of ocean waves under different sea conditions:

[0159] In actual marine environments, ocean waves can be considered as waves of different amplitudes, frequencies, and phases. The wave spectrum can characterize the internal structure and external features of ocean waves. The wave spectrum is related to wave height and period. In this embodiment, the PM spectrum is used to represent the spectrum of ocean waves. The PM spectrum is suitable for fully grown ocean waves. The spectrum formula is shown in equation (22):

[0160] (twenty two)

[0161] In the formula, a = 0.081, β = 0.74, and U is the wind speed at 19.5 m above the sea surface. Wind speeds at different heights can be calculated from the wind speed at 10 m using formula (23), where V... Z V represents the wind speed at a point z m above sea level. 10 Let Z0 be the wind speed at 10 m above the sea surface, and Z0 be the roughness length. When the wind speed is less than 15 m / s, lgZ0 = -3.8; when the wind speed exceeds 15 m / s, lgZ0 = -2.4. Assuming the wind speed at 10 m is 15 m / s, U is calculated to be 16.28 m / s. Substituting this into equation (22) yields the PM spectrum, as shown below. Figure 8 As shown.

[0162] (twenty three)

[0163] Sea state is a classification standard that characterizes the size and appearance of sea surface waves. Currently, there is no strict sea state standard, and the wave class and wind class are usually used to reflect the sea state level. In this embodiment, the wind class is used to calculate the sea state. The Beaufort scale is used to classify the wind speed from low to high into 12 levels, for a total of 13 levels, as shown in Table 2. The wind speed in the table is the wind speed at 10 m above sea level. The relationship between wind speed and Beaufort scale is given by equation (24), where V in equation (24) is the wind speed at 10 m above sea level and B is the Beaufort scale. The wind speed is calculated based on the sea state level, and the wave spectrum under different sea states is further obtained.

[0164] (twenty four)

[0165] Table 2

[0166]

[0167] Wave magnetic noise power spectrum at different wind speeds: According to Table 2, sea states can be distinguished by wave height and wind speed. The wind speed at 10 m above sea level under sea states 5-7 is calculated according to Equation (24). Then, the wind speeds at 19.5 m under different sea states 5-7 are calculated according to Equation (23) as 9.912 m / s, 13.029 m / s and 16.80 m / s, respectively. Substituting these values ​​into Equation (22) yields the PM spectrum for different sea state levels, as follows: Figure 9 As shown in the figure, taking the point in the South China Sea in Table 1 as an example, the subplots represent the wave spectra at sea level and at a depth of 10 m for sea states 5-7. It can be seen that the wave magnetic noise power increases with increasing sea state, i.e., increasing wind speed. At sea level, the center frequencies of the wave magnetic noise for sea states 5-7 are 0.127 Hz, 0.097 Hz, and 0.075 Hz, respectively. At a depth of 10 m, the center frequencies of the wave magnetic noise for sea states 5-7 are almost the same as those at sea level. Therefore, with increasing sea state, the wave frequency at the point of maximum magnetic noise decreases, and the peak frequency shifts towards lower frequencies.

[0168] Further analysis of PM spectra at different depths under sea state 7 was conducted. The air altitude was set to three points: h=0 m, h=50 m, and h=100 m. The upper seawater depth in the South China Sea was set to 100 m. The wave magnetic noise power spectra at three points in the upper seawater at depths of h=0 m, h=50 m, and h=100 m, and at three points in the lower seawater at depths of h=150 m, h=200 m, and h=250 m, were compared and analyzed. Figure 10As shown, the power spectrum is a narrow band spectrum, approaching zero above 100 m above the sea surface. Within the seawater, based on the non-uniform distribution of conductivity, the seawater is stratified according to conductivity. Analysis reveals that the power spectrum amplitude decreases with increasing depth, approaching zero after 200 m. This indicates that magnetic noise primarily occurs within 200 m of the sea surface. Furthermore, under the same wind speed, the bandwidth narrows and the peak frequency decreases with increasing depth.

