CN115165767B - A Groundwater Monitoring Device Based on Grating Cascade Composite Sensing Technology - Google Patents

Abstract

An underground water monitoring device based on a grating cascade composite sensing technology relates to an underground water monitoring device. The monitoring module is hung at the lower end of a cable, the upper cavity is provided with a through hole and is covered by a filter screen, the broadband light source, the coated long-period photonic crystal grating, the Bragg temperature grating and the spectrometer are respectively connected with the optical fiber coupler through optical fibers, an isolator is arranged between the broadband light source and the optical fiber coupler, the spectrometer is externally connected with a computer, the cascading long-period grating is connected with the spectrometer through optical fibers, the bottom of the lower cavity is provided with a piston cylinder, the Bragg temperature grating is connected with the Bragg temperature grating and the cascading long-period grating through optical fibers, a piston of a piston rod is arranged in the piston cylinder, the connecting rod extends out of the piston cylinder and is provided with a spring, and the piston rod and the piston cylinder are provided with clamping arms for fixing the Bragg temperature grating. The in-situ multi-index real-time monitoring can be realized, and the accuracy and stability of monitoring are improved, wherein the multi-index real-time monitoring comprises water level, water temperature, PH value and chloride ion concentration.

Description

A Groundwater Monitoring Device Based on Grating Cascade Composite Sensing Technology

technical field

The invention relates to a groundwater monitoring device, in particular to a groundwater monitoring device based on grating cascade composite sensing technology, belonging to the technical field of groundwater monitoring.

Background technique

Groundwater is an important source of water supply, and long-term monitoring of groundwater dynamics is an important basis for scientific management of groundwater resources. Traditional artificial water level measurement equipment mostly uses measuring ropes to judge the groundwater level based on the signal that the end touches the water surface. There are relatively large subjective factors and errors, and real-time monitoring cannot be performed.

The automation of groundwater information collection and transmission can solve the problems of backward manual groundwater monitoring methods and weak information management capabilities. At present, the common automatic monitoring instruments of groundwater level are divided into two categories: pressure water level gauge and float type water level gauge. The pressure sensor of the pressure water level gauge is suspended in the groundwater logging well through a special cable, the measuring device is encapsulated in the housing, and the power supply and data storage device are integrated in the housing as an integrated pressure water level gauge or installed on the ground as a pressure sensor + In the form of the host, because the measuring device usually uses an electric sensor, it has high requirements for the waterproofness of the package and the stability of the instrument; the measuring part of the float type water level gauge instrument is suspended above the logging water surface, and the measured data object is relatively single, and Common float-type water level gauges have a diameter of 6cm-10cm and cannot be used for monitoring wells with smaller apertures. In addition, for the water quality monitoring of groundwater, the method of regular sampling and laboratory analysis is usually adopted, and in-situ real-time monitoring is rarely carried out at present.

In view of the above reasons, it is urgent to optimize and improve the groundwater monitoring method, and realize in-situ multi-indicator real-time monitoring while improving the accuracy and stability.

Contents of the invention

In order to solve the deficiencies in the background technology, the present invention provides a groundwater monitoring device based on grating cascade composite sensing technology, which can realize real-time monitoring of multiple indicators of groundwater in situ, including water level, water temperature, pH value and chloride ion concentration, Improved monitoring accuracy and stability.

In order to achieve the above object, the present invention adopts the following technical solutions: a groundwater monitoring device based on grating cascade composite sensing technology, including a broadband light source, a monitoring module and a spectrometer, the monitoring module has a shell and is suspended and fixed on the lower end of the cable, The inside of the shell is divided into an upper chamber and a lower chamber by a partition. The upper chamber is provided with a plurality of through holes and covered by a filter. The upper chamber is arranged with a fiber coupler, a coated long-period photonic crystal grating, Bragg temperature grating and cascaded long-period grating, the broadband light source, the coated long-period photonic crystal grating, the Bragg temperature grating and the spectrometer are respectively connected to the fiber coupler through an optical fiber, and the broadband light source is coupled to the optical fiber An isolator is arranged between the spectrometers, a computer is connected outside the spectrometer, a layer of silver mirror is coated on the end of the coated long-period photonic crystal grating opposite to the fiber connection point, and a side wall of the internal microhole of the coated long-period photonic crystal grating is coated with a A sensitive film whose refractive index is only affected by the concentration of chloride ions. A sealed cavity is set outside the Bragg temperature grating. The cascaded long-period grating is connected to the spectrometer through an optical fiber. The cascaded long-period grating is composed of two identical coated pH-sensitive The long-period gratings of hydrogel are connected in series. The bottom of the lower chamber is provided with a piston cylinder to communicate with the outside world. A Bragg strain grating and a piston rod matching it are arranged in the lower chamber. Both ends of the Bragg strain grating are respectively connected to the Bragg The temperature grating is connected to the cascaded long-period grating, the piston of the piston rod is arranged in the piston cylinder, the connecting rod of the piston rod extends out of the piston cylinder and a spring is arranged between its end and the partition plate, the piston rod and the The piston cylinders are respectively provided with clamping support arms, and the Bragg strain grating is fixed between the two clamping support arms.

