A method for ground-based nuclear magnetic resonance frequency domain inversion imaging

Abstract

The present invention relates to the field of geophysical exploration technology, specifically to a ground nuclear magnetic resonance frequency domain inversion imaging method, which solves the problems of weak ground nuclear magnetic resonance signals, difficulty in extracting effective information, long inversion iteration time, and insufficient imaging accuracy. The method includes obtaining the time domain pseudo-seismic wave field values ​​of each receiving coil; using time-frequency conversion to convert the obtained time domain pseudo-seismic wave field values ​​to the frequency domain; in the frequency domain, synthesizing the aperture of the pseudo-seismic wave field values ​​in the frequency domain to obtain the synthesized pseudo-seismic wave field; based on the Born approximate imaging method, inverting the synthesized pseudo-seismic wave field to achieve frequency domain inversion imaging of groundwater. The inversion imaging can be achieved while achieving ground nuclear magnetic resonance signal enhancement.

Description

Technical Field

[0001] The invention relates to the technical field of geophysical exploration, and in particular to a ground nuclear magnetic resonance frequency domain inversion imaging method. Background Art

[0002] Groundwater is not only an important component of water resources, but also a key source of water for human survival, agricultural irrigation and industrial production. Therefore, finding and evaluating groundwater resources is crucial to maintaining ecological balance, ensuring the sustainable use of water resources and supporting the country's socio-economic development.

[0003] Traditional geophysical methods for finding water, such as induced polarization and transient electromagnetic methods, are commonly used techniques. These methods indirectly search for groundwater by investigating physical anomalies in water-bearing structures or layers, but they cannot determine whether the anomalies are caused by water and the specific water content, among other basic issues. In contrast, ground-based nuclear magnetic resonance (MRS) technology has been widely used in groundwater exploration and water-related geological disaster prediction due to its non-destructive, in-situ and unique direct detection advantages. MRS technology uses an artificially generated magnetic field to excite hydrogen nuclei in groundwater to form macroscopic magnetic moments and perform precession motion in the geomagnetic field. When the excitation field stops, the relaxation phenomenon generated by the spin of the hydrogen nuclei is recorded by the ground coil, thereby generating an MRS signal, which can be used to make qualitative and quantitative estimates of groundwater and directly obtain relevant parameters of the aquifer.

[0004] However, the signal of ground-based nuclear magnetic resonance is very weak, only at the nanovolt level, and is particularly easy to be drowned by noise. It is difficult to obtain effective signals from the time domain perspective, which also limits the accuracy of subsequent inversion imaging. From the frequency domain perspective, the strength and distribution of each frequency component can be intuitively observed, and multi-dimensional information can be displayed, with strong anti-interference capabilities. Converting the signal of ground-based nuclear magnetic resonance into a pseudo-seismic wave field signal and then performing synthetic aperture can also enhance the signal to a certain extent, suppress noise, and improve the signal-to-noise ratio. In addition, the pseudo-seismic wave field signal is more sensitive to the boundary of the aquifer, which can improve the resolution and speed of ground-based nuclear magnetic resonance inversion imaging.

[0005] In the "ground nuclear magnetic resonance inversion method based on dynamic contour model" disclosed in CN104008249A, the number of aquifers in the detection area is set, the thickness and water content of each layer are initialized, the aquifer is divided into MN microelements, the matrix equation of the dynamic model is satisfied, and the matrix equation is iteratively solved. During the iteration process, the thickness and water content of each aquifer are dynamically adjusted to search for the optimal solution that satisfies the matrix equation. This method has a fast convergence speed and a relatively stable inversion result, but it is easily interfered by noise, which affects the inversion accuracy.

