CN106019393A - Method for detecting ground nuclear magnetic off-resonance in Larmor frequency unknown condition - Google Patents
Method for detecting ground nuclear magnetic off-resonance in Larmor frequency unknown condition Download PDFInfo
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Abstract
The invention relates to a method for detecting the ground nuclear magnetic off-resonance in the Larmor frequency unknown condition. According to the conventional nuclear magnetic resonance detection method, the transmitting frequency must be the same with the Larmor frequency. However, due to the non-uniform space-time distribution of the earth 's magnetic field, the interference of the magnetic field and the influence of strong electromagnetic noises, the Larmor frequency is difficult to be accurately obtained. Therefore, the off-resonance phenomenon exists widely. When the Larmor frequency is unknown, the frequency offset is also unknown. In this way, off-resonance signals cannot be effectively acquired. According to the technical scheme of the invention, two transmission pulses form a triggered sequence for the detection of the off-resonance phenomenon. The differences of two transmission frequencies fT1 and fT2 between the pre-estimated value of the Larmor frequency are +/- delta fset (preset frequency offset) respectively. The real parts of off-resonance signals obtained during the two transmitting processes are added up, while the imaginary parts of the off-resonance signals obtained during the two transmitting processes are subtracted. The superposed result corrects the influence of the unknown Larmor frequency, so that off-resonance signals with the frequency offset delta fset are obtained. According to the technical scheme of the invention, off-resonance signals are accurately obtained when the Larmor frequency is unknown. Therefore, the problem in the prior art that the conventional resonance detection method cannot be applied in the complex environment can be solved.
Description
Technical field
The present invention relates to the fields of measurement of nuclear magnetic resonance, NMR off resonance signal, be specifically related to a kind of unknown Larmor's frequency
Rate carries out ground nuclear-magnetism off resonance detection method.
Background technology
Nuclear magnetic resonance, NMR (MRS) technology is widely used in water money with the advantage of its direct quantitative Underground water
Source is evaluated and disaster water such as detects at the field.The principle of subsoil water nuclear magnetic resonance, NMR detection method is: by launching people
Industry and traffic varying magnetic field, excites the Hydrogen Proton in subsoil water, produce relaxation signals directly determine the water-bearing layer degree of depth,
The groundwater occurrence status informations such as thickness, water content and pore size.The tranmitting frequency of artificial magnetic field is by earth magnetism
Field B0Determine, when tranmitting frequency is equal with Larmor frequency, Hydrogen Proton generation covibration；When launching frequency
When rate is unequal with Larmor frequency, Hydrogen Proton generation off resonance (off-resonance) phenomenon generally exists.
Traditional subsoil water nuclear magnetic resonance, NMR detection method magnetic field B hypothetically0It is constant, utilizes magnetometer to measure B0
Rear calculating Larmor frequency, as the tranmitting frequency of artificial magnetic field.Earth's magnetic field B0Can be by following several factors
Impact: 1, the inhomogeneities in earth's magnetic field self and time variation；2, there is magnetic interference in measured zone；3、
Strong electromagnetic noise circumstance.Accordingly, it is difficult to obtain the exact value of Larmor frequency, can only be with its approximation conduct
Tranmitting frequency, causes tranmitting frequency to certainly exist frequency deviator, thus acts on Hydrogen Proton from screwing on no longer
It is covibration strict in physical significance, but off resonance closely-related with frequency deviator excites, and draw
During More's frequency the unknown, frequency deviator is the most unknown, it is impossible to the real part of accurate expression off resonance signal and imaginary part,
Hydrographic information is caused to explain the problems such as inaccurate and deep layer resolution critical constraints.
