CN105785457B - The measuring method of ground nuclear magnetic resonance T2 based on bipolar pulse - Google Patents

Abstract

The present invention relates to the measurement field of nuclear magnetic resonance transverse relaxation time, in particular to the measurement method of ground nuclear magnetic resonance transverse relaxation time based on bipolar pulses, adopting bipolar spin echo to measure transverse relaxation time _T2 , by Send No. 1 positive polarity 90° pulse and No. 1 positive polarity 180° pulse to collect the first measured signal and the second measured signal, and send No. 2 negative polarity 90° pulse and No. 2 positive polarity 180° pulse again to collect the third measured signal and For the fourth actual measurement signal, the second actual measurement signal and the fourth actual measurement signal are superimposed to obtain a spin echo signal without interference from free induction fading signals. By changing the time interval τ between the 90° pulse and the 180° pulse, and performing multiple repeated measurements, multiple spin echo signals can be obtained. The invention improves the measurement accuracy of the ground nuclear magnetic resonance transverse relaxation time T ₂ , and avoids the problem that when the T ₂ is small, the number of spin echo signals collected by the CPMG sequence is small and the measurement accuracy is limited.

Description

Measuring Method of Transverse Relaxation Time of Terrestrial NMR Based on Bipolar Pulse

technical field

The invention relates to the field of measuring the transverse relaxation time of nuclear magnetic resonance, in particular to a method for measuring the transverse relaxation time of ground nuclear magnetic resonance based on bipolar spin echo.

Background technique

The distribution of transverse relaxation time T ₂ , the characteristic parameter of surface NMR (Magnetic Resonnance Sounding, MRS) signal, includes very rich formation information, directly reflects the size of medium particles and pores, and can be used to determine porosity, irreducible water saturation, permeability Hydrogeological parameters such as flow rate, pore size distribution, and fluid properties and water content. However, the current international commercial ground NMR water detectors generally use a single pulse sequence to obtain the free induction decay FID signal, so they can only obtain the approximate value of the transverse relaxation time T ₂ - the average relaxation time T ₂ ^* , in the external magnetic field In the case of inhomogeneity, the average relaxation time T ₂ ^* of large pores is smaller than the transverse relaxation time T ₂ , so formation information such as porosity, irreducible water saturation, permeability and pore size distribution cannot be accurately calculated, which seriously restricts the ground The development and application of nuclear magnetic resonance technology in groundwater resources exploration and other fields.

At present, in the process of NMR detection, there are mainly two methods for measuring the transverse relaxation time T ₂ : spin echo SE sequence and CPMG sequence. The NMR measurement scheme based on the spin echo sequence is based on the single pulse excitation, adding a 180° pulse to observe the spin echo signal SE signal, so as to measure the transverse relaxation time T ₂ . This method is easy to implement, but it has the following disadvantages: transmitting a 90° pulse generates a FID signal, and transmitting a 180° pulse not only generates a spin echo SE signal, but also generates a second FID signal, which will interfere with the spin echo Wave SE signal, the measured signal is the superposition signal of FID signal and spin echo SE signal, which will cover the amplitude and shape of the real spin echo SE signal. Meanwhile, the spin echo SE sequence only acquires one spin echo SE signal, and the measurement accuracy of the transverse relaxation time T2 is low. The CPMG sequence is based on the spin echo SE sequence, and multiple 180° hard pulses are applied to obtain multiple spin echo signals. However, there are the following disadvantages. The CPMG sequence needs to transmit 180° pulses multiple times, which consumes a lot of time; when T ₂ is small, the CPMG sequence collects fewer spin echo signals, resulting in low T ₂ measurement accuracy. At the same time, there is a problem that the 180° pulse generates the FID signal and interferes with the real spin echo signal.

CN102096112A discloses a nuclear magnetic resonance groundwater detector and field detection method based on array coils. This invention is composed of a computer connected to a control unit, a transmitting coil and a receiving coil through a serial port line or a network port line. The receiving coil is composed of 25 receiving units Connect to form an array coil, use the array coil as the antenna of the receiving unit, and equip each antenna with an independent receiving unit. The advantages of this invention are: it can realize two-dimensional and three-dimensional groundwater imaging, not only can realize high-sensitivity acquisition and long-distance data transmission, but also can be laid on complex terrain and landforms, which improves the accuracy of nuclear magnetic resonance detection on the horizontal plane, and can Efficiently and accurately determine the location of the well to reduce the risk of drilling a dry well. However, the disadvantage of this invention is: the emission pulse of the nuclear magnetic resonance groundwater detector is a conventional hard pulse sequence, and only the average relaxation time T ₂ ^* can be obtained. In the case of an uneven external magnetic field, the error in calculating the hydrogeological parameters is large, Reduced water search efficiency.

