A deep seabed gas leakage flux in-situ monitoring device and measurement method

Abstract

The present invention relates to the technical field of marine engineering observation equipment, and more specifically to an in-situ monitoring device and measurement method for deep-sea seabed gas leakage flux. The device comprises a fixed chamber, a mounting frame, and a counterweight seat. A gas release port and a gas inlet port are respectively provided at the upper and lower ends of the fixed chamber, and electromagnetic valves are installed in each. A gas measuring device is installed in the fixed chamber. The top of the mounting frame is fixedly connected to the upper portion of the fixed chamber via a sealing plate, and the bottom of the mounting frame is connected to the counterweight seat via an acoustic releaser. A buoyant block forms a receiving chamber between the sealing plate and the counterweight seat within the mounting frame. The mounting frame is connected to a pressure-resistant chamber and a battery chamber within the receiving chamber. A controller is provided within the pressure-resistant chamber, and a lithium battery pack is provided within the battery chamber. The controller is electrically connected to two electromagnetic valves and the acoustic releaser via cables. The mounting frame also has an underwater acoustic communication system installed within the receiving chamber that is electrically connected to the controller and capable of transmitting data to the outside world. The present invention achieves high-precision deep-sea seabed gas leakage flux measurement based on fiber Bragg grating (FBG), enabling continuous, real-time, and long-term observation.

Description

Technical Field

[0001] The present invention relates to the technical field of marine engineering geological observation equipment, and in particular to an in-situ monitoring device and a measurement method for deep-sea seabed gas leakage flux. Background Art

[0002] Submarine gas seepage is the release of gas from within the seabed into the ocean water. It is the result of a variety of chemical, biological, geological, and physical processes occurring within the seabed and is widespread and diverse across continental margins. In addition to the geomorphological manifestations of submarine gas seeps (e.g., authigenic carbonate crusts on the cratered seafloor surface, deep-water corals, and submarine domes), fluid conduit features (e.g., landslides, faults, and mud volcanoes) also play a significant role in facilitating gas escape. The scale of submarine gas seeps is difficult to determine because seep characteristics, including bubble size, velocity, and concentration, typically exhibit significant temporal variability on timescales ranging from seconds to decades. Therefore, in situ observations of submarine gas seep fluxes can provide insights into seafloor geological changes and potential submarine geohazards.

[0003] Currently, there are two methods for measuring seafloor gas seepage flux: acoustic inversion and direct seafloor observation. Acoustic inversion methods (such as Chinese invention patent applications CN108931777A, CN113624639B, and Chinese utility model patent application CN214334779U) are limited to single-shot detection and cannot capture long-term changes in seafloor gas seepage. Furthermore, the seafloor gas seepage flux derived from acoustic inversion suffers from large errors, thus presenting significant limitations. In contrast, direct seafloor observation offers a significant advantage by providing a time series of changes in seafloor gas seepage flux. Among similar devices currently available, Chinese invention patent application CN102012246B uses a quantitative volumetric emptying method (measuring the number of times a quantitative volume of gas is generated) to determine the average in-situ flow rate of seafloor cold seep natural gas seepage. This method can also determine the total amount of natural gas seeping upward from the seafloor. However, this device takes a long time to perform a single measurement, and its observation method is not continuous, resulting in low temporal resolution of the measured data. For short, rapid gas eruptions, determining the pattern of gas release is difficult. Chinese invention patent application CN113624639B employs a method for rapidly capturing and obtaining gas volume to calculate real-time flux. However, this method requires the use of an underwater robot, preventing continuous in-situ observation and limiting its practical application. Clearly, current technical methods and measurement devices are still deficient in accurately measuring long-term variations in submarine gas seepage flux, failing to meet current practical application requirements. This presents an urgent problem that needs to be addressed. Summary of the Invention

[0004] The present invention provides a high-precision deep-sea seabed gas leakage flux in-situ monitoring device and measurement method for continuous real-time long-term in-situ observation of the seabed based on fiber grating to overcome the shortcomings of the existing technology.

[0005] The present invention is achieved through the following technical solutions:

[0006] Provided is an in-situ monitoring device for deep seabed gas leakage flux, comprising a fixed cavity, a mounting frame with a circumferential surface connected to a floating block, and a counterweight seat with a gas channel.

[0007] A gas release port is provided at the upper end of the fixed cavity and a gas inlet port is provided at the lower end facing the gas channel. Electromagnetic valves are installed in the gas release port and the inlet port respectively. A gas measuring device for measuring gas leakage flux is provided in the fixed cavity.

[0008] The top of the mounting frame is fixedly connected to the upper part of the fixed cavity through a sealing plate, and the bottom of the mounting frame is detachably connected to the counterweight seat through an acoustic releaser. A accommodating cavity is formed between the sealing plate and the counterweight seat in the mounting frame through a floating block. The mounting frame is connected to a pressure-resistant chamber and a battery chamber in the accommodating cavity. A controller is provided in the pressure-resistant chamber, and a high-density lithium battery pack is provided in the battery chamber. The controller is electrically connected to two solenoid valves and an acoustic releaser through cables respectively. The mounting frame is also equipped with an underwater acoustic communication system in the accommodating cavity that is electrically connected to the controller and can transmit data with the outside world.

[0009] Furthermore, a trumpet-shaped collecting cover is connected to the gas inlet at the bottom of the fixed cavity.

