A method for detecting groundwater using magnetic resonance longitudinal relaxation time

Abstract

The present invention belongs to the field of geophysical exploration technology and is a method for detecting the longitudinal relaxation time of groundwater using magnetic resonance. The method comprises: laying a magnetic resonance detection coil on the ground, fixing a short emission time, varying the current amplitude to emit multiple short pulses, and collecting the short pulse observation signal generated by each short pulse; fixing a long emission time, re-emitting a series of long pulses with different current amplitudes, and collecting the long pulse observation signal generated by each long pulse; inverting the short pulse observation signal to obtain the water content and transverse relaxation time distribution of groundwater at different depths; using the water content and transverse relaxation time distribution of groundwater at different depths as prior data for Bayesian inversion, and employing Bayesian inversion to analyze the long pulse observation signal to obtain the longitudinal relaxation time distribution. The present invention can capture the longitudinal relaxation signal of samples with fast decay rates and short relaxation without the need for complex excitation sequences. The method is suitable for high-power scenarios in field ground magnetic resonance detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of geophysical exploration, and in particular relates to a method for detecting groundwater magnetic resonance longitudinal relaxation time. Background Art

[0002] Ground magnetic resonance technology is a non-invasive geophysical method for direct quantitative detection of groundwater and evaluation of pore size distribution. The magnetic resonance response has three time constants that reflect the relaxation characteristics of the aquifer: longitudinal relaxation time , transverse relaxation time and observed relaxation time In porous media, relaxation time is approximately proportional to the average pore size and can be used to infer aquifer pore properties. Ground-based magnetic resonance (GMR) has extensively studied methods for obtaining transverse and observed relaxation times. To fully characterize groundwater pore properties, it is necessary to implement longitudinal relaxation time detection in GMR.

[0003] In existing technologies, complex excitation sequences are often used and relaxation times are observed. The parameters are sensitive to the inhomogeneity of the geomagnetic field between formation rocks and cannot directly reflect the pore characteristics of groundwater. It is also difficult to capture the longitudinal relaxation signal of samples with fast decay speed and short relaxation. Summary of the Invention

[0004] The embodiment of the present application provides a method for detecting the longitudinal relaxation time of groundwater magnetic resonance, which solves the problem of difficulty in capturing the longitudinal relaxation signal of samples with fast decay speed and short relaxation.

[0005] The present invention is achieved in this way:

[0006] A method for detecting groundwater magnetic resonance longitudinal relaxation time, the method comprising:

[0007] A magnetic resonance detection coil is laid on the ground, a short emission time is fixed, multiple short pulses are emitted by changing the current amplitude, and the short pulse observation signal generated by each short pulse is collected;

[0008] A long emission time is fixed, a series of long pulses with different current amplitudes are re-emitted, and the long pulse observation signal generated by each long pulse is collected;

[0009] Invert short pulse observation signals to obtain the water content and lateral relaxation time distribution of groundwater at different depths;

[0010] The water content and transverse relaxation time distribution of groundwater at different depths are used as prior data for Bayesian inversion. The long pulse observation signal is analyzed using Bayesian inversion to obtain the longitudinal relaxation time distribution.

[0011] Furthermore, the inversion of the short pulse observation signal to obtain the water content and lateral relaxation time distribution of groundwater at different depths includes:

[0012] The objective function of regularized inversion is established as:

[0013] ,

[0014] Where, It is a short pulse inversion model that represents the distribution of groundwater water content and lateral relaxation time at different depths. is the short pulse magnetic resonance FID signal obtained by forward calculation, is the smoothness matrix describing the depth correlation, is a regularization parameter used to balance data fitting and model smoothness, is the data normalization item, Observe the signal for a short pulse;

[0015] The objective function is solved iteratively to obtain the distribution of groundwater content and lateral relaxation time at different depths.

[0016] Furthermore, the calculation process of the short pulse magnetic resonance FID signal includes:

[0017] Calculate the transverse magnetization vector under short pulses: ,in, It is a spatial point The transverse magnetization vector under a short pulse is is the net magnetization vector, For spatial points The magnetization vector flip angle at ;

[0018] The short pulse magnetic resonance FID signal in the time domain is calculated based on the transverse magnetization vector under the short pulse.

