A deep-sea laser-induced plasma photo-acoustic combined detection system and method

Abstract

The application belongs to the field of ocean exploration technology and equipment, and discloses a deep-sea laser-induced plasma photo-acoustic combined detection system and method. The application realizes qualitative and quantitative analysis of target ore elements by collecting and analyzing characteristic spectral signals of plasma. The application realizes millimeter-level resolution thickness measurement by using high-frequency (up to 10MHz level) characteristics of reflection signals of wideband, short pulse sound waves excited by plasma expansion in rock layers. The excitation, collection and processing units of light and sound signals are highly integrated in a single system, and finally, in-situ, synchronous, same-point and high-precision integrated detection of seabed ore composition and thickness in deep-sea environment is realized. The system has high integration degree, good miniaturization and platform adaptability, high detection efficiency, low cost and good reliability, and has high-precision composition analysis and high-resolution thickness detection capabilities, and realizes real in-situ synchronous detection.

Description

Technical Field

[0001] This invention belongs to the field of marine exploration technology and equipment, and relates to a deep-sea laser-induced plasma photoacoustic joint detection system and method, especially to an underwater photoacoustic joint detection device and method based on laser-induced breakdown plasma, which is used to realize in-situ, synchronous, and same-point measurement of seabed ore composition and thickness. Background Technology

[0002] Refined exploration of key minerals such as iron-manganese crusts urgently requires detection technologies capable of simultaneously obtaining elemental composition and stratigraphic thickness data for the same micro-area. However, existing technologies have significant limitations in meeting this need. The most representative method is drilling sampling. This method obtains core samples through mechanical drilling, which are then returned to the laboratory for destructive analysis. Although it can simultaneously obtain composition and thickness data, it is a discrete, non-in-situ detection method with inherent drawbacks such as extremely low efficiency, high cost, and complex operation. Furthermore, the destructive sampling method makes it impossible to conduct large-scale, rapid resource assessment.

[0003] To overcome the inefficiency of sampling techniques, various in-situ detection technologies have been developed, but they are all limited in function and can only obtain partial information.

[0004] Composition detection methods: Represented by laser-induced breakdown spectroscopy (LIBS). This technology generates plasma by ablating the sample with a laser and analyzes its emission spectrum to achieve in-situ analysis of elemental composition. However, its inherent limitation is that it can only provide chemical composition information and cannot detect or measure the physical structure of the target (such as thickness).

[0005] Thickness detection schemes: represented by acoustic detection methods (such as shallow seismic profilometers and parametric array sonar). These technologies estimate stratigraphic thickness by delaying the echo of sound waves. Their limitations are: spatial resolution is limited by the wavelength of the sound waves (usually only on the order of decimeters), making it difficult to meet the requirements for fine detection of centimeter-level crusts; while parametric array technology can provide sound waves up to 1MHz, its strong directivity and narrow bandgap characteristics require strict cooperation from a submersible to achieve thickness detection; furthermore, acoustic-based methods completely lack the capability for chemical composition analysis.

[0006] Faced with the need to simultaneously obtain composition and thickness, the most direct and conventional approach for those skilled in the art, based on existing technology libraries, is to assemble and combine a functionally complementary LIBS system with an acoustic thickness measurement system. However, even with this conventional approach, the following fundamental and insurmountable drawbacks arise:

[0007] (1) System assembly and non-homogeneous excitation: The essence of this approach is to forcibly couple two independent physical processes (laser breakdown and electroacoustic conversion). This inevitably results in the system consisting of two independent core components (laser and acoustic wave transmitter / receiver), making the equipment bulky, power-consuming, and low-integrated, making it difficult to achieve the miniaturization and high efficiency required for deep-sea equipment.

[0008] (2) Inherent spatiotemporal mismatch problem: Since the composition and thickness information come from two different excitation sources and detection time sequences, it is impossible to achieve true synchronous measurement and detection of the same micro-area. This results in difficult-to-calibrate errors in the spatiotemporal reference of the acquired composition and thickness data, and the accuracy of subsequent data fusion cannot be guaranteed, thus seriously affecting the reliability of resource assessment.

[0009] In summary, the core technological challenge in this field lies in the contradiction between the unity of the detection target (composition and thickness) and the singularity and fragmentation of existing technological methods. Whether it's traditional drilling sampling, single-function in-situ techniques, or even the conventional approach of simply assembling them, all fail to achieve efficient and accurate integrated synchronous detection due to the separation of physical principles. Therefore, there is an urgent need for a novel technological solution that innovates from the physical source, utilizing the same excitation event to simultaneously generate composition and thickness signals, in order to fundamentally solve the aforementioned problems.

[0010] Based on the above background technology, existing solutions have the following main technical problems and disadvantages in achieving in-situ, synchronous, and same-point measurement of seabed mineral composition and thickness, which are the focus of this invention:

[0011] (1) Existing in-situ detection technologies can only obtain single-type parameters (composition or thickness), which cannot meet the need for comprehensive characterization of the target. Specifically: LIBS technology: its physical principle is based on the analysis of laser plasma spectrum, and it inherently cannot obtain the physical structure information of the target (such as layer thickness). Acoustic thickness measurement technology (including high-frequency parametric arrays, etc.): its physical principle is based on the analysis of mechanical acoustic wave reflection signals, and it inherently cannot obtain the chemical composition information of the target. Even if high-frequency parametric array technology (such as 1MHz main frequency) can improve the thickness detection resolution to the centimeter level, its single-function limitation of only measuring thickness remains unchanged.

