A method for detecting the reflection coefficient of sound waves between the sediment layers of a river bed

Abstract

The application discloses a method for detecting the sound wave reflection coefficient between river bottom silt layers, and comprises the following steps: step one, obtaining the silt parameters of the first layer of silt through mechanical sampling or calculation; step two, setting the towfish of a shallow ground profile instrument at 2 meters below the water surface, and making the towfish emit sound waves upward to obtain the amplitude of the reflection wave between water and air; and step three, calculating the reflection coefficient of the bottom of the first layer of silt according to the silt parameters of the first layer of silt. The application can obtain the wave impedance parameters of the second layer of silt and enrich the silt information; the wave impedance parameters of the second layer of silt at the bottom of water can be obtained by only using the shallow ground profile instrument, so that the manpower and material resources are saved, and the work efficiency is improved; and data support is provided for the researches on river dynamics and silt resource utilization.

Description

Technical Field

[0001] This invention belongs to the fields of acoustics and geophysics, and specifically relates to a method for detecting the sound wave reflection coefficient between layers of sediment at the bottom of a river. Background Technology

[0002] Currently, the physical properties (density, permeability, porosity, etc.) of surface sediment in riverbeds are mainly obtained through mechanical sampling. However, the disturbance inherent in mechanical sampling leads to deviations in the test results. Furthermore, mechanical sampling is labor-intensive and resource-intensive, resulting in low efficiency. Acoustic methods are also commonly used to detect the characteristics of riverbed sediments, but currently, they primarily rely on sonar images to determine the sediment deposition structure, and effective methods for extracting sediment physical properties are still lacking.

[0003] The acoustic reflection coefficient is the ratio of the reflected intensity to the incident wave intensity when a sound wave encounters the interface between two media during propagation. In riverbed sediment layer detection, given the density and wave velocity of the upper sediment layer, the density and wave velocity of the lower sediment layer, among other related parameters, can be deduced from the acoustic reflection coefficients of the upper and lower layers. Therefore, the acoustic reflection coefficient can characterize information such as the hardness and softness of the lower sediment layer, providing crucial data support for river dynamics studies such as riverbed scouring. Conventional detection methods rely on elastic medium models to study sediment reflection coefficients, but these methods have significant errors. Porous medium models can more accurately describe the propagation characteristics of sound waves in sediment layers. This invention, based on a porous medium model, proposes a method for extracting the acoustic reflection coefficient between sediment layers, improving the accuracy of the detection results. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for detecting the acoustic reflection coefficient between riverbed sediment layers using a shallow seismic profiler. This method is based on the acoustic theory of porous media, and by analyzing the signals received by the shallow seismic profiler, the acoustic information of the shallow seismic profiler is extracted to obtain the reflection coefficient between riverbed sediment layers.

[0005] The technical solution adopted in this invention is as follows:

[0006] A method for detecting the acoustic wave reflection coefficient between layers of sediment at the bottom of a river includes the following steps:

[0007] Step 1: Obtain the sediment parameters of the first layer of sediment through mechanical sampling or calculation;

[0008] Step 2: Set the towed fish of the shallow seismic profiler 2 meters below the water surface and make the towed fish emit sound waves upward to obtain the amplitude A(f) of the reflected wave between water and air.

[0009] Step 3: Calculate the reflection coefficient at the bottom of the first layer of sediment based on the parameters of the first layer of sediment.

[0010] Let T be the transmission coefficient of the sound wave incident from the water to the first layer of sediment. ws Then, through the following system of linear equations

[0011]

[0012] Calculate the transmission coefficient T ws ;

[0013] In the formula: and These are the transmission coefficients for the fast longitudinal wave P1 and the slow longitudinal wave P2, respectively. ρ is the reflection coefficient of the fast longitudinal wave; k1 and k2 are the wave numbers of P1 and P2 waves, respectively; f k is the density of water. w ω is the wave number of water; C is the circular frequency; and C is the porosity parameter.

