CN106019393B - A kind of unknown Larmor frequency carries out ground nuclear-magnetism off resonance detection method - Google Patents
A kind of unknown Larmor frequency carries out ground nuclear-magnetism off resonance detection method Download PDFInfo
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- 238000004364 calculation method Methods 0.000 description 5
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Abstract
The invention belongs to a kind of unknown Larmor frequency to carry out ground nuclear-magnetism off resonance detection method.Traditional core magnetic resonance detection necessarily requires tranmitting frequency identical with Larmor frequency.Due to the influence of earth's magnetic field spatial and temporal distributions inequality, magnetic interference and forceful electric power magnetic noise, Larmor frequency is difficult accurately to obtain, therefore off resonance phenomenon generally existing, and when Larmor frequency is unknown, frequency deviator is also unknown, can not obtain effective off resonance signal.The present invention carries out off resonance detection, tranmitting frequency f using two transmitting pulse composition excitation sequencesT1And fT2Respectively with Larmor frequency discreet value fLDifference ± Δ fset(the frequency deviator of setting), the real part for the off resonance signal that twice emitting is obtained is added and imaginary part is subtracted each other, and the influence of unknown Larmor frequency of the modified result of superposition, obtains frequency deviator Δ fsetUnder off resonance signal.The present invention can realize that off resonance signal accurately obtains under unknown Larmor frequency, solve the problem that traditional resonant probe method can not carry out application under complex environment.
Description
Technical field
The present invention relates to the fields of measurement of nuclear magnetic resonance off resonance signal, and in particular to a kind of unknown Larmor frequency is carried out
Ground nuclear-magnetism off resonance detection method.
Background technology
Nuclear magnetic resonance (MRS) technology with the advantage of its direct quantitative Underground water be widely used in water resources assessment and
Disaster water such as detects at the field.The principle of underground water nuclear magnetic resonance detection method is：By launching artificial alternating magnetic field, underground is excited
Hydrogen Proton in water, relaxation signals are produced directly to determine that the underground water such as aqueous layer depth, thickness, water content and pore size are assigned
Deposit status information.The tranmitting frequency of artificial magnetic field is by earth's magnetic field B0Determine, when tranmitting frequency is equal with Larmor frequency, hydrogen matter
Covibration occurs for son；When tranmitting frequency and Larmor frequency are unequal, off resonance (off-resonance) occurs for Hydrogen Proton
Phenomenon generally existing.Traditional underground water nuclear magnetic resonance detection method assumes earth's magnetic field B0It is constant, B is measured using magnetometer0
After calculate Larmor frequency, the tranmitting frequency as artificial magnetic field.Earth's magnetic field B0It can be influenceed by following several factors：1st,
The inhomogeneities and time variation in magnetic field itself；2nd, there is magnetic interference in measured zone；3rd, strong electromagnetic noise circumstance.Accordingly, it is difficult to
The exact value of Larmor frequency is obtained, tranmitting frequency can only be caused to certainly exist frequency inclined by the use of its approximation as tranmitting frequency
Amount, thus it is no longer covibration strict in physical significance to act on Hydrogen Proton from what is screwed on, but it is close with frequency deviator
Related off resonance excites, and when Larmor frequency is unknown, frequency deviator is also unknown, can not accurate expression off resonance signal reality
Portion and imaginary part, cause the problems such as hydrographic information explanation inaccuracy is with deep layer resolution ratio critical constraints.
