Method for high-precision detection of organic pollution by nuclear magnetic resonance

Abstract

This application relates to the field of geophysical exploration technology and provides a high-precision nuclear magnetic resonance (NMR) detection method for organic pollution. The method includes: transmitting a DC pulse P1; applying an amplitude-sweeping AC pulse AFP1 after turning off the DC pulse P1; turning off the amplitude-sweeping AC pulse AFP1 and transmitting a DC pulse P2; after turning off the DC pulse P2, applying an adiabatic half-wave pulse AHP1 and acquiring a first induced voltage signal; applying an adiabatic half-wave pulse AHP2 at the moment after turning off the adiabatic half-wave pulse AHP1 and acquiring a second induced voltage signal; applying an amplitude-sweeping AC pulse AFP2 after turning off the adiabatic half-wave pulse AHP2, and repeatedly applying the amplitude-sweeping AC pulse AFP2 after each time it is turned off, acquiring a third induced voltage signal. This application can selectively suppress groundwater signals and enhance the sensitivity of organic pollution signals.

Description

Technical Field

[0001] This application relates to the field of geophysical exploration technology, specifically a high-precision nuclear magnetic resonance detection method for organic pollution. Background Technology

[0002] Groundwater is a crucial source of water for agricultural irrigation, industrial production, and domestic use, especially in arid or semi-arid regions where it may even be the sole source of water for sustaining ecosystems and human survival. However, with rapid industrialization and urbanization, groundwater pollution has become increasingly severe. Human activities such as industrial wastewater leakage and the seepage of agricultural fertilizers and pesticides lead to the intrusion of organic pollutants into aquifers. This type of pollution is insidious, cumulative, and difficult to remediate; once it occurs, it can persist for decades or even longer, posing a long-term threat to the ecological environment and public health. Therefore, research on the detection of organic pollution in groundwater is of paramount importance.

[0003] Groundwater pollution is characterized by its slow process and difficulty in detection. Nuclear magnetic resonance (NMR) technology can directly locate the concentration and spatial distribution of organic pollutants in aquifers by measuring the relaxation time differences of hydrogen nuclei, and has become an important tool for groundwater pollution detection in recent years. Compared with traditional sampling and analysis methods, it has the advantages of being non-destructive and allowing for in-situ detection.

[0004] However, inorganic hydrogen nuclei in groundwater can easily mask the response of organic pollution, and the relaxation times of organic pollutants and water may be similar, making it difficult for traditional nuclear magnetic resonance to distinguish them, resulting in blurred inversion results. Furthermore, since the intensity of nuclear magnetic resonance signals is positively correlated with the content of hydrogen nuclei, low concentrations of pollution may not be detected.

[0005] In existing methods for detecting organic pollution using nuclear magnetic resonance (NMR), traditional NMR technology is inaccurate in modeling the response to organic pollution, fails to effectively distinguish the NMR signals of groundwater from those of organic pollutants, resulting in significant interference in the detection results. Furthermore, it lacks sensitivity for detecting low concentrations of organic pollution, and the methods for extracting characteristic parameters are not perfect, making it difficult to achieve accurate identification and quantitative analysis of organic pollution. Summary of the Invention

[0006] This application provides a high-precision nuclear magnetic resonance (NMR) detection method for organic pollution, which solves the problems in the prior art that fail to effectively distinguish between the NMR signals of groundwater and organic pollutants, resulting in significant interference in the detection results, insufficient sensitivity for detecting low concentrations of organic pollution, and imperfect feature parameter extraction methods, making it difficult to achieve accurate identification and quantitative analysis of organic pollution.

