CN117804390A - A method for detecting seabed ore thickness based on laser-induced plasma sound source - Google Patents

Abstract

The present invention belongs to the field of geophysical detection technology, and discloses a method for detecting the thickness of seabed ore based on a laser-induced plasma sound source. The method uses an experimental device for collecting plasma sound waves to collect reflected sound signals of underwater target thickness; pre-processes the obtained n sound signals; extracts the reflected waves of the upper and lower surfaces and calculates the thickness. The present invention focuses a beam of pulsed laser into a liquid to generate plasma. This physical process will generate plasma sound wave signals, and the sound wave signals can be collected using a hydrophone. By extracting position and intensity characteristic information from the sound wave signal, filtering and feature recognition of the sound signal, the reflected sound signals of the upper and lower surfaces of the seabed ore can be extracted, and the thickness information of the seabed ore can be obtained. Due to the wide bandwidth (10MHz) and short pulse (100ns) of the laser sound signal, it is very suitable for measuring the thickness of the seabed cobalt-rich crust, which is helpful for estimating the abundance of the seabed cobalt-rich crust.

Description

A method for detecting seabed ore thickness based on laser-induced plasma sound source

Technical Field

The invention belongs to the technical field of geophysical detection, and in particular relates to a method for detecting the thickness of seabed ores based on a laser-induced plasma sound source.

Background technique

In recent years, with the development of the economy, the demand for mineral resources has continued to increase. As land mineral resources are increasingly depleted, the direction of mineral exploration must gradually shift to the deep sea. Among seabed mineral resources, cobalt-rich crusts are another important deep-sea resource that has attracted much attention from all countries after polymetallic nodules. In the offshore investigation of cobalt-rich crusts, crust thickness is one of the most important and basic parameters. .

The thickness of seabed ore generally refers to the true thickness of seabed ore, that is, the vertical distance between the two parallel top and bottom interfaces of seabed ore. At present, the most common and effective methods for deep-sea ore detection include drilling sampling and shallow stratigraphic profilers. Among them, drilling sampling is the most direct and reliable measurement method. However, as a "point operation" method, this method can only obtain measurement results from a few stations, and sampling is difficult, time-consuming, and costly. Limited by the number of spatial sampling points, there will be large errors when using drilling sampling methods to evaluate crustal resources in a large area. Most of the current solutions to this problem use shallow formation profilers based on acoustic parametric array technology. Acoustic parametric array technology can use difference frequency and sum frequency sound fields at the same time to ensure high-precision detection of crust thickness. However, acoustic parametric array technology can The energy conversion efficiency is low, the acoustic signal bandwidth is limited, and the measurement results are easily interfered by external noise.

The prior art discloses a system for detecting mud thickness based on a plasma source. However, this system requires a ship-borne platform and is only applicable to shallow water mud thickness measurement. At the same time, due to the limitation of the source frequency, the resolution can only reach 0.1m. Therefore, in order to meet the technical requirements of formation thickness measurement, a new technical method is urgently needed.

China’s invention patent is a system for detecting mud thickness based on plasma source, (publication number CN113093283A, publication date 2021.07.09). The sound source used in this solution is an electric spark sound source, which has shortcomings compared with the laser breakdown sound source in this solution. The reason is that the sound is unstable, and the positions and sound pressure amplitudes of each two sound sources are quite different. At the same time, since the electric spark source device requires a considerable supply of electrical energy and can only be mounted on a ship, the solution described in this patent can only be limited to shallow water operations and cannot meet deep water applications.

Through the above analysis, the problems and defects of the prior art are as follows:

(1) Drilling sampling: It needs to be returned to the indoor processing and cannot be measured in situ.

(2) Shallow stratum profiler: External sound source signals within the system bandwidth can be connected to cause interference signal images. The rocking motion of the ship results in poor image quality. The thickness resolution is only 0.1m.

(3) Although existing technologies can measure the thickness of seabed ores to a certain extent, the acquisition of thickness information is limited by the mounting platform. The acoustic wave signal (laser acoustic signal) of underwater laser-induced plasma is not used for detection, resulting in low accuracy in detecting the thickness of seabed ore.

