Fresh water lenticular body resource monitoring method based on weak reflection fiber grating

Abstract

The invention discloses a fresh water lenticular resource monitoring method based on weak reflection fiber bragg grating, and relates to the technical field of marine environment monitoring and geological engineering, and the method comprises the following steps: planning a monitoring section along the earth surface, and drilling a plurality of vertical monitoring holes in sequence from the central area to the offshore side edge; a weak reflection fiber grating sensing optical cable is adopted to construct a sensing array, and sensor monitoring units are arranged at a high density according to a preset interval; according to the vertical salinity profile data fed back by the sensor monitoring unit, the overall contour of the salt-fresh water transition zone is inverted, and the change of a fresh water lenticular body is analyzed. By adopting the freshwater lenticular resource monitoring method based on the weak reflection fiber grating, the problem of long-term survival in an underground high-corrosion environment is solved, the risk that key information of a transition zone is missed due to sparse measuring points is avoided, the water level of a single point is monitored, the advancing and retreating rule of the edge of the lenticular along with tide fluctuation is completely reflected, and the survival rate of the lenticular resource is improved. And a visual and reliable decision basis is provided for protective exploitation of fresh water resources.

Description

Technical Field

[0001] This invention relates to the fields of marine environmental monitoring and geological engineering technology, and in particular to a method for monitoring freshwater lens resources based on weak reflection fiber optic gratings. Background Technology

[0002] Freshwater lenses are extremely valuable underground freshwater resources. Their spatial morphology and thickness are not static but are controlled by factors such as rainfall replenishment, tidal disturbances, and differences in geological structure and permeability, exhibiting significant spatiotemporal dynamic evolution characteristics. Accurate and continuous in-situ monitoring of the thickness and morphological changes of freshwater lenses, as well as the location of seawater intrusion, is a crucial foundation for supporting ecological protection, sustainable development of freshwater resources, and disaster risk early warning.

[0003] Currently, freshwater lens monitoring mainly relies on traditional electrical sensors such as conductivity meters and water level gauges, or periodic manual sampling and analysis. However, in high-salt and high-humidity underground environments, conventional electronic sensors are prone to corrosion and sealing failure due to prolonged immersion, resulting in short lifespan, poor data stability, and high frequency of on-site maintenance and replacement, thus significantly increasing long-term operation and maintenance costs. Furthermore, electrical sensors typically require underground power supply and signal cable laying, which are more susceptible to short circuits, leakage, and signal drift in humid and high-salt environments, increasing the difficulty of construction organization and safety risks, thus limiting large-scale engineering deployment.

[0004] In terms of monitoring capabilities, traditional methods often only allow for the deployment of a small number of discrete measuring points in a single borehole, making it difficult to form a high-density longitudinal profile observation. In addition, multi-well synchronous network monitoring under traditional electrical systems is limited by factors such as power supply, wiring, waterproofing, and long-term reliability, often resulting in large-scale, continuous real-time monitoring, which restricts a systematic understanding of the overall structure and evolution mechanism of the lens body. Summary of the Invention

[0005] The purpose of this invention is to provide a freshwater lens resource monitoring method based on a weak reflection fiber optic grating, which aims to overcome the problems of poor corrosion resistance, low spatial resolution, and difficulty in long-term online monitoring of electronic sensors in existing freshwater lens monitoring technologies.

[0006] To achieve the above objectives, this invention provides a method for monitoring freshwater lens resources based on weak-reflection fiber Bragg gratings, comprising the following steps: S1. Monitoring route planning: Along the planned monitoring profile on the surface, several vertical monitoring holes are drilled sequentially from the central area to the near-sea edge. The depth of the monitoring holes penetrates the underground freshwater layer and extends to the deep saline layer. S2. Construction and deployment of weak reflection sensor array: The sensor array is constructed using weak reflection fiber optic grating sensing cables. Sensor monitoring units are deployed in a high density along the axial direction on a single sensing cable at a preset spacing. Then, the sensing cable is vertically placed into each monitoring hole. S3. Data demodulation and parameter inversion: Based on the vertical salinity profile data fed back by the sensor monitoring unit, identify the depth range where the salinity value changes drastically, define the center point of the depth range as the brackish water interface point at the monitoring well, and invert the overall contour of the brackish water transition zone based on the data to analyze the changes in the freshwater lens.

[0007] Preferably, the sensor monitoring unit in S2 includes a fiber optic salinity sensor and a temperature compensation sensor.