[0019] Furthermore, Bayesian inversion is used to analyze the long pulse observation signal and obtain the longitudinal relaxation time distribution, including:

[0020] Calculate the fitting error , represents the long pulse magnetic resonance FID signal obtained by forward calculation, Observe the signal for a long pulse;

[0021] The longitudinal relaxation time distribution in the current Bayesian inversion model is randomly perturbed in one region and re-sampled to obtain a new Bayesian inversion model;

[0022] Calculate the fitting error of the new Bayesian inversion model , using simulated annealing to determine whether to accept the new Bayesian inversion model;

[0023] The longitudinal relaxation time distribution of the current Bayesian inversion model is continuously perturbed and re-valued to form a state jump of the Markov chain until the maximum number of iterations is reached.

[0024] Furthermore, Bayesian inversion is used to analyze the long pulse observation signal to obtain the longitudinal relaxation time distribution while keeping the water content and transverse relaxation time distribution of groundwater at different depths unchanged.

[0025] Furthermore, the calculation process of the long pulse magnetic resonance FID signal includes: searching for the slice with the closest combination of emission time and relaxation parameter in the interpolation library based on the emission time and the relaxation parameter obtained by Bayesian inversion, obtaining the transverse magnetization vector under the long pulse based on the one-dimensional interpolation of the slice; and calculating the long pulse magnetic resonance FID signal in the time domain based on the transverse magnetization vector under the long pulse.

[0026] Furthermore, the interpolation library is formed by calculating the Bloch equation under different emission time, longitudinal relaxation time and transverse relaxation time combinations with dense parameter intervals through the Runge-Kutta method to obtain the interpolation library formed by the lookup table relationship between the transverse magnetization vector and the excitation field strength.

[0027] Furthermore, the short emission time refers to that the emission current duration in one pulse cycle is less than 50 ms; the long emission time refers to that the emission current duration in one pulse cycle is greater than 200 ms.

[0028] Compared with existing technologies, this invention offers the following advantages: Based on the relaxation effect of the hydrogen proton magnetization vector during the excitation phase, this method can invert the longitudinal relaxation time distribution of groundwater using only the free induction decay (FID) signal obtained from a single pulse excitation. This method can capture the longitudinal relaxation signal of samples with fast decay rates and short relaxation times without requiring a complex excitation sequence. This method is suitable for high-power field ground magnetic resonance detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a flow chart of a method for detecting groundwater magnetic resonance longitudinal relaxation time provided in an embodiment of the present application;

[0030] Figure 2 These are the FID detection data under the excitation of short pulse (a) and long pulse (b) provided in the embodiments of the present application;

[0031] Figure 3 1 is the groundwater inversion result provided in the embodiment of the present application, (a) is the water content inversion result, (b) is the lateral relaxation time inversion result, and (c) is the longitudinal relaxation time inversion result; DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0033] Groundwater magnetic resonance detection uses the principle of nuclear magnetic resonance to detect groundwater. Its working principle is based on the magnetic resonance phenomenon of hydrogen nuclei in water. When the hydrogen nuclei in groundwater are excited by radio frequency pulses in a specific magnetic field environment, magnetic resonance signals are generated. By receiving and analyzing these signals, the equipment can obtain relevant information about groundwater, such as the location, thickness, and water content of the aquifer. Groundwater magnetic resonance detection equipment generally includes: a transmitting system: used to generate and transmit radio frequency pulses of specific frequency and intensity to the underground to stimulate the magnetic resonance of hydrogen nuclei in groundwater. A receiving system: responsible for receiving the signals generated by the nuclear magnetic resonance of underground hydrogen atoms, and amplifying, filtering, and other processing for subsequent analysis. A control system: controls the transmitting system and the receiving system, coordinates the work of each part, and ensures the smooth progress of the detection process. The transmitting system is connected to a transmitting coil or a long wire, and the receiving system is connected to a receiving coil. The embodiment of the present application uses a groundwater magnetic resonance detection system to achieve the transmission of pulses and the reception of observation signals.

[0034] See also Figure 1 The flowchart of a method for detecting groundwater magnetic resonance longitudinal relaxation time is shown, and a method for detecting groundwater magnetic resonance longitudinal relaxation time is provided, comprising:

[0035] S1 lays a magnetic resonance detection coil on the ground, fixes a short emission time, changes the current amplitude to emit multiple short pulses, and collects the short pulse observation signal generated by each short pulse;

[0036] The detection coils here include receiving coils and transmitting coils, and there are no restrictions on the layout of the receiving coils and transmitting coils. Short transmission time refers to the duration of the transmission current in a pulse cycle, relative to the long transmission time. In the embodiment of the present application, the short transmission time is controlled within 50ms, and the current amplitude of multiple short pulses varies. The current amplitude is distributed logarithmically within a certain range, for example, controlled within a range of 1A-200A.