[0010] By setting up a trumpet-shaped collection cover, it is convenient to collect seabed gas and concentrate it into a fixed cavity.

[0011] Furthermore, the gas measuring device includes a pressure-resistant shell, inside which are respectively provided an LED light source emitter, a photodiode and a multi-channel fiber Bragg grating demodulator, as well as a high-density lithium battery pack for powering the three. The photodiode and the multi-channel fiber Bragg grating demodulator are respectively connected to the combination socket on the top of the pressure-resistant shell through optical fibers; it also includes a plurality of fiber Bragg grating gas sensing devices located at the bottom of the pressure-resistant shell, each fiber Bragg grating gas sensing device includes a multi-stage coupling prism and transmission optical fibers respectively connected to the two ends of the multi-stage coupling prism, the transmission optical fiber at the upper end of each multi-stage coupling prism is connected to the LED light source emitter in the pressure-resistant shell through a photoelectric watertight connector, and the end of the transmission optical fiber at the lower end of each multi-stage coupling prism is connected to a sensor assembly, and each sensor assembly is plugged into the combination socket on the top of the pressure-resistant shell through a double-headed cable assembly and is respectively connected to the photodiode and the multi-channel fiber Bragg grating demodulator.

[0012] The multi-stage coupling prism serves as the sensing section. Two gratings are engraved on the transmission fiber of the fiber Bragg grating gas sensing device: one for measuring temperature and the other for measuring pressure. The temperature sensor corrects the pressure sensor's measurements, ensuring accurate pressure data. Fiber Bragg gratings utilize the reflection properties of light. As temperature and pressure change, the spectral characteristics of the grating change. By comparing the wavelength shift of the reflected light from the grating, changes in temperature and pressure can be accurately measured. An LED emitter emits a light beam into the fiber system, stimulating the reflected light from the grating in the fiber. This light beam is then transmitted through the transmission fiber to the multi-stage coupling prism, which serves as the measuring section. A photodiode receives the light signal reflected from the multi-stage coupling prism. The photodiode converts the received light signal into an electrical signal for further analysis by a signal processing system. The transmission fiber connects the LED emitter and the photodiode, transmitting the excitation light and the reflected light. This bifurcated design allows the light signal to be distributed along different paths, allowing the light passing through the multi-stage coupling prism to be transmitted to a multi-channel fiber optic interrogator and the photodiode, respectively. It ensures the accuracy of signal transmission and measurement; the multi-stage coupling prism is the key sensing segment in the system, and its main function is to reflect light signals through changes in the refractive index. When the multi-stage coupling prism is located in the gas, due to the low refractive index of the gas, the light undergoes total internal reflection and returns to the photodiode. When the multi-stage coupling prism comes into contact with seawater, due to the difference in the refractive index of seawater and gas, the total internal reflection of the light is destroyed, and part of the light passes through the prism into the seawater, resulting in a decrease in the intensity of the reflected light. By monitoring the changes in the intensity of the returned light, the volume change of the gas can be inferred, and thus the leakage flux of the gas can be calculated; the function of the multi-channel fiber optic demodulator is to demodulate the changes in light waves. It can determine the temperature, pressure and light intensity changes by measuring the wavelength offset of the light beam, thereby further inferring the volume and leakage flux of the gas.

[0013] The overall working process:

[0014] When the device starts working, the LED light source transmitter emits laser light into the transmission fiber. The laser propagates through the transmission fiber and reaches the multi-stage coupling prism. When the multi-stage coupling prism is in the gas, due to the low refractive index of the gas, the light undergoes total internal reflection in the prism and returns to the photodiode. The light signal received by the photodiode is transmitted to the multi-channel fiber optic demodulator through the optical fiber. The demodulator obtains data related to the gas volume and pressure by analyzing the changes in light intensity and wavelength offset.

[0015] When the multi-stage coupled prism comes into contact with seawater, due to the different refractive index of seawater, some light passes through the prism and enters the seawater, weakening the light intensity received by the photodiode. Based on this change in light intensity, the amount of gas released can be inferred. Furthermore, the system's temperature and pressure sensors simultaneously monitor environmental conditions and correct temperature and pressure data to ensure measurement accuracy.

[0016] In this way, the fiber optic measurement system can capture the volume changes of gas in real time and continuously and calculate the gas leakage flux. The measurement data can be transmitted to the surface buoy via the underwater acoustic communication system and further transmitted to the laboratory for real-time analysis.

[0017] Furthermore, it also includes a coupled communication buoy, which transmits data with the underwater acoustic communication system through wireless communication, and the coupled communication buoy is connected to the surface buoy through a cable, and the surface buoy transmits data with the land laboratory through satellite communication.

[0018] By setting up a coupled communication buoy and a surface buoy in conjunction with satellite communication, the real-time data monitored by the monitoring device can be transmitted to the land laboratory.

[0019] Furthermore, a sensor interface for reading data stored in the gas measuring device is provided on the sealing plate.

[0020] The sensor interface can be connected to internal sensors to measure the temperature, pressure, salinity, methane concentration, carbon dioxide concentration and other indicators of seawater affected by submarine gas release, further determine the situation of gas release, and further provide data support for submarine geological disaster monitoring.

[0021] Furthermore, a guardrail is installed on the sealing plate.

[0022] By setting up guardrails, on the one hand, it can play a protective role, and on the other hand, it can be used for the manipulator of the ROV robot to grasp during deployment.