[0006] In the "array SQUID nuclear magnetic resonance groundwater detection device and imaging method" disclosed in CN106873044A, the detected water body is polarized by a prepolarization field, which improves the macroscopic magnetization intensity of the water body. At the same time, the array SQUID receiving mode is adopted to detect extremely weak geoelectric signals, thereby obtaining a larger initial amplitude of the nuclear magnetic resonance signal and obtaining an underground nuclear magnetic resonance detection image in a strong noise environment. This method enhances the ground nuclear magnetic resonance signal, which is conducive to groundwater detection in the detection area under complex terrain and strong noise environment, but it is not sensitive enough to the boundary of the aquifer, and the error is large at the boundary.

[0007] In the "high-resolution ground nuclear magnetic resonance imaging method" disclosed in CN108897051A, wave field transformation is performed in the time domain, and the correlation coefficient is used to perform synthetic aperture to achieve signal enhancement, and then based on the Kirchhoff migration imaging theory, underground water-bearing structure migration imaging is achieved. Although this method achieves signal enhancement from the perspective of the time domain, and realizes imaging through a virtual wave field, and is more sensitive to the boundary of the aquifer, it is difficult to intuitively extract effective nuclear magnetic resonance signals and the frequency distribution of each phase of the signal in the time domain, and the Kirchhoff imaging method has a complex calculation process, which is time-consuming and cannot meet the needs of rapid imaging.

[0008] In the above-mentioned traditional time-domain ground nuclear magnetic resonance inversion method, the information dimension displayed is single, the signal is relatively weak, it is easily interfered by noise, and it is not sensitive enough to the boundary of the aquifer. The calculation amount is large and the inversion time is long, making it difficult to achieve fast and high-precision inversion imaging of ground nuclear magnetic resonance. Summary of the invention

[0009] The purpose of the present invention is to solve the problems of weak ground nuclear magnetic resonance signals, difficulty in extracting effective information, long inversion iteration time, insufficient imaging accuracy, etc., and to provide a ground nuclear magnetic resonance frequency domain inversion imaging method that can achieve inversion imaging while enhancing the ground nuclear magnetic resonance signal.

[0010] The present invention is achieved in this way.

[0011] A ground nuclear magnetic resonance frequency domain inversion imaging method, the method comprising:

[0012] S1. Based on the classical transformation formula of diffusion field and wave field, the transformation equation of nuclear magnetic resonance response field and time domain pseudo-seismic wave field is obtained. The time domain pseudo-seismic wave field has high resolution at the edge of the aquifer. Based on the collected nuclear magnetic resonance signal, the time domain pseudo-seismic wave field value of each receiving coil is obtained.

[0013] S2, based on S transformation, using time-frequency conversion, convert the obtained time domain pseudo-seismic wave field value into the frequency domain;

[0014] S3, according to the correlation of echo signals of the same geological body on adjacent receiving coils, synthesizing the aperture of the pseudo-seismic wave field value in the frequency domain in the frequency domain to obtain a synthesized pseudo-seismic wave field;

[0015] S4. Based on the Born approximate imaging method, the synthesized pseudo-seismic wave field is inverted to achieve frequency domain inversion imaging of groundwater.

[0016] Furthermore, S1 specifically includes:

[0017] Collecting NMR signals: Alternating current is passed through the transmitting coil to generate an alternating magnetic field, which deflects the spin direction of the hydrogen nuclei in the groundwater and deviates from the direction of the geomagnetic field. After the transmission is completed, the voltage signal induced in the receiving coil is the NMR signal. The relationship between the NMR signal and the NMR response field is:

[0018] (1)

[0019] in, For time, is the NMR signal, is the NMR response field;

[0020] The NMR response field belongs to the diffusion field. According to the classical transformation formula between the diffusion field and the wave field, the NMR response field is converted into the pseudo-seismic wave field in the time domain:

[0021] (2)

[0022] in, is the pseudo-seismic wave field time, It is the pseudo seismic wave field in time domain;

[0023] From equations (1) and (2), we get the transformation equation between the NMR signal and the pseudo-seismic wave field in the time domain:

[0024] (3)