[0169] Wave noise power spectra at different wind speeds: Comparison of wave noise power spectra under sea states 5-7, such as... Figure 11 As shown, the subplots represent the wave spectra at sea level and at a depth of 5 m for sea states 5–7. It can be seen that the power spectrum amplitude of the electrical noise is relatively small, and the power of the wave electrical noise increases with increasing sea state. At sea level, the center frequencies of the wave electrical noise for sea states 5–7 are 0.157 Hz, 0.119 Hz, and 0.093 Hz, respectively. At a depth of 5 m, the center frequencies are 0.141 Hz, 0.111 Hz, and 0.088 Hz, respectively. Therefore, with increasing sea state and water depth, the center frequency of the wave electrical noise decreases and also shifts towards lower frequencies.

[0170] The power spectra of wave electrical noise at different sea depths under sea state 7 were analyzed. The conductivity of air was set to 0, thus the electrical noise value in the air was 0. The power spectra of wave electrical noise at four points in the sea at depths of h=0 m, h=20 m, h=50 m, and h=100 m were compared and analyzed. Figure 12 As shown, the amplitude of the electrical noise power spectrum is relatively small, ranging from 0 to 10 μV. 2 / m 2 Within the / Hz range, it approaches 0 after a seawater depth of 50m, and the bandwidth narrows as the seawater depth increases.

[0171] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A method for analyzing the wave noise characteristics in conductive stratified seawater, characterized in that, Includes the following steps: S1. Establish a calculation model for wave-induced electromagnetic field based on stratified seawater with finite conductivity at a certain depth; S2. Solve for the electromagnetic field induced by ocean waves using the conditions of Maxwell's differential equation and the ocean wave equation. S3. Based on the obtained wave-induced electromagnetic field, analyze the noise generated by the wave motion cutting the geomagnetic field and derive the spatial distribution of wave magnetic noise and electrical noise per unit amplitude. S4. Combine wave spectrum analysis to analyze the electromagnetic noise power spectrum of waves under different sea conditions; The wave-induced electromagnetic field calculation model for finite-depth conductive stratified seawater described in S1 includes four media layers: air, two layers of seawater, and the seabed / land. The specific modeling process is as follows: Assuming seawater is an incompressible, nonviscous liquid, meaning its velocity vector is irrotational and divergent, a velocity potential satisfying Laplace's equations is introduced to calculate the velocity vector. ,satisfy Ocean waves propagate along the x-axis; assuming the waves are sine waves with a wave height of a, the wave amplitude is very small relative to the wavelength, the sea surface is approximately a plane, and the velocity potential is as shown in (1): (1) in, The wave angular frequency, For time, For transmission distance, For depth distance, The imaginary unit; At this point, the wave number m satisfies the dispersion equation (2): (2) Where, g = 9.8 m / For gravitational acceleration, ω = 2π / T. For the wave cycle, This represents the total depth of the seawater. The equation for the free water surface is shown in equation (3): (3) The velocities satisfy the boundary conditions of the sea surface and seabed media, respectively, as shown in equations (4) and (5): (4) (5) in, The z-axis component of the seawater velocity field; Therefore, the obtained velocity potential is given by equation (6): (6) The velocity vector is further obtained as shown in equation (7): (7) Assuming the geomagnetic field in the study area remains constant, the geomagnetic vector is given by equation (8): (8) Where F is the total magnitude of the geomagnetic field, which is the angle between the magnetic north direction and the direction of ocean wave propagation. Unit vectors along the X, Y, and Z axes. It is the geomagnetic tilt angle. It is the magnetic declination; Compared to the Earth's magnetic field, the amplitude of the changing magnetic field caused by the induced current is very small and can be ignored. The corrected Ampere circuital law in seawater is shown in equation (9): (9) in, To induce a magnetic field in ocean waves within stratified seawater. These represent the upper and lower layers of seawater, respectively. The electric field induced by ocean waves in stratified seawater; Combining Faraday's law of electromagnetic induction, the wave equation satisfied by the magnetic field in seawater is shown in equation (10): (10) The magnetic permeability is the same in air and seawater, which is μ=4π×10-7 H / m.