CN203759267U discloses a kind of Larmor frequency measuring instrument, is to be connected respectively by master control system
Position machine, button, display, power supply and three-component dynamometer module, master control system through polarized circuit, probe,
Joining humorous circuit, amplifying circuit and frequency measurement circuit to be connected with master control system, master control system is through relay switching circuit
Connect polarized circuit respectively and join humorous circuit composition.It is high that this utility model measures efficiency, just could at several seconds
Measure result；Certainty of measurement is high；All automatic measurement, easy to use succinctly；Small volume, with low cost,
It is advantageously integrated inside all kinds nuclear magnetic resonance apparatus.But, there is following shortcoming in this utility model:
Under strong electromagnetic noise circumstance, three-component magnetometer module can not normally work, and is unable to Rameau
Your frequency measuring instrument carries out joining humorous, has electric arc and produces, affect Larmor frequency in relay handoff procedure
Certainty of measurement.In the case, the Larmor frequency of measurement is unequal with actual value, there is unknown frequency
Deviator, Hydrogen Proton generation off resonance phenomenon, traditional resonance model cannot obtain MRS signal accurately
Amplitude and phase information, and then had a strong impact on the accuracy and deep sea water layer that underground hydrographic information is explained
Resolution.
CN103955004A discloses a kind of four-way NMR signal all-wave acquisition system and collection side
Method, is to be connected with power management module by computer via controller, high speed digital I/O card and controller, control
The acquired card of device processed connects broadband amplifier respectively, and computer via controller connects and composes with GPS module.Before
Putting amplifier, effectively to have resisted amplifier saturated, shortens Dead Time with Q_SWITCH, improves signal to noise ratio,
Improve synchronization accuracy, prevent false triggering, noise signal electricity consumption stream from carrying out teletransmission and effectively suppressing transmitting procedure
In signal attenuation, carry out data process by the auto-adaptive parameter denoising algorithm NMR signal to collecting,
Improve the capacity of resisting disturbance of instrument, improve the dynamic range of instrument, make magnetic nuclear resonance method at big noise
The application in region is possibly realized, and is effectively improved the lateral resolution to body of groundwater distribution measuring and accuracy.
Multi-channel measurement improves work efficiency, is accurately positioned body of groundwater.But this invention has the disadvantage in that
This invention uses traditional nuclear magnetic resonance, NMR detection mode, i.e. necessarily requires tranmitting frequency equal with Larmor frequency,
Due to magnetic field spatial and temporal distributions inequality, magnetic interference and strong electromagnetic effect of noise, Larmor frequency is difficult to obtain,
Causing off resonance phenomenon generally to exist, in the case, traditional nuclear magnetic resonance, NMR detection mode cannot obtain standard
The amplitude of true MRS detectable signal and phase place, and then had a strong impact on the standard that underground hydrographic information is explained
Really property and the resolution in deep layer water-bearing layer.
Accordingly, it would be desirable to invent a kind of new off resonance excitation sequence and detection method, it is achieved unknown Larmor's frequency
In the case of rate, effective acquisition of off resonance signal, breaks through the tradition resonant probe dependence to accurate Larmor frequency.
Summary of the invention
It is inclined that the technical problem to be solved is to provide a kind of unknown Larmor frequency to carry out ground nuclear-magnetism
Resonant probe method is to solve under Complicated Geologic Condition, owing to off resonance phenomenon generally exists, traditional
The big problem of MRS signal phase error that resonant probe method obtains, and when unknown Larmor frequency
Off resonance signal accurately obtain a difficult problem.
The present invention is achieved in that a kind of unknown Larmor frequency carries out ground nuclear-magnetism off resonance detection method
Comprise the following steps that,
Step 1: estimate Larmor frequency fL；
Step 2: setpoint frequency deviator Δ fset；
Step 3: positive bias tranmitting frequency f is setT1=fL+Δfset；
Step 4: by positive bias tranmitting frequency fT1Launch, gather signal S1=x1+iy1；
Step 5: negative bias tranmitting frequency f is setT2=fL-Δfset；
Step 6: by negative bias tranmitting frequency fT2Launch, gather signal S2=x2+iy2；
Step 7: Signal averaging
Step 8: obtain and differ Δ f with true Larmor frequencysetSignal.
The present invention compared with prior art, has the beneficial effects that: when carrying out traditional nuclear magnetic resonance, NMR detection,
Necessarily require tranmitting frequency equal with Larmor frequency, owing to earth's magnetic field spatial and temporal distributions is uneven, magnetic interference and
Strong electromagnetic effect of noise, causes off resonance phenomenon generally to exist, and traditional nuclear magnetic resonance, NMR detection method cannot
Obtain MRS signal amplitude and phase information accurately.The present invention breaches tradition resonant probe and has to rely on
The accurately restriction of Larmor frequency, launches two transmitting pulse composition excitation sequence by using, can realize
Know frequency deviator Δ fsetThe detection of lower off resonance signal, obtains MRS amplitude and phase information accurately.