[0012] (2) To address the issue of limited functionality, those skilled in the art might readily conceive of combining an independent LIBS system with an acoustic thickness measurement system. However, this approach suffers from the following inherent drawbacks: Spatiotemporal mismatch: Since the composition and thickness information originate from two independent physical processes and non-homogeneous excitation signals, the two systems cannot achieve true synchronous triggering and detection of the same micro-area. This results in spatiotemporal reference errors in the acquired data, severely impacting the accurate correlation and fusion of composition and thickness information. System integration challenges and poor operational flexibility: The two independent systems result in bulky equipment, complex structures, and high power consumption. In particular, high-performance acoustic thickness measurement systems (such as parametric arrays) typically possess extremely strong directivity, with stringent requirements for detection distance and attitude angle. This significantly limits the operational flexibility of the mounting platform (such as ROV), increasing operational difficulty and risk.

[0013] (3) Although drilling sampling can obtain composition and thickness information, it is a non-in-situ destructive sampling method, which is inefficient, costly, and cannot achieve large-scale and rapid exploration. Previous difficulties: In seeking solutions to the above problems, the field has long been plagued by a core technical contradiction: the contradiction between the unity of the detection target (composition and thickness) and the fragmentation of the physical principles of existing detection technologies. Because laser spectroscopy and acoustic detection belong to different categories in terms of physical mechanisms, traditional technical approaches cannot deeply integrate the two at the physical source and system level. Even with the development of high-performance specialized technologies (such as high-frequency acoustic thickness measurement), a systematic solution cannot be formed; instead, performance improvements may exacerbate the contradictions in system size, power consumption, and operational flexibility during assembly. Therefore, it has been impossible to develop measurement equipment that can achieve true homogeneity, synchronization, integration, and ease of operation. Summary of the Invention

[0014] To overcome the problems of limited functionality, spatiotemporal mismatch and operational difficulties caused by system assembly, and insufficient detection performance and efficiency in related technologies, this invention discloses an integrated deep-sea laser-induced plasma optical-acoustic joint detection system and method. This fundamentally changes the existing approach of simply assembling devices based on different physical principles. It creatively utilizes the single physical event of a single-beam pulsed laser penetrating an underwater target to generate plasma, simultaneously exciting spectral signals for composition analysis and acoustic signals for thickness detection. The technical solution is as follows:

[0015] This invention is implemented as follows: a deep-sea laser-induced plasma optical-acoustic joint detection system, comprising:

[0016] Pressure chambers are used to provide protection in deep-sea environments;

[0017] A pulsed laser, installed inside the pressure chamber, is used to generate high-energy pulsed lasers;

[0018] The laser focusing unit is used to focus a high-energy pulsed laser onto the surface of the underwater target to induce the generation of plasma.

[0019] The spectral signal acquisition unit is used to collect characteristic spectral signals emitted by the plasma.

[0020] The acoustic signal acquisition unit is used to acquire acoustic signals excited by plasma expansion and propagated and reflected by underwater targets;

[0021] The control and processing unit, connected to the pulsed laser, the spectral signal acquisition unit, and the acoustic signal acquisition unit, is used to control laser excitation and simultaneously process characteristic spectral signals and acoustic signals, thereby achieving integrated, simultaneous, and synchronous detection of elemental composition analysis and layered structure thickness measurement of underwater targets.

[0022] Furthermore, the spectral signal acquisition unit includes: a focusing lens for collecting plasma optical signals; an optical path guiding component for collimating and guiding the collected optical signals to a grating spectrometer; and a grating spectrometer for performing spectral processing on the optical signals.

[0023] The acoustic signal acquisition unit includes: a ring-shaped hydrophone for receiving acoustic signals; and a digital acquisition card for performing analog-to-digital conversion of acoustic signals; both the grating spectrometer and the digital acquisition card are triggered by the same laser output synchronization signal.

[0024] The control and processing unit is an industrial control computer.

[0025] Furthermore, the laser focusing unit includes a focusing lens disposed inside the pressure chamber and a pressure-resistant window disposed on the wall of the pressure chamber; wherein, the pressure-resistant window is a plano-convex lens, used to supplement the focusing of the pulsed laser and achieve sealing;

[0026] The optical path guiding assembly includes a dichroic mirror for reflecting the characteristic spectral signal and a parabolic mirror for focusing the reflected light to the entrance of the grating spectrometer.

[0027] Furthermore, the annular hydrophone is coaxially arranged around the light-emitting probe outside the pressure-resistant window to receive direct and reflected sound waves from underwater targets.

[0028] The front of the light-emitting probe is provided with a pressure-resistant and watertight quartz window. The light-emitting surface of the quartz window is flat, and the incident surface facing the laser surface is curved. A plano-convex lens with a focal length of 10mm is provided on the incident surface. A plano-convex lens with a focal length of 100mm is also provided behind the quartz window. The laser is emitted and focused through the two lenses, and the focusing position is located on the axis of the light-emitting probe.

[0029] Furthermore, the system is connected to the underwater vehicle via a watertight cable for power supply and communication, and the underwater vehicle provides power and data communication with the surface control terminal.

[0030] Another objective of this invention is to provide a method for deep-sea laser-induced plasma optical-acoustic joint detection based on the aforementioned deep-sea laser-induced plasma optical-acoustic joint detection system, the method comprising the following steps:

[0031] S1, System Initialization: The control and processing unit controls the pulsed laser to emit a high-energy pulsed laser, which is focused by the laser focusing unit and penetrates the surface of the underwater target to generate plasma.

[0032] S2, Synchronous signal acquisition: The characteristic spectral signal emitted by the plasma is collected using the spectral signal acquisition unit, and the broadband acoustic signal excited by the plasma expansion is collected using the acoustic signal acquisition unit. The broadband acoustic signal propagates inside the underwater target and is reflected due to the impedance difference of the layered structure interface.

