Method for monitoring thickness of ice layer by using air coupling bending waves

Abstract

The invention belongs to the field of acoustic measurement, and relates to an ice layer thickness measurement method, in particular to a method for monitoring the ice layer thickness by using air coupling bending waves. A sensor arranged on the ice surface is used for collecting air coupling bending waves propagated in the ice layer; calculating the characteristic frequency of the air coupling bending wave; performing frequency spectrum solving or time-frequency spectrum solving on the air-coupled bending waves collected in the step 1, and determining the characteristic frequency f0 of the air-coupled bending waves. And calculating characteristic frequencies of different ice thicknesses. When different ice layer thicknesses d are set, solving the characteristic equation of the air coupling bending wave to obtain a change function f (d) of the characteristic frequency f of the air coupling bending wave along with the ice layer thicknesses d; according to f0 and f (d), f (d) is made to be equal to f0, and finally the thickness d of the ice layer is obtained. Collection can be performed by using a single sensor, and the method has the characteristics of economy and high efficiency; the activity safety of people on ice can be effectively ensured; finally, the effectiveness of the method is verified through an external field test.

Description

Technical Field

[0001] The present invention belongs to the field of acoustic measurement, and relates to a method for measuring ice layer thickness, in particular to a method for monitoring ice layer thickness by using air-coupled flexural waves. Background Art

[0002] When there is pressure on the ice surface, flexural stress will be generated. The upper surface is under pressure and the bottom is under tension. Since ice is weaker under tension than under compression, when the critical stress exceeds the tensile strength of the ice, cracking will occur. If the pressure increases, the bottom crack will develop into a series of radial cracks penetrating the ice thickness, propagate outward and completely crack.

[0003] In the ice-covered area, having an ice layer with sufficient thickness is a condition for supporting ice machinery and allowing personnel to move safely. Accidents often occur when activities are carried out on thin ice with insufficient bearing capacity, resulting in unfortunate events. If there is timely and accurate information about the ice thickness and bearing capacity, the vast majority of these accidents should be considered avoidable.

[0004] The impact on the ice layer will cause the propagation of flexural waves, and its propagation characteristics are strongly affected by the flexural stiffness of the ice. The flexural waves propagating in floating ice belong to a kind of guided wave, similar to the flexural waves of a rod and the Lamb waves in a free plate. Some studies have used the dispersion of ice flexural waves to estimate the elastic parameters of ice, such as "A Method for Measuring Ice Thickness Based on the Dispersion Curve of A0 Mode".

[0005] It is found that there is an air-coupled flexural wave between the air and the ice surface. The air-coupled flexural wave appears in the form of a constant frequency and arrives before the air acoustic wave. In this case, using a cheap microphone or a geophone on the ice surface to collect the air-coupled flexural wave may promote the development of a convenient method to estimate the ice thickness and avoid people from moving on unsafe ice. Summary of the Invention

[0006] Aiming at the above-mentioned existing research and technologies, the technical problem to be solved by the present invention is to provide a method for monitoring ice layer thickness by using air-coupled flexural waves.

[0007] To solve the above technical problem, a method for monitoring ice layer thickness by using air-coupled flexural waves of the present invention includes the following steps:

[0008] Step 1: Use a sensor arranged on the ice surface to collect the air-coupled flexural waves propagating in the ice layer; the sensors here include but are not limited to collection instruments such as microphones and geophones.

[0009] Step 2: Calculate the characteristic frequency of the air-coupled flexural wave; solve the frequency spectrum or time-frequency spectrum of the air-coupled flexural wave collected in Step 1 to determine the characteristic frequency f of the air-coupled flexural wave0 。

[0010] Step 3: Calculate the characteristic frequencies for different ice thicknesses. The characteristic equation of the air-coupled flexural wave is as follows:

[0011]

[0012] where f is the characteristic frequency, and the flexural rigidity D of ice = Ed 3 / 12(1 - v 2 ), Young's modulus Poisson's ratio v = [0.5(c s / c t ) 2 - 1] / [(c s / c t ) 2 - 1], d and ρ are the thickness and density of ice respectively, c s and c t are the longitudinal wave velocity and transverse wave velocity of ice respectively. The air sound velocity is c = 331.6 + 0.6T, where T is the Celsius temperature. ρ w and H are the density and depth of water respectively. g is the acceleration due to gravity.

[0013] When different ice layer thicknesses d are set, solving this equation can obtain the variation function f(d) of the characteristic frequency f of the air-coupled flexural wave with the ice layer thickness d.

[0014] Step 4: According to the characteristic frequency f of the air-coupled flexural wave measured in Step 2 0 and the theoretical characteristic frequency f(d) calculated in Step 3, let f(d) = f 0 , and finally obtain the ice layer thickness d.

[0015] Advantages of the present invention: The present invention proposes a method for monitoring ice layer thickness using air-coupled flexural waves, which can be collected using a single sensor, featuring economy and high efficiency; it can effectively ensure the safety of personnel activities on ice; and finally, the effectiveness of the method is verified through field tests. Description of the Drawings

[0016] Figure 1 is the flowchart of ice thickness measurement in the present invention;

[0017] Figure 2 is the collected signal in the present invention;

[0018] Figure 3 is the time-frequency spectrum of the collected signal in the present invention;

[0019] Figure 4 is the frequency spectrum of the collected signal in the present invention;

[0020] Figure 5is the characteristic frequency of different ice thicknesses in the present invention. Detailed implementation manners

[0021] The present invention will be further described below in conjunction with the accompanying drawings of the specification and embodiments.