CN203759267U discloses a kind of Larmor frequency measuring instrument, be connected respectively by master control system host computer, by
Key, display, power supply and three-component dynamometer module, master control system through polarized circuit, probe, with humorous circuit, amplifying circuit and
Frequency measurement circuit is connected with master control system, and master control system connects polarized circuit and with humorous circuit structure respectively through relay switching circuit
Into.The utility model measurement efficiency is high, and result can be just measured at several seconds；Measurement accuracy is high；All automatic measurement, user
Just it is succinct；Small volume, cost is cheap, is advantageously integrated inside all kinds nuclear magnetic resonance apparatus.But the utility model is deposited
Following the shortcomings that：Under strong electromagnetic noise circumstance, three-component magnetometer module can not normal work, therefore can not to draw
More's frequency measuring instrument is carried out with humorous, and electric arc generation is had in relay handoff procedure, influences the measurement essence of Larmor frequency
Degree.In the case, the Larmor frequency of measurement is unequal with actual value, unknown frequency deviator be present, and Hydrogen Proton occurs partially common
Shake phenomenon, traditional resonance model can not obtain the amplitude and phase information of accurate MRS signals, and then have a strong impact on over the ground
The accuracy and the resolution ratio of deep sea water layer that lower hydrographic information is explained.
CN103955004A discloses a kind of four-way NMR signal all-wave acquisition system and acquisition method, is by counting
Calculation machine via controller, high speed digital I/O cards and controller are connected with power management module, and connection is wide respectively for the acquired card of controller
Band amplifier, computer via controller connect and compose with GPS module.Preamplifier has effectively resisted amplifier saturation, uses
Q_SWITCH shortens dead time, improves signal to noise ratio, improves synchronization accuracy, prevents false triggering, and noise signal electricity consumption stream is carried out
Teletransmission effectively suppresses the signal attenuation in transmitting procedure, with auto-adaptive parameter denoising algorithm to the NMR signal that collects
Data processing is carried out, the antijamming capability of instrument is improved, improves the dynamic range of instrument, magnetic nuclear resonance method is being made an uproar greatly
The application in sound area domain is possibly realized, and effectively improves lateral resolution and accuracy to body of groundwater distribution measuring.Multichannel is surveyed
Amount improves operating efficiency, and body of groundwater is accurately positioned.But this invention has the following disadvantages：This invention uses traditional core
Magnetic resonance detection mode, that is, necessarily require tranmitting frequency equal with Larmor frequency, because magnetic field spatial and temporal distributions are uneven, magnetic field is done
The influence with forceful electric power magnetic noise is disturbed, Larmor frequency is difficult to obtain, and causes off resonance phenomenon generally existing, in the case, is passed
The nuclear magnetic resonance detection mode of system can not obtain the amplitude and phase of accurate MRS detectable signals, and then have a strong impact on over the ground
The accuracy and the resolution ratio in deep layer water-bearing layer that lower hydrographic information is explained.
Therefore, it is necessary to invent a kind of new off resonance excitation sequence and detection method, unknown Larmor frequency situation is realized
Effective acquisition of lower off resonance signal, break through dependence of traditional resonant probe to accurate Larmor frequency.
The content of the invention
The technical problems to be solved by the invention are that providing a kind of unknown Larmor frequency carries out ground nuclear-magnetism off resonance
Detection method, to solve under Complicated Geologic Condition, due to off resonance phenomenon generally existing, traditional resonant probe method obtains
MRS signal phase errors it is big the problem of, and the accurate acquisition problem of the off resonance signal in unknown Larmor frequency.
The present invention is achieved in that a kind of unknown Larmor frequency carries out ground nuclear-magnetism off resonance detection method and specifically walked
It is rapid as follows,
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：With positive bias tranmitting frequency fT1Transmitting, collection signal S1=x1+iy1；
Step 5：Negative bias tranmitting frequency f is setT2=fL-Δfset；
Step 6：With negative bias tranmitting frequency fT2Transmitting, collection signal S2=x2+iy2；
Step 7：Signal averaging
Step 8：Obtain and differ Δ f with true Larmor frequencysetSignal.
Compared with prior art, beneficial effect is the present invention：When carrying out traditional nuclear magnetic resonance detection, must be requested that
Tranmitting frequency is equal with Larmor frequency, due to the influence of earth's magnetic field spatial and temporal distributions inequality, magnetic interference and forceful electric power magnetic noise, leads
Deflection covibration generally existing, traditional nuclear magnetic resonance detection method can not obtain accurate MRS signal amplitudes and phase letter
Breath.The present invention breaches the limitation that traditional resonant probe has to rely on accurate Larmor frequency, by using two transmittings of transmitting
Pulse forms excitation sequence, and given frequency deviator Δ f can be achievedsetThe detection of lower off resonance signal, obtain accurate MRS amplitudes
And phase information.