CN103852794A discloses a hydrocarbon-polluted shallow groundwater magnetic resonance detection device and detection method. The device consists of a transmission logic and control unit, an MCU, a signal acquisition card, a 24V battery, a DCDC module, a signal conditioning circuit, an energy storage transmission unit, and a voltage The current acquisition unit consists of a transmitting coil and a receiving unit. The advantages of this invention are: non-invasive quantitative and qualitative measurement is realized, the test results can be quickly obtained on site, the strength of the local geomagnetic field can be increased by using permanent magnets, nuclear magnetic resonance measurement can be carried out in places with serious power interference, and the signal-to-noise ratio can be effectively improved. Breaking the shackles of nuclear magnetic resonance detection that cannot be implemented due to severe power interference, the use of spin echo pulses can effectively overcome the shortcomings of inaccurate results caused by inhomogeneous magnetic fields, and can quickly and accurately detect hydrocarbon pollution within 5 meters underground. However, the disadvantage of this invention is: the spin echo pulse emitted by this invention is CPMG pulse, therefore, this invention has the disadvantage of CPMG sequence. At the same time, the device of this invention needs to transmit multiple 180° pulses without charging, and the power supply voltage drops, which will cause the emission pulses to not reach 180°, and the transverse relaxation time T ₂ cannot be accurately measured.

Therefore, it is necessary to invent a method that can measure the transverse relaxation time _T2 quickly and effectively, so as to solve the problem of large error in estimating hydrogeological parameters of _T2 ^* from conventional single-pulse measurement, and _T2 in the case of inhomogeneous external magnetic field effective measurement.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for measuring the transverse relaxation time of terrestrial nuclear magnetic resonance based on bipolar pulses, so as to solve the problem of estimating the average relaxation time T ₂ ^* of conventional single pulse measurement under the condition of inhomogeneous external magnetic field. The problem of large errors in hydrological parameters and the difficulty in measuring the transverse relaxation time T ₂ .

The present invention is achieved like this, a kind of measurement method based on the ground nuclear magnetic resonance transverse relaxation time of bipolar pulse,

Transmit No. 1 positive polarity 90° pulse, collect the first measured signal,

After the time interval τ, the No. 1 positive polarity 180° pulse is emitted, and the second measured signal is collected;

After the time is restored, the No. 2 negative polarity 90° pulse is emitted to collect the third measured signal;

After the same time interval τ, the No. 2 positive polarity 180° pulse is emitted to collect the fourth measured signal;

Superimposing the second measured signal and the fourth measured signal to obtain a spin echo SE signal that eliminates the interference of the free induction FID signal;

superimposing the first measured signal and the third measured signal to obtain the first free induction FID signal;

Repeat the above steps by changing the time interval τ between the 90° pulse and the 180° pulse, and obtain multiple spin echo SE signals during repeated measurements;

The initial amplitude of the first free induction FID signal is connected with the peak value of each spin echo SE signal to form an exponential decay curve, and the transverse relaxation time T2 is obtained by fitting.

Further, superimposing the second actual measurement signal and the fourth actual measurement signal is: subtracting the second actual measurement signal and the fourth actual measurement signal by 2.

Further, the superposition of the first measured signal and the third measured signal specifically includes: subtracting and dividing the first measured signal and the third measured signal by 2.

Further, to transmit No. 1 positive polarity 90° pulse is: in the three-dimensional Cartesian coordinate system, the static magnetic field Bo is along the Z-axis direction of the three-dimensional Cartesian coordinate system, and the transmitting coil is placed on the ground, at the XY direction perpendicular to the static magnetic field Bo direction. The No. 1 positive polarity 90° pulse is emitted underground in the direction of the X axis in the plane.

Further, the No. 1 positive polarity 180° pulse is transmitted as follows: in the three-dimensional rectangular coordinate system, the static magnetic field Bo is along the Z-axis direction of the three-dimensional rectangular coordinate system, the transmitting coil is placed on the ground, and it is emitted underground in the Z-axis direction. No. 1 positive polarity 180° pulse.