[0007] A high-precision nuclear magnetic resonance detection method for organic pollution according to an embodiment of this application includes: A DC pulse P1 is emitted to excite hydrogen nuclei in groundwater and organic pollutants, causing a change in magnetization. After the DC pulse P1 is turned off, an amplitude-sweeping AC pulse AFP1 is applied to reverse the magnetization. Turn off the amplitude-sweeping AC pulse AFP1, transmit a DC pulse P2 with the same polarization current as the DC pulse P1, turn off the DC pulse P2, apply an adiabatic half-wave pulse AHP1, and collect the first induced voltage signal. After shutting down the adiabatic half-wave pulse AHP1 Adiabatic half-wave pulse AHP2 is applied continuously, and the second induced voltage signal is acquired. After shutting down the adiabatic half-wave pulse AHP2 The amplitude-sweep / frequency-sweep AC pulse AFP2 is applied at all times, and after each time the amplitude-sweep / frequency-sweep AC pulse AFP2 is turned off... A variable amplitude / sweep frequency AC pulse AFP2 is repeatedly applied to collect the third induced voltage signal.

[0008] Furthermore, envelope extraction is performed on the first induced voltage signal to obtain the relationship between the nuclear magnetic resonance FID response signal and the hydrogen nucleus content and average relaxation time of the pollutants, which is expressed as follows: , in, This is the nuclear magnetic resonance FID response signal. The excitation pulse moment of the adiabatic half-wave pulse AHP1, To correspond to different average relaxation times underground location The hydrogen nucleus content of the pollutants, To form a virtual receiving field corresponding to a unit received induced current, This represents the decomposition phase of the elliptic polarization corresponding to the emission field. The phase decomposition of the received field corresponding to the elliptic polarization. For the effective unit vector, Let be the unit vector of the receiving field. is the unit vector of the Earth's magnetic field. The average relaxation time, This is the nuclear magnetic resonance FID response signal. After the activation process, Perpendicular to Magnetization components: , The nuclear magnetization intensity of hydrogen atoms in organic pollution. It is a natural geomagnetic field. For the Lamo frequency, For time, The imaginary unit, The first tilt angle is generated by the adiabatic half-wave pulse AHP1 excitation.

[0009] Furthermore, an inversion objective function for the nuclear magnetic resonance FID response signal is constructed, and the inversion objective function for the nuclear magnetic resonance FID response signal is solved to obtain the spatial distribution information of the hydrogen nucleus content and average relaxation time of the pollutants. The objective function for inverting the nuclear magnetic resonance FID response signal for: , in, For regularization parameters, The smoothness matrix, For the FID response sensitivity kernel function of pollution detection, for The abbreviation of .

[0010] Furthermore, envelope extraction is performed on the second induced voltage signal to obtain the relationship between the inversion recovery FID response signal and the longitudinal relaxation time, expressed as: , , in, The excitation pulse distance of the adiabatic half-wave pulse AHP2 is... The time interval between the adiabatic half-wave pulse AHP2 and the adiabatic half-wave pulse AHP1. For longitudinal relaxation time, To reverse and recover the FID response signal, To invert the FID response sensitivity kernel function; Transverse magnetization generated by the adiabatic half-wave pulse AHP2: , in, The second tilt angle is generated by the adiabatic half-wave pulse AHP2 excitation.

[0011] Furthermore, an inversion objective function for recovering the FID response signal is established. The spatial distribution of the longitudinal relaxation time is obtained by solving the inversion objective function for recovering the FID response signal. for: , The longitudinal relaxation time is updated iteratively, and the contamination detection inversion recovery FID response sensitivity kernel function is recalculated. The spatial distribution of the longitudinal relaxation time is successively approximated and finally solved.

[0012] Furthermore, envelope extraction is performed on the third induced voltage signal to obtain the relationship between the spin echo response signal and the transverse relaxation time, expressed as: , , in, This is the spin echo response signal. The excitation pulse distance of the adiabatic half-wave pulse AFP2 is... The time interval between the adiabatic half-wave pulse AHP2 and the amplitude-sweep AC pulse AFP2. For the lateral relaxation time, To accommodate different lateral relaxation times underground location The hydrogen nucleus content of the pollutants, For the corresponding number The peak time of the spin echo response signal This is the spin echo response sensitivity kernel function. The transverse magnetization at the peak of the spin echo response is: , in, The third tilt angle is generated by the adiabatic half-wave pulse AFP2 excitation.

[0013] Furthermore, an objective function for the inversion of the spin echo signal is established. The spatial distribution of the transverse relaxation time is obtained by solving the equation, and the objective function for the inversion of the spin echo signal is... for: , in, For regularization parameters, This is the smoothness matrix.