Accompanying drawing explanation
Accompanying drawing is for providing a further understanding of the present invention, and constitutes a part for description, with this
Bright embodiment is used for explaining that the present invention is not intended that limitation of the present invention together.In the accompanying drawings:
The unknown Larmor frequency that Fig. 1 embodiment of the present invention provides carries out ground nuclear-magnetism off resonance detection method
Flow chart；
Resonant excitation (a) and off resonance that Fig. 2 embodiment of the present invention provides excite (b) sequence diagram；
It is when pulse square q=10As that Fig. 3 frequency deviator optimizes procedure chart (a), different Δ fsetThe MRS calculated
The analogous diagram of signal real part, imaginary part and mould, (b) be MRS signal real part absolute value, imaginary part absolute value and both
Sum and Δ fsetGraph of a relation；
Fig. 4 resonant probe MRS signal simulation figure lower with off resonance detection, wherein (a) is the imitative of signal real part
True figure, (b) is the analogous diagram of signal imaginary part, and (b) is the analogous diagram of the mould of signal；
Fig. 5 is off resonance excitation sequence schematic diagram when Larmor frequency instability；
Fig. 6 is off resonance excitation sequence schematic diagram when measured zone earth's magnetic field spatial and temporal distributions is uneven；
Detailed description of the invention
In order to make the purpose of the present invention, technical scheme and advantage clearer, below in conjunction with embodiment,
The present invention is further elaborated.Should be appreciated that specific embodiment described herein only in order to
Explain the present invention, be not intended to limit the present invention.
A kind of unknown Larmor frequency of the present invention carries out block diagram such as Fig. 1 of ground nuclear-magnetism off resonance detection method
Shown in 2.
A kind of unknown Larmor frequency of the present invention carries out the concrete steps of ground nuclear-magnetism off resonance detection method such as
Under:
Step 1: estimate Larmor frequency fL；
When complex geologic conditions, Larmor frequency is difficult to accurately estimate, though the present invention need not true Rameau
That frequency, but its estimated value can not differ too big with actual value, estimates to this end, the present invention proposes the following two kinds
Larmor frequency fLScheme as follows:
Scheme 11: utilize magnetometer, at different time, the multiple positions measuring place are carried out geomagnetic field measuring,
Then averaged is as the discreet value f of Larmor frequencyL；
Scheme 12: when magnetometer cannot be measured, according to data with existing storehouse (such as China and U.S. geological Survey
Office) carry out the mathematic interpolation discreet value f as Larmor frequencyL；
In conjunction with Fig. 2 (a) resonant excitation sequence diagram, Larmor discreet value fLWith Larmor frequency actual value
fLarmorBetween there is unknown frequency deviator Δ fuk=fL-fLarmor。
Step 2: setpoint frequency deviator Δ fset；
By using Larmor frequency discreet value fLAs frequency deviator Δ fsetSelection gist, it is ensured that two send out
Radio frequency rate, in real Larmor frequency both sides, makes the real part of MRS signal and imaginary part all reach the most simultaneously
Good value.
With reference to the optimization procedure chart of Fig. 3 frequency deviator, frequency deviator Δ fsetCalculation procedure is as follows:
A) setting search Δ fsetIn the range of [-20,20] Hz and step-size in search 1Hz；
B) calculate according to nuclear magnetic resonance principle, and according to emulation water-bearing layer model calculate MRS signal real part and
Imaginary part；
C) MRS signal real part absolute value and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) the frequency deviator Δ f that the real part of MRS signal is corresponding with the maximum of the L1 norm of imaginary part is foundset's
Optimal value, real part and the imaginary part of the MRS signal under the most all pulse squares have all reached optimum.
