A highly efficient and high signal-to-noise ratio ground magnetic resonance detection method

Abstract

The present invention is applicable to the field of geophysical exploration technology, and provides a high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method. The present invention adopts a DC transmitting coil and an AC transmitting coil arranged concentrically to form a transmitting coil group, which replaces the traditional single AC transmitting coil. Through the change of magnetization intensity caused by alternating excitation, the DC current emission time, the AC current emission time and the emission interval time are adjusted, and the magnetization intensity is controlled to gradually accumulate and stabilize within a certain range, thereby improving the macroscopic magnetization vector of hydrogen atoms; the single group pulse emission time of the alternating excitation emission mode proposed by the present invention is within 1s, which can effectively improve the detection efficiency. The present invention achieves the effect of both improving the signal-to-noise ratio of the ground magnetic resonance detection signal and improving the detection efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of geophysical exploration, and in particular relates to a high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method. Background Art

[0002] The ground magnetic resonance groundwater detection method is a non-invasive geophysical method for direct detection of groundwater. The traditional ground magnetic resonance detection method uses the geomagnetic field as the background detection magnetic field, and uses an AC transmitting coil to transmit radio frequency pulses to excite the hydrogen atoms in the groundwater to produce magnetic resonance, thereby detecting groundwater. However, this method has two problems: first, the signal-to-noise ratio is low. In a strong interference environment, the effective signal is easily submerged by noise; second, the long pulse emission interval leads to low detection efficiency. A single measuring point generally needs to transmit more than 10 groups of detection pulses. Therefore, a single measuring point usually takes several hours to detect, which makes it impossible to carry out a large-scale regional survey.

[0003] In response to the above-mentioned problem of low signal-to-noise ratio of the detection signal, relevant practitioners have developed a prepolarization method. This method increases the detection background field strength by applying a DC prepolarization field before the detection pulse, which can greatly improve the detection signal amplitude compared to the traditional method. However, this method adds a prepolarization pulse of more than 4 to 7 seconds on the basis of the traditional method, which reduces the detection efficiency. In response to the above-mentioned problem of low detection efficiency, relevant practitioners have developed a steady-state pulse detection method. This method can greatly improve the detection efficiency by shortening the pulse interval and reducing the emission time of a single group of pulses. However, this method only improves the detection efficiency and does not improve the signal-to-noise ratio. Therefore, in a complex noise environment, the effective signal is still difficult to extract.

[0004] At present, there is no method at home and abroad that can improve the signal-to-noise ratio and detection efficiency of ground magnetic resonance signals by adjusting the DC and AC transmission pulse time and transmission interval. Therefore, an efficient and high signal-to-noise ratio ground magnetic resonance detection method is proposed. Summary of the invention

[0005] The object of the present invention is to provide a high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method, aiming to solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method, the method is based on a transmitting coil group consisting of a DC transmitting coil and an AC transmitting coil, the DC transmitting coil and the AC transmitting coil are arranged in a concentric pattern; the method comprises the following steps:

[0008] Step 1, calculating the DC prepolarization magnetic field generated by the DC transmitting coil in the underground space and the radio frequency excitation magnetic field generated by the AC transmitting coil in the underground space;

[0009] Step 2, calculating the change of underground magnetization intensity under the alternating action of DC prepolarization magnetic field and radio frequency excitation magnetic field;

[0010] The step 1 comprises the following steps:

[0011] Step 11, calculate the magnetic field generated when the coil is fed with current;

[0012] Step 12, calculate the overall magnetic field generated by the coil; the DC transmitting coil generates a constant magnetic field to enhance the magnetization vector of hydrogen atoms in the underground space, and the AC transmitting coil generates a radio frequency magnetic field to reverse the spin direction of hydrogen atoms in the underground space;

[0013] The step 2 comprises the following steps:

[0014] Step 21, calculating the enhancement of the magnetization intensity of hydrogen atoms by the DC prepolarization magnetic field;

[0015] Step 22, calculating the knockdown of hydrogen atoms by the radio frequency excitation magnetic field;

[0016] Step 23, calculating the change of underground magnetization intensity: according to the different DC emission current time and AC emission current time, respectively calculate the enhancement degree of the DC prepolarization magnetic field on the underground magnetization intensity and the knockdown of hydrogen atoms by the RF excitation magnetic field, and obtain the underground magnetization intensity change curve; by adjusting the DC current emission time, the AC current emission time and the emission interval time, control the magnetization intensity to gradually accumulate and stabilize within a certain range.