Contents of the invention

In order to overcome the problems existing in related technologies, the disclosed embodiments of the present invention provide a method for detecting the thickness of seabed ore based on a laser-induced plasma sound source, specifically related to a multi-platform seabed ore thickness measurement based on a laser-induced plasma sound source. method.

The technical solution is as follows: A method for detecting the thickness of seabed ore based on a laser-induced plasma acoustic source, comprising:

S1, use the experimental device to collect plasma acoustic waves to collect the reflected sound signal of the thickness of the underwater target;

S2, preprocess the obtained n acoustic signals;

S3, extract the reflected waves from the upper and lower surfaces and calculate the thickness.

In step S1, the experimental device for collecting plasma acoustic waves includes an energy attenuator, a laser beam expander, a meniscus lens, a doublet lens, and a lens barrel;

The energy attenuator transmits the energy-adjusted laser to the laser beam expander;

The meniscus lens and the doublet lens are combined to form a focusing lens, and the focusing lens is placed in the lens barrel to form a focusing lens group;

The laser beam expander expands the laser beam and emits it to the focusing lens group.

In step S1, an experimental device for collecting plasma acoustic waves is used to collect an underwater target thickness reflected acoustic signal, including:

The laser beam expander of the experimental device for collecting plasma acoustic waves expands the laser beam and emits it to the focusing lens group. The focused laser is emitted to the sample to be measured; the laser focusing position is 2 to 10cm above the sample to be measured; a hydrophone is used The acoustic wave signals reflected from the upper and lower surfaces of the sample to be tested are collected and sent to the oscilloscope.

In step S2, preprocess the obtained n acoustic signals, including:

(1) Extract the amplitude A0 and arrival time T0 of the plasma expansion acoustic signal Pn; n represents the total number of acoustic signals obtained at the measurement position;

(2) Calculate the amplitude standard deviation Astd, the arrival time standard deviation Tstd, the amplitude average Amean, and the arrival time average Tmean;

(3) Calculate the amplitude deviation Ae and arrival time deviation Te;

(4) Eliminate values with a deviation greater than 2 times the standard deviation, specifically: Ae>2×Astd, Te>2×Tstd, and obtain m acoustic signals Pm after eliminating outliers;

(5) Average the acoustic signal Pm to get the average value Pa;

(6) Perform band-pass filtering on the average value Pa to remove low-frequency noise to obtain the characteristic sound signal Pb.

Further, the extraction of plasma expansion acoustic signal Pn amplitude A0 and arrival time T0 includes: using matlab function max to find the maximum value, the expression is: [A0, T0]=max(Pn).

Furthermore, the expression for calculating the amplitude average Amean is:

Amean=sum(Ai)/n

When measuring thickness, several infrasound signals are collected at the same position. n represents the total number of acoustic signals obtained at the measurement position; Ai is the amplitude of the i-th acoustic signal, where i is an integer from 1 to n.

Furthermore, the expression for calculating the amplitude standard deviation Astd is:

Astd = sqrt(sum((Ai-Amean)^2)/n)

Among them, i is a positive integer from 1 to n;

The expressions for calculating the amplitude deviation Ae and arrival time deviation Te are:

Ae＝|A0-Amean|

Te=|T0-Tmean|.

In step S3, the reflected waves of the upper and lower surfaces are extracted and the thickness is calculated, including:

Perform feature identification on the obtained characteristic sound signal Pb, and extract the peak arrival times t1 and t2 of the upper and lower reflected sound signals; the thickness of the sample to be measured is:

d＝c×|t1-t2|/2

Where c is the speed of sound in the sample.

Further, the characteristic sound signal Pb is used for feature recognition, including:

Obtain the pulse signal whose pulse width is lower than the threshold, which is 2us;

Obtain the pulse signal with a peak value higher than the threshold. The threshold depends on the amplitude A0. According to the acoustic wave reflection and transmission formula of different media, the upper surface reflection wave threshold is calculated as:

0.5×|(z2-z1)/(z1+z2)|×A0

The lower surface reflection wave threshold is:

0.5×|(4×z1×z2×(z1-z2)/(z1+z2)^3)|A0

Among them, z1 and z2 are the impedance between water and the target medium, z=ρv, ρ is the density of the medium, and v is the speed of sound in the medium.

