Method for determining sound velocity and attenuation coefficient of infrasonic wave of unsaturated soil slope

Abstract

The invention provides an unsaturated soil slope infrasonic wave sound velocity and attenuation coefficient determination method, which comprises the following steps of 1, extracting hypothesis based on a Biot-Gertsma low-frequency limit theory and a boundary layer viscosity loss model, and obtaining basic parameters; 2, adopting a Brutsaert correction'equal strain 'model to determine a three-phase composite fluid modulus; 3, calculating equivalent medium parameters such as equivalent density and dry skeleton modulus; 4, calculating a complex wave number after viscosity loss correction; and 5, respectively determining the sound velocity c and the attenuation coefficient alpha according to the real part and the imaginary part of the complex wave number. According to the method, double models are coupled to synchronously output the sound velocity and the attenuation coefficient, the slope instability early warning threshold value can be calibrated more accurately, and a more comprehensive sound characteristic theoretical basis is provided for slope instability early warning; the three-phase coupling effect (skeleton deformation, fluid viscosity and gas-liquid interface action) is fully considered, it is ensured that acoustic characteristic parameters can truly reflect the internal physical state of unsaturated soil, and high-reliability support is provided for monitoring data analysis.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of engineering geology and disaster monitoring, and particularly relates to a method for determining the speed and attenuation coefficient of infrasound waves in unsaturated soil slopes. BACKGROUND

[0002] The instability and destruction of soil slopes (such as landslides and collapses) are frequent geological disasters in the fields of transportation and water conservancy, which not only damage roadbeds, dams and other engineering structures, but also can cause casualties and huge economic losses. Infrasound monitoring technology captures low-frequency acoustic signals (1-100 Hz) generated in the process of micro-damage and sliding evolution in soil, thereby identifying early signs of slope instability. This technology has the characteristics of long-distance propagation, wide monitoring range, strong anti-interference ability, and broad prospects in the field of soil slope disaster monitoring. However, soil slopes in engineering are usually in a non-saturated state, which is essentially a "solid-liquid-gas" three-phase coexisting porous medium. The propagation behavior of infrasound waves in this medium is affected by the three-phase coupling effect, which makes it difficult to accurately quantify the core acoustic characteristic parameters (speed and attenuation coefficient) of infrasound waves, directly restricting the transformation of infrasound monitoring from laboratory research to engineering application.

[0003] Currently, some classic models and calculation methods have been developed for the study of acoustic wave propagation in unsaturated soil. The most representative of these include the Brutsaert static sound speed model, the Biot three-phase medium acoustic theory, and numerical simulation-based calculation schemes. For example, the Brutsaert model, as an early widely used three-phase medium sound speed calculation model, can only calculate the static sound speed and ignores the skeleton shear modulus and fluid viscous loss, making it impossible to output the attenuation coefficient. The single parameter cannot meet the demand for accurate monitoring, greatly limiting the engineering value of the model. At the same time, the existing acoustic characteristic calculation methods in the prior art rely on numerical simulation and require a large number of iterations and complex parameter inputs, such as local jet flow coefficients and turbulence correction terms. These methods cannot directly calculate acoustic characteristics from conventional soil force parameters, making them difficult to quickly apply in engineering sites. In addition, existing models do not explicitly state the simplification conditions for low-frequency sound waves, resulting in a calculation error of more than 20% when high-frequency assumptions are used, and a large deviation from field measurement data.

[0004] In summary, the existing calculation of infrasound wave characteristics in unsaturated soil slopes has the core defects of single function (only calculating sound speed, no attenuation coefficient), non-closed calculation system (relying on complex parameters and iterations), and poor frequency adaptability (large error at low frequencies), which makes it difficult to quantify acoustic characteristics and obtain parameters in engineering.

[0005] Therefore, there is a need to provide an improved technical solution to address the above-mentioned deficiencies in the prior art. SUMMARY

[0006] The application aims to provide a method for determining the speed and attenuation coefficient of infrasonic waves of unsaturated soil slope, so as to solve or alleviate the problems in the prior art.

