Method for Measuring Shallow Shear Wave Velocity of Seafloor Sedimentary Layer Based on Single Vector Hydrophone

Abstract

The present invention discloses a method for measuring the shear wave velocity of the shallow seafloor sediment layer based on a single vector hydrophone. The measurement method includes: collecting underwater acoustic signals and seafloor interface wave signals excited by a very low frequency sound source near the seafloor using a single vector hydrophone; obtaining the horizontal component of the particle velocity by rotating the quasi-source direction using the horizontal transverse component and the horizontal longitudinal component of the particle velocity; calculating the amplitude ratios between the sound pressure, the vertical component of the particle velocity, and the horizontal component of the particle velocity pairwise, and calculating the interface wave velocity using any one of the amplitude ratios; and finally calculating the shear wave velocity according to the linear relationship between the interface wave and the shear wave. The method of the present invention can accurately estimate the shear wave velocity of the shallow seafloor sediment layer, and the detection device only requires a single vector hydrophone, providing a feasible technical approach for solving long-term problems such as difficult seafloor shear wave measurement, high cost, and low efficiency.

Description

Technical Field

[0001] The present invention relates to the field of hydrophone acquisition and ocean exploration, and in particular to a method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone. Background Art

[0002] The longitudinal wave (P-wave) and shear wave (S-wave) velocities of the seabed sediment layer are crucial seabed geoacoustic parameters. Understanding the S-wave velocity profile of the seabed sediment layer is very important for seabed geotechnical engineering applications, because the S-wave velocity can not only well indicate the hardness of the sediment layer, but also be used to characterize the sediment layer characteristics, seismic exploration and geological disaster assessment. In addition, when the seabed S-wave velocity is high, the S-wave conversion of the seabed P-wave is an important underwater acoustic propagation loss mechanism, which must be considered in the propagation model, sonar performance prediction and sonar equipment design applications. Traditionally, the large-area seabed S-wave velocity field measurement relies on deploying geophones (arrays) on the seabed to collect geoacoustic data, supplemented by core drilling sampling with various seabed samplers or in-situ measurement of local S-wave velocity. The engineering implementation is difficult, costly and inefficient.

[0003] Therefore, there is a need for a high-efficiency, simple and feasible method for estimating the shallow shear wave velocity of the seabed sediment layer. Summary of the Invention

[0004] The purpose of the present invention is to propose a method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone in view of the deficiencies of the prior art. This method is a new method for measuring the shallow seabed S-wave velocity based on the polarization ellipticity of the particle vibration velocity and the curl of the sound intensity of a single vector hydrophone. Only one vector hydrophone is required, and it can be carried on an underwater unmanned platform to realize the rapid measurement of the seabed S-wave velocity in a large area.

[0005] The purpose of the present invention is achieved through the following technical solutions: A method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone, comprising the following steps:

[0006] Step 1: Excite interface waves with a near-seabed sound source;

[0007] Step 2: Use a single vector hydrophone to collect the underwater acoustic signals excited by the near-seabed sound source;

[0008] Step 3: Calculate the sound pressure p, the vertical component v of the particle vibration velocity z and the horizontal component v of the particle vibration velocity r The amplitude ratios between each pair;

[0009] Step 4: Calculate the interface wave velocity using the amplitude ratios;

[0010] Step 5: Calculate the shear wave velocity using the linear relationship between the interface wave and the shear wave.

[0011] Further, in the step 1, the sound source for exciting the interface wave is a very low frequency point source, and the grazing angle of the sound source The excited interface wave is a Scholte wave, c0 is the sound speed of the water body, v p is the phase velocity of the Scholte wave on the seabed, k is the wave number, the angular frequency ω = 2πf, and f is the signal frequency.

[0012] Further, in the step 2, the single vector hydrophone can be carried on an underwater unmanned platform to realize the rapid measurement of the shear wave velocity of the seabed in a large area in a mobile manner.

[0013] Further, in the step 2, the deployment depth of the very low frequency sound source and the single vector hydrophone should be within the water depth less than or equal to 1 / 2 of the Scholte wave wavelength from the seabed, and the horizontal distance between the very low frequency sound source and the single vector hydrophone should be greater than 100 m.

[0014] Further, in the step 2, the obtained underwater acoustic signal is the sound pressure information p, the horizontal transverse component v x of the particle velocity, the horizontal longitudinal component v y of the particle velocity, and the vertical component v z of the particle velocity.

[0015] Further, in the step 3, the horizontal component v r of the particle velocity is obtained by rotating the quasi-source direction from the horizontal transverse component v x of the particle velocity and the horizontal longitudinal component v y of the particle velocity.