Further, the No. 2 negative polarity pulse of 90° is transmitted as follows: in the three-dimensional Cartesian coordinate system, the static magnetic field Bo is along the Z-axis direction of the three-dimensional Cartesian coordinate system, and the transmitting coil is placed on the ground, at the XY direction perpendicular to the direction of the static magnetic field Bo. The No. 2 negative polarity 90° pulse is emitted underground in the direction of the negative X-axis in the plane.

Further, the transmission of No. 2 positive polarity 180° pulse is: in the three-dimensional rectangular coordinate system, the static magnetic field Bo is along the Z-axis direction of the three-dimensional rectangular coordinate system, the transmitting coil is placed on the ground, and the No. 2 pulse is launched in the Z-axis direction. Positive polarity 180° pulse.

Compared with the prior art, _the present invention has the beneficial effect that: since the magnetic field distribution in the actual measurement area usually has inhomogeneity, the measured T ₂ * is less than the transverse relaxation time T ₂ , so the bound water, Formation information such as permeability and pore size distribution have large errors with theoretical results. However, the transverse relaxation time in the spin echo signal will not be affected by the inhomogeneity of the magnetic field, so the measurement of the transverse relaxation time T ₂ can be realized by using a double-pulse spin echo sequence. By transmitting a bipolar 90° pulse, the interference of the FID signal generated by the 180° pulse on the spin echo SE signal can be eliminated, thereby obtaining a real spin echo SE signal. By changing the time interval τ between the 90° pulse and the 180° pulse and performing multiple shots, multiple spin echo signals can be obtained, thereby improving the measurement accuracy of the transverse relaxation time _T2 .

Description of drawings

The accompanying drawings are used to provide a further understanding of the present invention, and constitute a part of the description, and are used together with the embodiments of the present invention to explain the present invention and not to limit the present invention. In the attached picture:

Fig. 1 is a flow chart of the method for measuring the transverse relaxation time _T2 of terrestrial NMR based on bipolar spin echo;

Fig. 2 The phase diagram of the FID signal and the spin echo SE signal generated by transmitting bipolar spin echo; a is the phase diagram of the FID signal and SE signal collected by transmitting a positive 90° pulse and a positive 180° pulse, b is The phase diagram of the FID signal and SE signal collected by emitting a negative 90° pulse and a positive 180° pulse, c is the FID signal and spin echo SE signal obtained by superposition of a and b.

Fig. 3 is the pulse timing diagram of the ground-based NMR transverse relaxation time _T measurement method based on bipolar spin echo, a is the first measurement with a time interval of τ ₁ , and b is the second measurement with a time interval of τ ₂ measurement, c is the third measurement with a time interval of τ ₃ , and d is the nth measurement with a time interval of τ _n .

Fig. 4 Comparison diagram of transverse relaxation time _T2 obtained by GPMG sequence (a) and bipolar spin echo (b);

Figure 5. Effect diagram of bipolar spin echo eliminating free induction attenuation signal interference, a is the first measured signal, b is the second measured signal, c is the third measured signal, d is the fourth measured signal, e is the FID signal , f is the spin echo SE signal.

Detailed ways

In order to make the object, technical solution and advantages of the present invention more clear, the present invention will be further described in detail below in conjunction with the examples. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

The block diagrams of Embodiment 1 of a method for measuring the transverse relaxation time T ₂ of the terrestrial nuclear magnetic resonance based on bipolar pulses of the present invention are shown in FIGS. 1 and 3 . When the magnetic field in the measured area is not uniform, it is necessary to use a bipolar pulse to measure the transverse relaxation time T ₂ . The present invention is based on the ground MRS transverse relaxation time _T of bipolar pulse The measuring method steps are as follows:

Step 10: Send No. 1 positive polarity 90° pulse;

In the three-dimensional Cartesian coordinate system, the static magnetic field B _o is along the Z-axis direction of the three-dimensional Cartesian coordinate system, and the transmitting coil is placed on the ground _, and the No. The positive polarity 90° pulse excites the hydrogen nuclei in the groundwater to make them precess in the geomagnetic field. When the excitation stops, the hydrogen nuclei undergo spin relaxation (out-of-phase). The signal collected by the ground receiving coil is The first measured signal 11;

Step 20: Send No. 1 positive polarity 180° pulse;