[0023] An observation method using a deep-sea seabed gas leakage flux in-situ monitoring device comprises the following steps:

[0024] S1. Deployment: The manipulator of the underwater robot (ROV) grasps the guardrail of the device and drives the device down. During deployment, the upper and lower solenoid valves are opened. The ROV uses an underwater camera to determine the exact location of the seabed gas leakage. The device is then placed over the leakage point. The manipulator detaches from the device, completing the fixed-point deployment on the seabed.

[0025] S2. Measurement: The lower solenoid valve is opened and the upper solenoid valve is closed. Gas and seawater enter the fixed cavity through the gas channel and the collection cover. When there is no gas in the fixed cavity, the light intensity received by the photodiode is the weakest. As the amount of captured gas gradually increases, the light intensity received by the photodiode gradually increases.

[0026] By calculating the change in gas volume based on the change in light intensity, the gas temperature can be measured T and pressure P , the change in gas volume can be calculated using the ideal gas equation:

[0027] ;

[0028] in: V is the volume of the gas, n is the number of moles of gas, R is the ideal gas constant, T is the temperature of the gas, P is the pressure of the gas.

[0029] According to the change in gas volume, the flux formula is used to calculate the gas leakage flux: ;

[0030] in: Q is the gas flux, Δ V is the change in gas volume per unit time, A is the area where the gas flux is measured, Δ t is the time interval.

[0031] During the measurement, the gas volume changes Δ V The calculation can be performed based on the real-time monitoring data of temperature and pressure. The gas flux can be obtained by accumulating multiple measurement cycles. Q long-term changes.

[0032] S3. Transmission: An underwater acoustic communication system is installed on the monitoring device, and in conjunction with a coupled communication buoy, the controller sends the measurement data to the receiving end of the coupled communication buoy through the underwater acoustic communication system. The receiving end of the coupled communication buoy transmits the data upward along the cable to the surface buoy, and the buoy transmits the data to the land laboratory via satellite communication.

[0033] S4. Recovery: The upper and lower solenoid valves are opened to control the acoustic releaser to separate the mounting frame and the counterweight seat. The buoyancy of the floating block on the mounting frame can drive the entire vessel to float upward. The controller sends distance data in real time during the floating process until it reaches the sea surface to complete the recovery.

[0034] Furthermore, before the measurement in step S2, an in-situ zero drift correction is performed: the upper solenoid valve is opened to allow the gas in the fixed cavity to overflow, and then the lower solenoid valve is closed to prevent the gas released from the seabed from entering the fixed cavity. The upper solenoid valve is then closed to seal the fixed cavity so that the internal temperature and pressure are stabilized, which is used as a reference to correct the sensor measurement value.

[0035] Furthermore, after the zero drift is corrected in situ, the output signals of the fiber Bragg grating gas sensing device for measuring temperature and pressure are calibrated respectively.

[0036] Temperature calibration: The wavelength shift of the fiber Bragg grating is used to measure temperature. The reflected wavelength λ of the fiber Bragg grating will shift with changes in temperature and can be expressed by a linear relationship:

[0037] ;

[0038] Where: Δλ is the change in the reflected wavelength of the fiber Bragg grating, α is the temperature sensitivity coefficient, which is a constant obtained through calibration experiments and represents the wavelength change caused by a unit temperature change; Δ T It's the temperature change.

[0039] Under known temperature conditions, the reflected wavelength of the fiber Bragg grating gas sensing device is recorded; multiple sets of temperature and reflected wavelength data are fitted to obtain the relationship between temperature and wavelength changes; this relationship is used to calculate the temperature online and infer the real-time temperature.

[0040] Pressure calibration: Since pressure changes affect the refractive index of the optical fiber, the reflection wavelength will change:

[0041] ;

[0042] Where: Δλ is the change in reflection wavelength; β is the pressure sensitivity coefficient, which is a constant obtained through experimental calibration, Δ P It's the pressure change.

[0043] The fiber Bragg grating (FBG) gas sensing device was calibrated using known pressure conditions, and the fiber Bragg grating reflection wavelength under different pressures was recorded. The measured pressure and reflection wavelength changes were fitted to obtain a linear relationship between the pressure and wavelength changes, and the pressure was calculated online using this relationship.

[0044] Furthermore, if temperature and pressure change simultaneously, the fiber Bragg grating gas sensing device will be affected by both temperature and pressure. The effects of temperature and pressure can be calibrated separately and corrected by combining the effects of both. The output wavelength change of the fiber Bragg grating gas sensing device can be expressed as a function of the combined effects of temperature and pressure:

[0045] ;

[0046] Δ T and Δ P represent the changes in temperature and pressure respectively; α and β It is the temperature and pressure sensitivity coefficient obtained through experiments; under known temperature and pressure conditions, a joint calibration experiment is carried out, the reflection wavelength under each set of conditions is recorded, and multiple linear regression or other fitting methods are used to determine the contribution of temperature and pressure to the wavelength change, and the calibration coefficient is obtained. α and β It can be used to calculate temperature and pressure in real time during actual measurement.

[0047] Beneficial effects of the present invention:

[0048] 1. Long-term real-time in-situ observation of seabed gas leakage flux has significantly improved the time resolution and data real-time performance of seabed gas leakage flux compared with existing technical equipment, which plays an important role in monitoring and early warning of seabed geological disasters.