Furthermore, the method is applied to the cable-free underwater robot AUV to achieve the measurement of seabed ore thickness in deep-sea environments.

Combining all the above technical solutions, the advantages and positive effects of the present invention are: the present invention uses the acoustic wave signal (laser acoustic signal) of underwater laser-induced plasma for detection, which can obtain the thickness of seabed ore at a higher resolution (1mm) , and is not limited to the mounting platform, and can realize deep-sea in-situ detection.

Regarding the deficiencies in the prior art, the purpose of the present invention is to propose a new method for measuring the depth of seabed ore, aiming at measuring the thickness of seabed ore. Focusing a pulsed laser beam into a liquid creates plasma, a physical process that generates plasma acoustic signals that can be collected using hydrophones. Extract the position and intensity characteristic information of the acoustic signal, and perform filtering and feature identification on the acoustic signal to extract the reflected acoustic signal from the upper and lower surfaces of the seabed ore, and then obtain the thickness information of the seabed ore. Due to the high frequency of the laser acoustic signal (10MHz) , short pulse (100ns), which can achieve higher resolution detection of seabed ore thickness.

Description of drawings

The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure;

Figure 1 is a flow chart of a method for detecting the thickness of seabed ore based on a laser-induced plasma sound source provided by an embodiment of the present invention;

FIG2 is a diagram of an experimental device for collecting plasma acoustic waves provided in an embodiment of the present invention;

Figure 3 is a physical diagram of five groups of aluminum block samples used in the embodiment of the present invention;

FIG4 is a physical picture of the stones used in the embodiment of the present invention;

Figure 5 is a typical plasma acoustic wave signal collected by the experimental device of Figure 2 provided by the embodiment of the present invention;

Figure 6 is a characteristic signal diagram of the acoustic wave reflected from the upper and lower surfaces of the left sample after processing in the actual measurement of the thickness of the five groups of aluminum blocks used in Figure 3 provided by the embodiment of the present invention;

Figure 7 is a characteristic signal diagram of the acoustic wave reflected from the upper and lower surfaces after processing of the second left sample in the actual measurement of the thickness of the five groups of aluminum blocks used in Figure 3 provided by the embodiment of the present invention;

Figure 8 is a characteristic signal diagram of the acoustic wave reflected from the upper and lower surfaces after processing of the third left sample in the actual measurement of the thickness of the five groups of aluminum blocks used in Figure 3 provided by the embodiment of the present invention;

9 is a diagram of characteristic signals of reflected acoustic waves on the upper and lower surfaces of the four left samples after being processed in the actual measurement of the thickness of the five groups of aluminum blocks used in FIG. 3 provided by an embodiment of the present invention;

10 is a diagram of characteristic signals of reflected acoustic waves on the upper and lower surfaces of the five left samples after being processed in the actual measurement of the thickness of the five groups of aluminum blocks used in FIG. 3 according to an embodiment of the present invention;

11 is a graph showing the thickness of stones used in the present invention;

Figure 12 is a sample diagram that needs to be detected during simulation verification provided by the embodiment of the present invention;

Figure 13 is an acoustic wave signal diagram collected during simulation verification provided by the embodiment of the present invention;

Figure 14 is a diagram of acoustic wave signals collected at different locations during simulation verification provided by the embodiment of the present invention;

Figure 15 is a sample thickness detection curve used in simulation verification provided by the embodiment of the present invention;

In the picture: 1. Energy attenuator; 2. Laser beam expander; 3. Meniscus lens; 4. Double cemented lens; 5. Lens tube.

Detailed ways

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

The principle of the method for detecting the thickness of seabed ore based on laser-induced plasma sound source provided by the embodiment of the present invention is as follows: high-power laser is focused on the water body to generate transient plasma after interaction, and the plasma expands outward to generate an acoustic wave signal A. Due to the large impedance difference between seawater and seabed ore, when the sound wave signal reaches the upper surface of the seabed ore, it will generate reflected sound wave B and transmit part of the sound wave into the interior of the seabed ore. There is still an impedance difference between the seabed ore and the soil or water below the seabed ore, so the sound waves transmitted into the seabed ore will also produce a reflected sound wave C on the lower surface of the seabed ore. The hydrophone can be used to receive sound waves A, B, and C in sequence. The thickness of the seabed ore can be calculated based on the time difference between sound waves B and C combined with the speed of sound in the seabed ore. According to the Ricker wavelet principle, the critical resolution layer thickness of the composite reflection wave with the time reflection sequence as a function is: d=λ/4.3. If the sound speed is c=5000m/s and the detection frequency is 5MHz, then d=0.23mm can be achieved. High-precision measurement of seafloor ore thickness.