[0014] The beneficial effects of this application are as follows: This application can realize the full-time-domain evolution process of the hydrogen nuclear magnetization vector of organic pollution through dynamic simulation. It can not only selectively suppress groundwater signals and enhance the sensitivity of organic pollution signals, but also solve the problems of inaccurate modeling of organic pollution response by traditional nuclear magnetic resonance technology, failure to effectively distinguish between the nuclear magnetic resonance signals of groundwater and organic pollutants, resulting in large interference in the detection results, insufficient sensitivity for detecting low concentrations of organic pollution, and imperfect characteristic parameter extraction methods, making it difficult to achieve accurate identification and quantitative analysis of organic pollution. Attached Figure Description

[0015] Figure 1 A flowchart illustrating the method provided in this application embodiment; Figure 2A timing diagram of the nuclear magnetic resonance detection sequence for organic pollution provided in the embodiments of this application; Figure 3 The graph shows the variation of the magnetization intensity of free water hydrogen nuclei and the magnetization intensity of organic pollutant hydrogen nuclei in the embodiments of this application. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0017] See Figure 1 Combination Figure 2 and Figure 3 As shown, a high-precision nuclear magnetic resonance detection method for organic pollution includes the following steps: S1. A DC pulse P1 is emitted to excite hydrogen nuclei in groundwater and organic pollutants, causing a change in magnetization. After the DC pulse P1 is turned off, an amplitude-sweeping AC pulse AFP1 is applied to reverse the magnetization. The amplitude-sweeping AC pulse sweeps the frequency from a frequency far away from the Larmor frequency to the Larmor frequency, while the pulse amplitude gradually increases from zero until it equals the Larmor frequency. Then, it continues to sweep the frequency in the opposite direction from the Larmor frequency, moving away from the Larmor frequency, while the pulse amplitude gradually decreases to zero.

[0018] S2, turn off the amplitude / sweep AC pulse AFP1, emit a DC pulse P2 with the same polarization current as the DC pulse P1, turn off the DC pulse P2, apply an adiabatic half-wave pulse AHP1, and collect the first induced voltage signal; the adiabatic half-wave pulse refers to the pulse sweeping from a frequency far away from the Larmor frequency to the Larmor frequency, while the pulse amplitude gradually increases from zero.

[0019] Turning off the amplitude-sweeping AC pulse AFP1 and emitting a DC pulse P2 with the same polarization current as the DC pulse P1 is used to further selectively cancel the macroscopic magnetic moment of groundwater. After turning off the DC pulse P2, the longitudinal relaxation time of organic pollutants is shorter. Applying an adiabatic half-wave pulse AHP1 and collecting the first induced voltage signal can be used to obtain the hydrogen nucleus content and average relaxation time of pollutants at different underground locations. S3, after turning off the adiabatic half-wave pulse AHP1 Adiabatic half-wave pulse AHP2 is applied continuously, and the second induced voltage signal is collected. At this time, the hydrogen nuclei in the organic pollutant are flipped over again. The longitudinal relaxation time of the hydrogen nuclei in the pollutant can be obtained from the second induced voltage signal. S4, after turning off the adiabatic half-wave pulse AHP2 A variable amplitude / sweep frequency AC pulse AFP2 is applied at all times, and after each adiabatic half-wave pulse AHP2 is turned off... A variable amplitude / sweep frequency AC pulse AFP2 is repeatedly applied to collect the third induced voltage signal. The transverse relaxation time of the hydrogen nuclei of the pollutants can be obtained through the third induced voltage signal.

[0020] See one example. Figure 3 As shown, the magnetization intensity of hydrogen nuclei in free water and in organic pollutants is represented. In S1, the hydrogen nuclei undergo a process from steady state to positive polarization to enhance the magnetization intensity of all hydrogen nuclei, and then a 180° flip occurs.