[0037] The obtained short pulse observation signal is the free induction decay signal (FID).

[0038] S2 fixes a long emission time, re-emits a series of long pulses with different current amplitudes, and collects the long pulse observation signal generated by each long pulse;

[0039] In the embodiment of the present application, the long emission time is controlled to be greater than 200 ms, that is, the emission current duration in one pulse cycle is controlled to be greater than 200 ms.

[0040] S3 inverts short pulse observation signals to obtain the water content and lateral relaxation time distribution of groundwater at different depths;

[0041] S4 uses the water content and transverse relaxation time distribution of groundwater at different depths as prior data for Bayesian inversion, and uses Bayesian inversion to analyze long pulse observation signals to obtain the longitudinal relaxation time distribution.

[0042] The evolution law of groundwater hydrogen protons at different emission times is:

[0043] The evolution process of groundwater hydrogen protons obeys the Bloch equation and can be expressed as:

[0044] ,

[0045] Where, is the gyromagnetic ratio, is the net magnetization vector, is the longitudinal relaxation time, is the transverse relaxation time, is the external magnetic field, 、 and is the magnetization vector The directional component of

[0046] The relaxation effect of the magnetization vector upon external excitation causes its modulus to decay, and the degree of decay is affected by the longitudinal and transverse relaxation times. When the relaxation characteristics and external excitation conditions are fixed, a longer emission time leads to a greater degree of modulus decay. This characteristic is exploited in the embodiments of the present application to invert the short-pulse observation signal to obtain the water content and transverse relaxation time distribution of groundwater at different depths.

[0047] The water content and transverse relaxation time distribution of groundwater at different depths are used as prior data for Bayesian inversion. That is, the Bayesian inversion includes the water content, longitudinal relaxation time and transverse relaxation time of groundwater at different depths. The long pulse observation signal is analyzed using Bayesian inversion to obtain the longitudinal relaxation time distribution.

[0048] In one embodiment, inverting the short pulse observation signal to obtain the water content and lateral relaxation time distribution of groundwater at different depths includes:

[0049] The objective function of regularized inversion is established as:

[0050] ,

[0051] Where, It is a short pulse inversion model that represents the distribution of groundwater water content and lateral relaxation time at different depths. is the short pulse magnetic resonance FID signal obtained by forward calculation, is the smoothness matrix describing the depth correlation, is a regularization parameter used to balance data fitting and model smoothness, is the data normalization item, Observe the signal for a short pulse;

[0052] The objective function is solved iteratively to obtain the distribution of groundwater content and lateral relaxation time at different depths.

[0053] For short pulse excitation, there is an explicit analytical solution for the transverse magnetization vector at the end of excitation:

[0054] ,

[0055] Where, It is a spatial point The transverse magnetization vector under a short pulse is is the net magnetization vector, For spatial points The magnetization vector at the flip angle.

[0056] The above formula can be used to obtain the transverse magnetization vector under a short pulse, and the short pulse magnetic resonance FID signal in the time domain can be calculated based on the transverse magnetization vector under a short pulse.

[0057] In one embodiment, since there is no explicit analytical solution between the transverse magnetization vector and the relaxation parameter under long pulse excitation, Bayesian inversion is used to analyze the long pulse observation signal to obtain the longitudinal relaxation time distribution of groundwater at different depths.

[0058] The Bayesian inversion framework can be expressed as:

[0059] ,

[0060] Where, is the prior probability density function of the Bayesian inversion model, represents the likelihood function, is the probability density function of the long pulse observation signal, The posterior probability density function of the longitudinal relaxation time representing the Bayesian inversion model.

[0061] The Markov chain Monte Carlo sampling method is used to solve the Bayesian problem. The specific iterative process includes: calculating the fitting error , represents the long pulse magnetic resonance FID signal obtained by forward calculation, Observe the signal for a long pulse;

[0062] The longitudinal relaxation time distribution in the current Bayesian inversion model is randomly perturbed in one region and re-sampled to obtain a new Bayesian inversion model.

[0063] Calculate the fitting error of the new Bayesian inversion model , using simulated annealing to determine whether to accept the new Bayesian inversion model;

[0064] The longitudinal relaxation time distribution of the current Bayesian inversion model is continuously perturbed and re-valued to form a state jump of the Markov chain until the maximum number of iterations is reached.