[0007] In order to achieve the above-mentioned purpose, the application provides the following technical scheme: The application provides a method for determining the speed and attenuation coefficient of infrasonic waves of unsaturated soil slope, which is based on the Biot-Geertsma low-frequency limit theory of three-phase porous medium and the boundary layer viscous loss model, realizes the rapid and accurate determination of the speed and attenuation coefficient of infrasonic waves of unsaturated soil slope by constructing a closed-form calculation link of "composite fluid modulus→ equivalent medium parameter→ complex wave number→ speed / attenuation coefficient", and the steps are as follows: Step one, basic assumptions are proposed based on the core applicable conditions of the Biot-Geertsma low-frequency limit theory of three-phase porous medium and the boundary layer viscous loss model, and basic parameters are obtained through field test and specification value; Step two, the three-phase composite fluid modulus is determined based on the Brutsaert modified "equal strain" model; Step three, the equivalent medium parameters are calculated, including the equivalent density, dry skeleton bulk modulus, skeleton shear modulus and equivalent bulk modulus; Step four, viscous loss correction is performed, and the complex wave number is calculated; Step five, the speed c and attenuation coefficient a of infrasonic waves are determined; the speed c is calculated from the real part of the complex wave number, and the attenuation coefficient a is calculated from the imaginary part of the complex wave number.

[0008] Further, the basic assumptions include: (1) the unsaturated soil slope shallow soil is regarded as a locally uniform and transversely isotropic "solid-liquid-gas" three-phase porous medium, which is convenient for estimation by volume ratio and uniform pore size. However, if the site is obviously layered or has large-scale heterogeneity (such as interlayer cracks), the model prediction may have errors; (2) the frequency of infrasonic waves satisfies 1Hz≤ ≤100Hz, which is much lower than the Biot characteristic frequency ( >1kHz), which meets the "low-frequency limit" assumption; under the low-frequency limit, the fluid inertia term can be ignored, so that the Biot equation is simplified to an algebraic form, thereby obtaining a closed-form analytical expression. However, if the actual frequency of infrasonic waves is close to or exceeds , ignoring the fluid inertia term will result in significant errors; (3) the soil is in a small strain state (strain ε<10 -4), the skeleton is only considered to be coupled with the fluid in terms of mass, and the effects of local injection, turbulence and surface tension are ignored. The small strain assumption, which only considers mass coupling, ignores local injection, turbulence, surface tension, ensures linear elastic skeleton, avoids nonlinear flow dissipation terms, and realizes a simplified model. However, when microcrack propagation, bubble vibration or liquid-gas interface dynamics occur before collapse (such as low saturation), injection, turbulence and interface tension effects may be significant. The slope characteristics applicable to the present application are: unsaturated soil subgrade slopes that are still in the stable period or the initial stage of creep deformation, have not developed macroscopic through cracks or entered the large deformation failure stage, and have relatively uniform soil structure. Such subgrade slopes meet the prerequisites for linear elastic skeleton and laminar flow, and meet the low-frequency limit and small strain assumption of the present method.

[0009] Further, in step one, the basic parameters include skeleton parameters, fluid parameters, soil structure parameters, subsonic wave parameters and water saturation; the skeleton parameters include skeleton solid bulk modulus, skeleton solid density, soil Poisson's ratio, skeleton viscoelastic loss factor; the fluid parameters include water bulk modulus, air bulk modulus, water density, air density, water dynamic viscosity, air dynamic viscosity; the soil structure parameters include soil porosity, soil absolute permeability; the subsonic wave parameters include subsonic wave angular frequency. The symbols, definitions, units and value ranges / sources of the basic parameters are shown in Table 1.

[0010] Table 1 Information table of basic parameters

[0011] Further, in step two, the Brutsaert modified "equal strain" model (ignoring surface tension at low frequency limit) is used to directly calculate the complex fluid modulus through water and gas saturation and the bulk modulus of each phase, and the formula is as follows: (1) ; Physical rationality verification: when tends to 1 (complete saturation), the value of is close to ; when tends to 0 (complete dryness), the value of is close to , which is consistent with the physical test law.

[0012] Further, in step three, the calculation methods of each basic parameter are as follows: Equivalent density : Considering the mass distribution of three phases of unsaturated soil, the equivalent density is the weighted sum of the densities of solid, water and gas according to the volume fraction, and the formula is as follows: (2) ; Dry skeleton bulk modulus : Using the commonly used empirical formula (applicable to sandy soil, loess unsaturated soil), the porosity is directly calculated, and the formula is as follows: (3) ; In the formula, is an empirical coefficient, sandy soil takes =4, loess soil takes =3.5, and cohesive soil takes =3, Calibrated by indoor compaction test.