[0017] Furthermore, the specific steps of step 11 and step 12 are as follows:

[0018] Step 11: Calculate the magnetic field distribution according to the Biot-Savart law, with a side length of 2 a The magnetic field formed by the side current in the DC transmitting coil at any point r in space is expressed as:

[0019] ;

[0020] in, B Pr is the prepolarization magnetic field intensity at point r in space, μ 0 is the magnetic permeability, I P is the excitation current of the DC prepolarization magnetic field, θ r and is the angle between point r and the two end points of the wire;

[0021] The side length is 2 c The magnetic field formed by the side current in the AC transmitting coil at any point r in space is expressed as:

[0022] ;

[0023] in, B Ar is the excitation field intensity at point r in space, I A is the excitation current of the RF excitation magnetic field, α and is the angle between point r and the two end points of the wire;

[0024] Step 12: Based on the result calculated in step 11, the DC prepolarization magnetic field excited by the DC transmitting coil at a point r underground is expressed as:

[0025] ;

[0026] ;

[0027] ;

[0028] in, B Prx , B Pry and B Prz are the DC and polarized magnetic field components in the x, y, and z directions at point r underground, B Pr1 ~ B Pr4 are the magnetic fields generated by the four sides of the DC transmitting coil, e xr , e yr and e zr are the unit flux in the x, y, and z directions respectively;

[0029] The RF excitation magnetic field excited by the AC transmitting coil at a point r underground is expressed as:

[0030] ;

[0031] ;

[0032] ;

[0033] in, B Arx , BAry and B Arz is the AC excitation magnetic field component in the x, y, and z directions at point r underground, B Ar1 ~ B Ar4 They are the magnetic fields generated by the four sides of the AC transmitting coil.

[0034] Furthermore, the specific steps of step 21 and step 22 are as follows:

[0035] Step 21: Obtain the DC prepolarization magnetic field B of the underground space from step 12 P , find B P After distribution, the fully polarized underground M Bp distributed:

[0036] ;

[0037] in, N is the number of hydrogen atoms per unit volume, G m is the Lande factor, h q is Planck's constant, k is the Boltzmann constant, T is the absolute temperature;

[0038] According to the obtained net magnetization intensity enhanced by the DC transmitting coil excitation, the total magnetization intensity M of underground hydrogen atoms is calculated:

[0039] ;

[0040] in, M 0 is the magnetization intensity of the Earth's magnetic field;

[0041] Step 22: Obtain the radio frequency excitation magnetic field B of the underground space from step 12 A , find B A After distribution, calculate the vertical component of the effective RF excitation magnetic field:

[0042] ;

[0043] ;

[0044] ;

[0045] in, α L is the major axis component of the elliptically polarized field of the RF excitation field, β is the short-axis component of the elliptical polarization field of the RF excitation field, is the vertical component of the effective RF excitation magnetic field, i is a unit vector, is the direction vector of the geomagnetic field, for The adjoint matrix of I A is the excitation current of the RF excitation magnetic field;

[0046] Then, according to the relationship between the flipping angle of hydrogen atoms and the vertical component of the effective RF excitation magnetic field, the excitation current and the gyromagnetic ratio of hydrogen atoms, the flipping angle of hydrogen atoms is calculated:

[0047] ;

[0048] Where γ is the gyromagnetic ratio of hydrogen atoms, is the rate of change of the flip angle of the hydrogen atom.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] The present invention adopts a DC transmitting coil and an AC transmitting coil arranged concentrically to form a transmitting coil group, replacing the traditional single AC transmitting coil, and adjusts the DC current transmitting time, the AC current transmitting time and the transmitting interval time through the change of magnetization intensity caused by alternating excitation, and controls the magnetization intensity to gradually accumulate and stabilize within a certain range, thereby improving the macroscopic magnetization vector of hydrogen atoms; the single group pulse transmitting time of the alternating excitation transmitting mode proposed by the present invention is within 1s, which can effectively improve the detection efficiency. The present invention achieves the effect of both improving the signal-to-noise ratio of the ground magnetic resonance detection signal and improving the detection efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic diagram of detection of AC and DC coil groups in the present invention.

[0052] Figure 2 It is a schematic diagram of the calculation of the three-dimensional magnetic field of AC and DC coils in the present invention.

[0053] Figure 3 It is a schematic diagram of the decomposition of the coil side current in the present invention.

[0054] Figure 4 Schematic diagram of the alternating pulse sequence in the present invention.