Brief description of the drawings
Accompanying drawing is used for providing a further understanding of the present invention, and a part for constitution instruction, the reality with the present invention
Example is applied to be used to explain that the present invention is not construed as limiting the invention together.In the accompanying drawings：
Fig. 1 unknown Larmor frequencies provided in an embodiment of the present invention carry out the flow of ground nuclear-magnetism off resonance detection method
Figure；
Fig. 2 resonant excitations (a) provided in an embodiment of the present invention and off resonance excite (b) sequence diagram；
Fig. 3 frequency deviator optimization process figures (a) are the different Δ f in pulse square q=10AssetThe MRS signals of calculating are real
The analogous diagram in portion, imaginary part and mould, (b) are MRS signal real parts absolute value, imaginary part absolute value and both sum and Δ fsetRelation
Figure；
Fig. 4 resonant probes and the lower MRS signal simulation figures of off resonance detection, wherein (a) is the analogous diagram of signal real part, (b)
For the analogous diagram of signal imaginary part, (b) is the analogous diagram of the mould of signal；
Fig. 5 off resonance excitation sequence schematic diagrames when Larmor frequency is unstable；
Fig. 6 off resonance excitation sequence schematic diagrames when measured zone earth's magnetic field spatial and temporal distributions are uneven；
Embodiment
In order to make the purpose , technical scheme and advantage of the present invention be clearer, with reference to embodiments, to the present invention
It is further elaborated.It should be appreciated that the specific embodiments described herein are merely illustrative of the present invention, it is not used to
Limit the present invention.
A kind of unknown Larmor frequency of the present invention carries out block diagram such as Fig. 1 and 2 institutes of ground nuclear-magnetism off resonance detection method
Show.
A kind of unknown Larmor frequency of the present invention carries out comprising the following steps that for ground nuclear-magnetism off resonance detection method：
Step 1：Estimate Larmor frequency fL；
When complex geologic conditions, Larmor frequency is difficult to accurately estimate, though the present invention does not need true Larmor frequency,
But its estimate can not differ too big with actual value, therefore, the present invention proposes that the following two kinds estimates Larmor frequency fLScheme
It is as follows：
Scheme 11：Geomagnetic field measuring, Ran Houqiu are carried out to the multiple positions for measuring place in different time using magnetometer
The discreet value f to average as Larmor frequencyL；
Scheme 12：When magnetometer can not measure, carried out according to data with existing storehouse (such as China and US Geological Survey)
Discreet value f of the mathematic interpolation as Larmor frequencyL；
With reference to Fig. 2 (a) resonant excitation sequence diagrams, Larmor's discreet value fLWith Larmor frequency actual value fLarmorIt
Between unknown frequency deviator Δ f be presentuk=fL-fLarmor。
Step 2：Setpoint frequency deviator Δ fset；
By using Larmor frequency discreet value fLAs frequency deviator Δ fsetSelection gist, ensure two transmitting frequency
Rate causes the real and imaginary parts of MRS signals all to reach optimum value in real Larmor frequency both sides.
With reference to the optimization process figure of the frequency deviator of figure 3, frequency deviator Δ fsetCalculation procedure is as follows：
A) setting search Δ fsetScope be [- 20,20] Hz and step-size in search 1Hz；
B) calculated according to nuclear magnetic resonance principle, and according to the real and imaginary parts for emulating aqueous layer model calculating MRS signals；
C) MRS signal real part absolute values and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) frequency deviator Δ f corresponding to the maximum of the L1 norms of the real and imaginary parts of MRS signals is foundsetOptimal value,
Now the real and imaginary parts of the MRS signals under all pulse squares have reached optimum value.