Embodiment 1, as shown in FIG1 , the method for detecting the thickness of seabed ore based on a laser-induced plasma acoustic source provided by the embodiment of the present invention comprises:

S1, using the experimental device for collecting plasma acoustic waves to collect the reflected acoustic signal of the underwater target thickness.

Change the thickness of the sample to be measured, and collect the acoustic signal reflected from the underwater target thickness and the acoustic wave signal reflected from the upper and lower surfaces of the sample under a laser energy of 30mJ.

For example, a short-pulse solid-state laser is used to generate a laser beam (the model of the laser is Leibao Optoelectronics Dawa-200). When the laser is focused by the focusing device, the medium at the focus (about 30-50 μm in diameter and 30 μm in length) is A columnar area of hundreds of μm (the length here will change due to focusing parameters) will be heated to a temperature of several thousand K within the laser pulse time (10ns). The medium will be ionized at a short period of extremely high temperature, and the ionization will transform water. It is plasma and generates bubbles. The expansion of bubbles and plasma will produce an acoustic signal (the expansion of bubbles and plasma can be understood as hitting the medium at a very fast speed, and the medium vibrates and makes sound. The medium in this work is water).

In order to generate a suitable acoustic signal in the present invention, a focusing combination device of a beam expander, a meniscus lens, and a doublet lens is used, that is, the experimental device for collecting plasma acoustic waves shown in Figure 2. Under this setting, the acoustic signal can be ensured. Signal stability and strength.

It includes an energy attenuator 1, a laser beam expander 2, a meniscus lens 3, a double cemented lens 4, and a lens barrel 5;

The energy attenuator 1 transmits the energy-adjusted laser to the laser beam expander 2;

The meniscus lens 3 and the doublet lens 4 are combined into a focusing lens, and the focusing lens is placed in the lens barrel 5 to form a focusing lens group;

The laser beam expander 2 expands the laser beam and emits it to the focusing lens group;

However, if the energy attenuator 1 and the beam expander 2 are not added, the acoustic signal generated by directly focusing the laser and the focusing lens group cannot meet the acoustic signal requirements of the present invention.

S2, signal processing: preprocess the obtained n acoustic signals. The specific process is:

Extract the plasma expansion acoustic signal Pn amplitude A0 and arrival time T0. The extraction method is to use the matlab function max to find the maximum value [A0, T0] = max (Pn); n represents the total number of acoustic signals obtained at the measurement position;

Calculate the standard deviation (Astd, Tstd) and mean (Amean, Tmean) of the amplitude and arrival time. The mean is calculated by Amean=sum(Ai)/n, and the standard deviation is calculated by Astd=sqrt(sum((Ai-Amean)^2)/n);

When measuring thickness, several infrasound signals are collected at the same position, n represents the total number of acoustic signals obtained at the measurement position; Ai is the amplitude of the i-th acoustic signal, where i is an integer from 1 to n.

Calculate the degree of deviation Ae and Te between amplitude and arrival time, Ae=|A0-Amean|, Te=|T0-Tmean|;

Eliminate values with a deviation greater than 2 times the standard deviation, that is, Ae>2×Astd, Te>2×Tstd, and obtain m acoustic signals Pm after eliminating abnormal values;

Average Pm to get Pa;

Bandpass filter Pa is performed to remove low-frequency noise to obtain the characteristic sound signal Pb.

S3, extract the reflected waves from the upper and lower surfaces and calculate the thickness.

Perform feature recognition on the characteristic sound signal Pb obtained in step S2, and extract the peak arrival times t1 and t2 of the upper and lower reflected sound signals. The thickness of the sample to be measured is d=c×|t1-t2|/2, and c is the sound speed in the sample.