[0021] In S1, under the action of a DC pulse P1 with a polarization current greater than 240A, the magnetization intensity of organic pollutants and groundwater changes with the duration of the magnetic field action, and its vector process conforms to the DC Bloch equation: , in, The nuclear magnetization intensity of hydrogen atoms in organic pollution. For time, It is the gyromagnetic ratio. The magnetic field generated by the DC pulse P1 This is the natural geomagnetic field. After the DC pulse P1 is turned off, an amplitude-sweeping / frequency-sweeping AC pulse AFP1 is applied to effectively excite the field. for: , in, The excitation field formed by the modulation pulse amplitude, This is an additional excitation component formed in the direction of the geomagnetic field through polarization. This is the frequency offset. and These are the Larmor frequency and the pulse time-varying transmission frequency, respectively. Represents the Earth's magnetic field The direction vector of the direction.

[0022] Organic pollution hydrogen nuclei magnetization intensity The reverse direction occurs under the action of the amplitude / frequency sweep AC pulse AFP1, and this process conforms to the AC adiabatic conditions: , in, To effectively stimulate the field With the natural geomagnetic field Direction angle: , Due to organic pollution during this process, the nuclear magnetization intensity of hydrogen atoms... Always focusing on effectively stimulating the field Rotate it forward and tilt it 180°. The nuclear magnetization intensity of hydrogen atoms in the organic pollutants will then be... Vector processes can be estimated using the AC form of the Bloch equations: , See one example. Figure 3 As shown, the hydrogen nuclei in S2 undergo a process of flipping from 180° to reverse polarization, canceling the magnetization of hydrogen nuclei in free water, then undergoing a 90° flip, and finally releasing a nuclear magnetic resonance FID response signal (FID) to measure the average relaxation time. .

[0023] Excitation in S2 turns off the amplitude-sweeping AC pulse AFP1, and a DC pulse P2 with the same polarization current as the DC pulse P1 is emitted. The dynamic response process also conforms to the DC Bloch equation. After turning off the DC pulse P2, an adiabatic half-wave pulse AHP1 is applied, and the first induced voltage signal is collected. When the DC pulse P2 is cut off, the macroscopic magnetic moment of the hydrogen nuclei in the water has been fully canceled under the influence of the magnetic field generated by the DC pulse P2 (no subsequent nuclear magnetic resonance response is generated). At the same time, due to the shorter longitudinal relaxation time of organic pollutants, they have re-entered the positive polarization state from the reverse polarization state.

[0024] An adiabatic half-wave pulse AHP1 is applied to obtain the first induced voltage signal. Envelope extraction of the first induced voltage signal yields the relationship between the nuclear magnetic resonance FID response signal and the hydrogen nucleus content and average relaxation time of the pollutants, expressed as follows: , in, This is the nuclear magnetic resonance FID response signal. The excitation pulse moment of the adiabatic half-wave pulse AHP1, To correspond to different average relaxation times underground location The hydrogen nucleus content of the pollutants, To form a virtual receiving field corresponding to a unit received induced current, This represents the decomposition phase of the elliptic polarization corresponding to the emission field. The phase decomposition of the received field corresponding to the elliptic polarization. For the effective unit vector, Let be the unit vector of the receiving field. is the unit vector of the Earth's magnetic field. The average relaxation time, This is the nuclear magnetic resonance FID response signal. After the activation process, Perpendicular to Magnetization components: , The nuclear magnetization intensity of hydrogen atoms in organic pollution. It is a natural geomagnetic field. For the Lamo frequency, It is time. It is the imaginary unit. The first tilt angle is the adiabatic half-wave pulse AHP1 excitation angle.

[0025] For ease of modeling and solving, it can be further expressed as the FID response sensitivity kernel function for pollution detection. Corresponding to different average relaxation times underground location Hydrogen nucleus content of pollutants With average relaxation time The product of: , , Construct the inversion objective function of the nuclear magnetic resonance FID response signal, solve the inversion objective function of the nuclear magnetic resonance FID response signal, and obtain the spatial distribution information of the hydrogen nucleus content and average relaxation time of pollutants; Constructing the inversion objective function of the nuclear magnetic resonance FID response signal for: , in, For regularization parameters, The smoothness matrix, The kernel function for the FID response sensitivity of pollution detection.