[0049] Second, a fiber Bragg grating (FBG) gas sensing device has achieved quasi-distributed measurement to determine submarine gas leakage flux. Compared with traditional methods, this device establishes a precise mathematical relationship between the fiber Bragg grating sensor's output signal and actual physical quantities (such as temperature and pressure), making gas leakage flux measurements more accurate and reliable. In deep-sea environments, this sensor, due to its superior corrosion resistance, electromagnetic interference resistance, and stability, enables long-term, precise monitoring, significantly improving the measurement accuracy of submarine gas leakage flux.

[0050] 3. Based on the quasi-distributed fiber Bragg grating sensor array, it has the advantages of resistance to electromagnetic and atomic radiation interference, thin diameter, soft texture, light weight mechanical properties, insulation, non-inductive electrical properties, water resistance, high temperature resistance, and corrosion resistance. For the in-situ long-term monitoring of high pressure and high corrosion in the deep seabed, the advantages are very prominent.

[0051] Fourth, by regularly connecting to external seawater, the sensor's in-situ zero drift correction is achieved, which enhances the sensor's stability in in-situ long-term measurement, improves the quality of observation data, and indirectly improves the sensor's measurement accuracy.

[0052] 5. The quasi-distributed fiber Bragg grating sensor array is set inside the observation device, which is less affected by the external environment and has high stability.

[0053] 6. The device features a simple structure with few moving parts, high overall stability, and high reliability, ensuring both structural reliability and sensor efficiency. The device is self-recovering by jettisoning its load, and the bottom counterweight can be discarded, allowing for rapid recovery without relying on an underwater robot. An ROV is required for deployment to the release point. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a schematic diagram of the overall structure of the present invention.

[0055] Figure 2 for Figure 1 side view.

[0056] Figure 3 It is a schematic diagram of the vertical cross section inside the mounting frame of the present invention.

[0057] Figure 4 It is a vertical cross-sectional schematic diagram of the fixed cavity in the present invention.

[0058] Figure 5 It is a top view of the present invention.

[0059] Figure 6 It is a top perspective view of the present invention.

[0060] Figure 7 It is a bottom view of the present invention.

[0061] Figure 8 This is a simplified diagram of the structure inside the fixed cavity in the present invention.

[0062] Figure 9 Schematic diagram of the structure of the gas measuring device in the present invention.

[0063] Figure 10 Schematic diagram of the measurement principle of the present invention.

[0064] Figure 11 This is a schematic diagram of the gas measuring device of the present invention during measurement.

[0065] Figure 12 Schematic diagram of the in-situ zero drift correction in the present invention.

[0066] Figure 13 This is a diagram of the deployment and data transmission structure of the present invention on the seabed.

[0067] As shown in the figure:

[0068] 1. Fixed cavity, 2. Floating block, 3. Counterweight seat, 4. Gas release port, 5. Guardrail, 6. Sensor interface, 7. Pressure-resistant chamber, 8. Battery compartment, 9. Gas inlet, 10. Mounting bracket, 11. Free gas methane sensor, 12. Accommodating chamber, 13. Collection cover, 14. Lower solenoid valve, 15. Upper solenoid valve, 16. Fiber Bragg grating gas sensing device, 17. LED light source transmitter, 18. Transmission optical fiber, 19. Multi-stage coupling prism, 20. Multi-channel optical fiber demodulator, 21. Photodiode, 22. Sealing plate, 23. High-density lithium battery pack, 24. Buoy, 25. Satellite, 26. Land laboratory, 27. Coupled communication buoy, 28. Underwater robot (ROV), 29. Pressure-resistant housing, 30. Combination socket, 31. Sensor end assembly, 32. Double-headed cable assembly, 33. Optoelectronic watertight connector. DETAILED DESCRIPTION

[0069] In order to clearly illustrate the technical features of this solution, this solution is described below through specific implementation methods.

[0070] Example 1:

[0071] like Figures 1-9 As shown, a deep seabed gas leakage flux in-situ monitoring device comprises a fixed cavity 1, a mounting frame 10 connected to a buoy 2 on its periphery, and a counterweight seat 3 with a gas passage. In this embodiment, the fixed cavity 1 is a cylindrical cavity.

[0072] A gas release port 4 is located at the top of the fixed chamber 1, and a gas inlet port 9 is located at the bottom, facing the gas passage. A free methane sensor 11 is also installed in the fixed chamber 1 to detect free methane gas. A trumpet-shaped collection hood 13 is connected to the bottom of the fixed chamber 1 at the gas inlet port 9. Solenoid valves are installed at the gas release port 4 and the gas inlet port 9, respectively: an upper solenoid valve 15 is installed at the gas release port 4, and a lower solenoid valve 14 is installed at the gas inlet port 9. A gas measurement device for measuring flux is also installed in the fixed chamber 1.

[0073] The top of the mounting frame 10 is fixedly connected to the upper portion of the fixed chamber 1 via a sealing plate 22. A guardrail 5 is mounted on the sealing plate 22, which also houses a sensor interface 6 capable of reading data from the gas measuring device. The bottom of the mounting frame 10 is detachably connected to the counterweight seat 3 via an acoustic release. Within the mounting frame 10, a buoyant block 2 forms a housing 12 between the sealing plate 22 and the counterweight seat 3. Within the housing 12, the mounting frame 10 is connected a pressure-resistant chamber 7 and a battery chamber 8. The pressure-resistant chamber 7 houses a controller, while the battery chamber 8 houses a high-density lithium battery pack. The controller is electrically connected to two solenoid valves and the acoustic release via cables. The mounting frame 10 also houses an underwater acoustic communication system within the housing 12, which is electrically connected to the controller and capable of transmitting data to the outside world.