As can be seen from the above embodiments, the present invention uses a beam of pulsed laser to focus on the physical process of generating plasma in water, and uses hydrophones to effectively collect the reflected sound wave signals generated by the generated plasma sound waves on the upper and lower surfaces of seabed ores. .

The present invention does not require a fixed platform to carry the transmitting/receiving device, and the carrying platform can be easily replaced to perform in-situ measurement, thus meeting the needs of deep-sea measurement.

The method proposed by the present invention is not limited by the mounting platform, does not require a fixed mounting platform, does not only measure the thickness of seabed ore in shallow water, and does not cause damage to the product, and the method proposed by the present invention can realize the original measurement of the thickness of seabed ore. Real-time high-precision measurement without the need to take samples back indoors for analysis or can only be applied in shallow seas. The method proposed by the present invention utilizes the plasma acoustic wave signal formed by focusing a short pulse laser. It requires low power (hundreds of watts to several kilowatts) and can be mounted on a cableless underwater vehicle (AUV) to realize the detection of seabed ores in a deep sea environment. Thickness measurement. In addition, because the laser sound source has the characteristics of high frequency width and short pulse, high resolution (millimeter level) measurement can be achieved. At the same time, emitting/receiving acoustic signals at near-seabed ore locations can reduce the interference caused by environmental noise on measurements. The method proposed by the present invention has the advantages of high precision, low cost, simple method, simple operation, and is not limited by the platform.

The invention can effectively improve the detection efficiency of deep sea mineral resources; the invention can also be applied to deep sea pipeline thickness detection. The invention uses laser sound sources to detect and diagnose underwater targets to solve the technical problem of measuring the thickness of deep-sea pipelines.

Embodiment 2, as another embodiment of the present invention, the equipment used to generate laser-induced plasma acoustic wave signals in water is a short-pulse laser, but other short-pulse lasers can also implement the present invention, so the solution of other laser generation equipment is adopted. It can be used as an alternative to the above-mentioned plasma acoustic wave generation.

Embodiment 3, as another embodiment of the present invention, the equipment used for collecting laser-induced plasma acoustic wave signals in water is an ultrasonic hydrophone and an oscilloscope, but other equipment that can detect and collect acoustic wave signals can also implement the present invention. Therefore, the solution of using other acoustic wave signal acquisition equipment can be used as an alternative to the above-mentioned plasma acoustic wave acquisition. This alternative is only a replacement for the previous data collection method. The core of the present invention is to use the collected plasma acoustic waves to achieve high-resolution measurement of seabed ore thickness information. As for the replacement of data collection experimental devices, in principle, it is enough to ensure the collection of acoustic signals and obtain good signal quality at the same time.

Embodiment 4, as another embodiment of the present invention, the present invention aims at high-resolution measurement of underwater seabed ore thickness, and calculates the seabed ore thickness by collecting the reflected sound signal of the underwater target thickness on the upper and lower surfaces of the seabed ore. At the same time, since laser-induced plasma is used to generate sound sources, and the power required by the laser is small, the transceiver equipment can be integrated into a smaller device, and the use of cableless underwater vehicles (AUV) as a carrying platform can achieve deep-sea environments. In situ measurement of seafloor ore thickness. In order to elucidate the advantages and key technologies of this invention in detail, each step of this invention will be further described in detail with reference to specific examples.

The method adopted by the present invention uses an underwater LIB experimental system to generate an underwater target thickness reflection sound signal, and a hydrophone to receive a reflection sound signal generated after the plasma sound wave signal is reflected by the upper and lower surfaces of the sample. After processing the data, outliers are first eliminated, and then band-pass filtering is performed to extract the characteristic acoustic signals of the upper and lower reflection waves of the sample, and the sample thickness is calculated through the time difference between the two acoustic signals. Next, this process will be further explained in detail through the accompanying drawings and examples to illustrate the feasibility and superiority of the correction method proposed in the present invention.