Step 3: positive bias tranmitting frequency f is setT1=fL+Δfset；
In conjunction with Fig. 2 (b) off resonance excitation sequence schematic diagram, the present invention uses two artificial magnetic field compositions of transmitting to swash
Send out sequence and carry out off resonance detection, first positive bias tranmitting frequency f is setT1With fLDifference Δ fset, i.e.
fT1=fL+Δfset, there is frequency deviator Δ fT1=fT1-fLarmor=Δ fset+Δfuk。
Step 4: by positive bias tranmitting frequency fT1Launch, gather signal S1=x1+iy1；
It is f by transmitting coil to underground tranmitting frequencyT1Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S1=x1+iy1；
Step 5: negative bias tranmitting frequency f is setT2=fL-Δfset；
With reference to Fig. 2 (b) off resonance excitation sequence schematic diagram, negative bias tranmitting frequency f is setT2With fLDifference
-Δfset, i.e. fT2=fL-Δfset, there is frequency deviator Δ fT2=fT2-fLarmor=Δ fset-Δfuk。
Step 6: by negative bias tranmitting frequency fT2Launch, gather signal S2=x2+iy2；
It is f by transmitting coil to underground tranmitting frequencyT2Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S2=x2+iy2。
Step 7: Signal averaging
MRS signal real part x twice emitting gathered is added divided by 2, and imaginary part y is subtracted each other divided by 2, it is thus achieved that
Signal
Step 8: obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, the MRS signal simulation under detecting in conjunction with Fig. 4 resonant probe and off resonance
Figure, the magnetic moment producing MRS signal is perpendicular to earth's magnetic field B0Component m on direction⊥, in off resonance situation
Under to excite magnetic moment be m⊥,off=my+imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequencies fT=fL±Δfset,
The off resonance produced excites magnetic moment m⊥,offMyComponent is identical, mxComponent amplitude is equal, but symbol is contrary.
In conjunction with Fig. 2 (b) off resonance excitation sequence schematic diagram, tranmitting frequency is fT1Artificial alternating magnetic field, produce
The magnetic moment of MRS signal is m⊥,off1=my+imx(as shown in Fig. 4 (a), 4 (b) and 4 (c)), then
Secondary tranmitting frequency is fT2Artificial alternating magnetic field, produce MRS signal magnetic moment be m⊥,off2=my-imx(such as figure
Shown in 4 (a), 4 (b) and 4 (c)), therefore the MRS signal real part that twice emitting gathers is added
Divided by 2, imaginary part is subtracted each other divided by 2, is equivalent to twice off resonance Δ fT1With Δ fT2The positive and negative counteracting of result of detection
The Δ f of original the unknownukImpact on MRS signal, it is thus achieved that only exist setpoint frequency deviator Δ fsetUnder the most common
Shake detectable signal.
Embodiment 1
A kind of unknown Larmor frequency of the present invention carries out the embodiment 1 of ground nuclear-magnetism off resonance detection method
Block diagram is as illustrated in fig. 1 and 2.When complex geologic conditions, Larmor frequency is unknown, this embodiment concrete
Step is as follows:
Step 1: estimate Larmor frequency fL；
When complex geologic conditions, Larmor frequency is difficult to accurately estimate, though the present invention need not true Rameau
That frequency, but its estimated value can not differ too big with actual value, estimates to this end, the present invention proposes the following two kinds
Larmor frequency fLScheme as follows:
Scheme 11: utilize magnetometer, at different time, the multiple positions measuring place are carried out geomagnetic field measuring,
Then averaged is as the discreet value f of Larmor frequencyL；
Scheme 12: when magnetometer cannot be measured, according to data with existing storehouse (such as China and U.S. geological Survey
Office) carry out the mathematic interpolation discreet value f as Larmor frequencyL；
In conjunction with Fig. 2 (a) resonant excitation sequence diagram, Larmor discreet value fLWith actual value fLarmorBetween exist
Unknown frequency deviator Δ fuk=fL-fLarmor。
Step 2: setpoint frequency deviator Δ fset；
By using Larmor frequency discreet value fLAs frequency deviator Δ fsetSelection gist, it is ensured that two send out
Radio frequency rate is in real Larmor frequency both sides, simultaneously so that the real part x of MRS signal and imaginary part y all reach
To optimum.