[0016] Further, in the step 3, according to the displacement potential function of the Scholte wave in the elastic seabed ideal waveguide water, the analytical expression of the sound pressure the analytical expression of the particle velocity and

[0017] are obtained. Among them, the subscript r represents the horizontal component, the subscript z represents the vertical component, i is the imaginary number, t is the time interval, v r0 is the horizontal velocity of the Scholte wave, v z0 is the vertical velocity of the Scholte wave, A is an arbitrary coefficient, r is the transceiver distance, z is the receiving depth, the attenuation coefficient the wave number where c sch is the Scholte wave velocity; the angular frequency ω = 2πf, f is the signal frequency, the wave number in water c0 is the sound speed in water, and ρ0 is the density of the seawater body.

[0018] Further, in the step 3, the ratio of the sound pressure to the horizontal component of the particle velocity is The ratio of the sound pressure to the vertical component of the particle velocity is The ratio of the amplitude of the vertical component of the particle velocity to the horizontal component of the particle velocity is

[0019] Further, in the step 4, the Scholte wave velocity is given by the following formula:

[0020] Further, in the step 5, the linear relationship between the interface wave and the shear wave is obtained from the following dispersion equation

[0021]

[0022] where c s is the shear wave velocity in water, c p is the longitudinal wave sound velocity of the seabed, D is the water depth, ρ1 is the elastic seabed density, and ρ0 is the sea water density;

[0023] The Scholte wave velocity is slightly less than 90% of the shear wave velocity of the seabed.

[0024] Advantages of the present invention: The present invention is directed to extracting the shear wave velocity in the shallow part of the seabed sediment layer applied to a large area. By using an underwater unmanned platform equipped with a single vector hydrophone and a very low frequency sound source, the equipment and deployment costs in the process of extracting the shear wave velocity in the shallow part of the seabed sediment layer are greatly reduced; and compared with the traditional method of collecting geoacoustic data by deploying geophones (arrays) on the seabed supplemented by core drilling sampling with various seabed samplers or in-situ measurement methods of local S-wave velocity, the present invention can realize the extraction of the shear wave velocity in the shallow part of the seabed sediment layer in a large area in a mobile manner. Compared with the traditional method, the present invention has lower cost, higher working efficiency, and can provide accurate shear wave velocity. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic flow chart of the method provided by the embodiment of the present invention;

[0026] Figure 2 is a schematic geometric model diagram of the received signal applied by the embodiment of the present invention;

[0027] Figure 3 is a schematic diagram of the main energy propagation limitation near the seabed interface of the Scholte wave provided by the embodiment of the present invention;

[0028] Figure 4 is a schematic diagram of the phase velocity and group velocity dispersion of the Scholte wave with finite water depth provided by the embodiment of the present invention;

[0029] Figure 5 is the simulation result of the Scholte wave dispersion curve under the sea trial environment parameters provided by the embodiment of the present invention;

[0030] Figure 6 Schematic diagram of the grazing angle for Scholte wave excitation provided by the embodiments of the present invention. Specific embodiments

[0031] The following further elaborates on the specific embodiments of the present invention with reference to the accompanying drawings.

[0032] As Figure 1 shown, this embodiment provides a method for measuring the shear wave velocity in the shallow layer of the seabed sediment layer using a single vector hydrophone, including the following steps:

[0033] Step 1: As Figure 2 shown, an interface wave is excited by a near-seabed sound source;

[0034] Since all vertical wave numbers of Scholte waves are imaginary numbers, resulting in their inability to be excited by incident plane waves, a very low-frequency sound source is used to excite the Scholte wave at the seabed interface. As Figure 6 shown, the grazing angle θ0 of the sound source takes a value of where c0 is the sound velocity in the seawater body, and v p is the phase velocity of the Scholte wave at the seabed; k is the wave number, the angular frequency ω = 2πf, and f is the signal frequency.

[0035] Since the amplitude of the Scholte wave decays exponentially as it moves further away from the interface, as Figure 3 shown, under normal circumstances, the main energy propagation of the Scholte wave is restricted within a flat waveguide with half a wavelength of the water body above the seabed interface and one wavelength of the sediment layer below. Therefore, the deployment depths of the very low-frequency sound source and the single vector hydrophone should be within the water depth less than or equal to 1 / 2 of the Scholte wave wavelength from the seabed, approximately 5m.

[0036] Since too short a receiving distance will cause waveform aliasing and interference, the deployment horizontal distance of the very low-frequency sound source and the single vector hydrophone should be at least greater than 100m.