[0008] Preferably, the sensor monitoring units in S2 are distributed at 0.5m intervals to form a vertical sensing array that spans the entire well depth.

[0009] Preferably, the specific steps of S3 are as follows: S31. Use the dual-wavelength matrix method or temperature compensation method to remove temperature crosstalk and obtain the salinity value of the monitoring point. S32. Map the physical location of each fiber Bragg grating salinity sensor on the optical cable to the absolute underground depth to obtain discrete depth-salinity data points for a single monitoring well. Zi,Si ); S33. A nonlinear fitting algorithm is used to perform regression analysis on the vertical salinity profile data; S34. Solve the inflection point of the nonlinear fitting algorithm by iteratively solving the least squares method, and define the inflection point as the precise depth of the brackish water interface at the monitoring hole. S35. Calculate the interface depth set of all monitoring wells. And based on the horizontal coordinates of each monitoring hole X k Using cubic spline interpolation as nodes, a continuous and smooth brackish water interface curve is generated by connecting the feature points. I(x) ; S36. By comparing the brackish water interface curves generated at different times. I t1(x) and I t2(x) The average vertical migration rate of the computing interface V To determine the evolution trend of the lens body.

[0010] Preferably, the calculation formula for removing temperature crosstalk in S31 is as follows: ; in, This represents the wavelength shift of the salinity sensor. This represents the wavelength shift of the temperature sensor. These are the sensitivity coefficients of the salinity sensor to salinity and temperature, respectively. These are the sensitivity coefficients of the temperature sensor to salinity and temperature, respectively. The determinant value of the coefficient matrix. The change in temperature This represents the change in salinity.

[0011] Preferably, the specific formula for the salinity value of the monitoring point in S31 is as follows: ; in, S 0 This is the initial calibrated salinity value.

[0012] Preferably, the nonlinear fitting algorithm in S33 uses the Boltzmann Sigmoid function, with the specific formula as follows: ; in, S ( z () represents the fitted salinity distribution function with depth. S 1 Background salinity in freshwater areas, S 2 For deep salinity background, Z For depth variables, Z m The center depth of the brackish water interface. d z The slope factor characterizes the thickness of the transition zone.

[0013] Preferably, the brackish water interface curve in S35 I(x) The specific formula is as follows: ; in, A k , B k , C k , D k These are the spline interpolation coefficients, ensuring that the curve maintains continuity of its second derivative while passing through each monitoring point. x The coordinates represent the horizontal distance along the monitoring profile.

[0014] Preferably, the average vertical migration rate in S36 V The specific formula is as follows: ; in, for t The brackish water interface curve generated at time 1 for t The brackish water interface curve generated at time 2, t 1. t 2 represents different times. X To monitor the horizontal coordinates of the endpoint of the effective integration region of the profile, X 0 To monitor the starting horizontal coordinates of the effective integration region of the profile, d x Let be the differential variable of the horizontal distance.

[0015] Preferably, the specific criteria for judging the evolution trend of the lens body in S36 are as follows: like V >0 indicates that the interface has shifted upwards as a whole, and the freshwater lens is shrinking or thinning. like V <0 indicates that the interface has shifted downwards and the freshwater lens is recovering or filling.

[0016] Therefore, the above-mentioned freshwater lens resource monitoring method based on weak reflection fiber gratings in this invention has the following beneficial effects: (1) The present invention achieves a high-density sensor array in a single monitoring hole. This dense layout can completely cover the entire longitudinal profile from the freshwater layer, the brackish water-freshwater mixed transition zone to the brackish water layer, avoiding the risk of missing key information of the transition zone due to sparse measuring points.

[0017] (2) The present invention adopts a deployment method from the center of the island to the near sea side, which not only monitors the water level at a single point, but also fully reflects the advance and retreat pattern of the lens edge with the rise and fall of the tide.

[0018] (3) This invention, through a strategy combining transverse multi-hole joint measurement and longitudinal high-density sensing, can reconstruct discrete measurement point data into a continuous two-dimensional profile. This allows managers to intuitively see the complete lens-shaped form of the freshwater lens and its dynamic expansion and contraction process over time, providing an intuitive and reliable basis for the protective exploitation of freshwater resources.