[0013] Skeleton shear modulus G: Based on the elastic assumption under small strain, the Poisson's ratio v of the soil and the dry skeleton bulk modulus are derived, and the formula is as follows: (4) ; Equivalent bulk modulus : Based on the Biot-Geertsma low-frequency limit theory (ignoring the fluid inertia term), the coupling effect of dry skeleton and composite fluid is considered, and the formula is as follows: (5) ; Further, in step four, the calculation link of viscous loss correction and complex wave number is effective permeability correction→dynamic viscous length→dissipation function→complex density→complex fluid modulus→skeleton viscoelastic additional term→complex wave number, and the specific calculation steps are as follows: Effective permeability correction: The effective permeability of water and gas in unsaturated soil changes with the saturation in a cubic relationship, and the formula is as follows: (6) ; In the formula, is the effective permeability of water phase, is the effective permeability of gas phase.

[0014] Dynamic viscous length : The thickness of the viscous boundary layer of fluid flowing in the pore, and the formula is as follows: (7) ; Where, =w is the water phase, =a is the gas phase.

[0015] Further, the boundary layer theory is introduced into fluid mechanics, and the Bessel function dissipation model commonly used in high-frequency acoustics is transplanted to rock and soil low-frequency acoustic analysis, which can accurately capture the phase difference between the viscous resistance and inertia effect of fluid on the pore wall, thereby realizing the analytical description of the mechanism of subsonic wave energy attenuation in unsaturated soil at the theoretical level. Boundary layer dissipation function : When the pore size Sticky length At comparable time, the boundary layer dissipation should be considered. Pore size By absolute permeability With porosity Derivation: (8); The dissipation function is as follows: (9); In the formula, And are the zeroth-order and first-order first-kind Bessel functions, respectively.

[0016] Complex density : The Bessel function can automatically adjust the weight of fluid inertial force and viscous force according to the frequency. This makes the three-phase equivalent complex density considering viscous dissipation not only applicable to very low frequency, but also automatically corrected when the frequency is slightly higher, avoiding the calculation error caused by the narrow frequency range of the traditional model. The formula is as follows: (10); In the formula, is the porosity of the soil, is the water saturation, is the angular frequency of the infrasound, is the density of the skeleton solid, is the density of water, is the density of air, is the boundary layer dissipation function of the water phase, is the boundary layer dissipation function of the gas phase.

[0017] Complex fluid modulus : Considering the phase loss caused by fluid viscosity, the complex fluid modulus is corrected to avoid the phase error caused by the simple linear resistance model. The formula is as follows: (11); In the formula, is the complex fluid bulk modulus of unsaturated soil, is the viscous phase factor, is the angular frequency of the infrasound, is the dynamic viscosity of water, is the water saturation, is the dynamic viscosity of air, is the porosity of the soil.

[0018] Skeleton viscoelastic additional term : The constant Q model suitable for low-frequency sound waves is used to describe the energy loss caused by the skeleton viscoelasticity. The formula is as follows: (12); Complex wave number The complex wave number of infrasound propagating in unsaturated soil (the real part reflects the wave propagation speed, and the imaginary part reflects energy attenuation) is calculated using the following formula: (13).

[0019] Furthermore, in step five, the speed of sound c is determined by the real part of the complex wavenumber. The calculation formula is as follows: (14); Attenuation coefficient From the imaginary part of the complex wavenumber The calculation, converting the unit to dB / m, uses a conversion factor of 20 / ln10 ≈ 8.686, as shown in the following formula: (15).

[0020] Furthermore, the sound velocity c and attenuation coefficient α calculated by this method are used for the analysis of infrasound monitoring data of unsaturated soil slopes, including but not limited to: (1) Key parameters of soil inversion: based on infrasound velocity measured on site attenuation coefficient Minimizing the residuals from the values ​​calculated by this method allows us to infer the saturation of the slope on-site. and soil porosity ; (2) Instability early warning threshold calibration: based on Correlation with α ( When α reaches 0.85~0.9, a spike appears. Combined with other sensor parameters (such as deformation), a warning threshold is jointly defined. (3) Monitoring data verification: used to verify the measurement accuracy of the infrasound sensor on site and eliminate abnormal data.