[0026] By inverting the objective function of the nuclear magnetic resonance FID response signal Solving the equation yields spatial distribution information on the hydrogen nucleus content and average relaxation time of pollutants.

[0027] See one example. Figure 3 As shown, the hydrogen nucleus in S3 undergoes a process from releasing the nuclear magnetic resonance (NMR) FID response signal to tilting at 90°, and finally releasing the inversion recovery FID response signal, which is used to measure the longitudinal relaxation time. .

[0028] In one embodiment, the envelope of the second induced voltage signal is extracted to obtain the relationship between the inversion recovery FID response signal and the longitudinal relaxation time, which is expressed as: , , in, The excitation pulse distance of the adiabatic half-wave pulse AHP2 is... The time interval between the adiabatic half-wave pulse AHP2 and the adiabatic half-wave pulse AHP1. For longitudinal relaxation time, To reverse and recover the FID response signal, To invert the FID response sensitivity kernel function; Transverse magnetization generated by the adiabatic half-wave pulse AHP2: , in, The second tilt angle is the one excited by the adiabatic half-wave pulse AHP2.

[0029] Establish the inversion objective function for recovering the FID response signal. The spatial distribution of the longitudinal relaxation time is obtained by solving the inversion objective function for recovering the FID response signal. for: , in, The longitudinal relaxation time is updated iteratively at intervals, and the contamination detection inversion recovery FID response sensitivity kernel function is recalculated. The spatial distribution of the longitudinal relaxation time is successively approximated and finally solved.

[0030] See one example. Figure 3 As shown, the hydrogen atom in S4 undergoes a process from releasing the reverse recovery FID response signal (R-FID) to flipping 180°, and then releasing the spin echo response signal (SE), which is used to measure the transverse relaxation time. Finally, the hydrogen nucleus returns to a steady state, and the magnetization intensity of the hydrogen nucleus in organic pollution gradually decreases.

[0031] Envelope extraction is performed on the third induced voltage signal to obtain the relationship between the spin echo response signal and the transverse relaxation time, which is expressed as: , , in, This is the spin echo response signal. The excitation pulse distance of the adiabatic half-wave pulse AFP2 is... The time interval between the two adiabatic half-wave pulses AFP2 For the lateral relaxation time, To accommodate different lateral relaxation times underground location The hydrogen nucleus content of the pollutants, For the corresponding number The peak time of the spin echo response signal This is the spin echo response sensitivity kernel function. The transverse magnetization at the peak of the spin echo response is: , in, The third tilt angle is generated by the adiabatic half-wave pulse AFP2 excitation.

[0032] Establish the objective function for inverting the spin echo signal. The spatial distribution of the transverse relaxation time is obtained by solving the equation, and the objective function for the inversion of the spin echo signal is... for: .

[0033] Based on the spatial distribution information of pollutant hydrogen nucleus content, longitudinal relaxation time, and transverse relaxation time, an inversion model can be developed, thus enabling the solution of various characteristic parameters of the organic pollution response.

[0034] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A high-precision nuclear magnetic resonance detection method for organic pollution, characterized in that, The method includes: A DC pulse P1 is emitted to excite hydrogen nuclei in groundwater and organic pollutants, causing a change in magnetization. After the DC pulse P1 is turned off, an amplitude-sweeping AC pulse AFP1 is applied to reverse the magnetization. Turn off the amplitude-sweeping AC pulse AFP1, transmit a DC pulse P2 with the same polarization current as the DC pulse P1, turn off the DC pulse P2, apply an adiabatic half-wave pulse AHP1, and collect the first induced voltage signal. After shutting down the adiabatic half-wave pulse AHP1 Adiabatic half-wave pulse AHP2 is applied continuously, and the second induced voltage signal is acquired. After shutting down the adiabatic half-wave pulse AHP2 The amplitude-sweep / frequency-sweep AC pulse AFP2 is applied at all times, and after each time the amplitude-sweep / frequency-sweep AC pulse AFP2 is turned off... A variable amplitude / sweep frequency AC pulse AFP2 is repeatedly applied to collect the third induced voltage signal.