[0055] Figure 5 Schematic diagram of the change in macroscopic magnetization intensity of hydrogen atoms in the present invention.

[0056] Figure 6 Detection curve diagram of the traditional detection method and the high-efficiency and high signal-to-noise ratio detection method proposed by the present invention. DETAILED DESCRIPTION

[0057] 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 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 used to limit the present invention.

[0058] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0059] An embodiment of the present invention provides a high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method, the method is based on a transmitting coil group consisting of a DC transmitting coil and an AC transmitting coil, such as Figure 1 As shown, two coils are laid on the ground in a center-overlapping manner, the target water body is placed in the coil group arrangement area, and the DC transmitting coil and the AC transmitting coil are arranged in a concentric pattern. When performing detection, the method proposed by the present invention is used to control the duration of the DC current and the AC current in the transmitting coil group and the interval time of the current emission, thereby controlling the macroscopic magnetization intensity of the target water body to increase and dynamically stabilize within a certain range, and finally achieving the purpose of taking into account the detection signal amplitude and thus improving the signal-to-noise ratio. The method comprises the following steps:

[0060] Step 1, calculating the DC prepolarization magnetic field generated by the DC transmitting coil in the underground space and the radio frequency excitation magnetic field generated by the AC transmitting coil in the underground space;

[0061] Step 2, calculating the change of underground magnetization intensity under the alternating action of DC prepolarization magnetic field and radio frequency excitation magnetic field;

[0062] The step 1 comprises the following steps:

[0063] Step 11: Calculate the magnetic field generated by the coil after the current is passed through it: Calculate the magnetic field distribution according to the Biot-Savart law, such as Figure 2 and Figure 3 As shown, Figure 2 It is a schematic diagram of the three-dimensional magnetic field calculation of the coil group. When current is passed through the DC coil and the AC coil, the magnetic field at any point underground can be calculated. r The magnetic induction intensity B P and B A . Figure 3 This is a schematic diagram of the decomposition of the current on the coil side. For the DC line transmitting coil and the AC transmitting coil, the field strengths generated by the four sides are superimposed in space, and the field strength of the coil at any point underground can be calculated. r The magnetic induction intensity is then Figure 2 Calculate the magnetic induction intensity at any point underground B P and B A .

[0064] DC transmitting coil (side length is 2 a ) The magnetic field formed by the edge current at any point r in space is expressed as:

[0065] ;

[0066] in, B Pr is the prepolarization magnetic field intensity at point r in space, μ 0 is the magnetic permeability, I P is the excitation current of the DC prepolarization magnetic field, θ r and is the angle between point r and the two end points of the wire;

[0067] AC transmitting coil (side length is 2 c ) The magnetic field formed by the edge current at any point r in space is expressed as:

[0068] ;

[0069] in, B Ar is the excitation field intensity at point r in space, I A is the excitation current of the RF excitation magnetic field, α and is the angle between point r and the two end points of the wire;

[0070] Step 12, calculate the overall magnetic field generated by the coil: Based on the result calculated in step 11, the DC prepolarization magnetic field excited by the DC transmitting coil at a point r underground is expressed as:

[0071] ;

[0072] ;

[0073] ;

[0074] in, B Prx , B Pry and B Prz are the DC and polarized magnetic field components in the x, y, and z directions at point r underground, B Pr1 ~ B Pr4 are the magnetic fields generated by the four sides of the DC transmitting coil, e xr ,e yr and e zr are the unit flux in the x, y, and z directions respectively;

[0075] The RF excitation magnetic field excited by the AC transmitting coil at a point r underground is expressed as:

[0076] ;

[0077] ;

[0078] ;

[0079] in, B Arx , B Ary and B Arz is the AC excitation magnetic field component in the x, y, and z directions at point r underground, B Ar1 ~ B Ar4 They are the magnetic fields generated by the four sides of the AC transmitting coil;

[0080] The DC transmitting coil enhances the magnetization vector of hydrogen atoms in the underground space by generating a constant magnetic field, while the AC transmitting coil reverses the spin direction of hydrogen atoms in the underground space by generating a radio frequency magnetic field.