[0074] The gas measuring device includes a pressure-resistant shell 29 made of 316L stainless steel. The pressure-resistant shell 29 is provided with an LED light source emitter 17, a photodiode 21, and a multi-channel fiber Bragg grating demodulator 20, as well as a high-density lithium battery pack 23 for powering the three. The photodiode 21 and the multi-channel fiber Bragg grating demodulator 20 are connected to a combination socket 30 at the top of the pressure-resistant shell 29 through a transmission optical fiber 18. The pressure-resistant shell 29 also includes a plurality of fiber Bragg grating gas sensing devices 16 located at the bottom of the pressure-resistant shell 29. Each fiber Bragg grating gas sensing device 16 is It includes a multi-stage coupling prism 19 and transmission optical fibers 18 connected to both ends of the multi-stage coupling prism 19. The transmission optical fibers 18 at the upper end of each multi-stage coupling prism 19 are connected to the LED light source emitter 17 in the pressure-resistant shell 29 through an optoelectronic watertight connector 33. The ends of the transmission optical fibers 18 at the lower end of each multi-stage coupling prism 19 are connected to a sensor assembly 31. Each sensor assembly 31 is plugged into a combination socket 30 at the top of the pressure-resistant shell 29 through a double-headed cable assembly 32 and is respectively connected to a photodiode 21 and a multi-channel fiber Bragg grating demodulator 20.

[0075] In this embodiment, if Figure 9As shown, the gas measurement device includes six fiber Bragg grating (FBG) gas sensing devices 16. Each transmission fiber is engraved with two gratings for measuring temperature and pressure, respectively. A multi-stage coupled prism 19 serves as the sensing segment. A multi-channel fiber demodulator 20 can demodulate the wavelength variation of light to further calculate the synchronized temperature and pressure variations. A photodiode 21 can sense the intensity variation of light to further calculate the synchronized gas volume variation. If only the intensity variation of light sensed by the photodiode 21 is used to infer the gas volume variation, errors will occur because the effects of temperature and pressure on gas volume are not considered. Relying solely on laboratory calibration functions for inference can result in significant errors. The laboratory calibration function is derived from empirical linear fit values ​​obtained from multiple measurements at different gas positions at room temperature and pressure. Therefore, using a multi-channel fiber demodulator 20 to demodulate the wavelength variation of light to further calculate the synchronized temperature and pressure variations, and using temperature and pressure data to correct the calculated gas volume, can significantly improve measurement accuracy.

[0076] Because the spatial resolution of the entire measurement system depends on the number of stages of the multi-stage coupled prisms 19, which can reach millimeter levels, the present invention significantly improves both measurement accuracy and resolution compared to existing technologies. The measurement results of the six fiber Bragg grating gas sensing devices 16 are mutually verified and can be averaged, thereby reducing data errors and improving data quality.

[0077] like Figure 12 As shown, it also includes a coupling communication buoy 27, which transmits data with the underwater acoustic communication system via wireless communication. The coupling communication buoy 27 is connected to the surface buoy via a cable, and the surface buoy 24 transmits data with the land laboratory 26 via satellite 25 communication.

[0078] Example 2:

[0079] An observation method using a deep-sea seabed gas leakage flux in-situ monitoring device comprises the following steps:

[0080] S1. Deployment: The manipulator of the underwater robot ROV28 grasps the guardrail 5 of the device and drives the device down. During deployment, the upper solenoid valve 15 and the lower solenoid valve 14 are opened. The ROV28 uses an underwater camera to determine the exact location of the seabed gas leakage. The device is then placed over the leakage point. The manipulator detaches from the device, completing the seabed fixed-point deployment.

[0081] S2. Measurement: The lower solenoid valve 14 is opened and the upper solenoid valve 15 is closed. Gas and seawater enter the fixed chamber 1 through the gas channel and the collection cover. When there is no gas in the fixed chamber 1, the light intensity received by the photodiode 21 is the weakest. As the amount of captured gas gradually increases, the light intensity received by the photodiode 21 gradually increases.

[0082] like Figure 11 As shown, before measurement, in-situ zero drift correction is performed: the upper solenoid valve 15 is opened to allow the gas in the fixed chamber 1 to overflow, and then the lower solenoid valve 14 is closed to prevent the gas released from the seabed from entering the fixed chamber 1. The upper solenoid valve 15 is then closed to seal the fixed chamber 1 to stabilize the internal temperature and pressure, which are used as a reference to correct the sensor measurement value.

[0083] After zero drift correction in situ, the output signals of the fiber Bragg grating gas sensing device for measuring temperature and pressure are calibrated respectively.

[0084] Temperature calibration: The wavelength shift of the fiber Bragg grating is used to measure temperature. The reflected wavelength λ of the fiber Bragg grating will shift with changes in temperature and can be expressed by a linear relationship:

[0085] ;

[0086] Where: Δλ is the change in the reflected wavelength of the fiber Bragg grating, α is the temperature sensitivity coefficient, which is a constant obtained through calibration experiments and represents the wavelength change caused by a unit temperature change; Δ T It's the temperature change.