[0065] Here, the calculation method for short pulse magnetic resonance FID signals cannot be used. For the long pulse excitation of this application, the magnetization vector is affected by the relaxation effect during the excitation phase and cannot be solved explicitly. This application uses the "slice-interpolation" magnetization vector calculation strategy to quickly obtain the transverse magnetization vector. . Specifically including:

[0066] The Runge-Kutta method is used to calculate the Bloch equation for different combinations of emission time, longitudinal relaxation time, and transverse relaxation time at dense parameter intervals, obtaining a lookup table relationship between the transverse magnetization vector and the excitation field strength as a large interpolation library.

[0067] For the forward response solution, the transverse magnetization vectors of all spatial points can be searched in parallel for the slice of the closest combination of emission time and relaxation parameters in the interpolation library, and then one-dimensionally interpolated from the slice according to the excitation field strength at the spatial point.

[0068] The long pulse magnetic resonance FID signal in the time domain is calculated based on the transverse magnetization vector under long pulse.

[0069] The method for calculating a long pulse magnetic resonance FID signal or a short pulse magnetic resonance FID signal in a time domain according to a transverse magnetization vector under a long pulse or a short pulse comprises:

[0070] Spatial point kernel function It is related to the transverse magnetization vector and is expressed as:

[0071] ,

[0072] Where, represents the Larmor frequency, The term represents the spatial sensitivity of the receiving coil, and are the phase components of the transmitting field and the receiving field generated under elliptical excitation, represents the net magnetization vector;

[0073] Assumptions and Represents spatial points The water content at and the observed relaxation time, according to Kernel function forward calculation can obtain short pulse or long pulse magnetic resonance FID signal , expressed as:

[0074] ,

[0075] in, Indicates time.

[0076] The following is a specific application to explain the implementation process and effects of the embodiment of the present application.

[0077] A rectangular magnetic resonance detection coil with a size of 100m×100m and 1 turn is laid on the ground;

[0078] Fixed a short launch time , 40 short pulses are emitted with varying current amplitudes, with the current amplitudes distributed logarithmically between 1A and 200A;

[0079] After completing 40 short pulse excitations, a long emission time is fixed , re-emit a series of long pulses of different current amplitudes;

[0080] Figure 2 (a) is a short pulse observation signal of 40ms short pulse, and the short pulse observation signal is represented by real part, imaginary part and modulus value. Figure 2 (b) is a long pulse observation signal of a 600ms long pulse, and the long pulse observation signal is represented by the real part, imaginary part, and modulus value. It can be seen that the long pulse observation signal has obvious amplitude attenuation compared to the short pulse observation signal.

[0081] According to the evolution law of groundwater hydrogen protons at different emission times, the relaxation effect of the magnetization vector when it is excited by the outside world will cause the modulus of the magnetization vector to decay, and the degree of decay is affected by the longitudinal relaxation time and transverse relaxation time parameters;

[0082] When the relaxation characteristics and external excitation conditions are fixed, a longer emission time will result in a greater degree of attenuation of the modulus of the magnetization vector;

[0083] The emission time is 40ms and 600ms respectively, and the longitudinal relaxation time is Take 100 samples at even intervals within the interval 0.01s-1s;

[0084] For each longitudinal relaxation time Value, transverse relaxation time In the interval Take 100 at even intervals to meet ;

[0085] According to the evolution law of groundwater hydrogen protons under different emission times, the initial amplitude of the short pulse observation signal obtained by the 40ms short pulse is mainly related to the water content of the formation, while the initial amplitude of the long pulse observation signal obtained by the 600ms long pulse is related to the attenuation of the long pulse observation signal compared with the short pulse observation signal, which is related to the longitudinal relaxation time at different depths.

[0086] By establishing the objective function of regularized inversion and iteratively solving the objective function, the water content and lateral relaxation time distribution of groundwater at different depths are obtained.

[0087] Since there is no explicit analytical solution between the transverse magnetization vector and the relaxation parameter under long pulse excitation, the Bayesian inversion method is used to analyze the long pulse observation signal to obtain the longitudinal relaxation time distribution of groundwater at different depths.

[0088] The Bayesian inversion model includes transverse relaxation time, longitudinal relaxation time and water content;

[0089] Assign the water content and transverse relaxation time distributions of the short pulse inversion model to the Bayesian inversion model;

[0090] According to the iterative process of the Markov chain mentioned above, the longitudinal relaxation time distribution of groundwater at different depths within 60 m underground is obtained.