Compared with the prior art, the beneficial effect of the present invention is that the present invention can realize in-situ multi-indicator real-time monitoring through the coordinated cooperation of Bragg strain gratings, Bragg temperature gratings, cascaded long-period gratings and coated long-period photonic crystal gratings The indicators include water level, water temperature, pH value and chloride ion concentration. The grating sensor does not need power supply, and the signal is transmitted through the optical fiber. Compared with the traditional electrical sensor, the measurement form is more stable. The cascaded long-period grating improves the pH value measurement. Accuracy, the coated long-period photonic crystal grating realizes the high sensitivity measurement of chloride ion concentration, and provides a more comprehensive in-situ real-time measurement form for groundwater monitoring.

Description of drawings

Fig. 1 is the overall structural representation of the present invention;

Fig. 2 is a schematic diagram of the optical path principle of the present invention;

Fig. 3 is the working optical path diagram of the long-period photonic crystal grating of coating of the present invention;

Fig. 4 is the working optical path figure of Bragg temperature grating and Bragg strain grating of the present invention;

Fig. 5 is a working optical path diagram of the cascaded long-period gratings of the present invention.

Detailed ways

The technical solutions in the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the invention, not all of them. Based on the present invention All other embodiments obtained by persons of ordinary skill in the art without creative efforts, all belong to the scope of protection of the present invention.

As shown in FIGS. 1-2 , a groundwater monitoring device based on grating cascade composite sensing technology includes a broadband light source 1 , a monitoring module 3 and a spectrometer 9 .

The broadband light source 1 and the spectrometer 9 are arranged outside the well logging. The broadband light source 1 provides a light source signal and an isolator 2 is installed to prevent reflected light from affecting the broadband light source 1. The spectrometer 9 is connected with a computer 10 to display monitoring data in real time.

The monitoring module 3 has a shell and is hung and fixed on the lower end of the cable 33. The monitoring module 3 is put into the groundwater to a certain depth along the well logging through the cable 33. The cable 33 should be provided with a scale line so as to read the lowering depth. The shell should be It is made of a material with high density and the density hardly changes with the temperature. It can be submerged in water and maintain a vertical posture. The partition inside the shell is divided into an upper chamber and a lower chamber.

The upper chamber is provided with a plurality of through holes 31 and is covered by a filter screen to prevent debris such as earth and gravel from entering the interior of the housing. Fiber couplers 4, coated long-period photonic crystal gratings 5, and Braggs are arranged in the upper chamber. Temperature grating 6 and cascaded long-period grating 8 .

The fiber coupler 4 uses a 2×2 fiber coupler to respectively connect the broadband light source 1, the coated long-period photonic crystal grating 5, the Bragg temperature grating 6 and the spectrometer 9 through optical fibers.

The coated long-period photonic crystal grating 5 adopts a photonic crystal grating and is connected with an optical fiber through fiber optic fusion technology. Compared with an ordinary long-period grating, the photonic crystal grating helps to increase the contact area with groundwater due to the existence of micropores. The side wall of the microhole is coated with a sensitive film whose refractive index is only affected by the concentration of chloride ions. The concentration of chloride ions will cause the change of the refractive index of the coating, resulting in the change of the effective refractive index of the cladding mode of the long-period photonic crystal. The effective refractive index of the cladding mode changes. According to the phase matching condition and wavelength expression of the long-period grating, the wavelength shift value, that is, the center wavelength of the resonance wave of the transmission spectrum, will change accordingly. The chloride ion can be obtained through the previous calibration experiment. The corresponding relationship between concentration and wavelength drift value, and then realize the measurement of chloride ion concentration in groundwater. In addition, a layer of silver mirror is coated on the end of the coated long-period photonic crystal grating 5 opposite to the fiber connection point, which can reflect the transmitted light source signal back, so that the light source signal passes through the grating twice, further improving the measurement accuracy.