[0037] Step 2: As Figure 2 shown, a single vector hydrophone is used to collect the underwater acoustic signal excited by the near-seabed sound source; the vector hydrophone collects the acoustic pressure information p, the horizontal transverse component v x of the particle velocity, the horizontal longitudinal component v y of the particle velocity, and the vertical component v z of the particle velocity, a total of four components of underwater acoustic information.

[0038] Step 3: Perform operations on the obtained acoustic pressure p, the particle velocity v x , v y , v z according to the theoretical formula to obtain the amplitude ratio between the components.

[0039] Rotate the particle velocity components \(v\) x and \(v\) y in the direction of the rotation quasi-source to obtain the horizontal component \(v\) of the particle velocity r ; According to the displacement potential function of Scholte waves in water in an elastic seabed ideal waveguide: z ≤ 0;

[0040] where the attenuation coefficient the wave number the wave number in water \(c_0\) is the sound speed in water, \(\rho_0\) is the water body density. The subscript \(z\) represents the vertical component, \(i\) is the imaginary number, \(t\) is the time interval, \(A\) is an arbitrary constant coefficient determined by the sound source conditions, \(r\) is the transceiver distance, \(z\) is the receiving depth, the attenuation coefficient the wave number where \(c\) Sch is the Scholte wave velocity; the angular frequency \(\omega = 2\pi f\), \(f\) is the signal frequency, the wave number in water \(c_0\) is the sound speed in water, \(\rho_0\) is the seawater density.

[0041] According to the relationship between the displacement potential function and the sound pressure \(p\) and the particle velocity \(v\):

[0042] The analytical expressions of the sound pressure \(p\) and the particle velocity \(v\) can be obtained:

[0043]

[0044] The ratios of the sound pressure to the particle velocity components are respectively:

[0045] where the subscript \(r\) represents the horizontal component and the subscript \(z\) represents the vertical component.

[0046] In fact, the velocity channels of vector hydrophones generally use equivalent plane wave sound pressure calibration and have the same physical dimension of Pascal as the sound pressure. Therefore, the following dimensionless ratios are obtained:

[0047] \(\eta\) r is the ratio of the sound pressure of the vector hydrophone to the output amplitude of the horizontal component of the particle velocity, \(\eta\) z is the ratio of the sound pressure of the vector hydrophone to the output amplitude of the vertical component of the particle velocity. At this time, the dimension of the particle velocity adopts the same dimension as the water body sound pressure, both being Pascal. \(v\) r0 is the horizontal velocity of the Scholte wave, \(v\) z0 is the vertical velocity of the Scholte wave.

[0048] Since the horizontal component and the vertical component of the particle velocity have the same dimension, the ratio of the amplitude of the vertical component to the horizontal component of the particle velocity can be obtained:

[0049] Step 4: Using the ratio of the amplitudes of each component obtained in Step 3, the velocity estimate of the Scholte wave can be obtained according to the following formula:

[0050] c sch = η r c0

[0051]

[0052] Step 5: Calculate the shear wave velocity using the linear relationship between the surface wave and the shear wave.

[0053] In the case of finite water depth D, the sound wave propagates through the reflection of the sea surface and the seabed in the waveguide, describing the velocity c of the Scholte wave in water sch and the shear wave velocity c s of the seabed. The dispersion equation of the relationship is as follows:

[0054]

[0055] where c s is the shear wave velocity in water, c p is the longitudinal wave velocity of the seabed, D is the water depth, ρ1 is the density of the elastic seabed, and ρ0 is the density of the seawater body; since the Scholte wave velocity is 90% of the shear wave velocity, using the Scholte wave velocity c obtained in Step 3 sch According to the linear relationship between the Scholte wave velocity and the shear wave velocity, the final shear wave velocity is obtained, that is, the shear wave velocity is extracted.

[0056] Figure 4 is the schematic diagram of the relationship between the theoretical Scholte wave velocity and the shear wave velocity c s of the seabed under different f×D conditions. The abscissa in the figure is the product of the frequency f and the water depth D, and the ordinate is the ratio of the Scholte wave velocity to the shear wave velocity of the seabed, defined as the normalized velocity. It can be seen that when f×D≥150, the Scholte wave velocity tends to be stable and is slightly less than 90% of the shear wave velocity c s of the seabed; Figure 5 is the Scholte wave dispersion curve obtained by using the finite element method provided by ComSol multi-physics software. When the frequency of the Scholte wave is greater than 15 Hz, the Scholte wave velocity stably tends to 360 m / s, which is about 90% of the shear wave velocity of the seabed. At this time, f×D≥150, which is consistent with Figure 4 the theoretical result (simulated shear wave velocity 400 m / s). It shows that the present invention can extract the shear wave velocity that conforms to the actual situation.