[0014] m1 and m2 are respectively derived from the following formulas:

[0015]

[0016]

[0017] After solving the system of equations, That is, T ws ;

[0018] Let T be the transmittance coefficient of the first layer of sediment incident into the water. sw ,

[0019]

[0020] After solving the above system of linear equations, That is, T ws ;

[0021] Let R be the reflection coefficient of the sediment incident from the first layer to the second layer. ss If the distance between the shallow seismic profiler's towed fish and the seabed is r1, the thickness of the first layer of sediment is h1, and the acoustic attenuation coefficient is α, then the signal S(f) received by the sonar after reflection from the bottom of the first layer of sediment can be expressed as:

[0022]

[0023] Therefore, the reflectance at the bottom of the first layer of sediment can be obtained as follows:

[0024]

[0025] Furthermore, the wave numbers k1 and k2 of the P1 wave and P2 wave are respectively

[0026]

[0027]

[0028] in

[0029] a = C 2 -HM

[0030]

[0031]

[0032] In the above formula

[0033] ρ=βρ f +(1-β)ρ s

[0034]

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041]

[0042]

[0043]

[0044] τ x =K0τ z

[0045] τ y =K0τ z

[0046] τ z =(1-β)(ρ s -ρ f )gz

[0047] In the formula: ρ is the density of the sediment; h is the viscosity of the pore water, taken as 0.001 kg / m³ / s; κ is the permeability; F is the deviation of the pore fluid as the frequency increases; ρ f and ρ s Let K be the pore fluid density and the sediment particle density, respectively, and both be known constants. band K f These are the sediment skeleton modulus and the bulk modulus of pore fluid, respectively, where K f The value is 2.395 × 10⁹ Pa; K γ The bulk modulus of the sediment particles is taken as 3.2 × 10⁻⁶. 10 Pa; μ is the shear modulus of the sediment framework; δ is the dynamic Poisson's ratio of the sediment framework, ranging from 0.1 to 0.3, with an intermediate parameter K0 = 0.5; g is the gravitational acceleration; z is the depth below the water-sediment interface; and the volume logarithmic decay rate δ b =0.1, shear logarithmic decay rate δ s =0.1; j is the imaginary unit; β is porosity; τ is the weight of the overlying sediment; τ x ,τ y ,τ z These represent the forces in the x, y, and z directions, respectively; K br This represents the bulk modulus of the skeleton.

[0048] Furthermore, the deviation F of the pore fluid as the frequency increases is determined by the following formula:

[0049]

[0050]

[0051]

[0052]

[0053] Furthermore, the sound wave attenuation coefficient α = Im{k1}, where Im is the imaginary part.

[0054] The beneficial effects of this invention are:

[0055] 1. This invention can obtain parameters such as the wave impedance of the second layer of sediment, enriching the information on silted sediment;

[0056] 2. The wave impedance and other parameters of the second layer of sediment at the bottom of the water can be obtained using only a shallow bottom profiler, saving manpower and resources and improving work efficiency.

[0057] 3. This invention provides data support for research on river dynamics and sediment resource utilization. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of sound wave propagation in the present invention;

[0059] Figure 2 This is a cross-sectional view of sonar detection. Detailed Implementation

[0060] This invention discloses a method for detecting the acoustic reflection coefficient between layers of sediment at the bottom of a river. It requires mechanical sampling and calculation results as a basis, and the relevant sediment reflection coefficient can be obtained by receiving signals using a shallow seismic profiler. The wave impedance and other parameters of the lower sediment layers can then be calculated using the parameters of the upper sediment layers. Compared with mechanical sampling, this method is faster and more efficient.