With reference to the optimization procedure chart of Fig. 3 (a) with Fig. 3 (b) frequency deviator, frequency deviator Δ fsetCalculation procedure
As follows:
A) setting search Δ fsetIn the range of [-20,20] Hz and step-size in search 1Hz；
B) calculate according to nuclear magnetic resonance principle, and according to emulation water-bearing layer model calculate MRS signal real part and
Imaginary part；
C) MRS signal real part absolute value and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) the frequency deviator Δ f that the real part of MRS signal is corresponding with the maximum of the L1 norm of imaginary part is foundset's
Optimal value, real part and the imaginary part of the MRS signal under the most all pulse squares have all reached optimum.
Step 3: positive bias tranmitting frequency f is setT1=fL+Δfset；
In conjunction with Fig. 2 (b) off resonance excitation sequence schematic diagram, the present invention uses two artificial magnetic field compositions of transmitting to swash
Send out sequence and carry out off resonance detection, first positive bias tranmitting frequency f is setT1With fLDifference Δ fset, i.e.
fT1=fL+Δfset, there is frequency deviator Δ fT1=fT1-fLarmor=Δ fset+Δfuk。
Step 4: by positive bias tranmitting frequency fT1Launch, gather signal S1=x1+iy1；
It is f by transmitting coil to underground tranmitting frequencyT1Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S1=x1+iy1；
Step 5: negative bias tranmitting frequency f is setT2=fL-Δfset；
In conjunction with Fig. 2 (b) off resonance excitation sequence schematic diagram, negative bias tranmitting frequency f is setT2With fLDifference
-Δfset, i.e. fT2=fL-Δfset, there is frequency deviator Δ fT2=fT2-fLarmor=Δ fset-Δfuk。
Step 6: by negative bias tranmitting frequency fT2Launch, gather signal S2=x2+iy2；
It is f by transmitting coil to underground tranmitting frequencyT2Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S2=x2+iy2。
Step 7: Signal averaging
MRS signal real part x twice emitting gathered is added divided by 2, and imaginary part y is subtracted each other divided by 2, it is thus achieved that
Signal
Step 8: obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, the MRS signal simulation under detecting in conjunction with Fig. 4 resonant probe and off resonance
Figure, the magnetic moment producing MRS signal is perpendicular to earth's magnetic field B0Component m on direction⊥, in off resonance situation
Under to excite magnetic moment be m⊥,off=my+imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequencies fT=fL±Δfset,
The off resonance produced excites magnetic moment m⊥,offMyComponent is identical, mxComponent amplitude is equal, but symbol is contrary.
In conjunction with Fig. 2 (b) off resonance excitation sequence schematic diagram, tranmitting frequency is fT1Artificial alternating magnetic field, produce
The magnetic moment of MRS signal is m⊥,off,1=my+imx(as shown in Fig. 4 (a), 4 (b) and 4 (c)), then
Secondary tranmitting frequency is fT2Artificial alternating magnetic field, produce MRS signal magnetic moment be m⊥,off,2=my-imx(as
Shown in Fig. 4 (a), 4 (b) and 4 (c)), the MRS signal real part phase therefore twice emitting gathered
Adding divided by 2, imaginary part is subtracted each other divided by 2, is equivalent to twice off resonance Δ fT1With Δ fT2Result of detection positive and negative support
Disappeared the Δ f of original the unknownukImpact on MRS signal, it is thus achieved that only exist setpoint frequency deviator Δ fsetUnder inclined
Resonant probe signal.