[0081] The step 2 comprises the following steps:

[0082] Step 21, calculate the enhancement of the magnetization intensity of hydrogen atoms by the DC prepolarization magnetic field: the DC prepolarization magnetic field B in the underground space is obtained by step 12. P , find B P After distribution, the fully polarized underground M Bp distributed:

[0083] ;

[0084] in, N is the number of hydrogen atoms per unit volume, G m is the Lande factor, h q is Planck's constant, k is the Boltzmann constant, T is the absolute temperature;

[0085] According to the obtained net magnetization intensity enhanced by the DC transmitting coil excitation, the total magnetization intensity M of underground hydrogen atoms is calculated:

[0086] ;

[0087] in, M 0 is the magnetization intensity of the Earth's magnetic field;

[0088] Step 22: Calculate the knockdown of hydrogen atoms by the radio frequency excitation magnetic field: Obtain the radio frequency excitation magnetic field B in the underground space from step 12. A , find B A After distribution, calculate the vertical component of the effective RF excitation magnetic field:

[0089] ;

[0090] ;

[0091] ;

[0092] in, α L is the major axis component of the elliptically polarized field of the RF excitation field, β is the short-axis component of the elliptical polarization field of the RF excitation field, is the vertical component of the effective RF excitation magnetic field, i is a unit vector, is the direction vector of the geomagnetic field, for The adjoint matrix of I A is the excitation current of the RF excitation magnetic field;

[0093] Then, according to the relationship between the flipping angle of hydrogen atoms and the vertical component of the effective RF excitation magnetic field, the excitation current and the gyromagnetic ratio of hydrogen atoms, the flipping angle of hydrogen atoms is calculated:

[0094] ;

[0095] Where γ is the gyromagnetic ratio of hydrogen atoms, is the rate of change of the flip angle of the hydrogen atom;

[0096] Step 23: Calculate the change of underground magnetization intensity:

[0097] Figure 4 This is a schematic diagram of the alternating pulses in the present invention. In the entire pulse sequence, DC pulses, AC pulses and pulse intervals are performed alternately, wherein the duration of the DC pulse is at the level of several hundred ms, the duration of the AC pulse is at the level of several tens of ms, the pulse interval is at the level of several tens of ms, and the duration of a single group of pulses is within 1 second. Figure 4As shown, according to the different DC emission current time and AC emission current time, the enhancement degree of the DC prepolarization magnetic field on the underground magnetization intensity and the knockdown of the hydrogen atoms by the RF excitation magnetic field are calculated, and the underground magnetization intensity change curve is obtained ( Figure 5 ). Figure 5 The results show that the magnetization intensity of underground hydrogen atoms can be enhanced by using the alternating detection pulses in the present invention. By controlling the DC current emission time, AC current emission time and pulse emission interval time, the macroscopic magnetization intensity can be made to reach the maximum within 2 seconds and dynamically stabilized within a certain range.

[0098] Example 1: In Changchun Shaoguo Town, the high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method proposed by the present invention was used for field practical application. The specific implementation steps are as follows:

[0099] In the test area, press Figure 1 As shown in the figure, a DC transmitting coil and an AC transmitting coil are arranged concentrically, and the corresponding receiving coils are arranged at the same time, and the instrument is debugged. The hydrogeological conditions of the measuring point are known. According to the existing geological information, the magnetization intensity change is calculated according to the set DC pulse transmission time, AC pulse transmission time and pulse interval time according to the above steps 1 and 2, and the signal reception is estimated.

[0100] After calculating the change curve of underground hydrogen atom magnetization intensity, detect the groundwater under the measuring point. Set the emission pulse moment range from 0.10As to 10A.s, a total of 32 pulse moments, 32 superposition times, and a collection frequency of 31.25kHz. After the settings are completed, start the ground magnetic resonance detector until the test at the measuring point is completed and save the detected data.

[0101] After the test, the data collected by the receiver is processed to obtain the initial amplitude of each pulse moment, and compared with the signal obtained by the traditional method to draw the detection curve ( Figure 6 ), comparing the initial signal amplitudes shown by the two curves, it can be seen that the signal amplitude detected by the method proposed in the present invention is much higher than that of the traditional method, and the pulse emission time of the present invention is much shorter than that of the traditional detection method, which proves that the method proposed in the present invention can effectively improve the amplitude of groundwater detection signals and thus improve the signal-to-noise ratio of detection signals, and the detection efficiency is also higher than that of traditional methods.