[0087] Under known temperature conditions, the reflected wavelength of the fiber Bragg grating gas sensing device is recorded; multiple sets of temperature and reflected wavelength data are fitted to obtain the relationship between temperature and wavelength changes; this relationship is used to calculate the temperature online and infer the real-time temperature.

[0088] Pressure calibration: Since pressure changes affect the refractive index of the optical fiber, the reflection wavelength will change:

[0089] ;

[0090] Where: Δλ is the change in reflected wavelength; β is the pressure sensitivity coefficient, which is a constant obtained through experimental calibration, Δ P It's the pressure change.

[0091] The fiber Bragg grating (FBG) gas sensing device was calibrated using known pressure conditions, and the fiber Bragg grating reflection wavelength under different pressures was recorded. The measured pressure and reflection wavelength changes were fitted to obtain a linear relationship between the pressure and wavelength changes, and the pressure was calculated online using this relationship.

[0092] If the temperature and pressure change simultaneously, the fiber Bragg grating gas sensing device will be affected by both temperature and pressure. The effects of temperature and pressure can be calibrated separately and corrected by combining the effects of both. The output wavelength change of the fiber Bragg grating gas sensing device can be expressed as a function of the combined effects of temperature and pressure:

[0093] ;

[0094] Δ T and Δ P represent the changes in temperature and pressure respectively; α and β It is the temperature and pressure sensitivity coefficient obtained through experiments; under known temperature and pressure conditions, a joint calibration experiment is carried out, the reflection wavelength under each set of conditions is recorded, and multiple linear regression or other fitting methods are used to determine the contribution of temperature and pressure to the wavelength change, and the calibration coefficient is obtained. α and β It can be used to calculate temperature and pressure in real time during actual measurement.

[0095] By calculating the change in gas volume based on the change in light intensity, the gas temperature can be measured T and pressure P , the change in gas volume can be calculated using the ideal gas equation:

[0096] ;

[0097] in: V is the volume of the gas, n is the number of moles of gas, R is the ideal gas constant, T is the temperature of the gas, P is the pressure of the gas.

[0098] According to the change in gas volume, the flux formula is used to calculate the gas leakage flux:

[0099] ;

[0100] in: Q is the gas flux, Δ V is the change in gas volume per unit time, A is the area where the gas flux is measured, Δ t is the time interval.

[0101] During the measurement, the gas volume changes Δ V The calculation can be performed based on the real-time monitoring data of temperature and pressure. The gas flux can be obtained by accumulating multiple measurement cycles. Q long-term changes.

[0102] S3. Transmission: An underwater acoustic communication system is installed on the monitoring device, and in conjunction with the coupled communication buoy 27, the controller sends the measurement data to the receiving end of the coupled communication buoy 27 through the underwater acoustic communication system. The receiving end of the coupled communication buoy 27 transmits the data upward along the cable to the surface buoy 24. The buoy 24 transmits the data to the land laboratory 26 via satellite 25 communication.

[0103] S4. Recovery: The upper solenoid valve 15 and the lower solenoid valve 14 are opened, controlling the acoustic releaser to separate the mounting frame 10 and the counterweight seat 3. The buoyancy of the floating block 2 on the mounting frame 10 can drive the entire structure to float upward. The controller sends distance data in real time during the floating process until it floats to the sea surface to complete the recovery.

[0104] To more clearly illustrate the workflow of a gas measurement device, the following is a calculation example based on a fiber Bragg grating temperature and pressure sensor. The volume change of the gas is measured through the optical fiber, and the gas flux is calculated using a calibration formula.

[0105] Sensor data: Fiber Bragg grating gas sensing device: Measured wavelength change Δλ T =0.25 nm, fiber Bragg grating pressure sensor: measured wavelength change Δλ P = 1.50 nm, temperature change Δ T = 5°C; pressure change Δ P =1000Pa.

[0106] Calibration coefficient: Temperature sensitivity coefficient: α =0.05 nm / ℃; Pressure sensitivity coefficient: β =0.002nm / Pa.

[0107] Gas leakage measurement area: A=0.1m 2 (Assume that the sensing area of ​​the sensor is 0.1m 2 ),

[0108] Observation time interval: Δ t= 1s.

[0109] The conversion between temperature and pressure is based on the calibration formula of fiber Bragg grating, and the changes in temperature and pressure are first calculated.

[0110] Temperature changes:

[0111] Δλ т = α·Δ T ;

[0112] 0.25 nm = 0.05 nm / °C·Δ T ;

[0113] Δ T=0.25 nm / 0.05 nm / ℃=5℃.

[0114] This result shows that a temperature change of 5°C is consistent with a measured wavelength change of 0.25 nm.

[0115] Pressure changes:

[0116] Δλ P = β ·Δ P ;

[0117] 1.50nm=0.002nm / Pa·Δ P ;

[0118] Δ P =1.50 nm / 0.002 nm / Pa=750 Pa.

[0119] This result shows that the pressure change of 750 Pa is consistent with the measured wavelength change of 1.50 nm.

[0120] Calculate the change in gas volume:

[0121] According to the ideal gas equation (assuming the gas is an ideal gas), we can calculate the volume change of the gas. The volume change of the gas is related to the temperature and pressure changes:

[0122] ;

[0123] For simplicity, assume that the number of moles of gas is n Keeping the same, we use the experimental calibration data to estimate the volume change. For the estimation of volume change, we use the empirical relationship between wavelength change and volume change:

[0124] ;

[0125] in, k is a constant obtained through calibration, and Δλ is the change in the optical fiber reflection wavelength. k =0.1m 3 / nm, then:

[0126] ΔV T =0.1m 3 / nm 0.25nm=0.025 m 3 ;ΔV P =0.1m 3 / nm·1.50nm=0.15m 3 .