Embodiment 2
A kind of unknown Larmor frequency of the present invention carries out the embodiment 2 of ground nuclear-magnetism off resonance detection method
Block diagram is as described in figures 1 and 5.Because earth's magnetic field changes over, Larmor frequency changes the most therewith, therefore
When Larmor frequency instability, specifically comprising the following steps that of this embodiment
Step 1: estimate Larmor frequency fL；
Because earth's magnetic field changes over, Larmor frequency changes the most therewith, and Larmor frequency is unstable,
Though the present invention need not true Larmor frequency, but its estimated value can not differ too big with actual value, to this end,
The present invention proposes the following two kinds and estimates Larmor frequency fLScheme as follows:
Scheme 11: utilize magnetometer, at different time, the multiple positions measuring place are carried out geomagnetic field measuring,
Then averaged is as the discreet value f of Larmor frequencyL；
Scheme 12: when magnetometer cannot be measured, according to data with existing storehouse (such as China and U.S. geological Survey
Office) carry out the mathematic interpolation discreet value f as Larmor frequencyL；
In conjunction with Fig. 2 (a) resonant excitation sequence diagram, Larmor discreet value fLWith Larmor actual value fLarmor(t)
Between there is unknown frequency deviator Δ fuk(t)=fL-fLarmor(t)。
Step 2: setpoint frequency deviator Δ fset；
By using Larmor frequency discreet value fLAs frequency deviator Δ fsetSelection gist, it is ensured that two send out
Radio frequency rate is in real Larmor frequency both sides, simultaneously so that real part and the imaginary part of MRS signal all reach
Optimum.
With reference to the optimization procedure chart of Fig. 3 (a) with Fig. 3 (b) frequency deviator, frequency deviator Δ fsetCalculation procedure
As follows:
A) setting search Δ fsetIn the range of [-20,20] Hz and step-size in search 1Hz；
B) calculate according to nuclear magnetic resonance principle, and according to emulation water-bearing layer model calculate MRS signal real part and
Imaginary part；
C) MRS signal real part absolute value and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) the frequency deviator Δ f that the real part of MRS signal is corresponding with the maximum of the L1 norm of imaginary part is foundset's
Optimal value, real part and the imaginary part of the MRS signal under the most all pulse squares have all reached optimum.
Step 3: positive bias tranmitting frequency f is setT1=fL+Δfset；
In conjunction with Fig. 5 off resonance excitation sequence schematic diagram when Larmor frequency instability, the present invention proposes to use
Launch two artificial magnetic field composition excitation sequence and carry out off resonance detection, positive bias tranmitting frequency f is setT1With fL
Difference Δ fset, i.e. fT1=fL+Δfset, there is frequency deviator Δ fT1(t)=fT1-fLarmor(t)
(ΔfT1(t)=Δ fset+Δfuk(t))。
Step 4: by positive bias tranmitting frequency fT1Launch, gather signal S1=x1+iy1；
It is f by transmitting coil to underground tranmitting frequencyT1Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S1=x1+iy1；
Step 5: negative bias tranmitting frequency f is setT2=fL-Δfset；
In conjunction with Fig. 5 off resonance excitation sequence schematic diagram when Larmor frequency instability, negative bias is set and launches
Frequency fT2With fLDifference-Δ fset, i.e. fT2=fL-Δfset, there is frequency deviator Δ fT2(t)=fT2-fLarmor(t)
(ΔfT2(t)=Δ fset-Δfuk(t))。
Step 6: by negative bias tranmitting frequency fT2Launch, gather signal S2=x2+iy2；
It is f by transmitting coil to underground tranmitting frequencyT2Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S2=x2+iy2。
Step 7: Signal averaging
MRS signal real part twice emitting gathered is added divided by 2, and imaginary part is subtracted each other divided by 2, it is thus achieved that signal
Step 8: obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, the MRS signal simulation under detecting in conjunction with Fig. 4 resonant probe and off resonance
Figure, the magnetic moment producing MRS signal is perpendicular to earth's magnetic field B0Component m on direction⊥, in off resonance situation
Under to excite magnetic moment be m⊥,off=my+imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequencies fT=fL±Δfset,
The off resonance produced excites magnetic moment m⊥,offMyComponent is identical, mxComponent amplitude is equal, but symbol is contrary.