[0033] S3, Integrated Processing and Analysis: The control and processing unit performs qualitative and quantitative analysis on the synchronously acquired characteristic spectral signals to obtain the elemental composition and content of the underwater target; it processes the synchronously acquired acoustic signals to identify the time difference of reflected waves at different interfaces, and calculates the thickness of the target layered structure by combining the known propagation speed of acoustic waves in the target medium.

[0034] In step S3, the quantitative analysis of the characteristic spectral signal is performed by inverting the element concentration from the measured spectral line intensity based on the quantitative relationship model between the intensity and concentration of the characteristic spectral lines of a specific element.

[0035] In step S3, the acoustic signal is processed to achieve thickness measurement: the center frequency of the high-frequency acoustic wave is between 0.1MHz and 10MHz, and the thickness d is calculated using the relationship between the reflected wave time difference Δt and the thickness d: d=(c*Δt) / 2, where c is the propagation speed of the acoustic wave in the target medium.

[0036] Furthermore, the center frequency of the high-frequency sound wave is on the order of 0.5 MHz, which, combined with the sound velocity in the rock strata, enables thickness measurement with millimeter-level resolution.

[0037] The laser breakdown in step S1 and the optical and acoustic signal acquisition in step S2 are both completed during the same laser pulse and at the same detection point, achieving in-situ, synchronous, and same-point detection.

[0038] Combining all the above technical solutions, the beneficial effects of this invention are as follows:

[0039] First, this invention achieves qualitative and quantitative analysis of target ore elements by collecting and analyzing the characteristic spectral signals of plasma; by collecting and processing the reflection signals of broadband, short-pulse sound waves excited by plasma expansion in rock strata, it utilizes the high-frequency characteristics (up to 10MHz) to achieve millimeter-level resolution thickness measurement; by highly integrating the excitation, collection, and processing units of optical and acoustic signals into a single system, it overcomes the inherent defects of discrete systems such as spatiotemporal mismatch, complex equipment, and inflexible operation, and ultimately achieves in-situ, synchronous, same-point, and high-precision integrated detection of seabed ore composition and thickness in deep-sea environments.

[0040] Secondly, compared with the existing technology, the deep-sea laser-induced plasma optical-acoustic joint detection system provided by the present invention has the following significant beneficial effects: The present invention achieves true integrated and in-situ synchronous detection: by using the physical event of inducing plasma with the same laser pulse, spectral signals for composition analysis and acoustic signals for thickness detection are generated synchronously, fundamentally solving the problems of spatiotemporal mismatch and non-homogeneous excitation in traditional assembly schemes, ensuring that the composition and thickness data originate from the same time and the same detection micro-area, and the data fusion accuracy is high.

[0041] This invention combines high-precision component analysis with high-resolution thickness detection capabilities: it inherits and leverages the advantages of LIBS technology, such as no complex sample preparation, in-situ, and rapid elemental analysis. Utilizing a high-frequency (e.g., 10MHz level) and short-pulse (e.g., 100ns level) sound source generated by laser plasma, it achieves millimeter-level high-resolution detection of rock layer thickness. Its performance surpasses that of traditional shallow seismic profilometers and is comparable to the thickness measurement resolution of high-frequency parametric arrays, but the system is much simpler.

[0042] This invention features high system integration, miniaturization, and excellent platform adaptability: since both optical and acoustic signals originate from the same laser excitation system, a separate acoustic wave emitting device (such as a parametric array transducer) is eliminated, significantly simplifying the system structure and reducing overall power consumption (only tens to hundreds of watts) and size. This highly integrated and low-power design allows the system to be flexibly mounted on various deep-sea operation platforms, such as tethered underwater robots (ROVs), untethered underwater robots (AUVs), and manned submersibles (HOVs), without being limited by the platform. This solves the problems of bulky and cumbersome assembly systems and the excessively high requirements for platform operational flexibility in high-frequency acoustic systems.

[0043] This invention offers high detection efficiency, low cost, and high reliability: overcoming the drawbacks of low efficiency and high cost associated with drilling and sampling, it achieves in-situ, non-destructive, and rapid continuous profile measurement, significantly improving the efficiency of deep-sea exploration. The close-range detection method effectively reduces the interference of environmental noise on acoustic signals, improving the signal-to-noise ratio and measurement reliability. The development, maintenance, and operation costs of a single system are far lower than the combined costs of maintaining two independent systems. The method proposed in this invention is not limited by a platform and does not require a fixed platform, enabling in-situ measurement of the composition and thickness of seabed minerals without the need for sample collection and analysis indoors.

[0044] Third, this invention can be directly applied to the refined exploration of minerals such as deep-sea iron-manganese crusts and polymetallic nodules, solving the core pain points of traditional exploration such as "low efficiency, high cost, and asynchronous data". After industrialization, on the one hand, it can improve the efficiency of deep-sea mineral exploration and significantly reduce the manpower, equipment, and time costs of drilling and sampling (taking the exploration of a 10-kilometer-long mining area at a water depth of 2000 meters as an example: with the TV grab method, it can only detect a single point and a single detection takes about 100 minutes. If the minimum resolution grid is 1 kilometer, it requires 10 detections. Not counting the mother ship's travel time, the detection time alone takes 1000 minutes; while the prototype of this invention allows continuous detection at a detection frequency of up to 10Hz. Calculated with a submersible with a speed of 1 knot, the spatial resolution can reach about 0.05 meters, and it takes 324 minutes to explore a 10-kilometer-long mining area. Both the resolution and the detection time are significantly reduced). On the other hand, its miniaturization and low power consumption characteristics can be adapted to various deep-sea operation platforms to form a standardized detection module, meeting the needs of marine resource development enterprises, research institutions, marine engineering operation and maintenance, and other scenarios.