[0019] (4) The deployment method of this invention, combined with passive optical fiber devices, solves the problem of long-term survival in underground highly corrosive environments, and one deployment can achieve online monitoring for several years.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1This is a schematic flowchart of an embodiment of a freshwater lens resource monitoring method based on a weakly reflective fiber Bragg grating according to the present invention. Figure 2 This is a monitoring method diagram of an embodiment of the freshwater lens body resource monitoring method based on a weak reflection fiber optic grating according to the present invention; Reference numerals: 1. Soil; 2. Freshwater lens area; 3. Saltwater area; 4. Freshwater-saltwater transition zone; 5. Sensor monitoring unit; 6. Monitoring hole. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0024] Example Please see Figures 1-2 This invention provides a method for monitoring freshwater lens resources based on weak-reflection fiber Bragg gratings (FBGs). Fiber optic sensing technology possesses inherent advantages such as resistance to electromagnetic interference, corrosion resistance, passive detection, and long-distance transmission, providing a more suitable technical path for long-term in-situ monitoring of freshwater lenses. In particular, compared to traditional high-reflectivity FBGs, weak-reflection FBG technology can inscribe a larger number of grating arrays on a single fiber, enabling ultra-long-distance, ultra-high-density quasi-distributed measurements and forming high-resolution, multi-point continuous profile data in deep well environments. Based on this, applying weak-reflection FBGs to high-density monitoring of hydrological parameters such as salinity is expected to overcome the bottlenecks of traditional monitoring in terms of durability, power supply cabling, point density, and networking capabilities, providing a more reliable data foundation for interface identification, morphological evolution analysis, and seawater intrusion early warning of freshwater lenses.

[0025] This embodiment provides a freshwater lens monitoring method based on a weak-reflection fiber Bragg grating. This method utilizes an all-fiber passive sensing network to perform high-density, long-term real-time monitoring of salinity at different layers of the freshwater lens, thereby accurately monitoring changes in the freshwater lens. The method includes the following steps: (1) Monitoring route planning.

[0026] A profile was selected on the surface that spanned the main distribution area of ​​the freshwater lens. Along this profile, multiple vertical monitoring boreholes were drilled sequentially from one edge through the central region to the other edge. The depth of the monitoring boreholes was set to penetrate the estimated freshwater layer and brackish water transition zone, reaching the deep brackish water layer, thus forming a monitoring array covering the entire freshwater lens profile. Specifically: A monitoring profile is planned across the freshwater lens on the surface. Nine monitoring wells are arranged along this profile. One main monitoring well is located at the center, and four auxiliary monitoring wells are symmetrically arranged on each side edge. The spacing between the wells is set according to the width of the freshwater lens, and the well depth is adjusted according to the size of the freshwater lens at the monitoring site to ensure penetration of the bottom boundary of the freshwater lens.

[0027] A sensor array is constructed using low-reflection fiber Bragg grating sensing cables. High-density sensing units, including fiber Bragg grating salinity sensors and temperature compensation sensors, are integrated axially on a single fiber. These sensing units are distributed at predetermined intervals, forming a vertical sensing array that spans the entire well depth. The aforementioned sensing cable chain is vertically inserted into each monitoring borehole, and the borehole is backfilled with well-permeable graded sand and gravel to ensure that groundwater can freely permeate to the sensor surface.

[0028] In this embodiment, the salinity monitoring nodes on the sensing optical cable chain are spaced 0.5 meters apart, with one monitoring unit installed every 0.5 meters along the cable. Each monitoring unit includes a fiber Bragg grating salinity sensor and a temperature compensation sensor. The number of monitoring units is adjusted according to the depth of the monitoring well. The salinity sensor is fabricated using a weak-reflection fiber Bragg grating to achieve multi-point series connection on the same optical cable.

[0029] (2) Construction and layout.

[0030] Nine monitoring wells were drilled using a drilling rig. Fiber optic cables were then lowered to the bottom of the wells while being kept taut to prevent kinking and localized stress that could cause sensor node misalignment. After lowering, the wellbores were backfilled with quartz sand to ensure tight coupling between the sensors and the formation water, improving the accuracy and stability of the monitoring data. The fiber optic cables extending from the nine monitoring wells were then connected to a transmission cable buried in trenches at the ground level and routed to the control center. This enabled centralized data acquisition, transmission, and management across multiple wells and depths, providing fundamental data support for dynamic assessment and safety early warning of freshwater lens resources.

[0031] like Figure 2 As shown, the upper layer is soil 1, and below soil 1 is freshwater lens area 2, including saline water area 3 and saline-freshwater transition zone 4, drilling monitoring holes 6, and high-density sensor monitoring units 5 integrated along the axis on the weak reflection fiber optic grating sensing cable.

[0032] (3) Monitoring methods and data processing procedures.