2. The method for high-precision detection of organic pollution using nuclear magnetic resonance according to claim 1, characterized in that, Envelope extraction of the first induced voltage signal yields the relationship between the nuclear magnetic resonance (FID) response signal and the hydrogen nucleus content and average relaxation time of the pollutants, expressed as follows: , in, This is the nuclear magnetic resonance FID response signal. The excitation pulse moment of the adiabatic half-wave pulse AHP1, To correspond to different average relaxation times underground location The hydrogen nucleus content of the pollutants, To form a virtual receiving field corresponding to a unit received induced current, This represents the decomposition phase of the elliptic polarization corresponding to the emission field. The phase decomposition of the received field corresponding to the elliptic polarization. For the effective unit vector, Let be the unit vector of the receiving field. is the unit vector of the Earth's magnetic field. The average relaxation time, This is the nuclear magnetic resonance FID response signal. After the activation process, Perpendicular to Magnetization components: , The nuclear magnetization intensity of hydrogen atoms in organic pollution. It is a natural geomagnetic field. For the Lamo frequency, For time, The imaginary unit, The first tilt angle is generated by the adiabatic half-wave pulse AHP1 excitation.

3. The method for high-precision nuclear magnetic resonance detection of organic pollution according to claim 2, characterized in that, Construct the inversion objective function of the nuclear magnetic resonance FID response signal, solve the inversion objective function of the nuclear magnetic resonance FID response signal, and obtain the spatial distribution information of the hydrogen nucleus content and average relaxation time of pollutants; The objective function for inverting the nuclear magnetic resonance FID response signal for: ， in, For regularization parameters, The smoothness matrix, For the FID response sensitivity kernel function of pollution detection, for The abbreviation of .

4. The method for high-precision nuclear magnetic resonance detection of organic pollution according to claim 2, characterized in that, Envelope extraction is performed on the second induced voltage signal to obtain the relationship between the inversion recovery FID response signal and the longitudinal relaxation time, which is expressed as: ， ， in, The excitation pulse distance of the adiabatic half-wave pulse AHP2 is... The time interval between the adiabatic half-wave pulse AHP2 and the adiabatic half-wave pulse AHP1. For longitudinal relaxation time, To reverse and recover the FID response signal, To invert the FID response sensitivity kernel function; Transverse magnetization generated by the adiabatic half-wave pulse AHP2: ， in, The second tilt angle is generated by the adiabatic half-wave pulse AHP2 excitation.

5. The method for high-precision nuclear magnetic resonance detection of organic pollution according to claim 4, characterized in that, Establish the inversion objective function for recovering the FID response signal. The spatial distribution of the longitudinal relaxation time is obtained by solving the inversion objective function for recovering the FID response signal. for: ， The longitudinal relaxation time is updated iteratively, and the contamination detection inversion recovery FID response sensitivity kernel function is recalculated. The spatial distribution of the longitudinal relaxation time is successively approximated and finally solved.

6. The method for high-precision nuclear magnetic resonance detection of organic pollution according to claim 4, characterized in that, Envelope extraction is performed on the third induced voltage signal to obtain the relationship between the spin echo response signal and the transverse relaxation time, which is expressed as: ， ， in, This is the spin echo response signal. The excitation pulse distance of the adiabatic half-wave pulse AFP2 is... The time interval between the adiabatic half-wave pulse AHP2 and the amplitude-sweep AC pulse AFP2. For the lateral relaxation time, To accommodate different lateral relaxation times underground location The hydrogen nucleus content of the pollutants, For the corresponding number The peak time of the spin echo response signal This is the spin echo response sensitivity kernel function. The transverse magnetization at the peak of the spin echo response is: ， in, The third tilt angle is generated by the adiabatic half-wave pulse AFP2 excitation.

7. The method for high-precision nuclear magnetic resonance detection of organic pollution according to claim 6, characterized in that, Establish the objective function for inverting the spin echo signal. The spatial distribution of the transverse relaxation time is obtained by solving the equation, and the objective function for the inversion of the spin echo signal is... for: ， in, For regularization parameters, This is the smoothness matrix.

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