[0045] Fourth, existing in-situ detection technologies, both domestically and internationally, either only enable in-situ composition detection (such as LIBS technology) or only enable thickness measurement (such as acoustic thickness measurement and parametric array technology), lacking a technical solution for acquiring composition and thickness data through "co-source excitation, synchronous and simultaneous point, and integrated" methods. This invention creatively utilizes a single laser plasma event to simultaneously generate spectral and acoustic signals, achieving for the first time in-situ synchronous detection of "composition-thickness" in deep-sea ores, filling a technological gap in the field of "multifunctional integrated deep-sea in-situ detection equipment" both domestically and internationally. This solution eliminates the need for assembling independent systems, achieving synchronous transmission and acquisition of photoacoustic signals from a physical mechanism perspective. Its technical approach and integration effect are pioneering in the industry, and no similar technologies or products have been publicly reported. Attached Figure Description

[0046] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure;

[0047] Figure 1This is a structural diagram of the deep-sea laser-induced plasma optical-acoustic joint detection system provided in an embodiment of the present invention;

[0048] Figure 2 This is a schematic diagram of the prototype of the deep-sea laser-induced plasma optical-acoustic joint detection system provided in an embodiment of the present invention;

[0049] Figure 3 This is a schematic diagram of the internal structure of the prototype of the deep-sea laser-induced plasma optical-acoustic joint detection system provided in an embodiment of the present invention;

[0050] Figure 4 This is an image of the result obtained by measuring the width (200mm) of marble sample 1 using the deep-sea laser-induced plasma optical-acoustic joint detection system provided in this embodiment of the invention;

[0051] Figure 5 This is an image showing the result obtained by measuring the length (300mm) of marble sample 1 using the deep-sea laser-induced plasma optical-acoustic joint detection system provided in this embodiment of the invention.

[0052] Figure 6 The image shows the spectral image of the zinc alloy composition of sample 2 measured by the deep-sea laser-induced plasma optical-acoustic joint detection system provided in this embodiment of the invention.

[0053] In the diagram: 1. Pressure chamber; 2. Pulsed laser; 3. Industrial computer; 4. Laser control line; 5. Focusing lens; 6. Pressure window; 7. Dichroic mirror; 8. Parabolic mirror; 9. Grating spectrometer; 10. Spectral data line; 11. Ring hydrophone; 12. Hydrophone watertight cable; 13. Digital acquisition card; 14. Acoustic data line; 15. System power supply and communication watertight cable; 16. Light emission probe. Detailed Implementation

[0054] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0055] A high-power laser, focused on the target ore, interacts to generate transient plasma. The characteristic radiation of this plasma can be collected as a spectral signal, while the plasma's outward expansion generates acoustic signals. The spectral signal is used to measure the ore grade, and the acoustic signal is used to measure the ore thickness. The spectral signal is the spatiotemporal integral of the plasma radiation, directly reflecting the elemental composition of the ore. Its quantitative analysis principle is that the higher the atomic concentration of a specific element in the sample, the stronger the intensity of the characteristic spectral lines emitted after being excited by the laser plasma. By establishing a quantitative relationship model (calibration curve) between spectral line intensity and elemental concentration using a series of standard samples, the accurate concentration of the element can be deduced from the spectral line intensity of the sample under test. After acoustic signal A is generated on the ore surface, a portion propagates inside the ore. Due to the impedance difference between the ore and the underlying bedrock, a reflected acoustic wave B is generated at the contact surface between the ore and the bedrock. A hydrophone can sequentially receive acoustic waves A and B. The thickness of the bedrock layer can be calculated from the time difference between A and B and the sound velocity within the bedrock. According to Ricker's wavelet principle, for composite reflected waves with time reflection sequence as a function, the critical resolution layer thickness is: d=λ / 3.4. Taking the sound velocity c=5000m / s and the detection frequency as 0.5MHz, then d=2.3mm, which can realize high-precision measurement of rock layer thickness.

[0056] The innovation of this invention lies in:

[0057] 1. This invention uses the optical and acoustic signals generated by the physical process of laser-induced breakdown as a common signal source for spectral and acoustic detection, realizing the simultaneous measurement of target components and structural thickness in time and space, and avoiding the problem of asynchronous detection caused by joint detection of different signal sources.

[0058] 2. This invention, based on the aforementioned physical event, simultaneously acquires and utilizes two signals generated during the process: atomic emission spectra and high-frequency short-pulse acoustic waves. Elemental composition is identified through analysis of the spectral signals, and thickness is measured through processing of the acoustic wave reflection signals. Thus, a single system enables simultaneous, in-situ detection of the target's chemical composition and physical structure.

[0059] 3. Based on the above principles, the excitation, collection, and processing units of optical and acoustic signals are highly integrated into a unified system architecture. This architecture eliminates the need for a separate acoustic wave emitting device, and features a compact structure, low power consumption, and strong platform adaptability. It can be directly mounted on various deep-sea submersibles, solving the problems of large size and complex operation caused by assembled systems.

[0060] Example 1: The deep-sea laser-induced plasma optical-acoustic joint detection method provided in this embodiment of the invention includes the following steps:

[0061] S1, synchronous detection from the same source: qualitative and quantitative analysis of target ore elements is achieved by collecting and analyzing the characteristic spectral signals of plasma;

[0062] S2, High-resolution thickness measurement: By acquiring and processing the reflection signal of broadband, short-pulse acoustic waves excited by plasma expansion in the rock strata, the thickness measurement with millimeter-level resolution is achieved by utilizing its high-frequency (up to 10MHz) characteristics.