[0021] The technical solution of this application has the following beneficial effects: This application, by coupling the Biot-Geertsma low-frequency limit theory with the boundary layer viscous loss model, and simultaneously outputting the sound velocity and attenuation coefficient, can more accurately calibrate the early warning threshold for slope instability, providing a more comprehensive theoretical basis for the acoustic characteristics of slope instability early warning.

[0022] The input parameters of the computational model established in this application can all be obtained through conventional experiments, without the need for numerical simulation iteration and complex parameter assumptions, which greatly improves the computational efficiency and parameter acquisition convenience in engineering field.

[0023] This application clarifies the simplified calculation conditions for low-frequency infrasound (1~100Hz), and clarifies that the infrasound frequency f≤100Hz is much lower than the Biot characteristic frequency f. cThe low-frequency limit assumption is introduced, and key correction terms such as pore fluid viscous boundary layer dissipation and skeleton viscoelastic loss are introduced, and the three-phase coupling effect (skeleton deformation, fluid viscosity, and gas-liquid interface effect) is fully considered, so that the acoustic characteristic parameters can truly reflect the internal physical state of the unsaturated soil, and provide high reliability support for monitoring data analysis.

[0024] The computing logic of the present application does not depend on special hardware devices, and the closed formula can be integrated into the existing slope monitoring Internet of Things platform through simple programming, without the need for additional hardware modification of existing sensors (infrasound microphones and three-component detectors), significantly reducing application costs. BRIEF DESCRIPTION OF DRAWINGS

[0025] The drawings accompanying the specification of the present application serve to provide further understanding of the present application, and the illustrative embodiments of the present application and their descriptions serve to explain the present application and do not constitute an improper limitation on the present application. Among them: Figure 1 is a schematic diagram of the size and monitoring position of the subgrade slope; Figure 2 is a schematic diagram of the comparison of the infrasound wave speed between the calculated value and the measured value using the method of the present application; Figure 3 is a schematic diagram of the comparison of the infrasound wave attenuation coefficient between the calculated value and the measured value using the method of the present application. DETAILED DESCRIPTION

[0026] The present application will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0027] Taking an unsaturated soil slope of a subgrade of a certain highway in the south as an example, the slope height is 10 m, the slope ratio is 1:1.5, the slope top width is 15 m, the slope length is 18 m, and the longitudinal length of the test slope is selected to be 50 m. A 10Hz three-component detector is arranged at the top, bottom and middle of the slope for assisting in capturing the vibration signal, and a 1Hz infrasound microphone is arranged at the center of the slope top for low-frequency signal collection.

[0028] A method for determining the infrasound wave speed and attenuation coefficient of an unsaturated soil slope, specifically comprising the following steps: Step 1: Obtain the following basic parameters: Through field tests and specification values, the following basic parameters are obtained: Skeleton parameters: =2.6×10 10 Pa, =2650kg / m 3 , v=0.3, Q=25; Fluid parameters: =2.2×10 9 Pa, =1.4×105 Pa, = 1000 kg / m 3 , = 1.2 kg / m 3 , = 1.0 x 10 -3 Pa s, = 1.8 x 10 -5 Pa s; Soil structure parameters: porosity of soil body determined by field static sounding test = 0.35, determined by indoor permeability test = 5 x 10 -15 m 2 ; Infrasonic wave parameters: = 20 Hz (field sensor operating frequency), ; Water saturation: = 0.7 (measured by field time domain reflectometer TDR).

[0029] Step two: determination of modulus of three-phase composite fluid : ; Step three: calculation of equivalent medium parameters: Equivalent density p:

[0030] Dry skeleton volume modulus : Since clay = 3.0, then: ; Thus, the skeleton shear modulus G is calculated: ; Therefore, the equivalent bulk modulus : ; Step four: calculation of viscous loss correction and complex wave number : Effective permeability: ; ; Dynamic viscous length: ; ; Pore size: ; Dissipation function: ; ; Wherein, the Bessel function approximation is: , ; Similarly, ; Complex density: ; Substituting the values, we obtain the viscous phase factor: ; Complex fluid modulus: ; Skeletal viscoelasticity additional features: ; Substituting the values, we obtain the complex wave number:

[0031] Step 5: Determining the infrasound velocity c and attenuation coefficient α.