[0022] As Figure 1 shown, the present invention includes the following steps:

[0023] As Figure 2 shown, in step 1, a sensor disposed on the ice surface is used to collect air-coupled flexural waves propagating in the ice layer; the sensors here include but are not limited to collection instruments such as microphones and geophones. The sampling frequency is 25,600 Hz, and the collection time length is 1 s. Various components of the collected signal are marked in the figure, including the ice flexural wave with a faster propagation speed, the air acoustic wave with a slower propagation speed, and the air-coupled flexural wave with a medium propagation speed and a single frequency.

[0024] As Figure 3 shown, in step 2, the characteristic frequency of the air-coupled flexural wave is calculated; when solving the time-frequency spectrum of the air-coupled flexural wave collected in step 1, the characteristic frequency f 0 = 43 Hz.

[0025] As Figure 4 shown, it is the frequency spectrum of the air flexural wave, which is consistent with the time-frequency spectrum. The amplitude of the air-coupled flexural wave frequency spectrum is strong, and the characteristic frequency f 0 = 43 Hz.

[0026] Step 3: Calculate the characteristic frequencies of different ice thicknesses. The characteristic equation of the air-coupled flexural wave is:

[0027]

[0028] where f is the characteristic frequency, the flexural rigidity D of the ice = Ed 3 / 12(1 - v 2 ), the Young's modulus Poisson's ratio v = [0.5(c s / c t ) 2 - 1] / [(c s / c t ) 2 - 1], d and ρ are the thickness and density of the ice respectively, c s and are the longitudinal wave velocity and transverse wave velocity of the ice respectively. The air sound velocity is c = 331.6 + 0.6T, where T is the Celsius temperature. ρ w and H are the density and depth of water respectively. g is the acceleration due to gravity.

[0029] Set the ice layer thickness d to be in the range of 0.1 m - 5.0 m. According to the actual environment of ice operations, the longitudinal wave velocity of ice is c s = 3800 m / s, and the transverse wave velocity of ice is c t = 1900 m / s. The density of water ρ w = 1000 kg / m 3 , and the depth H = 3 m. The temperature in the field experiment is about -20 °C, and the air sound velocity c = 331.6 - 0.6×20 = 320 m / s. The Poisson's ratio of ice v = [0.5(c s / c t ) 2 - 1] / [(c s / c t ) 2 - 1] = 0.33, and the Young's modulus

[0030] As Figure 5 shown, substitute the above parameters into the characteristic equation and solve to obtain the characteristic frequency f(d).

[0031] Step 4: According to the characteristic frequency f 0 = 43 Hz of the air-coupled bending wave measured in Step 2, and the theoretical characteristic frequency f(d) calculated in Step 3, let f(d) = f 0 = 43 Hz, and finally obtain the ice layer thickness d = 0.68 m, which is consistent with the actual result measured by ice chiseling, achieving the purpose of monitoring the ice thickness.

[0032] It should be understood that the application of the present invention is not limited to the above examples. For those of ordinary skill in the art, improvements or transformations can be made according to the above description, and all such improvements and transformations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for monitoring ice thickness using air-coupled bending waves, characterized in that: The following steps are involved: Step 1: Use sensors placed on the ice surface to collect air-coupled bending waves propagating in the ice layer; Step 2: Calculate the characteristic frequency of the air-coupled bending wave; solve the spectrum or time-frequency spectrum of the air-coupled bending wave collected in step 1 to determine the characteristic frequency f0 of the air-coupled bending wave; Step 3: Calculate the characteristic frequencies of different ice thicknesses; when setting different ice thicknesses d, solve the characteristic equation of air-coupled bending waves to obtain the function f(d) of the characteristic frequency f of air-coupled bending waves changing with the ice thickness d; Step 4: Based on the characteristic frequency f0 of the air-coupled bending wave measured in step 2 and the theoretical characteristic frequency f(d) calculated in step 3, let f(d) = f0, and finally obtain the thickness d of the ice layer.

2. The method for monitoring ice thickness using air-coupled bending waves according to claim 1, characterized in that: The sensors in step 1 are microphones and geophones.

3. The method for monitoring ice thickness using air-coupled bending waves according to claim 1, characterized in that: The characteristic equation of the air-coupled bending wave in step 3 is: Where f is the characteristic frequency, the bending stiffness of ice D = Ed 3 / 12(1-v 2 ), Young's modulus Poisson's ratio v = [0.5(c s / c t ) 2 -1] / [(c s / c t ) 2 -1], d and ρ are the thickness and density of ice, c s and c t are the longitudinal and transverse wave velocities of ice respectively; the air sound speed is c=331.6+0.6T, T is the temperature in Celsius; ρ w and H are the density and depth of water respectively; g is the acceleration due to gravity.

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