[0063] If the arrival time difference between two wavelets is greater than or equal to the interval between the two maximum steepness points on both sides of the main extremum of the wavelet, then the two wavelets are distinguishable; otherwise, they are not distinguishable. To simplify the calculation, we discuss a signal symmetrical about t=0, with its two zeros being t1=-1 / (f0*sqrt(3)) and t2=1 / (f0*sqrt(3)). According to Ricker's criterion, the distinguishable time interval is (t2-t1) / 2=1 / (f0*sqrt(3)). Let the target sound velocity be c1, then the distinguishable thickness is: h=c1*h=c1 / (2*f0*sqrt(3)), c1 / f0=, h=≈. The result of measuring a 200mm thick aluminum block using a 5MHz hydrophone was obtained. The thickness was measured to be 199.456mm, with an error of 0.544mm.

[0064] S3, System Integration: It highly integrates the excitation, collection and processing units of light and sound signals into a single system, thereby overcoming the inherent defects of discrete systems such as spatiotemporal mismatch, complex equipment and inflexible operation, and finally realizing in-situ, synchronous, same-point and high-precision integrated detection of seabed mineral composition and thickness in deep-sea environment.

[0065] This invention utilizes the physical event of plasma induced by the same laser pulse to simultaneously generate spectral signals for composition analysis and acoustic signals for thickness detection, specifically including:

[0066] When a high-power laser is focused on a target and the laser irradiance exceeds the target's breakdown threshold, a small amount of material will be ablated and excited to generate plasma. This process will produce both light and sound.

[0067] ①Luminescence process: After the laser pulse ends, the excited particles in the plasma will transition from a high energy level to a low energy level and emit characteristic spectral lines. These characteristic spectral lines are the spectral signals that can be used for analysis.

[0068] ② Sound generation process: The expansion of plasma is related to the laser pulse time. The laser pulse time used in this invention is 10ns and 100ns. When the laser pulse is applied, the plasma continuously absorbs laser energy and undergoes explosive expansion, compressing the surrounding liquid to generate shock waves. The propagation speed of the shock waves is supersonic, and the speed decreases nonlinearly. After propagating for about several hundred nanoseconds, the speed of the shock waves decays to the longitudinal wave speed of the liquid, that is, the speed of sound. After that, it radiates outward in the form of sound waves. This sound wave is the sound wave signal used for thickness detection.

[0069] Therefore, both signals are generated by the same physical process.

[0070] This invention utilizes a high-frequency (e.g., 10MHz level), short-pulse (e.g., 100ns level) sound source generated by laser plasma to achieve millimeter-level high-resolution detection of rock layer thickness. A detailed analysis follows:

[0071] After acoustic signal A is generated on the surface of the ore, a portion of it propagates within the ore. Due to the impedance difference between the ore and the underlying bedrock, a reflected acoustic wave B is generated at the contact surface between the ore and the bedrock. A hydrophone can sequentially receive acoustic waves A and B. The thickness of the rock layer can be calculated from the time difference between A and B and the sound velocity within the rock layer. According to Ricker's wavelet principle, for a composite reflected wave with the time reflection sequence as a function, the critical resolution layer thickness is d = λ / 3.4. Taking a rock sound velocity c = 5000 m / s and a detection frequency of 0.5 MHz as an example, the critical resolution thickness d = 2.3 mm. The laser acoustic signal has a frequency band covering 100 Hz to 10 MHz, enabling millimeter-level precision measurement of rock layer thickness.

[0072] This invention utilizes the optical and acoustic signals generated by the physical process of laser-induced breakdown as a common signal source for both spectral and acoustic detection, enabling simultaneous temporal and spatial measurement of target composition and structural thickness. The specific measurement process is as follows:

[0073] The spectral signal emitted by the plasma is used to detect the composition of the target, while the acoustic signal emitted by the same plasma is used to measure the thickness of the target. The use of the same plasma ensures that the spectral and acoustic signals are emitted simultaneously, thus achieving simultaneous measurement in time. Simultaneously, the use of the same plasma ensures that the detection locations of the spectral and acoustic signals are the same, thus achieving simultaneous measurement in space. Therefore, it can be said that the simultaneous measurement of the target's composition and structural thickness in both time and space is achieved.

[0074] Composition measurement is achieved by analyzing specific spectral lines in the target spectrum to identify specific elements; this process is a well-established technique known as LIBS. Structural thickness measurement is performed using the pulse-echo method. The focus of this invention is to achieve simultaneous measurement of composition and thickness.

[0075] Example 2, as Figure 1 As shown, the deep-sea laser-induced plasma optical-acoustic joint detection system provided in this embodiment of the invention uses a pressure-resistant chamber 1 to ensure the normal operation of the equipment underwater. A pulsed laser 2 is used as the excitation source, and the pulsed laser 2 is connected and controlled by an industrial control computer 3 via a laser control line 4. The laser emitted from the pulsed laser 2 is focused and broken down by a focusing lens 5 and a pressure-resistant window 6 to generate plasma. The point light source generated by the breakdown is converted into parallel light by the focusing lens 5, reflected by a dichroic mirror 7, and focused by a parabolic mirror 8 to the window of a grating spectrometer 9 for spectral acquisition. Finally, the spectral data is stored in the industrial control computer 3 via a spectral data line 10. The acoustic signal generated by the plasma expansion is collected by a ring hydrophone 11, transmitted through a watertight cable 12 to a digital acquisition card 13, converted into an electrical signal, and stored in the industrial control computer 3 via an acoustic data line 14. The equipment is connected to the underwater submersible via a system power supply and communication watertight cable 15 to achieve equipment power supply and communication with the ship. The light-emitting probe 16 is a replaceable accessory, and its dimensions are designed according to the ring hydrophone 11.