The method for detecting the thickness of seabed ore based on a laser-induced plasma acoustic source provided by an embodiment of the present invention includes:

Step 1: Change samples of different thicknesses and utilize the data acquisition device shown in Figure 2. A Q-switched laser is used to generate laser pulses with a wavelength of 1064nm. A half-wave plate (HWP) and a Glan prism (GP) are used to adjust the laser pulse energy to 30mJ. A part of each laser pulse (10%) passes through a cube beam splitter (BS). ) is reflected and sent to the photodiode (PD) connected to the oscilloscope. The other part of the laser pulse uses a laser beam expander (LBE) to expand the laser beam. The laser beam expander 2 expands the laser beam and emits it to the focusing lens group; the meniscus lens 3 and the double cemented lens 4 are combined into a focusing lens. The focusing lens is placed in the lens barrel 5 and combined into a focusing lens group;

Among them, the doublet lens 4 (achromatic doublet lens (L1)) in the focusing lens focuses the laser beam into a quartz water tank filled with samples for generating plasma. The plasma acoustic wave signals reflected from the upper and lower surfaces of the sample are collected by hydrophones placed perpendicular to the sample surface and stored in an oscilloscope. The reflected sound signals of samples with different thicknesses are collected.

Step 2: First, preprocess the obtained n acoustic wave signals to eliminate outliers. The specific process is divided into four steps: ① Extract the amplitude A0 and arrival time T0 of the plasma expansion acoustic signal Pn, ② Calculate the amplitude and arrival time standards Deviation (Astd, Tstd) and average value (Amean, Tmean), ③ Calculate the deviation degree Ae and Te between amplitude and arrival time, Ae = |A0-Amean|, Te = |T0-Tmean|, ④ Eliminate deviation degree greater than 2 times the standard deviation, that is, Ae>2×Astd, Te>2×Tstd, and m acoustic signals Pm are obtained after eliminating outliers. After obtaining the outlier-free data group Pm, average it to obtain Pa. According to the specific parameter settings of the sound source and the hydrophone, band-pass filtering is performed on Pa to obtain the filtered characteristic sound signal Pb. In the example of the present invention, 5 MHz to 12.5 MHz band-pass filtering is used.

Step 3: Perform feature recognition on the characteristic sound signal Pb obtained in Step 2, and extract the arrival times t1 and t2 of the reflected sound wave signals on the upper and lower surfaces. Then the thickness d of the sample to be measured can be calculated from the arrival time difference and the sound speed c in the sample, d=c×|t1-t2|/2.

In the embodiment of the present invention, characterizing the characteristic sound signal Pb includes:

① Look for a pulse signal with a pulse width (here the pulse width is the full width at half maximum) lower than a threshold (the threshold depends on the frequency of the detected sound source, which is 2us in the present invention); ② Look for a pulse signal with a peak value higher than the threshold, where the threshold depends on the amplitude A0 and the target material. According to the sound wave reflection and transmission formula of different media, the upper surface reflection wave threshold can be calculated to be 0.5×|(z2-z1)/(z1+z2)|×A0, and the lower surface reflection wave threshold can be calculated to be 0.5×|(4×z1×z2×(z1-z2)/(z1+z2)^3)|A0, where z1 and z2 are the impedances of water and the target medium (aluminum block and stone in the present invention), z=ρv, ρ is the medium density, and v is the sound speed in the medium.

Example detection effect: The method proposed by the present invention has been applied to the detection of underwater metal and seabed ore thickness in the laboratory. Figure 2 is a schematic diagram of the experimental device for collecting plasma acoustic waves in the laboratory, including: the laser emitted by the laser (DAWA200) is transmitted to The energy attenuator 1 transmits the energy-adjusted laser to the laser beam expander 2, expands the laser beam and launches it to the focusing lens group, where the focusing lens group includes a lens barrel 5, a meniscus lens 3, and a double cemented lens 4; so The meniscus lens 3 and the doublet lens 4 are combined into a focusing lens; the focusing lens is placed in the lens barrel 5 and combined into a focusing lens group;

The focused laser is emitted to the sample to be tested; the hydrophone is used to collect the reflected sound wave signals from the upper and lower surfaces of the sample to be tested. The laser focusing position is 2 to 10cm above the sample to be tested; at the same time, the hydrophone is connected to the signal of the energy attenuator, in which the focus The lens can be replaced by a reflector, a hydrophone can be used to collect the reflected sound wave signals from the upper and lower surfaces of the sample to be tested, and a signal sending oscilloscope can be used as a replacement. Furthermore, as long as it can meet the requirements of the generation and collection of laser-induced sound sources, it can be used as a substitute.