[0091] Figure 3 (a) in the figure is the water content inversion result. Figure 3 (b) is the inversion result of transverse relaxation time. Figure 3 (c) shows the inversion result of the longitudinal relaxation time. The two main aquifers are located at 20m and 40m below ground level, respectively. The longitudinal relaxation time of the shallow groundwater is approximately 0.5s, while the longitudinal relaxation time of the deep groundwater is relatively large, approximately 0.8s.

[0092] In the embodiment of the present application, the longitudinal relaxation time distribution of groundwater can be inverted by using only the observation signal obtained by one short pulse excitation and the observation signal obtained by one long pulse excitation, without the need for a complex excitation sequence, and is suitable for high-power scenarios of field ground magnetic resonance detection.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for detecting groundwater magnetic resonance longitudinal relaxation time, characterized in that: The method includes, A magnetic resonance detection coil is laid on the ground, a short emission time is fixed, multiple short pulses are emitted by changing the current amplitude, and the short pulse observation signal generated by each short pulse is collected; A long emission time is fixed, a series of long pulses with different current amplitudes are re-emitted, and the long pulse observation signal generated by each long pulse is collected; Invert short pulse observation signals to obtain the water content and lateral relaxation time distribution of groundwater at different depths, including: The objective function of regularized inversion is established as: ， Where, It is a short pulse inversion model that represents the distribution of groundwater water content and lateral relaxation time at different depths. is the short pulse magnetic resonance FID signal obtained by forward calculation, is the smoothness matrix describing the depth correlation, is a regularization parameter used to balance data fitting and model smoothness, is the data normalization item, Observe the signal for a short pulse; The objective function is solved iteratively to obtain the distribution of groundwater water content and lateral relaxation time at different depths; The water content and transverse relaxation time distribution of groundwater at different depths are used as prior data for Bayesian inversion. The long pulse observation signal is analyzed using Bayesian inversion to obtain the longitudinal relaxation time distribution, including: Calculate the fitting error , represents the long pulse magnetic resonance FID signal obtained by forward calculation, Observe the signal for a long pulse; The longitudinal relaxation time distribution in the current Bayesian inversion model is randomly perturbed in one region and re-sampled to obtain a new Bayesian inversion model; Calculate the fitting error of the new Bayesian inversion model , using simulated annealing to determine whether to accept the new Bayesian inversion model; The longitudinal relaxation time distribution of the current Bayesian inversion model is continuously perturbed and re-valued to form a state jump of the Markov chain until the maximum number of iterations is reached.

2. The method for detecting groundwater magnetic resonance longitudinal relaxation time according to claim 1, wherein: The calculation process of short pulse magnetic resonance FID signal includes: Calculate the transverse magnetization vector under short pulses: ,in, It is a spatial point The transverse magnetization vector under a short pulse is is the net magnetization vector, For spatial points The magnetization vector flip angle at ; The short pulse magnetic resonance FID signal in the time domain is calculated based on the transverse magnetization vector under the short pulse.

3. The method for detecting groundwater magnetic resonance longitudinal relaxation time according to claim 1, wherein: Bayesian inversion is used to analyze the long pulse observation signal to obtain the longitudinal relaxation time distribution, while keeping the water content and transverse relaxation time distribution of groundwater at different depths unchanged.

4. The method for detecting groundwater magnetic resonance longitudinal relaxation time according to claim 1, wherein: The calculation process of the long pulse magnetic resonance FID signal includes: searching the slice with the closest combination of emission time and relaxation parameters in the interpolation library based on the emission time and the relaxation parameters obtained by Bayesian inversion, obtaining the transverse magnetization vector under the long pulse based on the one-dimensional interpolation of the slice; and calculating the long pulse magnetic resonance FID signal in the time domain based on the transverse magnetization vector under the long pulse.

5. The method for detecting groundwater magnetic resonance longitudinal relaxation time according to claim 4, characterized in that: The interpolation library is formed by calculating the Bloch equation under different emission time, longitudinal relaxation time and transverse relaxation time combinations with dense parameter intervals through the Runge-Kutta method, and obtaining the interpolation library of the lookup table relationship between the transverse magnetization vector and the excitation field strength.

6. The method for detecting groundwater magnetic resonance longitudinal relaxation time according to claim 1, wherein: The short emission time means that the emission current duration in one pulse cycle is less than 50ms; the long emission time means that the emission current duration in one pulse cycle is greater than 200ms.

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