[0076] The front of the light-emitting probe 16 is provided with a pressure-resistant and watertight quartz window. The light-emitting surface of the quartz window is flat, while the incident surface facing the laser surface is curved. A plano-convex lens with a focal length of 10mm is provided on the incident surface. Behind the quartz window, another plano-convex lens with a focal length of 100mm is provided. The laser is emitted and focused through the two lenses, and the focusing position is located on the axis of the light-emitting probe 16. The custom-made annular hydrophone 11 is axially symmetrical. By fitting the hydrophone onto the light-emitting probe 16, it can be ensured that the hydrophone and the light-emitting probe 16 are coaxial. The hydrophone is fixed by three threaded holes evenly distributed on the circumference of the hydrophone base, and corresponding grooves are left at the corresponding positions of the probe. The hydrophone and the probe are fixed together by set screws. In addition, the sound source is excited by the expansion of plasma, and the detected spectrum is also the light emitted by the plasma. Therefore, the emission point of the sound source and the emission point of the spectrum are physically coincident.

[0077] Laser control line 4 is a USB interface for controlling pulsed laser 2, used to transmit control signals from industrial control computer 3 to pulsed laser 2; spectral data line 10 is a USB interface for controlling, powering, and receiving data from grating spectrometer 9, used to transmit the spectral digital signals acquired by grating spectrometer 9 to industrial control computer 3; acoustic data line 14 is a USB interface for controlling and receiving data from acquisition card, used to transmit the acoustic digital signals output by digital acquisition card 13 to industrial control computer 3. Hydrophone watertight cable 12 has a Hummer mini K-series 5-pin female connector at one end and a Subconn micro-circular 8-pin female connector at the other, used to transmit the analog acoustic signals received by the ring hydrophone 11 to the digital acquisition card 13 inside the pressure tank 1. System power supply and communication watertight cable 15 has Subconn micro-circular 13-pin male connectors at both ends, used to provide power to the entire system and to enable full-duplex data communication (including command issuance and data upload) between the system and the upper control terminal (via the submersible).

[0078] In Example 3, the device used to generate the laser-induced plasma acoustic wave signal in water is a short-pulse laser 2. However, other lasers that can be used to experiment with underwater photo-induced breakdown can also achieve the present invention. Therefore, the scheme of using other laser generation devices can be used as an alternative to the above-mentioned plasma acoustic wave generation.

[0079] Example 4: The equipment used for collecting laser-induced plasma acoustic wave signals in water is an ultrasonic hydrophone and a data acquisition card. However, other devices that can detect and collect acoustic wave signals can also realize the present invention. Therefore, the scheme of using other acoustic wave signal acquisition devices can be used as an alternative to the above-mentioned plasma acoustic wave acquisition.

[0080] Example 5: The optical path and equipment for collecting the spectral signal of laser-induced plasma in water are a dichroic mirror 7, a parabolic mirror 8, and a grating spectrometer 9. However, other devices that can effectively collect spectral signals can also realize the present invention. Therefore, the scheme of using other spectral signal acquisition devices can be used as an alternative to the above-mentioned spectral signal acquisition.

[0081] This alternative solution is merely a replacement for the initial data acquisition method. The core of this invention is to utilize the spectral and acoustic signals generated during laser-induced breakdown plasma to achieve in-situ measurement of the composition and thickness of seabed minerals. Regarding the replacement of the data acquisition device, in principle, it only needs to ensure the acquisition of spectral and acoustic signals while obtaining good signal quality.

[0082] To further demonstrate the positive effects of the above embodiments, the present invention conducts the following experiments based on the above technical solutions.

[0083] I. Principle Overview: The core of this invention lies in using a single-beam pulsed laser to penetrate an underwater target and generate plasma; by collecting and analyzing the atomic emission spectrum of this plasma, the composition analysis of the target elements can be achieved; simultaneously, by acquiring the reflection signal of the broadband acoustic wave excited by the expansion of this plasma in the layered structure of the target, the thickness of the target can be measured.

[0084] II. System Structure and Workflow; such as Figure 1 As shown in the figure, the deep-sea laser-induced plasma optical-acoustic joint detection system provided in this embodiment of the invention has the following working process:

[0085] 1. System Initialization and Laser Excitation: The entire system is protected from the deep-sea environment by the pressure chamber 1. The industrial control computer 3 controls the pulsed laser 2 to emit high-energy pulsed laser light via the laser control line 4. The laser beam is first initially focused by the focusing lens 5, and then passes through the pressure-resistant window 6, which functions as both a seal and a lens. The pressure-resistant window 6 is a plano-convex lens, which focuses the laser light and, together with the light-emitting probe 16, forms a pressure-resistant sealing structure to protect the internal optical path. The laser light is finally focused onto the surface of the mineral sample to be tested, forming plasma.

[0086] 2. Spectral Signal Acquisition and Processing: Plasma emission (i.e., spectral signal) is collected and collimated into parallel light by the pressure-resistant window 6 and focusing lens 5. The parallel light is reflected by the dichroic mirror 7, converged by the parabolic mirror 8, and enters the entrance slit of the grating spectrometer 9. The grating spectrometer 9 splits the light signal and converts it into spectral data, which is transmitted to the industrial control computer 3 via the spectral data line 10 for storage and analysis, used for subsequent qualitative and quantitative elemental analysis.

[0087] 3. Acoustic Signal Acquisition and Processing: The plasma expands rapidly, exciting broadband acoustic waves (i.e., acoustic signals) within the surrounding liquid. These signals propagate within the ore and are reflected at the "ore-seawater" interface and the "ore-bedrock" interface. A ring-shaped hydrophone 11 is fixed around the light-emitting probe 16 to receive acoustic signals, including direct waves and reflected waves from each interface. The acoustic signals are transmitted via the hydrophone's watertight cable 12 to the digital acquisition card 13, converted into digital signals, and then stored in the industrial control computer 3 via the acoustic data cable 14 for subsequent thickness calculations.