[0033] Data is collected using a demodulation device, and longitudinal analysis is performed on each monitoring well. Based on the vertical salinity profile data fed back by the salinity sensor, depth ranges where salinity values ​​change drastically are identified. The center point of this depth is defined as the brackish water interface point at that monitoring well. The overall contour of the brackish water transition zone is reconstructed from the data, thereby analyzing the changes in the freshwater lens. Specifically: The core of this embodiment lies in accurately retrieving the spatial location of the brackish water interface by using the wavelength shift obtained from the demodulator. Since fiber optic gratings are inherently sensitive to temperature, a dual-wavelength matrix method or temperature compensation method is required to remove temperature crosstalk in order to obtain accurate salinity values ​​at monitoring points. ; in, This represents the wavelength shift of the salinity sensor. This represents the wavelength shift of the temperature sensor. These are the sensitivity coefficients of the salinity sensor to salinity and temperature, respectively. These are the sensitivity coefficients of the temperature sensor to salinity and temperature, respectively. The determinant value of the coefficient matrix. The change in temperature This represents the change in salinity.

[0034] The salinity values ​​at the monitoring points can be obtained as follows: ; in, S 0 This is the initial calibrated salinity value.

[0035] By mapping the physical locations of each salinity sensor on the optical cable to absolute underground depths, discrete depth-salinity data points for a single monitoring well can be obtained. Zi,Si In actual geological formations, the salinity change from freshwater to seawater is not abrupt, but rather involves a mixing transition zone. To find the unique brackish water interface depth from discrete salinity points, this embodiment employs the Boltzmann Sigmoid function to perform nonlinear regression analysis on the vertical profile data. The fitting model formula is as follows: ; in, S ( z() represents the fitted salinity distribution function with depth. S 1 Background salinity in freshwater areas, S 2 For deep salinity background, Z For depth variables, d z The slope factor characterizes the thickness of the transition zone. Z m The depth at the center of the brackish water interface is the core parameter to be solved.

[0036] A nonlinear fitting algorithm was used to perform regression analysis on the vertical salinity profile. The inflection point of the function model was solved iteratively by the least squares method. This inflection point was defined as the precise depth of the brackish water interface at the monitoring well, so as to eliminate the depth resolution limitation caused by discrete measuring points.

[0037] In this embodiment, the horizontal coordinates of each monitoring well are used as nodes, and the calculated center depth of the brackish water interface is used as the function value. A piecewise cubic polynomial is constructed to connect each feature point, and the continuity of the first and second derivatives of the curve at the nodes is constrained, thereby generating a continuous and smooth interface curve that conforms to the natural physical morphology of groundwater fluids. Specifically: Set of interface depths calculated from all monitoring wells Combined with the horizontal coordinates of each well Using cubic spline interpolation function I ( x Connecting the feature points generates a continuous and smooth brackish water interface curve: ; in, A k , B k , C k , D k These are the spline interpolation coefficients, ensuring that the curve maintains continuity of its second derivative while passing through each monitoring point. x The coordinates represent the horizontal distance along the monitoring profile.

[0038] Based on the above conditions, a system of linear equations with N unknowns is constructed. The interpolation coefficients for all intervals can be quickly obtained through numerical computer solving. The final generated curve... I(x) Not only were all measuring points connected, but mathematical interpolation was also used to fill in the blind spots between wells, forming a complete bottom profile of the freshwater lens. Then, the interface curves generated at different times were compared. I t1(x) and I t2(x) The average vertical migration rate of the computing interfaceV This is used to determine the evolutionary trend of the lens body: ; in, for t The brackish water interface curve generated at time 1 for t The brackish water interface curve generated at time 2, t 1. t 2 represents different times. X To monitor the horizontal coordinates of the endpoint of the effective integration region of the profile, X 0 To monitor the starting horizontal coordinates of the effective integration region of the profile, d x Let be the differential variable of the horizontal distance.

[0039] like V A value >0 indicates that the interface has shifted upwards, and the freshwater lens is shrinking or thinning; if... V <0 indicates that the interface has shifted downwards and the freshwater lens is recovering or filling.