The working optical path is combined as shown in Figure 3: the broadband light source 1 sends out a light source signal, which is transmitted to the coated long-period photonic crystal grating 5 through the isolator 2 and fiber coupler 4, and the light source signal passes through the grating twice due to the existence of the terminal silver mirror, and then passes through the grating again. After the optical fiber coupler 4 is transmitted to the spectrometer 9, the computer 10 calculates and displays the chloride ion concentration in groundwater through the corresponding relationship between the chlorine ion concentration and the wavelength drift value calibrated in advance.

The outside of the Bragg temperature grating 6 is covered with a sealed cavity 61, and the sealed cavity 61 is filled with air to transfer the external temperature to the Bragg temperature grating 6. The Bragg temperature grating 6 is in a natural non-tensioned state, and the drift value of the reflected wavelength is only Affected by temperature, since the central wavelength drift value of the reflected light of Bragg temperature grating 6 has a good linear relationship with temperature, the corresponding relationship between wavelength drift value and temperature can be obtained through calibration experiments in advance, so as to realize the measurement of groundwater temperature.

A piston cylinder 32 is provided at the bottom of the lower chamber to communicate with the outside world, and a Bragg strain grating 7 and a piston rod 71 matching it are arranged in the lower chamber.

Both ends of the Bragg strain grating 7 are respectively connected to the Bragg temperature grating 6 and the cascaded long-period grating 8 through optical fibers. The piston of the piston rod 71 is arranged in the piston cylinder 32, and the connecting rod of the piston rod 71 extends out of the piston. Cylinder 32 and spring 72 is set between its end and the described dividing plate, and piston rod 71 should adopt light material to make and reduce quality, and described spring 72 should have larger stiffness coefficient to avoid that piston rod 71 displacements are bigger to make The Bragg strain grating 7 is disconnected, the piston rod 71 and the piston cylinder 32 are respectively provided with clamping support arms 73, the Bragg strain grating 7 is fixed between the two clamping support arms 73, and is in a natural non-tension state when no force is applied. In the tight state, and at this time, the spring 72 is in contact with the piston rod 71 but has no force effect. The spring 72 is in the original length. After the monitoring module 3 is put into the groundwater, the piston rod 71 is pressed upward so that the two clamping arms 73 are aligned with the Bragg strain grating. 7 for tensioning. The Bragg strain grating 7 and the Bragg temperature grating 6 have the same period. When measuring strain, in order to avoid the influence of temperature on the result, the Bragg temperature grating 6 is used to perform temperature compensation for the Bragg strain grating 7. The strain formula is as follows:

In the formula, ε represents the strain, Δλ _A represents the total wavelength shift of the Bragg strain grating caused by axial strain caused by temperature and pressure, Δλ _B represents the wavelength shift of the Bragg temperature grating caused by temperature, and k _ε represents the grating strain sensitivity coefficient.

The strain ε is multiplied by the spacing of the two clamping support arms 73 to obtain the displacement Δx of the spring 72 (the spacing of the two clamping support arms 73 is measured in advance, because the piston rod 71 moves very little to the two clamping support arms 73 The impact caused by the spacing can be ignored), according to Hooke’s law ΔF=KΔx can be calculated to obtain the pressure on the piston rod 71, according to P=F/S can be calculated to obtain the surface pressure of the piston rod 71, and because P=ρgh, known The pressure can be calculated to obtain the depth of the piston rod 71, and combined with the lowering depth of the cable 33 and the shell size of the monitoring module 3, the groundwater level can be measured.

The working optical path is combined as shown in Figure 4: the broadband light source 1 sends out a light source signal, which is transmitted to the Bragg temperature grating 6 and the Bragg strain grating 7 through the isolator 2 and the fiber coupler 4, and reflections occur at the two gratings respectively, and the two reflections The light passes through the fiber coupler 4 to the spectrometer 9 for demodulation.