In conjunction with Fig. 5 off resonance excitation sequence schematic diagram when Larmor frequency instability, tranmitting frequency is fT1's
Artificial alternating magnetic field, the magnetic moment producing MRS signal is m⊥,off,1=my+imx(as Fig. 4 (a), 4 (b) with
And shown in 4 (c)), tranmitting frequency is f againT2Artificial alternating magnetic field, produce MRS signal magnetic moment
For m⊥,off,2=my-imx(as shown in Fig. 4 (a), 4 (b) and 4 (c)), therefore by twice emitting collection
MRS signal real part be added divided by 2, imaginary part is subtracted each other divided by 2, is equivalent to twice off resonance Δ fT1(t) and
ΔfT2T the result of detection of () is positive and negative counteracts original unknown Δ fuk(t) impact on MRS signal, it is thus achieved that only deposit
At setpoint frequency deviator Δ fsetUnder off resonance detectable signal.
Embodiment 3
A kind of unknown Larmor frequency of the present invention carries out the embodiment 3 of ground nuclear-magnetism off resonance detection method
Block diagram is as shown in figs. 1 and 6.When measured zone earth's magnetic field spatial and temporal distributions is uneven, i.e. fLarmorExist simultaneously
Multiple values, and fLarmorPresent certain distribution, specifically comprising the following steps that of this embodiment
Step 1: estimate Larmor frequency fL；
When measured zone earth's magnetic field spatial and temporal distributions is uneven, i.e. fLarmorThere is multiple value, and f simultaneouslyLarmorIn
The most certain distribution, though the present invention need not true Larmor frequency, but its estimated value can not be with actual value phase
Difference is too big, to this end, the present invention proposes the following two kinds estimates Larmor frequency fLScheme as follows:
Scheme 11: utilize magnetometer, at different time, the multiple positions measuring place are carried out geomagnetic field measuring,
Then averaged is as the discreet value f of Larmor frequencyL；
Scheme 12: when magnetometer cannot be measured, according to data with existing storehouse (such as China and U.S. geological Survey
Office) carry out the mathematic interpolation discreet value f as Larmor frequencyL；
In conjunction with Fig. 2 (a) resonant excitation sequence diagram, Larmor discreet value fLWith multiple actual value fLarmorBetween
There is unknown frequency deviator Δ fuk(n)=fL-fLarmor(n) (n=1,2 ...).
Step 2: setpoint frequency deviator Δ fset；
By using Larmor frequency discreet value fLAs frequency deviator Δ fsetSelection gist, it is ensured that two send out
Radio frequency rate is in real Larmor frequency both sides, simultaneously so that the real part x of MRS signal and imaginary part y all reach
To optimum.
With reference to the optimization procedure chart of Fig. 3 (a) with Fig. 3 (b) frequency deviator, frequency deviator Δ fsetCalculation procedure
As follows:
A) setting search Δ fsetIn the range of [-20,20] Hz and step-size in search 1Hz；
B) calculate according to nuclear magnetic resonance principle, and according to emulation water-bearing layer model calculate MRS signal real part and
Imaginary part；
C) MRS signal real part absolute value and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) the frequency deviator Δ f that the real part of MRS signal is corresponding with the maximum of the L1 norm of imaginary part is foundset's
Optimal value, real part and the imaginary part of the MRS signal under the most all pulse squares have all reached optimum.
Step 3: positive bias tranmitting frequency f is setT1=fL+Δfset；
In conjunction with Fig. 6 off resonance excitation sequence schematic diagram when measured zone earth's magnetic field spatial and temporal distributions is uneven, this
Invention uses two artificial magnetic fields composition excitation sequence of transmitting to carry out off resonance detection, first arranges positive bias and sends out
Radio frequency rate fT1With fLDifference Δ fset, i.e. fT1=fL+Δfset, there is frequency deviator
ΔfT1(n)=fT1-fLarmor(n)=Δ fset+Δfuk(n) (n=1,2 ...).
Step 4: by positive bias tranmitting frequency fT1Launch, gather signal S1=x1+iy1；
It is f by transmitting coil to underground tranmitting frequencyT1Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S1=x1+iy1；
Step 5: negative bias tranmitting frequency f is setT2=fL-Δfset；
In conjunction with Fig. 6 off resonance excitation sequence schematic diagram when measured zone earth's magnetic field spatial and temporal distributions is uneven, if
Put negative bias tranmitting frequency fT2With fLDifference-Δ fset, i.e. fT2=fL-Δfset, there is frequency deviator
ΔfT2(n)=fT2-fLarmor(n)=Δ fset-Δfuk(n) (n=1,2 ...).