[0088] The sound wave frequency refers to the frequency of the sound wave generated by the expansion of plasma. This sound wave has a frequency coverage range below 10MHz (i.e., 1Hz-10MHz, determined by the physical form of the sound wave). The 0.5MHz example used in the calculation is for illustrative purposes, not because the center frequency of the emitted sound wave is 0.5MHz. It refers to the signal received by a hydrophone with a center frequency of 0.5MHz that can be used for calculation. In reality, the maximum resolution of this sound wave can reach sub-micrometers; however, due to the rapid attenuation of high-frequency components, it becomes difficult to detect a 10MHz echo when the thickness is large (generally >10mm). Broadband sound waves have a frequency coverage range between 1Hz and 10MHz.

[0089] The sound velocity c was obtained through a preliminary calibration experiment. The method used was the time-difference method, and the entire test environment was underwater. The process consisted of four steps: ① Measure the thickness d1 of the object to be tested; ② Place a hydrophone at a certain distance from the sound source (generally, it should be 1.5 times the thickness of the object to be tested, denoted as L, but the specific value of L is not used in the calculation), excite and receive the sound signal, record the arrival time t0 of the sound pulse, and denot the sound velocity in the water as c0 (the sound velocity in the water is a constant that can be looked up), then t0 = L / c0; ③ Keeping the positions of the hydrophone and the sound source unchanged, insert the object to be tested between the hydrophone and the sound source (the sound velocity in the object to be tested is denoted as c1, and the thickness of the object to be tested is denoted as d1), excite and receive the sound signal, and record the arrival time t1 of the sound pulse, then t1 = (L-d1) / c0 + d1 / c1; ④ Solve the equations for t0 and t1 simultaneously to obtain the sound velocity of the object to be tested, c1 = c0*d1 / (c0*(t1-t0)+d1). Where t0, t1, and d1 are all data measured in steps ①②③.

[0090] Interface recognition algorithm: ① Low-pass filtering is applied to the acquired signal s0 to obtain signal s1 (the low-pass filtering parameters depend on the hydrophone used; here, a Butterworth filter in third-order mode with a cutoff frequency of 200kHz is used); ② A detection threshold A0 is set (the value of A0 is related to the sensitivity of the hydrophone and the light output energy of the pulsed laser 2; here, it is set to 5V, and A0 is used to ensure that a direct sound signal is acquired); ③ Peak detection is performed, and values ​​less than A0 are discarded. The remaining peaks are sorted by time, and the first peak is the direct sound signal, with its arrival time recorded as t0. The sound signal acquired after t0+16μs is recorded as s2 (the 16μs is determined based on the distance between the hydrophone surface and the light-emitting surface, which is 25mm apart; therefore, 25 / 1.5≈16.7, and 16 is used here to ensure that when the light-emitting surface is close to the target, the subsequent acquired signal is not a signal obtained between the light-emitting surface and the target). ④ Set the detection threshold A1, A1=|(z2-z1) / (z1+z2)|*2*A0, and record the maximum value of s2 as As1. When As1 is less than A1, detect its pulse width. When the pulse width is less than 15μs, As1 is considered to be the upper surface wave. The arrival time ts1 is the arrival time (here z1 and z2 are impedances, z1 is the impedance in water, z2 is the impedance of the object to be measured, impedance z=ρc, ρ is the medium density, and c is the medium sound velocity. A1 is set to ensure that the detected signal is the upper surface reflection signal. When the signal is greater than A1, it can be physically ruled out as a reflected sound signal); ⑤ Set the detection threshold A2, A2=|(4*z1*z2*(z1-z2) / (z1+z2)^3)|*2*A0. Similarly, detect pulse signals less than A2 and record their peak time as ts2. This is the lower surface reflection sound signal.

[0091] 4. System power supply and communication: The entire system is connected to the underwater vehicle (such as ROV) via a watertight cable 15 for system power supply and communication. The underwater vehicle provides power and communicates with the surface mother ship via data.

[0092] III. Examples and Detection Results; To verify the feasibility and effectiveness of the present invention, the following experiments were conducted:

[0093] 1. Use as follows Figure 2 , Figure 3 The prototype shown was used to measure the underwater thickness of sample 1 (marble). The acquired acoustic signals were analyzed, such as... Figure 4 , Figure 5 As shown, the width (200mm) and length (300mm) of the marble sample were successfully calculated, verifying the accuracy of the thickness measurement of this invention. Figure 4 To measure the acoustic results of the marble width, the peak arrival time of the acoustic wave on the upper surface was 172.379 μs, the arrival time of the acoustic wave on the lower surface was 242.34 μs, the time interval was 69.961 μs, and the thickness was measured to be 206.455 mm. Figure 5 To measure the acoustic results of the marble length, the peak arrival time of the acoustic wave on the upper surface was 114.519 μs, and the arrival time of the acoustic wave on the lower surface was 219.637 μs, with a time interval of 105.118 μs. The measured thickness was 310.203 mm. The above measurement error is due to the fact that the hydrophone used in this experiment had a main frequency of 100 kHz, and the pulse width of the acoustic signal acquired by this hydrophone was 10 μs. Therefore, the error introduced by the hydrophone pulse width can reach ±30 mm.

[0094] 2. Underwater composition measurements were performed on sample 2 (zinc alloy) and sample 3 (iron-manganese crust). The acquired spectral signals were analyzed, such as... Figure 6 As shown, 0.5% cobalt was successfully detected in the zinc alloy, as well as the major elements iron and manganese in the iron-manganese crust, verifying the sensitivity and effectiveness of the component analysis of this invention.