[0127] Calculate gas flux:

[0128] Gas fluxQ The calculation formula is:

[0129] ;

[0130] Where: Δ V is the change in gas volume per unit time, A is the area where the gas flux is measured, Δ t is the time interval; in this embodiment, A =0.1m 2 , the time interval is 1s, then:

[0131] Q T =0.025 m 3 / 0.1m 2 1s = 0.25 m 3 / s;

[0132] Q P =0.15 m 3 / 0.1m 2 1s = 1.5 m 3 / s.

[0133] Finally, the total gas flux can be obtained by combining the effects of temperature and pressure changes on gas volume:

[0134] Q total = Q T +Q p =0.25 m 3 / s + 1.5 m 3 / s = 1.75 m 3 / s.

[0135] The workflow can be summarized as follows:

[0136] 1. Data collection:

[0137] Temperature and pressure sensors monitor gas volume changes in real time through fiber Bragg gratings.

[0138] Through the photodiode and LED light source transmitter system, the change in reflected wavelength corresponds to the temperature and pressure changes of the gas.

[0139] 2. Data calibration:

[0140] Use calibration factors α and β The measured wavelength changes are converted into actual temperature and pressure changes.

[0141] 3. Volume calculation:

[0142] Estimate the gas volume change Δ based on the calibration coefficient and wavelength change V .

[0143] Use empirical constants k Relate the wavelength change to the gas volume change.

[0144] 4. Gas flux calculation:

[0145] Calculate gas flux based on measured volume change and time interval Q .

[0146] Combines the contributions of different influencing factors (such as temperature and pressure changes) to the volume change.

[0147] The above calculations demonstrate the workflow of the gas measurement device. In practical applications, this process can be used to continuously monitor deep-sea gas seepage flux and support submarine geological disaster monitoring through real-time data transmission.

[0148] Of course, the above description is not limited to the above examples. Technical features not described in the present invention can be achieved by or by adopting existing technologies, which will not be described here. The above embodiments and drawings are only used to illustrate the technical solutions of the present invention and are not limitations of the present invention. The present invention is described in detail with reference to the preferred implementation methods. Ordinary technicians in this field should understand that changes, modifications, additions or substitutions made by ordinary technicians in this technical field within the essential scope of the present invention do not depart from the purpose of the present invention and should also fall within the scope of protection of the claims of the present invention.

Claims

1. An in-situ monitoring device for deep-sea seabed gas leakage flux, characterized by: The utility model comprises a fixed cavity, a mounting frame with a floating block connected to the circumference and a counterweight seat with a gas channel, a gas release port is provided at the upper end of the fixed cavity and a gas inlet port opposite to the gas channel is provided at the lower end, an upper solenoid valve is installed at the gas release port, a lower solenoid valve is installed at the gas inlet port, and a gas measuring device for measuring gas leakage flux is provided in the fixed cavity; the top of the mounting frame is fixedly connected to the upper part of the fixed cavity through a sealing plate, and the bottom of the mounting frame is detachably connected to the counterweight seat through an acoustic releaser, and a accommodating cavity is formed between the sealing plate and the counterweight seat through the floating block in the mounting frame, the mounting frame is connected to a pressure-resistant chamber and a battery chamber in the accommodating chamber, a controller is provided in the pressure-resistant chamber, a high-density lithium battery pack is provided in the battery chamber, the controller is electrically connected to the two solenoid valves and the acoustic releaser through cables respectively, and an underwater acoustic communication system electrically connected to the controller and capable of transmitting data to the outside world is also installed in the accommodating chamber; a trumpet-shaped collecting cover is connected to the gas inlet at the bottom of the fixed cavity; the gas measuring device comprises a pressure-resistant shell, and LED light source emitters are respectively provided inside the pressure-resistant shell The device comprises a device, a photodiode and a multi-channel fiber Bragg grating demodulator, and a high-density lithium battery pack for powering the three respectively. The photodiode and the multi-channel fiber Bragg grating demodulator are respectively connected to the combination socket on the top of the pressure-resistant shell through optical fibers; it also includes multiple fiber Bragg grating gas sensing devices located at the bottom of the pressure-resistant shell, each fiber Bragg grating gas sensing device includes a multi-stage coupling prism and transmission optical fibers connected to both ends of the multi-stage coupling prism, the transmission optical fiber at the upper end of each multi-stage coupling prism is connected to the LED light source emitter in the pressure-resistant shell through an optoelectronic watertight connector, the end of the transmission optical fiber at the lower end of each multi-stage coupling prism is connected to the sensor assembly, each sensor assembly is plugged into the combination socket on the top of the pressure-resistant shell through a double-headed cable assembly and is respectively connected to the photodiode and the multi-channel fiber Bragg grating demodulator; it also includes a coupled communication buoy, which transmits data with the underwater acoustic communication system through wireless communication, the coupled communication buoy is connected to the surface buoy through a cable, and the surface buoy transmits data with the land laboratory through satellite communication.

2. The deep seabed gas leakage flux in-situ monitoring device according to claim 1 is characterized in that: The sealing plate is provided with a sensor interface for reading the data stored in the gas measuring device.