[0025] Rewrite equation (3) into a linear equation system to express the conversion relationship between the pseudo-seismic wave field value in the time domain and the nuclear magnetic resonance signal:

[0026] (4)

[0027] in, is the NMR signal, is the integral kernel function value, is the pseudo-seismic wave field value in the time domain, and Both The matrix of yes The square array, is the number of sampling points; if you want to use the nuclear magnetic resonance signal Obtaining pseudo-seismic wave field values ​​in the time domain , then transform equation (4):

[0028] (5)

[0029] in, for The pseudo-inverse matrix of Perform SVD decomposition, ,get:

[0030] (6)

[0031] in, , and Both , and and is an orthogonal matrix.

[0032] Furthermore, based on S transform, S2 uses time-frequency conversion to convert the pseudo-seismic wave field value in the time domain to the frequency domain using the formula:

[0033] (7)

[0034] in, represents the time shift parameter, Centered at Gauss window function, is the pseudo-seismic wave field value in the frequency domain;

[0035] Gauss window function Defined as:

[0036] (8)

[0037] in, is the frequency.

[0038] Furthermore, S3 specifically includes:

[0039] Assume that the total number of receiving coils on a measuring line is , is an odd number, The receiving coil is the center. , using the correlation of the echo signals of the same geological body on adjacent receiving coils, the pseudo-seismic wave field values ​​of each receiving coil on the entire survey line except the central coil are synthetically apertured:

[0040] (9)

[0041] in, For the Receive coil and center coil The linear distance The synthetic aperture range is excluding the center coil The pseudo-seismic wave field values ​​in the frequency domain of other receiving coils other than Center coil The synthesized pseudo-seismic wave field, is the phase coefficient, is the amplitude coefficient, is the magnetic permeability, is the medium conductivity;

[0042] Furthermore, S4 includes:

[0043] The synthesized pseudo-seismic wave field is decomposed into the sum of the scattered field and the background field. The scattered field is represented by the velocity perturbation quantity, which is linearly solved by the integral equation and the inversion imaging of the ground nuclear magnetic resonance is realized by using the velocity perturbation quantity.

[0044] Furthermore, S4 specifically includes:

[0045] The frequency domain wave equation satisfied by the synthesized pseudo-seismic wave field is:

[0046] (10)

[0047] in, is the distance between any position in the underground medium and the field source point, is the coordinate of the receiving point, is the velocity of the pseudo-seismic wave field, Center coil The synthesized pseudo-seismic wave field is The meaning is the same;

[0048] Pseudo-seismic wave field velocity Expressed as a slowly varying global background velocity and a rapidly varying velocity perturbation:

[0049] (11)

[0050] in, is the global background velocity, is the velocity disturbance;

[0051] Substituting formula (11) into formula (10), we get:

[0052] (12)

[0053] The synthesized pseudo-seismic wave field Decomposed into the sum of background field and scattered field:

[0054] (13)

[0055] in, For the background field, is the scattered field; according to formula (12), the background field It is expressed as:

[0056] (14)

[0057] Scattered Field It is expressed as:

[0058] (15)

[0059] Introducing the field source Green's function, the wave equation satisfied is:

[0060] (16)

[0061] in, is the coordinate of the field source point, The source coordinates are The Green function of is the pulse function; from equation (14) and equation (16), we get:

[0062] (17)

[0063] Combining equation (15) with equation (16), we get:

[0064] (18)

[0065] Integrating both sides of equation (18) we get:

[0066] (19)

[0067] in, For the on-site source Ignore the scattered field According to formula (19), we can get the velocity disturbance Born approximate expression:

[0068] (20)

[0069] According to formula (17), under the conditions that the global background velocity is constant, self-excitation and self-reception, and the receiving point is located on the surface, that is, , Under the condition of , formula (20) can be rewritten as:

[0070] (twenty one),

[0071] Global background speed is a constant The expression of frequency domain Green's function under the condition is:

[0072] (twenty two),

[0073] in, is an imaginary unit;

[0074] Substituting formula (22) into formula (21), we get:

[0075] (twenty three),

[0076] Perform Fourier transform on equation (23) to obtain the velocity disturbance expression:

[0077] (twenty four),

[0078] According to the velocity disturbance, the position and shape of the underground aquifer are located, and the frequency domain inversion imaging of groundwater is achieved.