[0095] IV. Conclusion: The above embodiments demonstrate that the deep-sea laser-induced plasma optical-acoustic joint detection system provided by this invention successfully achieves in-situ, synchronous, and integrated measurement of the composition and thickness of underwater targets. The system has a compact structure and is easy to operate, possessing broad application prospects in the field of deep-sea mineral resource exploration.

[0096] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A deep-sea laser-induced plasma optical-acoustic joint detection system, characterized in that, The system includes: Pressure tank (1), used to provide protection in deep-sea environments; A pulsed laser (2) is installed inside the pressure chamber (1) to generate high-energy pulsed lasers; A laser focusing unit is used to focus a high-energy pulsed laser onto the surface of the underwater target to induce plasma generation. The laser focusing unit includes a focusing lens (5) disposed in the pressure chamber (1) and a pressure-resistant window (6) disposed on the wall of the pressure chamber (1). The pressure-resistant window (6) is a plano-convex lens used to supplement the focusing of the pulsed laser and achieve sealing. A spectral signal acquisition unit is used to collect characteristic spectral signals emitted by plasma. The spectral signal acquisition unit includes: a focusing lens (5) for collecting plasma light signals; an optical path guiding component for collimating and guiding the collected light signals to a grating spectrometer (9); and a grating spectrometer (9) for performing spectral processing on the light signals. The optical path guiding component includes a dichroic mirror (7) for reflecting the characteristic spectral signals and a parabolic mirror (8) for focusing the reflected light to the entrance of the grating spectrometer (9). The acoustic signal acquisition unit is used to acquire acoustic signals excited by plasma expansion and propagated and reflected by underwater targets; the acoustic signal acquisition unit includes a ring hydrophone (11) for receiving acoustic signals; and a digital acquisition card (13) for analog-to-digital conversion of acoustic signals; the grating spectrometer (9) and the digital acquisition card (13) are both triggered by the same laser output synchronization signal; the ring hydrophone (11) is coaxially arranged around the light-emitting probe (16) outside the pressure-resistant window (6) for receiving direct and reflected acoustic waves from underwater targets; the front of the light-emitting probe (16) is provided with a pressure-resistant and watertight quartz window, the light-emitting surface of the quartz window is flat, the incident surface facing the laser surface is provided with curvature, the incident surface is provided with a plano-convex lens with a focal length of 10mm, and a plano-convex lens with a focal length of 100mm is also provided behind the quartz window, the laser is emitted and focused through the two lenses, and the focusing position is located on the axis of the light-emitting probe (16). The control and processing unit is connected to the pulsed laser (2), the spectral signal acquisition unit and the acoustic signal acquisition unit. It is used to control laser excitation and synchronously process characteristic spectral signals and acoustic signals to realize integrated, simultaneous and synchronous detection of elemental composition analysis and layered structure thickness measurement of underwater targets. The control and processing unit is an industrial computer (3).

2. The deep-sea laser-induced plasma optical-acoustic joint detection system according to claim 1, characterized in that, The system is connected to the underwater vehicle via a watertight cable (15) for system power supply and communication, and is powered by the underwater vehicle and communicates with the surface control terminal.

3. A method for deep-sea laser-induced plasma optical-acoustic joint detection, wherein the method is implemented using the deep-sea laser-induced plasma optical-acoustic joint detection system described in any one of claims 1-2, characterized in that, The method includes the following steps: S1, System initialization: The pulse laser (2) is controlled by the control and processing unit to emit a high-energy pulse laser, which is focused by the laser focusing unit and penetrates the surface of the underwater target to generate plasma; S2, Synchronous signal acquisition: The characteristic spectral signal emitted by the plasma is collected using the spectral signal acquisition unit, and the broadband acoustic signal excited by the plasma expansion is collected using the acoustic signal acquisition unit. The broadband acoustic signal propagates inside the underwater target and is reflected due to the impedance difference of the layered structure interface. S3, Integrated Processing and Analysis: The control and processing unit performs qualitative and quantitative analysis on the synchronously acquired characteristic spectral signals to obtain the elemental composition and content of the underwater target; it processes the synchronously acquired acoustic signals to identify the time difference of reflected waves at different interfaces, and calculates the thickness of the target layered structure by combining the known propagation speed of acoustic waves in the target medium.

4. The deep-sea laser-induced plasma optical-acoustic joint detection method according to claim 3, characterized in that, In step S3, the quantitative analysis of the characteristic spectral signal is performed by inverting the element concentration from the measured spectral line intensity based on the quantitative relationship model between the intensity and concentration of the characteristic spectral lines of a specific element.

5. The deep-sea laser-induced plasma optical-acoustic joint detection method according to claim 3, characterized in that, In step S3, the acoustic signal is processed to achieve thickness measurement: the center frequency of the high-frequency acoustic wave is between 0.1MHz and 10MHz, and the thickness d is calculated using the relationship between the reflected wave time difference Δt and the thickness d: d=(c*Δt) / 2, where c is the propagation speed of the acoustic wave in the target medium.

6. The deep-sea laser-induced plasma optical-acoustic joint detection method according to claim 5, characterized in that, The center frequency of the high-frequency sound wave is on the order of 0.5 MHz. Combined with the sound velocity in the rock strata, it enables thickness measurement with millimeter-level resolution.

7. The deep-sea laser-induced plasma optical-acoustic joint detection method according to claim 3, characterized in that, The laser breakdown in step S1 and the optical and acoustic signal acquisition in step S2 are both completed during the same laser pulse and at the same detection point, achieving in-situ, synchronous, and same-point detection.

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