It can be understood that when the acoustic signal generated by the focused breakdown propagates to the upper surface of the sample to be measured, a part of the energy will be reflected in the form of a reflected wave, reflecting the acoustic wave signal to the upper surface.

The remaining energy will be transmitted into the sample to be measured in the form of a transmitted wave, and the transmitted wave will be reflected on the lower surface of the sample (this process is similar to the upper surface reflected sound signal, except that the value of the reflection coefficient is different) to form a lower surface reflection signal.

Therefore, the process of reflecting acoustic signals from the upper and lower surfaces includes: the acoustic signal generated by focused breakdown forms an upper surface reflection signal on the upper surface; at the same time, through upper surface projection, lower surface reflection, and upper surface transmission, a lower surface reflection signal is formed.

In addition, in the laboratory or shallow water stage, there is an electric breakdown plasma sound source that can be used as a substitute for the laser breakdown sound source.

To illustrate the advantages of the method proposed in the present invention in the application of underwater seabed ore thickness detection, so that the underwater laser induced plasma acoustic wave signal can play its advantages in other applications and show the practical significance of the present invention, the following is an example of the practical application effect of the present invention:

Five groups of aluminum block samples with different thicknesses are shown in Figure 3. The continuously changing seabed ore samples are shown in Figure 4, and their thickness ranges are shown in Table 1. The typical upper and lower surface reflected wave signals obtained are shown in Figure 5. The thickness reflected sound signals of five groups of aluminum block samples with different thicknesses are obtained, as shown in Figures 6 to 10, where d is the measurement data.

The evaluation parameters are shown in Table 2. The obtained stone thickness variation curve is shown in Figure 11.

Table 1 Thickness range

Table 2 Evaluation Parameters

In the simulation experiment, the sample with thickness variation is set as shown in Figure 12, and its thickness range is noted in Figure 12. The typical acoustic signal obtained in the simulation experiment is shown in Figure 13. Set the detection range to 1-10mm, with an interval of 1mm, detect the thickness of the sample, and obtain the sound field distribution as shown in Figure 14. Perform step 3 on the above results to obtain the thickness detection curve as shown in Figure 15.

The results show that the seabed ore thickness detection method based on laser-induced plasma sound source proposed by the present invention has high resolution. The detection resolution of aluminum block samples can reach 0.4mm, and the measurement error is less than 1%; it can be achieved Thickness measurements were made on seafloor ore samples with continuously varying thicknesses.

At the same time, since the power required by the laser is only a few hundred watts, and the laser and the hydrophone can be integrated into a whole, the method proposed in the present invention is not limited by the mounting platform and can achieve in-situ measurement at any location.

The above results show that the multi-platform seabed ore thickness measurement method based on laser-induced plasma sound source proposed by the present invention can achieve in-situ detection at any sea depth and has high detection accuracy, indicating that the method proposed by the present invention is effective in realizing seabed thickness measurement. In-situ measurement of ore thickness has good application potential and generalization.

In the above embodiments, each embodiment is described with its own emphasis. For parts that are not detailed or documented in a certain embodiment, please refer to the relevant descriptions of other embodiments.

The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person familiar with the technical field shall, within the technical scope disclosed in the present invention, Any modifications, equivalent substitutions and improvements made within the spirit and principles should be covered by the protection scope of the present invention.

Claims (10)