[0079] The beneficial effect of the present invention is that the present invention proposes a method for frequency domain inversion imaging of ground nuclear magnetic resonance, which solves the problems of weak ground nuclear magnetic resonance signals, difficulty in extracting effective information, long inversion iteration time, and insufficient imaging accuracy. Compared with the prior art, the present invention converts the nuclear magnetic resonance signal into a pseudo-seismic wave field, then converts the pseudo-seismic wave field into the frequency domain for synthetic aperture, and finally decomposes the synthesized pseudo-seismic wave field, based on the Born approximation, and uses the velocity disturbance amount to realize the inversion imaging of ground nuclear magnetic resonance. From the perspective of the frequency domain, the present invention can obtain the strength and distribution of each frequency component of the signal, while enhancing the ground nuclear magnetic resonance signal, it can display information in multiple dimensions, and also realizes rapid inversion imaging, which makes up for the weak ground nuclear magnetic resonance signal, the effective information is not intuitive enough, the resolution is low at the boundary of the aquifer, the inversion iteration process is complicated, and the time is long. The problem is of great significance to the application and promotion of ground nuclear magnetic resonance technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Figure 1 A flow chart of a method provided by an embodiment of the present invention;

[0081] Figure 2 A schematic diagram of a synthetic aperture of a ground nuclear magnetic resonance pseudo-seismic wave field provided by an embodiment of the present invention;

[0082] 1 Power supply, 2 Host computer, 3 Tuning capacitor box, 4 Transmitter box, 5 Receiving box, 6 First receiving coil, 7 Second receiving coil, 8 Third receiving coil, 9 Fourth receiving coil, 10 Fifth receiving coil, 11 Transmitter coil. DETAILED DESCRIPTION

[0083] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0084] See also Figure 1 Combination Figure 2 As shown, a ground nuclear magnetic resonance frequency domain inversion imaging method comprises the following steps:

[0085] S1. Based on the classical transformation formula of diffusion field and wave field, the transformation equation of NMR response field and time-domain pseudo-seismic wave field is obtained. The time-domain pseudo-seismic wave field has high resolution at the edge of the aquifer. Based on the collected NMR signal, the time-domain pseudo-seismic wave field value of each coil is obtained.

[0086] S2, based on S transformation, using time-frequency conversion, convert the obtained time domain pseudo-seismic wave field value into the frequency domain;

[0087] S3, according to the correlation of echo signals of the same geological body on adjacent receiving coils, synthesizing the aperture of the pseudo-seismic wave field value in the frequency domain in the frequency domain to obtain a synthesized pseudo-seismic wave field;

[0088] S4. Based on the Born approximate imaging method, the synthesized pseudo-seismic wave field is inverted to achieve frequency domain inversion imaging of groundwater.

[0089] Among them, S1 specifically includes:

[0090] The power supply 1 is used to power the entire system, and the transmitting frequency is configured through the host computer 2 to detect the underground of the water body, the number of pulse intervals, the number of superpositions and other parameters. The transmitting box 4 controls the matching capacitor box 3 to match the resonant frequency to the Larmor frequency, and an alternating current is passed into the transmitting coil 11 to generate an alternating magnetic field, so that the spin direction of the hydrogen nuclei in the groundwater is deflected and deviates from the direction of the geomagnetic field. After the transmission is completed, the hydrogen nuclei will return to a position consistent with the direction of the geomagnetic field under the action of the geomagnetic field. In this process, an alternating magnetic field will be radiated to the ground. The voltage signal induced by the receiving coil controlled by the receiving box 5 is the nuclear magnetic resonance signal. In this embodiment, the receiving coil includes: a first receiving coil 6, a second receiving coil 7, a third receiving coil 8, a fourth receiving coil 9, and a fifth receiving coil 10. The relationship between the nuclear magnetic resonance signal and the nuclear magnetic resonance response field is:

[0091] (1)

[0092] in, For time, is the NMR signal, is the NMR response field;

[0093] The NMR response field belongs to the diffusion field. According to the classical transformation formula between the diffusion field and the wave field, the NMR response field is converted into the pseudo-seismic wave field in the time domain:

[0094] (2)

[0095] in, is the pseudo-seismic wave field time, It is the pseudo seismic wave field in time domain;

[0096] From equations (1) and (2), we can get the NMR signal: Pseudo-seismic wave field in time domain The transformation relationship is:

[0097] (3)

[0098] Rewrite equation (3) into a linear equation system to express the pseudo-seismic wave field value in the time domain: NMR signal value The conversion relationship between them is:

[0099] (4)

[0100] in, is the NMR signal value, is the integral kernel function value, is the pseudo-seismic wave field value in the time domain, and Both The matrix of yes The square array, The number of sampling points is 30; if you want to use the NMR signal value Obtaining pseudo-seismic wave field values ​​in the time domain , then we need to transform equation (4):

[0101] (5)

[0102] in, for The pseudo-inverse matrix of Perform SVD decomposition, ,get:

[0103] (6)

[0104] in, , and Both , and and is an orthogonal matrix.

[0105] S2 specifically includes: In S2, based on S transformation, using time-frequency conversion, the formula used to convert the obtained time domain pseudo-seismic wave field value into the frequency domain is:

[0106] (7)

[0107] in, represents the time shift parameter, Centered at Gauss window function, is the pseudo-seismic wave field value in the frequency domain;

[0108] Gauss window function Defined as:

[0109] (8)

[0110] in, is the frequency.

[0111] See also Figure 2 , the S3 specifically includes:

[0112] The total number of receiving coils on a survey line is 5, the size of the receiving coil is 4m×4m, and the distance between two adjacent receiving coils is 10m. First, taking the first receiving coil 6 as the center, the echo signals of the same geological body on adjacent coils have good correlation, and the synthetic aperture is performed on the pseudo-seismic wave field values ​​of each receiving coil on the entire survey line except the center coil:

[0113] (9)

[0114] in, For the The linear distance between the first receiving coil and the first receiving coil 6, is the pseudo-seismic wave field value in the frequency domain of the other receiving coils except the first receiving coil 6 within the synthetic aperture range, is the pseudo-seismic wave field value synthesized with the first receiving coil 6 as the center, and the phase coefficient is 0.4225, the amplitude coefficient is 0.2386, is the magnetic permeability, is the medium conductivity;

[0115] S4 specifically includes:

[0116] The synthesized pseudo-seismic wave field is decomposed into the sum of the scattered field and the background field. The scattered field is represented by the velocity perturbation quantity, which is linearly solved by the integral equation and the inversion imaging of the ground nuclear magnetic resonance is realized by using the velocity perturbation quantity.

[0117] According to the frequency domain wave equation satisfied by the synthesized pseudo-seismic wave field:

[0118] (10)

[0119] in, is the distance between any position in the underground medium and the field source point, is the coordinate of the receiving point, is the velocity of the pseudo-seismic wave field, Center coil The synthesized pseudo-seismic wave field is The meaning is the same;

[0120] Pseudo-seismic wave field velocity Expressed as a slowly varying global background velocity and a rapidly varying velocity perturbation:

[0121] (11)

[0122] in, is the global background velocity, is the velocity disturbance; Substituting equation (11) into equation (10), we get:

[0123] (12)

[0124] The term of velocity disturbance is moved to the right side of the equation, making the pseudo-seismic wave field Decomposed into the sum of background field and scattered field:

[0125] (13)

[0126] in, For the background field, is the scattered field; from formula (12), we can know that the background field It can be expressed as:

[0127] (14)

[0128] Scattered Field It can be expressed as:

[0129] (15)

[0130] Introducing the field source Green's function, the wave equation it satisfies is:

[0131] (16)

[0132] in, is the coordinate of the field source point, The source coordinates are The Green function of is the pulse function; comparing equation (14) with equation (16), we can know that:

[0133] (17)

[0134] After combining equation (15) and equation (16), we can get:

[0135] (18)

[0136] Integrating both sides of equation (18) we get:

[0137] (19)

[0138] in, For the on-site source The scattered field at ; From equation (15), we know that if The value of is very small, then the scattered field is correspondingly small, so for smaller , the scattered field can be ignored , rewrite equation (19) to obtain the velocity disturbance Born approximate expression:

[0139] (20)

[0140] And, according to formula (17), under the conditions that the global background velocity is constant, self-excitation and self-reception, and the receiving point is located on the surface, that is, , Under the condition of , formula (20) can be rewritten as:

[0141] (twenty one),

[0142] Global background speed is a constant The expression of frequency domain Green's function under the condition is:

[0143] (twenty two),

[0144] Substituting formula (22) into formula (21), we get:

[0145] (twenty three),

[0146] Perform Fourier transform on equation (23) to obtain the velocity disturbance expression:

[0147] (twenty four),

[0148] According to the velocity disturbance, the position and shape of the underground aquifer at a measuring point can be accurately located. Then, S3 and S4 are repeated with the second receiving coil 7, the third receiving coil 8, the fourth receiving coil 9, and the fifth receiving coil 10 as the center, respectively, to achieve ground nuclear magnetic resonance inversion imaging.

[0149] 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 protection scope of the present invention.

Claims

1. A ground nuclear magnetic resonance frequency domain inversion imaging method, characterized in that: The method includes: S1. Based on the classical transformation formula of diffusion field and wave field, the transformation equation of nuclear magnetic resonance response field and time domain pseudo-seismic wave field is obtained. The time domain pseudo-seismic wave field has high resolution at the edge of the aquifer. Based on the collected nuclear magnetic resonance signal, the time domain pseudo-seismic wave field value of each receiving coil is obtained. S2, based on S transformation, using time-frequency conversion, convert the obtained time domain pseudo-seismic wave field value into the frequency domain; S3, according to the correlation of echo signals of the same geological body on adjacent receiving coils, synthesizing the aperture of the pseudo-seismic wave field value in the frequency domain in the frequency domain to obtain a synthesized pseudo-seismic wave field; S4. Based on the Born approximate imaging method, the synthesized pseudo-seismic wave field is inverted to achieve frequency domain inversion imaging of groundwater.

2. A ground nuclear magnetic resonance frequency domain inversion imaging method according to claim 1, characterized in that: S1 specifically includes: Collecting NMR signals: Alternating current is passed through the transmitting coil to generate an alternating magnetic field, which deflects the spin direction of the hydrogen nuclei in the groundwater and deviates from the direction of the geomagnetic field. After the transmission is completed, the voltage signal induced in the receiving coil is the NMR signal. The relationship between the NMR signal and the NMR response field is: (1)， in, For time, is the NMR signal, is the NMR response field; The NMR response field belongs to the diffusion field. According to the classical transformation formula between the diffusion field and the wave field, the NMR response field is converted into the pseudo-seismic wave field in the time domain: (2)， in, is the pseudo-seismic wave field time, It is the pseudo seismic wave field in time domain; From equations (1) and (2), we can obtain the transformation equation between the NMR signal and the pseudo-seismic wave field in the time domain: (3)， Rewrite equation (3) into a linear equation system to express the conversion relationship between the pseudo-seismic wave field value in the time domain and the nuclear magnetic resonance signal: (4)， in, is the NMR signal, is the integral kernel function value, is the pseudo-seismic wave field value in the time domain, and Both The matrix of yes The square array, is the number of sampling points; if you want to use the nuclear magnetic resonance signal Obtaining pseudo-seismic wave field values ​​in the time domain , then transform equation (4): (5)， in, for The pseudo-inverse matrix of Perform SVD decomposition, ,get: (6)， in, , and Both , and and is an orthogonal matrix.