The cascaded long-period grating 8 is connected to the spectrometer 9 through an optical fiber, and the cascaded long-period grating 8 is connected in series by two identical long-period gratings coated with pH-sensitive hydrogel (carried out by high-frequency carbon dioxide laser pulse writing method). Writing long-period gratings, which is convenient to control the distance between two long-period gratings), is in a non-tensioned state in the natural state, and the pH-sensitive hydrogel can be made of sodium alginate and N-isopropylacrylamide Therefore, the sodium alginate in the gel will be fixed in the three-dimensional network of N-isopropylacrylamide, the hydrogel network shrinks under acidic conditions, and the swelling degree of the hydrogel increases under alkaline conditions , and then cause the axial strain of the grating, the wavelength shift caused by the axial strain, that is, the center wavelength shift of the resonant wave of the transmission spectrum, can be obtained through calibration experiments in advance, and the corresponding functional relationship between the wavelength shift and pH can be obtained, so as to measure the pH value of the groundwater. Furthermore, a porogen can be added during the fabrication of the pH-sensitive hydrogel to increase the contact area of the hydrogel and reduce the response time of the measurement.

The advantage of the cascaded long-period grating 8 is that when the distance between the two gratings is much greater than the grating period, the number of loss peaks in the transmission spectrum increases, the bandwidth narrows, and the transmission intensity is greatly improved compared with a single long-period grating. Can provide better resolution.

The working optical path is combined as shown in Figure 5: the broadband light source 1 sends out a light source signal, which is transmitted to the cascaded long-period grating 8 through the isolator 2, fiber coupler 4, Bragg temperature grating 6 and Bragg strain grating 7, and the transmitted light continues to propagate to the spectrometer 9 for demodulation.

It will be obvious to a person skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, but that it can be implemented in other configurations without departing from the spirit or essential characteristics of the invention. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents are included in the invention. Any reference sign in a claim should not be construed as limiting the claim concerned.

In addition, it should be understood that although this specification is described according to implementation modes, not each implementation mode only includes an independent technical solution, and this description in the specification is only for clarity, and those skilled in the art should take the specification as a whole , the technical solutions in the various embodiments can also be properly combined to form other implementations that can be understood by those skilled in the art.

Claims (4)

1. Groundwater monitoring device based on grating cascade composite sensing technology, its characterized in that: the device comprises a broadband light source (1), a monitoring module (3) and a spectrometer (9), wherein the monitoring module (3) is provided with a shell and is suspended and fixed at the lower end of a cable (33), a partition plate is arranged in the shell and is divided into an upper cavity and a lower cavity, the upper cavity is provided with a plurality of through holes (31) and is covered by a filter screen, an optical fiber coupler (4), a coated long-period photonic crystal grating (5), a Bragg temperature grating (6) and a cascade long-period grating (8) are arranged in the upper cavity, the optical fiber coupler (4) adopts a 2X 2 optical fiber coupler, the broadband light source (1) the coated long-period photonic crystal grating (5) is connected with the optical fiber coupler (4) through optical fibers, the Bragg temperature grating (6) and the spectrometer (9) are respectively connected with the optical fiber coupler (4), an isolator (2) is arranged between the broadband light source (1) and the optical fiber coupler (4), one end, opposite to the optical fiber coupler, of the coated long-period photonic crystal grating (5) is coated with a silver layer, the long-period photonic crystal grating (5) is only connected with the optical fiber coupler (8) through the optical fiber, the concentration of the capillary grating is affected by the capillary grating (61) through the cascade long-period grating, the capillary grating (61 is connected with the capillary grating (61), the cascade long-period grating (8) is formed by connecting two identical long-period gratings coated with PH sensitive hydrogel in series, a piston cylinder (32) is arranged at the inner bottom of a lower cavity and communicated with the outside, a Bragg strain grating (7) and a piston rod (71) matched with the Bragg strain grating are arranged in the lower cavity, two ends of the Bragg strain grating (7) are respectively connected with the Bragg temperature grating (6) and the cascade long-period grating (8) through optical fibers, a piston of the piston rod (71) is arranged in the piston cylinder (32), a connecting rod of the piston rod (71) extends out of the piston cylinder (32) and a spring (72) is arranged between the end part of the connecting rod and the partition plate, the piston rod (71) and the piston cylinder (32) are respectively provided with a clamping support arm (73), and the Bragg strain grating (7) is fixed between the two clamping support arms (73).

2. The underground water monitoring device based on the grating cascade composite sensing technology as claimed in claim 1, wherein: the PH sensitive hydrogel is prepared from sodium alginate and N-isopropyl acrylamide, and is contracted under an acidic condition and swelled under an alkaline condition, so that axial strain of the grating is caused.

3. The underground water monitoring device based on the grating cascade composite sensing technology as claimed in claim 2, wherein: and adding a pore-forming agent in the process of preparing the PH sensitive hydrogel.

4. The underground water monitoring device based on the grating cascade composite sensing technology as claimed in claim 1, wherein: the cable (33) is provided with graduation marks.