[0040] Therefore, the present invention adopts the above-mentioned freshwater lens resource monitoring method based on weak reflection fiber optic grating, which solves the long-term survival problem in underground highly corrosive environments and avoids the risk of missing key information in the transition zone due to sparse measuring points. It not only monitors the water level at a single point, but also fully reflects the advance and retreat pattern of the lens edge with the rise and fall of tides, providing an intuitive and reliable decision-making basis for the protective exploitation of freshwater resources.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for monitoring freshwater lens resources based on weakly reflective fiber Bragg gratings, characterized in that, Includes the following steps: S1. Monitoring route planning: Along the planned monitoring profile on the surface, several vertical monitoring holes are drilled sequentially from the central area to the near-sea edge. The depth of the monitoring holes penetrates the underground freshwater layer and extends to the deep saline layer. S2. Construction and deployment of weak reflection sensor array: The sensor array is constructed using weak reflection fiber optic grating sensing cables. Sensor monitoring units are deployed in a high density along the axial direction on a single sensing cable at a preset spacing. Then, the sensing cable is vertically placed into each monitoring hole. S3. Data demodulation and parameter inversion: Based on the vertical salinity profile data fed back by the sensor monitoring unit, identify the depth range where the salinity value changes drastically, define the center point of the depth range as the brackish water interface point at the monitoring well, and invert the overall contour of the brackish water transition zone based on the data to analyze the changes in the freshwater lens.

2. The method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 1, characterized in that: The sensor monitoring unit in S2 includes a fiber optic salinity sensor and a temperature compensation sensor.

3. The method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 1, characterized in that: The sensor monitoring units in S2 are distributed at 0.5m intervals to form a vertical sensing array that spans the entire well depth.

4. The method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 1, characterized in that, The specific steps of S3 are as follows: S31. Use the dual-wavelength matrix method or temperature compensation method to remove temperature crosstalk and obtain the salinity value of the monitoring point. S32. Map the physical location of each fiber Bragg grating salinity sensor on the optical cable to the absolute underground depth to obtain discrete depth-salinity data points for a single monitoring well. Zi,Si ); S33. A nonlinear fitting algorithm is used to perform regression analysis on the vertical salinity profile data; S34. Solve the inflection point of the nonlinear fitting algorithm by iteratively solving the least squares method, and define the inflection point as the precise depth of the brackish water interface at the monitoring hole. S35. Calculate the interface depth set of all monitoring wells. And based on the horizontal coordinates of each monitoring hole X k Using cubic spline interpolation as nodes, a continuous and smooth brackish water interface curve is generated by connecting the feature points. I ( x ); S36. By comparing the brackish water interface curves generated at different times. I t1(x) and I t2(x) The average vertical migration rate of the computing interface V To determine the evolution trend of the lens body.

5. The method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 4, characterized in that, The calculation formula for removing temperature crosstalk in S31 is as follows: ； in, This represents the wavelength shift of the salinity sensor. This represents the wavelength shift of the temperature sensor. These are the sensitivity coefficients of the salinity sensor to salinity and temperature, respectively. These are the sensitivity coefficients of the temperature sensor to salinity and temperature, respectively. The determinant value of the coefficient matrix. The change in temperature This represents the change in salinity.

6. The method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 5, characterized in that, The specific formula for the salinity value of the monitoring point in S31 is as follows: ； in, S 0 This is the initial calibrated salinity value.

7. The method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 4, characterized in that, The nonlinear fitting algorithm in S33 uses the Boltzmann Sigmoid function, and the specific formula is as follows: ； in, S ( z () represents the fitted salinity distribution function with depth. S 1 Background salinity in freshwater areas, S 2 For deep salinity background, Z For depth variables, Z m The center depth of the brackish water interface. d z The slope factor characterizes the thickness of the transition zone.

8. A method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 4, characterized in that, The brackish water interface curve in S35 I(x) The specific formula is as follows: ； in, A k , B k , C k , D k These are the spline interpolation coefficients, ensuring that the curve maintains continuity of its second derivative while passing through each monitoring point. x The coordinates represent the horizontal distance along the monitoring profile.

9. A method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 4, characterized in that, The average vertical migration rate in S36 V The specific formula is as follows: ； in, for t The brackish water interface curve generated at time 1 for t The brackish water interface curve generated at time 2, t 1. t 2 represents different times. X To monitor the horizontal coordinates of the endpoint of the effective integration region of the profile, X 0 To monitor the starting horizontal coordinates of the effective integration region of the profile, d x Let be the differential variable of the horizontal distance.

10. A method for monitoring freshwater lens resources based on a weak-reflection fiber optic grating according to claim 9, characterized in that, The specific criteria for judging the evolution trend of the lens body in S36 are as follows: like V >0 indicates that the interface has shifted upwards as a whole, and the freshwater lens is shrinking or thinning. like V <0 indicates that the interface has shifted downwards and the freshwater lens is recovering or filling.

Images