3. The deep seabed gas leakage flux in-situ monitoring device according to claim 1 is characterized in that: A guardrail is installed on the sealing plate.

4. A method for measuring deep seabed gas leakage flux in-situ monitoring using the deep seabed gas leakage flux in-situ monitoring device according to claim 1, characterized in that: The following steps are involved: S1. Deployment: The manipulator of the underwater robot (ROV) grabs the guardrail of the monitoring device and drives the monitoring device down. During deployment, the upper and lower solenoid valves are opened. The ROV uses an underwater camera to determine the exact location of the seabed gas leak. The monitoring device is then placed over the leak point. The manipulator detaches from the device, completing the seabed fixed-point deployment. S2. Measurement: The lower solenoid valve is opened and the upper solenoid valve is closed. Gas and seawater enter the fixed cavity through the gas channel and the collection cover. When there is no gas in the fixed cavity, the light intensity received by the photodiode is the weakest. As the amount of captured gas gradually increases, the light intensity received by the photodiode gradually increases. By calculating the change in gas volume based on the change in light intensity, the gas temperature can be measured T and pressure P , the change in gas volume can be calculated using the ideal gas equation: ； in: V is the volume of the gas, n is the number of moles of gas, R is the ideal gas constant, T is the temperature of the gas, P is the pressure of the gas; According to the change in gas volume, the flux formula is used to calculate the gas leakage flux: ; in: Q is the gas flux, Δ V is the change in gas volume per unit time, A is the area where the gas flux is measured, Δ t is the time interval; During the measurement, the gas volume changes Δ V The calculation can be performed based on the real-time monitoring data of temperature and pressure. The gas flux can be obtained by accumulating multiple measurement cycles. Q Long-term changes in S3. Transmission: An underwater acoustic communication system is installed on the monitoring device, and in conjunction with a coupled communication buoy, the controller sends the measurement data via the underwater acoustic communication system to the receiving end of the coupled communication buoy. The receiving end of the coupled communication buoy transmits the data upward along the cable to the surface buoy. The buoy then transmits the data to the land laboratory via satellite communication. S4. Recovery: The upper and lower solenoid valves are opened to control the acoustic releaser to separate the mounting frame from the counterweight seat at the bottom. The buoyancy of the floating block on the mounting frame can drive the entire vessel to float upward. The controller sends distance data in real time during the floating process until it reaches the sea surface to complete the recovery.

5. The measurement method using the deep seabed gas leakage flux in-situ monitoring device according to claim 4 is characterized in that: Before the measurement in step S2, an in-situ zero drift correction is performed: the upper solenoid valve is opened to allow the gas in the fixed chamber to overflow, and then the lower solenoid valve is closed to prevent the gas released from the seabed from entering the fixed chamber. The upper solenoid valve is then closed to seal the fixed chamber to stabilize the internal temperature and pressure. This serves as a reference for calibrating the gas measurement device.

6. The measurement method using the deep seabed gas leakage flux in-situ monitoring device according to claim 5, characterized in that: After zero drift correction in situ, the output signals of the fiber Bragg grating gas sensing device used to measure temperature and pressure are calibrated respectively. Temperature calibration: The wavelength shift of the fiber Bragg grating is used to measure temperature. The reflected wavelength λ of the fiber Bragg grating will shift with changes in temperature and can be expressed by a linear relationship: ； Where: Δλ is the change in the reflected wavelength of the fiber Bragg grating, α is the temperature sensitivity coefficient, which is a constant obtained through calibration experiments and represents the wavelength change caused by a unit temperature change; Δ T It is the temperature change; Under known temperature conditions, the reflected wavelength of the fiber Bragg grating gas sensing device is recorded; multiple sets of temperature and reflected wavelength data are fitted to obtain the relationship between temperature and wavelength changes; this relationship is used to calculate the temperature online and infer the real-time temperature; Pressure calibration: Since pressure changes affect the refractive index of the optical fiber, the reflection wavelength will change: ； Where: Δλ is the change in reflection wavelength; β is the pressure sensitivity coefficient, which is a constant obtained through experimental calibration, Δ P It is the pressure change; The fiber Bragg grating (FBG) gas sensing device was calibrated using known pressure conditions, and the fiber Bragg grating reflection wavelength under different pressures was recorded. The measured pressure and reflection wavelength changes were fitted to obtain a linear relationship between the pressure and wavelength changes, and the pressure was calculated online using this relationship.

7. The measurement method using the deep seabed gas leakage flux in-situ monitoring device according to claim 6, characterized in that: If the temperature and pressure change simultaneously, the fiber Bragg grating gas sensing device will be affected by both temperature and pressure. The effects of temperature and pressure can be calibrated separately and corrected by combining the effects of both. The output wavelength change of the fiber Bragg grating gas sensing device can be expressed as a function of the combined effects of temperature and pressure: ； Δ T and Δ P represent the changes in temperature and pressure respectively; α and β It is the temperature and pressure sensitivity coefficient obtained through experiments; under known temperature and pressure conditions, a joint calibration experiment is carried out, the reflection wavelength under each set of conditions is recorded, and multiple linear regression or other fitting methods are used to determine the contribution of temperature and pressure to the wavelength change, and the calibration coefficient is obtained. α and β It can be used to calculate temperature and pressure in real time during actual measurement.

Images