3. A ground nuclear magnetic resonance frequency domain inversion imaging method according to claim 2, characterized in that: In S2, based on S transform, the time-frequency conversion is used to convert the pseudo-seismic wave field value in the time domain to the frequency domain using the formula: (7)， in, represents the time shift parameter, Centered at Gauss window function, is the pseudo-seismic wave field value in the frequency domain; Gauss window function Defined as: (8)， in, is the frequency.

4. A ground nuclear magnetic resonance frequency domain inversion imaging method according to claim 3, characterized in that: S3 specifically includes: Assume that the total number of receiving coils on a measuring line is , is an odd number, The receiving coil is the center. , using the correlation of the echo signals of the same geological body on adjacent receiving coils, the pseudo-seismic wave field values ​​of each receiving coil on the entire survey line except the central coil are synthetically apertured: (9)， in, For the Receive coil and center coil The linear distance The synthetic aperture range is excluding the center coil The pseudo-seismic wave field values ​​in the frequency domain of other receiving coils other than Center coil The synthesized pseudo-seismic wave field, is the phase coefficient, is the amplitude coefficient, is the magnetic permeability, is the conductivity of the medium.

5. A ground nuclear magnetic resonance frequency domain inversion imaging method according to claim 4, characterized in that: It is characterized in that S4 includes: The synthesized pseudo-seismic wave field is decomposed into the sum of the scattered field and the background field. The scattered field is represented by the velocity perturbation quantity, which is linearly solved by the integral equation and the inversion imaging of the ground nuclear magnetic resonance is realized by using the velocity perturbation quantity.

6. A ground nuclear magnetic resonance frequency domain inversion imaging method according to claim 5, characterized in that: S4 specifically includes: The frequency domain wave equation satisfied by the synthesized pseudo-seismic wave field is: (10)， in, is the distance between any position in the underground medium and the field source point, is the coordinate of the receiving point, is the velocity of the pseudo-seismic wave field, Center coil The synthesized pseudo-seismic wave field is The meaning is the same; Pseudo-seismic wave field velocity Expressed as a slowly varying global background velocity and a rapidly varying velocity perturbation: (11)， in, is the global background velocity, is the velocity disturbance; Substituting formula (11) into formula (10), we get: (12)， The synthesized pseudo-seismic wave field Decomposed into the sum of background field and scattered field: (13)， in, For the background field, is the scattered field; according to formula (12), the background field It is expressed as: (14)， Scattered Field It is expressed as: (15)， Introducing the field source Green's function, the wave equation satisfied is: (16)， in, is the coordinate of the field source point, The source coordinates are The Green function of is the pulse function; from equation (14) and equation (16), we get: (17)， Combining equation (15) with equation (16), we get: (18)， Integrating both sides of equation (18) we get: (19)， in, For the on-site source The scattered field at According to formula (19), we can get the velocity disturbance Born approximate expression: (20)， According to formula (17), under the conditions that the global background velocity is constant, self-excitation and self-reception, and the receiving point is located on the surface, that is, , Under the condition of , formula (20) can be rewritten as: (21)， Global background speed is a constant The expression of frequency domain Green's function under the condition is: (22)， in, is an imaginary unit; Substituting formula (22) into formula (21), we get: (23)， Perform Fourier transform on equation (23) to obtain the velocity disturbance expression: (24)， According to the velocity disturbance, the position and shape of the underground aquifer are located, and the frequency domain inversion imaging of groundwater is achieved.

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