[0061] The formulas and parameters involved in this invention are as follows:

[0062] k is the wave number. ρ is the angular frequency, f is the frequency, C is the wave velocity in the porous medium, and ρ is the wave velocity in the porous medium. f and ρ s Here, ρ represents the pore fluid density and sediment particle density, respectively, and are known constants; h represents the pore water viscosity, with a value of 0.001 kg / m³ / s; κ is the permeability; K... γ ρ is the bulk modulus of sediment particles; μ is the shear modulus of the sediment framework; K b and K f These are the sediment skeleton modulus and the bulk modulus of pore fluid, respectively, where K f The value is 2.395 × 10⁹ Pa; K γ The bulk modulus of the sediment particles is taken as 3.2 × 10⁻⁶. 10 Pa; F represents the deviation of the pore fluid as the frequency increases; the wave velocity of water is c. ω =1500kg / m 3 ,

[0063]

[0064] ρ=βρ f +(1-β)ρ s (2)

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075] τ x =K0τ z (13)

[0076] τ y =K0τ z (14)

[0077] τ z =(1-β)(ρ s -ρ f )gz (15)

[0078] The value of δ ranges from 0.1 to 0.3, with an intermediate parameter K0 = 0.5. g is the acceleration due to gravity, z is the depth below the water-sediment interface, and the volumetric logarithmic decay rate δ... b =0.1, shear logarithmic decay rate δ s =0.1; j is the imaginary unit; β is porosity; τ is the weight of the overlying sediment; τ x ,τ y ,τ z These represent the forces in the x, y, and z directions, respectively; K br This represents the bulk modulus of the skeleton.

[0079] F is determined by equations (16)-(19):

[0080]

[0081]

[0082]

[0083]

[0084] The attenuation coefficient α is determined by equations (20)-(24):

[0085] a = C 2 -HM (20)

[0086]

[0087]

[0088]

[0089] The attenuation coefficient α = Im{k1}, where Im represents the imaginary part (24).

[0090] The reflection coefficient R at the interface between the first and second sediment layers ss The calculation method is as follows.

[0091] The premise of this invention, Figure 1 The parameters of the first sediment layer are known. These parameters can be obtained through sampling tests or calculations.

[0092] like Figure 1 As shown, the sound waves emitted by the sonar are reflected at the bottom of the first layer of sediment and received by the sonar. This process involves two transmissions (in opposite directions) at the water-sediment interface and reflection at the bottom of the first layer of sediment, resulting in energy loss. Furthermore, the diffusion of the sound waves and their propagation within the first layer of sediment both contribute to energy attenuation. Let the transmission coefficient of the sound waves from the water to the first layer of sediment be T. ws The transmittance of the first layer of sediment into the water is T. sw The reflection coefficient of the sediment incident from the first layer to the second layer is R. ss If the distance between the shallow subsurface profiler's towed fish and the seabed is r1, the thickness of the first layer of sediment is h1, and the acoustic attenuation coefficient is α (unit: nepers / meter), then the signal S(f) received by the sonar after reflection from the bottom of the first layer of sediment can be expressed as:

[0093]

[0094] Therefore, the reflectance at the bottom of the first layer of sediment can be obtained as follows:

[0095]

[0096] The method for determining A(f) in equation (26) is as follows:

[0097] The towed fish of the shallow seismic profiler is placed in the water at a depth of 2m below the surface. The towed fish emits sound waves upwards, and the sound wave reflection interface is water-air. The sonar signal received by the shallow seismic profiler is then recorded. Due to the large difference in wave impedance between water and air, the sound wave can be considered as total internal reflection, and the amplitude of the reflected wave can be considered as A(f).

[0098] In equation (26), T ws The calculation method is as follows

[0099] After obtaining the parameters of the first layer of sediment, in equation (27), and is an unknown, while the coefficients of the other equations are known. Equation (27) is a system of linear equations.

[0100]

[0101] in and The transmission coefficients are the transmission coefficients of the fast longitudinal wave P1 and the slow longitudinal wave P2, respectively. ρ is the reflection coefficient of the fast longitudinal wave; k1 and k2 are the wave numbers of P1 and P2 waves, respectively.f k is the density of water. w The wave number of water.