Step 6: by negative bias tranmitting frequency fT2Launch, gather signal S2=x2+iy2；
It is f by transmitting coil to underground tranmitting frequencyT2Artificial alternating magnetic field, excite the hydrogen matter in subsoil water
Son, carries out off resonance detection, utilizes receiving coil to gather MRS off resonance signal S2=x2+iy2。
Step 7: Signal averaging
MRS signal real part x twice emitting gathered is added divided by 2, and imaginary part y is subtracted each other divided by 2, it is thus achieved that
Signal
Step 8: obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, the MRS signal simulation under detecting in conjunction with Fig. 4 resonant probe and off resonance
Figure, the magnetic moment producing MRS signal is perpendicular to earth's magnetic field B0Component m on direction⊥, in off resonance situation
Under to excite magnetic moment be m⊥,off=my+imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequencies fT=fL±Δfset,
The off resonance produced excites magnetic moment m⊥,offMyComponent is identical, mxComponent amplitude is equal, but symbol is contrary.
In conjunction with Fig. 6 off resonance excitation sequence schematic diagram when measured zone earth's magnetic field spatial and temporal distributions is uneven, send out
Radio frequency rate is fT1Artificial alternating magnetic field, produce MRS signal magnetic moment be m⊥,off,1=my+imx(such as Fig. 4
Shown in (a), 4 (b) and 4 (c)), tranmitting frequency is f againT2Artificial alternating magnetic field, produce MRS
The magnetic moment of signal is m⊥,off,2=my-imx(as shown in Fig. 4 (a), 4 (b) and 4 (c)), therefore by two
Secondary launch gather MRS signal real part be added divided by 2, imaginary part is subtracted each other divided by 2, be equivalent to twice the most common
Shake Δ fT1(n) and Δ fT2N the result of detection of () is positive and negative counteracts original unknown Δ fuk(n) shadow to MRS signal
Ring, it is thus achieved that only exist setpoint frequency deviator Δ fsetUnder off resonance detectable signal.
The foregoing is only presently preferred embodiments of the present invention, not in order to limit the present invention, all at this
Any amendment, equivalent and the improvement etc. made within bright spirit and principle, should be included in the present invention
Protection domain within.
Claims (4)
1. a unknown Larmor frequency carries out ground nuclear-magnetism off resonance detection method, it is characterised in that
Step 1: estimate the Larmor frequency discreet value f as Larmor frequencyL；
Step 2: setpoint frequency deviator Δ fset；
Step 3: positive bias tranmitting frequency f is setT1=fL+Δfset；
Step 4: by positive bias tranmitting frequency fT1Launch, gather signal S1=x1+iy1；
Step 5: negative bias tranmitting frequency f is setT2=fL-Δfset；
Step 6: by negative bias tranmitting frequency fT2Launch, gather signal S2=x2+iy2；
Step 7: Signal averaging
Step 8: obtain and differ Δ f with true Larmor frequencysetSignal.
Method the most according to claim 1, it is characterised in that carry out respectively in Larmor frequency both sides
The off resonance of setpoint frequency deviator excites.
Method the most according to claim 1, it is characterised in that
Estimate Larmor frequency fLMethod be: utilize magnetometer different time to measure place multiple positions
Putting and carry out geomagnetic field measuring, then averaged is as the discreet value f of Larmor frequencyL；
When magnetometer cannot be measured, carry out pre-as Larmor frequency of mathematic interpolation according to data with existing storehouse
Valuation fL。
Method the most according to claim 1, it is characterised in that setpoint frequency deviator Δ fsetStep such as
Under:
A) setting search frequency deviator Δ fsetScope and step-size in search；
B) calculate according to nuclear magnetic resonance principle, and according to emulation water-bearing layer model calculate MRS signal real part and
Imaginary part；
C) MRS signal real part absolute value and imaginary part absolute value sum are drawn with frequency deviator Δ fsetThe song of change
Line；
D) the frequency deviator Δ f that the real part of MRS signal is corresponding with the maximum of the L1 norm of imaginary part is foundset's
Optimal value.
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