1. A method for detecting the thickness of seabed ores based on laser-induced plasma sound source, characterized in that the method includes: S1, use the experimental device to collect plasma acoustic waves to collect the reflected sound signal of the thickness of the underwater target; S2, preprocessing the obtained n sound signals; S3, extract the reflected waves from the upper and lower surfaces and calculate the thickness. 2. The method for detecting the thickness of seabed ore based on laser-induced plasma acoustic source according to claim 1, characterized in that, in step S1, the experimental device for collecting plasma acoustic waves comprises an energy attenuator (1), a laser beam expander (2), a meniscus lens (3), a double cemented lens (4), and a lens barrel (5); The energy attenuator (1) transmits the energy-adjusted laser to the laser beam expander (2); The meniscus lens (3) and the doublet lens (4) are combined into a focusing lens, and the focusing lens is placed in the lens barrel (5) to form a focusing lens group; The laser beam expander (2) expands the laser beam and emits it to the focusing lens group. 3. The method for detecting seabed ore thickness based on laser-induced plasma sound source according to claim 2, characterized in that, in step S1, an experimental device for collecting plasma sound waves is used to collect the underwater target thickness reflected sound signal. ,include: The laser beam expander (2) of the experimental device for collecting plasma acoustic waves expands the laser beam and emits it to the focusing lens group, and then emits the focused laser to the sample to be measured; the laser focusing position is 2 to 10cm above the sample to be measured; using The hydrophone collects the reflected sound wave signals from the upper and lower surfaces of the sample to be tested and sends them to the oscilloscope. 4. The method for detecting seabed ore thickness based on laser-induced plasma sound source according to claim 1, characterized in that, in step S2, the obtained n acoustic signals are preprocessed, including: (1) Extract the amplitude A0 and arrival time T0 of the plasma expansion acoustic signal Pn; n represents the total number of acoustic signals obtained at the measurement position; (2) Calculate the amplitude standard deviation Astd, the arrival time standard deviation Tstd, the amplitude average Amean, and the arrival time average Tmean; (3) Calculate the amplitude deviation degree Ae and the arrival time deviation degree Te; (4) Eliminate values with a deviation greater than 2 times the standard deviation, specifically: Ae>2×Astd, Te>2×Tstd, and obtain m acoustic signals Pm after eliminating outliers; (5) Average the acoustic signal Pm to get the average value Pa; (6) Band-pass filtering is performed on the average value Pa to remove low-frequency noise and obtain the characteristic sound signal Pb. 5. The method for detecting the thickness of seabed ore based on laser-induced plasma sound source according to claim 4 is characterized in that the extraction of the amplitude A0 and arrival time T0 of the plasma expansion sound signal Pn includes: using the matlab function max to find the maximum value, and the expression is: [A0, T0] = max(Pn). 6. The method for detecting seabed ore thickness based on laser-induced plasma sound source according to claim 4, characterized in that the expression for calculating the amplitude average value Amean is: Amean=sum(Ai)/n When measuring thickness, several infrasound signals are collected at the same position. n represents the total number of acoustic signals obtained at the measurement position; Ai is the amplitude of the i-th acoustic signal, where i is an integer from 1 to n. 7. The method for detecting seabed ore thickness based on laser-induced plasma sound source according to claim 6, characterized in that the expression for calculating the amplitude standard deviation Astd is: Astd＝sqrt(sum((Ai-Amean)^2)/n) Among them, i is a positive integer from 1 to n; The expressions for calculating the amplitude deviation Ae and arrival time deviation Te are: Ae＝|A0-Amean| Te=|T0-Tmean|. 8. The method for detecting the thickness of seabed ore based on laser-induced plasma sound source according to claim 1, characterized in that, in step S3, extracting the upper and lower surface reflected waves and calculating the thickness includes: Perform feature identification on the obtained characteristic sound signal Pb, and extract the peak arrival times t1 and t2 of the upper and lower reflected sound signals; the thickness of the sample to be measured is: d＝c×|t1-t2|/2 Among them, c is the sound speed in the sample. 9. The method for detecting seabed ore thickness based on laser-induced plasma sound source according to claim 8, characterized in that the characteristic sound signal Pb performs feature identification, including: Get a pulse signal with a pulse width lower than the threshold, the threshold is 2us; Obtain the pulse signal with a peak value higher than the threshold. The threshold depends on the amplitude A0. According to the acoustic wave reflection and transmission formula of different media, the upper surface reflection wave threshold is calculated as: 0.5×|(z2-z1)/(z1+z2)|×A0 The lower surface reflection wave threshold is: 0.5×|(4×z1×z2×(z1-z2)/(z1+z2)^3)|A0 Among them, z1 and z2 are the impedances of water and the target medium, z=ρv, ρ is the medium density, and v is the sound speed in the medium. 10. The method for detecting the thickness of seabed ores based on laser-induced plasma sound source according to any one of claims 1 to 9, characterized in that the method is applied to a cableless underwater robot AUV to realize seabed detection in a deep sea environment. Ore thickness measurement.