[0032] Speed ​​of sound: ; Attenuation coefficient: .

[0033] This application clarifies the simplified conditions for low-frequency infrasound through basic assumptions, enabling the simultaneous calculation of low-frequency infrasound velocity and attenuation coefficient using conventional soil mechanics parameters and closed-form formulas without complex numerical iterations. It clarifies the simplified calculation conditions for low-frequency infrasound (1~100Hz), reducing model complexity and improving the reliability and accuracy of calculation results. It also enables the quantitative calculation of the attenuation coefficient, overcoming the limitation of existing models that can only calculate sound velocity, and providing dual-parameter support for soil parameter inversion, thereby promoting the large-scale and precise application of infrasound monitoring technology in the early warning of instability of unsaturated soil slopes.

[0034] Test result verification: (1) The infrasound velocity of the slope was measured on site using a 1Hz infrasound microphone and a 10Hz three-component detector. =1910m / s, attenuation coefficient =3.24×10 -3 dB / m; the calculated values ​​of infrasound velocity and attenuation coefficient using the method of this application deviate from the measured values ​​by 0.26% and 5.9% respectively, with an error ≤10%, which meets the engineering accuracy requirements.

[0035] (2) Standard infrasound signal attenuation tests were conducted indoors on slopes with different soil porosities and soil saturations. The test setup and process were as follows: A transparent plexiglass model with dimensions (length × width × height) of 2.0m × 1.0m × 1.5m was used for scaled-down physical simulation. The soil used in the tests was silty clay from the aforementioned site slope. Soil samples under different working conditions were prepared by controlling the filling density and moisture content. The specific setup was as follows: Model construction: The slope in the model trench was constructed using a layered compaction method, with each layer being 20cm thick. Precise control of the soil porosity was achieved by strictly controlling the quality and volume of each layer of fill. Saturation control: Before filling, the soil sample is air-dried and crushed, the required amount of water is calculated according to the design saturation, the water is sprayed evenly and stirred, and then sealed and left to stand for 24 hours to control the water saturation of the soil. Signal testing: A low-frequency infrasound loudspeaker was set up as the sound source at one end of the model tank. Infrasound sensors were buried every 0.5m along the wave propagation direction. Signals were collected in a relatively static indoor environment, and the sound velocity and attenuation coefficient under different working conditions were calculated. The soil porosity was designed with four gradients: 0.25, 0.3, 0.35, and 0.4. The soil water saturation was designed with four gradients: 0.2, 0.3, 0.4, and 0.5.

[0036] The test results are shown in Tables 1 and 2. Figure 2 , Figure 3 . Figure 2 , Figure 3 Tables 2 and 3 are schematic diagrams comparing the calculated and measured values ​​of infrasound velocity and infrasound attenuation coefficient using the method of this application. The original data of the calculated and measured values ​​are shown in Tables 2 and 3, respectively.

[0037] The experimental results show that: As soil saturation increases, the sound velocity decreases slowly, while the attenuation coefficient increases slowly; water saturation S w For every 0.1 increase, the speed of sound decreases by approximately 6 m / s; however, in water saturation... The change is gradual when the soil porosity is ≤0.40. As the soil porosity increases, the sound velocity decreases significantly, possibly due to a more porous structure and larger pores; at the same time, the attenuation coefficient decreases, which is due to reduced scattering or improved impedance matching.

[0038] water saturation =0.50 and soil porosity Mutation occurs when ≥0.35: the sound speed drops from 2150 m / s to 1770-1940 m / s, and the attenuation coefficient increases from 0-0.008 dB / m to >0.011 dB / m, indicating that the system enters a nonlinear or resonance dominated region, which may involve bubble vibration, local cavitation or two-phase flow instability. The above rules fully comply with the objective facts, indicating that this method has certain scientificity and can provide a theoretical basis for the large-scale and accurate application of infrasound monitoring technology in the early warning of unsaturated soil slope instability.

[0039] Under different test conditions, the error between the calculated infrasonic wave speed and attenuation coefficient and the laboratory test results is within 10%, fully meeting the engineering precision requirements.