2. The method for analyzing wave noise characteristics in stratified seawater with high conductivity according to claim 1, characterized in that, The solution to the wave-induced electromagnetic field described in S2, using the conditions of Maxwell's differential equation and the wave equation, is as follows: Since the magnetic field has a resonant form similar to the velocity in formula (6), the magnetic field vector generated by seawater is as shown in formula (11): (11) Among them, h x and h z These are the magnetic field components along the X and Z axes; Due to the divergence-free nature of the magnetic field Equation (12) is obtained: (12) At the same time h y =0; The electrical conductivity of air is 0, therefore the amount of air containing h z The scalar partial differential equation satisfied by the components is shown in equation (13): (13) The electrical conductivity of seawater and seabed is used to obtain h in seawater and seabed. z The scalar partial differential equations satisfied by the components are shown in equations (14) and (15), respectively: (14) (15) In the formula, , ; The boundary conditions of the electromagnetic field are given by equation (16) for the x and z components of the magnetic field on both sides of the interface at z=0, z=d1 and z=d. (16) The magnetic field components h in each medium are obtained from the wave equation of the magnetic field. z The component general solutions are shown in equations (17)-(20) respectively: (17) (18) (19) (20) The undetermined coefficients C1-C6 are obtained from the boundary condition (16), and the magnetic field component h is further obtained from equation (11). x The expression for this is used to obtain the magnetic field vector of each region; Further, the induced electric field vectors in each part of the medium are obtained from (21). Since the conductivity of air is set to 0, the electric field of air is 0. (21)。 3. The method for analyzing wave noise characteristics in stratified seawater with high conductivity according to claim 2, characterized in that, The spatial distribution of magnetic and electrical noise of unit amplitude ocean waves described in S3 is derived, and the magnetic noise generated by unit amplitude ocean waves with wave periods of T=5 s, 10 s, 20 s and 40 s is analyzed, as well as the electrical noise generated by unit amplitude ocean waves with wave periods of T=5 s, 6 s, 7 s, 8 s, 9 s and 10 s is analyzed.

4. The method for analyzing wave noise characteristics in stratified seawater with high conductivity according to claim 3, characterized in that, The wave electromagnetic noise power spectrum analysis described in S4, performed under different sea conditions, is represented by the PM spectrum. （22） The calculation method for wind speed at different altitudes is as follows: （23） Among them, V Z V represents the wind speed at a point z m above sea level. 10 Z0 represents the wind speed at 10 m above the sea surface, and Z0 is the roughness length. The sea state is calculated using wind speed, and the wind speed at 10 m above sea level is calculated according to formula (24): V 10 （24） B represents the Beaufort scale wind level.

5. The method for analyzing wave noise characteristics in stratified seawater with high conductivity according to claim 4, characterized in that, As described in S4, the electromagnetic noise power spectrum of ocean waves under different sea conditions is analyzed by combining ocean wave spectrum analysis. The air height is set to three points: 0 m, 50 m, and 100 m, and the depth of the upper seawater is set to 100 m. The magnetic noise power spectrum of ocean waves at depths of 0 m, 50 m, and 100 m in the upper seawater and at depths of 150 m, 200 m, and 250 m in the lower seawater is analyzed. The electromagnetic noise power spectrum of ocean waves at depths of 0 m, 20 m, 50 m, and 100 m in the seawater is also analyzed.

6. A system for analyzing the wave noise characteristics in stratified seawater with varying electrical conductivity, characterized in that, The system has a program module corresponding to the steps of any one of the claims 1 to 5 above, and executes the steps in the above-described method for analyzing the wave noise characteristics in conductive stratified seawater when it is run.

7. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program configured to, when invoked by a processor, implement the steps of the method for analyzing wave noise characteristics in conductive stratified seawater as described in any one of claims 1 to 5.

Images