After the time interval τ of step 12, the No. 1 positive polarity 180° pulse is launched in the direction of the Z axis (the excitation time is twice that of the 90° pulse), so that the out-of-phase hydrogen nuclei refocus and perform precession. When the excitation stops, the hydrogen The nuclear spin relaxation phenomenon occurs again, and the collected signal is the second measured signal 21;

Step 30: Send No. 2 negative polarity 90° pulse;

After time t in step 22, the hydrogen nucleus returns to its initial state, and the No. 2 negative polarity 90° pulse is emitted underground in the negative X-axis direction. When the excitation stops, the signal collected by the ground receiving coil is the third measured signal 31;

Step 40: Send No. 2 positive polarity 180° pulse;

After step 32 time interval τ (same as the time interval τ between No. 1 positive 90° pulse and No. 1 positive 180° pulse), transmit No. 2 positive 180° pulse in the Z-axis direction, when the excitation stops , the signal collected by the ground receiving coil is the fourth measured signal 41 .

Step 50: Obtain the free induction FID signal by superposition;

Referring to Fig. 2, the phase diagram of the FID signal and the spin echo SE signal generated by the bipolar spin echo of the present invention, according to the principle of nuclear magnetic resonance, a 90° pulse is emitted to generate the first FID1 signal, and a 180° pulse is emitted to generate a The spin echo SE signal generates the second FID2 signal at the same time. The FID2 signal interferes with the spin echo SE signal. The measured signal collected by the ground receiving coil is the superposition signal of the FID2 signal and the spin echo SE signal. This signal conceals the real Amplitude and shape of the spin echo SE signal. Therefore, the first measured signal and the third measured signal collected by the present invention are FID1 signals, the second measured signal and the fourth measured signal are superimposed signals of the FID2 signal and the spin echo SE signal, and the second measured signal and the fourth measured signal Masks the amplitude and shape of the true spin echo SE signal.

According to the principle of nuclear magnetic resonance, the phases of the collected FID1 signal and the spin echo SE signal are consistent with the phase of the 90° pulse, and the phase of the FID2 signal collected by transmitting the 180° pulse is consistent with the phase of the 180° pulse. Since the polarity of the No. 1 90° pulse and the No. 2 90° pulse are opposite, the phase difference of the FID1 signal of the first measured signal and the third measured signal is 180°, and the spin echo in the second measured signal and the fourth measured signal The phases of the SE signals differ by 180°, while the phases of the FID2 signals in the second measured signal and the fourth measured signal are the same. Therefore, the FID signal 51 can be obtained by subtracting the first measured signal and the third measured signal by 2;

Step 60: superimpose to obtain the spin echo SE signal;

According to the nuclear magnetic resonance principle described in step 50, the phase of the FID2 signal in the second actual measurement signal and the fourth actual measurement signal is the same, and the phase difference of the SE signal is 180°, so subtracting the second actual measurement signal and the fourth actual measurement signal by 2 is The spin echo SE signal 61 that eliminates the free induction FID signal can be obtained;

Step 70: change the time interval τ, and repeat the transmission multiple times;

See Figure 3 for the pulse timing diagram of _{the ground NMR transverse relaxation time T measurement method based on bipolar spin echo. In order to improve the measurement accuracy of the transverse relaxation time T 2 ,} by changing Time interval τ, repeat steps 10, 20, 30, 40, 50 and 60, and obtain a plurality of spin echo SE signals during repeated measurements, thereby realizing the measurement of accurate transverse relaxation time T ₂ ;

Step 80: fitting the transverse relaxation time T ₂ ;

Referring to Fig. 4(b) schematic diagram of bipolar pulse measuring transverse relaxation time _T2 , in the time domain, the envelope of the spin echo SE signal is approximately expressed as:

When applied in practice, the expression can be expressed as:

Where Δt _0.5 is the half-width of the spin echo SE signal, that is, the envelope width occupied by half of the maximum amplitude of the echo signal.

The peak value V _SE of the spin echo SE signal can be obtained by fitting the time-domain observation signal v _SE through expression (2). Concatenate the free induction FID signal and the peak V _SE of each spin echo signal to form an exponential decay curve It is the T ₂ decay curve, so this peak decay law can be used to fit the T ₂ value.