[0057] The above embodiments are used to explain the present invention rather than limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims of the present invention fall within the protection scope of the present invention.

Claims

1. A method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone, characterized in that, It includes the following steps: Step 1: Excite interface waves with a near-seabed sound source; Step 2: Use a single vector hydrophone to collect the underwater acoustic signals excited by the near-seabed sound source; Step 3: Calculate the sound pressure p, the vertical component v of the particle velocity z and the horizontal component v of the particle velocity r The amplitude ratios between each pair of them; Step 4: Calculate the interface wave velocity using the amplitude ratio; in the said Step 4, the Scholte wave velocity c Sch is given by the following formula: where γ r is the ratio of the sound pressure to the horizontal component of the particle vibration velocity, ρ0 is the density of the sea water body, c0 is the sound velocity of the water body, and γ z is the ratio of the sound pressure to the vertical component of the particle vibration velocity, and η is the ratio of the amplitude of the vertical component of the particle vibration velocity to the amplitude of the horizontal component of the particle vibration velocity; Step 5: Calculate the shear wave velocity by using the linear relationship between the interface wave and the shear wave.

2. The method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone according to claim 1, wherein In the above step 1, the sound source for exciting the interface wave is a very low frequency point source, and the grazing angle θ0 of the sound source takes a value of The excited interface wave is a Scholte wave, c0 is the sound speed in water, v p is the phase velocity of the Scholte wave at the seabed, k is the wave number, the circular frequency ω = 2πf, and f is the signal frequency.

3. A method for measuring the shallow shear wave velocity of a seabed sediment layer based on a single vector hydrophone according to claim 1, characterized in that In the said Step 2, the single vector hydrophone can be carried on an underwater unmanned platform to achieve rapid measurement of the shear wave velocity of the seabed in a large mobile area.

4. A method for measuring the shallow shear wave velocity of a seabed sediment layer based on a single vector hydrophone according to claim 2, characterized in that In the said Step 2, the deployment depths of the very low frequency sound source and the single vector hydrophone should be within the water depth where the height from the seabed is less than or equal to 1 / 2 of the Scholte wave wavelength, and the horizontal distance between the very low frequency sound source and the single vector hydrophone should be greater than 100 m.

5. A method for measuring the shallow shear wave velocity of a seabed sediment layer based on a single vector hydrophone according to claim 1, characterized in that, In the said step 2, the obtained underwater acoustic signal is the sound pressure p, the horizontal transverse component v x of the particle vibration velocity, the horizontal longitudinal component v y of the particle vibration velocity, and the vertical component v z of the particle vibration velocity.

6. The method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone according to claim 5, wherein In the said step 3, the horizontal component v of the particle vibration velocity r is obtained by rotating the quasi-source direction from the horizontal transverse component v x of the particle vibration velocity and the horizontal longitudinal component v y of the particle vibration velocity.

7. A method for measuring the shallow shear wave velocity of the seabed sediment layer based on a single vector hydrophone according to claim 2, wherein, In step 3, according to the displacement potential function of Scholte wave in water with an elastic seabed ideal waveguide the analytical expression of sound pressure is obtained the analytical expression of particle vibration velocity and where \(i\) is the imaginary number, \(t\) is the time interval, and \(v\) r0 is the horizontal vibration velocity of the Scholte wave, and \(v\) z0 is the vertical vibration velocity of the Scholte wave. \(A\) is an arbitrary constant coefficient determined by the sound source condition, \(r\) is the transceiver distance, \(z\) is the receiving depth, and the attenuation coefficient wave number where \(c\) Sch is the Scholte wave velocity; the circular frequency \(\omega = 2\pi f\), \(f\) is the signal frequency, and the wave number in water \(c_0\) is the sound velocity of the water body, and \(\rho_0\) is the density of the seawater body.

8. A method for measuring the shear wave velocity of shallow sea floor sediment layers based on a single vector hydrophone according to claim 7, characterized in that, In step 3, the ratio of the sound pressure to the horizontal component of the particle velocity is The ratio of the sound pressure to the vertical component of the particle velocity is The ratio of the amplitude of the vertical component of the particle velocity to the horizontal component of the particle velocity is 9. A method for measuring the shallow shear wave velocity of a seabed sediment layer based on a single vector hydrophone according to claim 8, characterized in that In the said Step 5, the linear relationship between the interface wave and the shear wave is obtained from the following dispersion equation: where c s is the shear wave velocity at the seabed, c p is the compressional wave velocity at the seabed, D is the water depth, ρ1 is the density of the elastic seabed, and ρ0 is the density of the sea water body.

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