[0102] m1 and m2 are determined by equations (28) and (29) respectively:

[0103]

[0104]

[0105] After solving the system of equations, That is, T ws .

[0106] In equation (26), T sw The calculation method is as follows

[0107]

[0108] After solving the linear system of equations (30), That is, T ws .

[0109] like Figure 2 The image shown is a shallow seismic profiler sonar image of section 27 in the Xiaolangdi Reservoir area of ​​the Yellow River.

[0110] The porosity of the first layer of sediment is 0.618, and the permeability is 2.02 × 10⁻⁶. -13 m 2 The wave velocity was 1597 m / s. The calculated reflection coefficient of the first and second layers of sediment at this measuring point was 0.28.

Claims

1. A method for detecting the acoustic wave reflection coefficient between layers of sediment at the bottom of a river, characterized in that, Includes the following steps: Step 1: Obtain the sediment parameters of the first layer of sediment through mechanical sampling or calculation; Step two: Set the towed fish of the shallow seismic profiler 2 meters below the water surface and make the towed fish emit sound waves upwards to obtain the amplitude of the reflected waves between the water and the air. ; Step 3: Calculate the reflection coefficient at the bottom of the first layer of sediment based on the parameters of the first layer of sediment. Let the transmission coefficient of the sound wave incident from the water to the first layer of sediment be... Then, through the following system of linear equations Calculate the transmission coefficient ; In the formula: and These are the transmission coefficients for the fast longitudinal wave P1 and the slow longitudinal wave P2, respectively. The reflection coefficient of a fast longitudinal wave; and These are the wave numbers of P1 and P2 waves, respectively. The density of water; The wave number of water; ω is the angular frequency; C is the porosity parameter of the medium. and They are derived from the following formulas: After solving the system of equations, That is ; Let the transmittance of the first layer of sediment incident into the water be... , After solving the above system of linear equations, That is ; Let the reflection coefficient of the sediment incident from the first layer to the second layer be... The distance between the shallow seismic profiler towed fish and the seabed is The thickness of the first layer of mud and sand is The sound wave attenuation coefficient is The signal received by the sonar after reflection from the bottom of the first layer of sediment It can be represented as Therefore, the reflectance at the bottom of the first layer of sediment can be obtained as follows:

2. The method for detecting the acoustic reflection coefficient between layers of riverbed sediment according to claim 1, characterized in that, The wave numbers of P1 and P2 waves and They are respectively in In the above formula In the formula: Density of the sediment mass; The viscosity of pore water is taken as 0.001 kg / m*s; For penetration rate; This refers to the deviation in pore fluid as frequency increases; and These are the pore fluid density and the sediment particle density, respectively, and are known constants; and These are the sediment skeleton modulus and the bulk modulus of pore fluids, respectively. The value is 2.395 × 10⁹ Pa; The bulk modulus of the sediment particles is taken as 3.2 × 10⁻⁶. 10 Pa; The shear modulus of the sediment-sand frame; The dynamic pine-pine ratio of the sediment skeleton ranges from 0.1 to 0.3, with intermediate parameters. , Let be the acceleration due to gravity, z be the depth below the water-sediment interface, and the volumetric logarithmic decay rate be... Log-shear decay rate j is the imaginary unit; Porosity The weight of the overlying silt and sand; , , These represent forces in the x, y, and z directions, respectively. This represents the bulk modulus of the skeleton.

3. The method for detecting the acoustic reflection coefficient between layers of riverbed sediment according to claim 2, characterized in that, The deviation of the pore fluid with increasing frequency It is determined by the following formula:

4. The method for detecting the acoustic reflection coefficient between layers of riverbed sediment according to claim 1, characterized in that, The sound wave attenuation coefficient In the formula, Im is the imaginary part.

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