As shown in Figure 5, the bipolar pulse of the present invention eliminates the interference effect of the free induction attenuation signal. According to the principle of nuclear magnetic resonance, the FID1 signal and the SE signal with opposite polarities can be obtained by transmitting a bipolar 90° pulse, and the transmission polarity is the same The 180° pulse of the FID2 signal with the same polarity can be obtained, and the signal can be superimposed to obtain the spin echo SE signal that eliminates the interference of the FID2 signal.

In summary, as shown in Figure 4, the GPMG sequence of the present invention and the bipolar pulse obtain the contrast figure of transverse relaxation time T ₂ , and figure a is the CPMG sequence acquisition transverse relaxation time T ₂ schematic diagram, when the transverse direction of the measurement region When the relaxation time _T2 is very small, as shown in the figure, because the interval between the 90° pulse and the 180° pulse of the CPMG sequence is fixed, only a few of the first few 180° pulses can be collected among the multiple 180° pulses transmitted The number of SE signals and spin echo SE signals is small, which affects the measurement accuracy of the transverse relaxation time T ₂ . Figure b is a schematic diagram of the acquisition of the transverse relaxation time _T2 by the spin echo sequence with double pulse variable interval. By changing the time interval τ between the 90° pulse and the 180° pulse, multiple spin echo SE signals can be collected, which can significantly improve The measurement accuracy of the transverse relaxation time _T2 is improved.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. within range.

Claims (7)

1. A kind of 1. measuring method of the ground nuclear magnetic resonance T2 based on bipolar pulse, it is characterised in that including：

   Launch the 90 ° of pulses of No. 1 positive polarity, gather the first measured signal,

   Elapsed time interval τ, launch the 180 ° of pulses of No. 1 positive polarity, gather the second measured signal；

   After elapsed time recovers, launch the 90 ° of pulses of No. 2 negative polarity, gather the 3rd measured signal；

   By same time interval τ, launch the 180 ° of pulses of No. 2 positive polarity, gather the 4th measured signal；

   Second measured signal and the 4th measured signal are overlapped, obtain the spin for eliminating the interference of free induction FID signal Echo SE signals；

   First measured signal and the 3rd measured signal are superimposed to obtain first free induction FID signal；

   By changing time interval τ between 90 ° of pulses and 180 ° of pulses, repeat above-mentioned steps, obtained during duplicate measurements Obtain multiple spin echo SE signals；

   The initial amplitude of first free induction FID signal is connected with the peak value of each spin echo SE signals, forms one Exponential decay curve, fitting obtain T2 T2.
2. 2. measuring method according to claim 1, it is characterised in that carried out to the second measured signal and the 4th measured signal Superposition is specially：Second measured signal and the 4th measured signal are subtracted each other divided by 2.
3. 3. measuring method according to claim 1, it is characterised in that be superimposed the first measured signal and the 3rd measured signal Specially：First measured signal and the 3rd measured signal are subtracted each other divided by 2.
4. 4. measuring method according to claim 1, it is characterised in that launching the 90 ° of pulses of No. 1 positive polarity is specially：Three To tie up in rectangular coordinate system, transmitting coil is positioned on ground by magnetostatic field Bo along in the Z-direction of three-dimensional cartesian coordinate system, X-direction in the X/Y plane in magnetostatic field Bo directions launches the 90 ° of pulses of No. 1 positive polarity to underground.
5. 5. measuring method according to claim 1, it is characterised in that launching the 180 ° of pulses of No. 1 positive polarity is specially：Three Tie up in rectangular coordinate system, transmitting coil is positioned on ground by magnetostatic field Bo along in the Z-direction of three-dimensional cartesian coordinate system, in Z Launch the 180 ° of pulses of No. 1 positive polarity to underground on direction of principal axis.
6. 6. measuring method according to claim 1, it is characterised in that launching the 90 ° of pulses of No. 2 negative polarity is specially：Three To tie up in rectangular coordinate system, transmitting coil is positioned on ground by magnetostatic field Bo along in the Z-direction of three-dimensional cartesian coordinate system, Negative X-direction in the X/Y plane in magnetostatic field Bo directions launches the 90 ° of pulses of No. 2 negative polarity to underground.
7. 7. measuring method according to claim 1, it is characterised in that launching the 180 ° of pulses of No. 2 positive polarity is specially：Three Tie up in rectangular coordinate system, transmitting coil is positioned on ground by magnetostatic field Bo along in the Z-direction of three-dimensional cartesian coordinate system, in Z Launch the 180 ° of pulses of No. 2 positive polarity on direction of principal axis.