[0040] Table 2 Comparison of calculated and measured infrasonic wave speed values

[0041]

[0042] Table 3 Comparison of calculated and measured infrasonic wave attenuation coefficient values

[0043] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining the speed of sound and attenuation coefficient of infrasonic waves in a non-saturated soil slope, characterized in that, The method comprises the following steps: Step one, basic assumptions are proposed based on the core applicable conditions of Biot-Geertsma low-frequency limit theory and boundary layer viscous loss model of three-phase porous medium, and basic parameters are obtained through field test and specification value; Step two, the modulus of three-phase composite fluid is determined based on the Brutsaert modified "equal strain" model; Step three, equivalent medium parameters are calculated, including equivalent density, dry skeleton bulk modulus, skeleton shear modulus and equivalent bulk modulus; Step four, viscous loss correction is carried out, and complex wave number is calculated; the calculated parameters include effective permeability, dynamic viscous length, pore size, dissipation function, complex density, complex fluid modulus, skeleton viscoelastic additional term, and complex wave number; the calculation link is: effective permeability correction→dynamic viscous length→dissipation function→complex density→complex fluid modulus→skeleton viscoelastic additional term→complex wave number; Step five, infrasonic wave speed c and attenuation coefficient a are determined; the speed c is calculated from the real part of the complex wave number, and the attenuation coefficient a is calculated from the imaginary part of the complex wave number.

2. The method of claim 1, wherein: The basic assumptions comprise: (1) the unsaturated soil slope shallow soil body is regarded as a local uniform and transversely isotropic "solid-liquid-gas" three-phase porous medium; (2) Infrasonic wave frequency satisfies 1 Hz ≤ ≤ 100 Hz, far lower than the Biot characteristic frequency , consistent with the "low-frequency limit" assumption; (3) The soil is in a small strain state, and the strain is less than 10 -4 The skeleton and fluid are only considered to be coupled in mass, and the effects of local jet, turbulence and surface tension are ignored.

3. The method of claim 1, wherein: In step one, the basic parameters include skeleton parameters, fluid parameters, soil structure parameters, infrasonic wave parameters and water saturation; the skeleton parameters include skeleton solid bulk modulus, skeleton solid density, soil Poisson's ratio and skeleton viscoelastic loss factor; the fluid parameters include water bulk modulus, air bulk modulus, water density, air density, water dynamic viscosity and air dynamic viscosity; the soil structure parameters include soil porosity and soil absolute permeability; the infrasonic wave parameters include infrasonic wave angular frequency.

4. The method of claim 1, wherein: In step three, the equivalent density is the weighted sum of the densities of solid, water and gas according to the volume ratio; the dry skeleton bulk modulus is directly calculated according to the porosity through an empirical formula; the skeleton shear modulus is derived through the soil Poisson's ratio and the dry skeleton bulk modulus; the equivalent bulk modulus is calculated based on the Biot-Geertsma low-frequency limit theory, ignoring the fluid inertia term and only considering the coupling effect of dry skeleton and composite fluid.

5. The method of claim 1, wherein: The effective permeability is calculated using the cubic relationship empirical model; the dynamic viscous length is calculated based on the boundary layer theory; the dissipation function is calculated using the dissipation model of Bessel function; The viscous dissipation is introduced into the calculation of the three-phase equivalent complex density; the calculation of the complex fluid modulus considers the phase loss caused by fluid viscosity; the constant Q model is used to calculate the skeleton viscoelastic additional term; based on the complex density and the complex modulus, the standard sound wave propagation model is used to calculate the complex wave number.

6. The method of claim 5, wherein: Complex density The calculation formula is as follows: ； wherein is the porosity of the soil, is the water saturation, is the angular frequency of the infrasonic wave, is the density of the skeleton solid, is the density of water, is the density of air, is the boundary layer dissipation function of the water phase, is the boundary layer dissipation function of the gas phase.

7. The method of claim 5, wherein: Complex fluid modulus The formula for calculating the complex fluid modulus is as follows: ； wherein is the complex fluid bulk modulus for unsaturated soil, is the viscous phase factor, is the angular frequency of the infrasonic wave, is the dynamic viscosity of water, is the water saturation, is the dynamic viscosity of air, is the soil porosity.

8. The method of claim 1, wherein: The infrasonic wave speed c and the attenuation coefficient a obtained in step five are used for unsaturated soil slope infrasonic wave monitoring data analysis, including at least one of the following: inversion of soil key parameters, instability early warning threshold calibration and monitoring data verification.

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