[0102] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. An efficient and high signal-to-noise ratio ground magnetic resonance detection method, characterized in that: The method is based on a transmitting coil group consisting of a DC transmitting coil and an AC transmitting coil, wherein the DC transmitting coil and the AC transmitting coil are arranged in a concentric pattern; the method comprises the following steps: Step 1, calculating the DC prepolarization magnetic field generated by the DC transmitting coil in the underground space and the radio frequency excitation magnetic field generated by the AC transmitting coil in the underground space; Step 2, calculating the change of underground magnetization intensity under the alternating action of DC prepolarization magnetic field and radio frequency excitation magnetic field; The step 1 comprises the following steps: Step 11, calculate the magnetic field generated when the coil is fed with current; Step 12, calculate the overall magnetic field generated by the coil; the DC transmitting coil generates a constant magnetic field to enhance the magnetization vector of hydrogen atoms in the underground space, and the AC transmitting coil generates a radio frequency magnetic field to reverse the spin direction of hydrogen atoms in the underground space; The step 2 comprises the following steps: Step 21, calculating the enhancement of the magnetization intensity of hydrogen atoms by the DC prepolarization magnetic field; Step 22, calculating the knockdown of hydrogen atoms by the radio frequency excitation magnetic field; Step 23, calculating the change of underground magnetization intensity: according to the different DC emission current time and AC emission current time, respectively calculate the enhancement degree of the DC prepolarization magnetic field on the underground magnetization intensity and the knockdown of hydrogen atoms by the RF excitation magnetic field, and obtain the underground magnetization intensity change curve; by adjusting the DC current emission time, the AC current emission time and the emission interval time, control the magnetization intensity to gradually accumulate and stabilize within a certain range.

2. The high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method according to claim 1 is characterized in that: The specific steps of step 11 and step 12 are as follows: Step 11: Calculate the magnetic field distribution according to the Biot-Savart law, with a side length of 2 a The magnetic field formed by the side current in the DC transmitting coil at any point r in space is expressed as: ； in, B Pr is the prepolarization magnetic field intensity at point r in space, μ 0 is the magnetic permeability, I P is the excitation current of the DC prepolarization magnetic field, θ r and is the angle between point r and the two end points of the wire; The side length is 2 c The magnetic field formed by the side current in the AC transmitting coil at any point r in space is expressed as: ； in, B Ar is the excitation field intensity at point r in space, I A is the excitation current of the RF excitation magnetic field, α and is the angle between point r and the two end points of the wire; Step 12: Based on the result calculated in step 11, the DC prepolarization magnetic field excited by the DC transmitting coil at a point r underground is expressed as: ； ； ； in, B Prx , B Pry and B Prz are the DC and polarized magnetic field components in the x, y, and z directions at point r underground, B Pr1 ~ B Pr4 are the magnetic fields generated by the four sides of the DC transmitting coil, e xr , e yr and e zr are the unit flux in the x, y, and z directions respectively; The RF excitation magnetic field excited by the AC transmitting coil at a point r underground is expressed as: ； ； ； in, B Arx , B Ary and B Arz is the AC excitation magnetic field component in the x, y, and z directions at point r underground, B Ar1 ~ B Ar4 They are the magnetic fields generated by the four sides of the AC transmitting coil.

3. The high-efficiency and high-signal-to-noise ratio ground magnetic resonance detection method according to claim 2 is characterized in that: The specific steps of step 21 and step 22 are as follows: Step 21: Obtain the DC prepolarization magnetic field B of the underground space from step 12 P , find B P After distribution, the fully polarized underground M Bp distributed: ； in, N is the number of hydrogen atoms per unit volume, G m is the Lande factor, h q is Planck's constant, k is the Boltzmann constant, T is the absolute temperature; According to the obtained net magnetization intensity enhanced by the DC transmitting coil excitation, the total magnetization intensity M of underground hydrogen atoms is calculated: ； in, M 0 is the magnetization intensity of the Earth's magnetic field; Step 22: Obtain the radio frequency excitation magnetic field B of the underground space from step 12 A , find B A After distribution, calculate the vertical component of the effective RF excitation magnetic field: ； ； ； in, α L is the major axis component of the elliptically polarized field of the RF excitation field, β is the short-axis component of the elliptical polarization field of the RF excitation field, is the vertical component of the effective RF excitation magnetic field, i is a unit vector, is the direction vector of the geomagnetic field, for The adjoint matrix of I A is the excitation current of the RF excitation magnetic field; Then, according to the relationship between the flipping angle of hydrogen atoms and the vertical component of the effective RF excitation magnetic field, the excitation current and the gyromagnetic ratio of hydrogen atoms, the flipping angle of hydrogen atoms is calculated: ； Where γ is the gyromagnetic ratio of hydrogen atoms, is the rate of change of the flip angle of the hydrogen atom.

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