Step 3：Positive bias tranmitting frequency f is setT1=fL+Δfset；
With reference to Fig. 2 (b) off resonance excitation sequence schematic diagrames, the present invention forms excitation sequences using two artificial magnetic fields are launched
Off resonance detection is carried out, positive bias tranmitting frequency f is set firstT1With fLDiffer Δ fset, i.e. fT1=fL+Δfset, frequency be present
Deviator Δ fT1=fT1-fLarmor=Δ fset+Δfuk。
Step 4：With positive bias tranmitting frequency fT1Transmitting, collection signal S1=x1+iy1；
By transmitting coil to underground tranmitting frequency be fT1Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil1=x1+iy1；
Step 5：Negative bias tranmitting frequency f is setT2=fL-Δfset；
With reference to figure 2 (b) off resonance excitation sequence schematic diagram, negative bias tranmitting frequency f is setT2With fLDifference-Δ fset, i.e.,
fT2=fL-Δfset, frequency deviator Δ f be presentT2=fT2-fLarmor=Δ fset-Δfuk。
Step 6：With negative bias tranmitting frequency fT2Transmitting, collection signal S2=x2+iy2；
By transmitting coil to underground tranmitting frequency be fT2Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil2=x2+iy2。
Step 7：Signal averaging
The MRS signal real parts x of twice emitting collection is added divided by 2, imaginary part y subtract each other divided by 2, obtains signal
Step 8：Obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, with reference to the MRS signal simulation figures under Fig. 4 resonant probes and off resonance detection, MRS is produced
The magnetic moment of signal is perpendicular to earth's magnetic field B0Component m on direction⊥, it is m that magnetic moment is excited in the case of off resonance⊥,off=my+
imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequency fT=fL±Δfset, caused off resonance excites magnetic moment m⊥,off's
myComponent is identical, mxComponent amplitude is equal, but symbol is opposite.
With reference to Fig. 2 (b) off resonance excitation sequence schematic diagrames, tranmitting frequency fT1Artificial alternating magnetic field, produce MRS signals
Magnetic moment be m⊥,off1=my+imx(shown in such as Fig. 4 (a), 4 (b) and 4 (c)), tranmitting frequency is f againT2Artificial alternation magnetic
, the magnetic moment for producing MRS signals is m⊥,off2=my-imx(such as Fig. 4 (a), 4 (b) and 4 (c) shown in), therefore by twice emitting
The MRS signals real part of collection is added divided by 2, and imaginary part is subtracted each other divided by 2, equivalent to off resonance Δ f twiceT1With Δ fT2Detection
As a result it is positive and negative to counteract original unknown Δ fukInfluence to MRS signals, acquisition only exist setpoint frequency deviator Δ fsetUnder it is inclined
Resonant probe signal.
Embodiment 1
A kind of unknown Larmor frequency of the present invention carries out the block diagram of the embodiment 1 of ground nuclear-magnetism off resonance detection method such as
Shown in Fig. 1 and 2.When complex geologic conditions, Larmor frequency is unknown, and the embodiment comprises the following steps that：
Step 1：Estimate Larmor frequency fL；
When complex geologic conditions, Larmor frequency is difficult to accurately estimate, though the present invention does not need true Larmor frequency,
But its estimate can not differ too big with actual value, therefore, the present invention proposes that the following two kinds estimates Larmor frequency fLScheme
It is as follows：
Scheme 11：Geomagnetic field measuring, Ran Houqiu are carried out to the multiple positions for measuring place in different time using magnetometer
The discreet value f to average as Larmor frequencyL；
Scheme 12：When magnetometer can not measure, carried out according to data with existing storehouse (such as China and US Geological Survey)
Discreet value f of the mathematic interpolation as Larmor frequencyL；
With reference to Fig. 2 (a) resonant excitation sequence diagrams, Larmor's discreet value fLWith actual value fLarmorBetween exist it 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, ensure two transmitting frequency
Rate is in real Larmor frequency both sides, simultaneously so that the real part x and imaginary part y of MRS signals reach optimum value.
With reference to figure 3 (a) and the optimization process figure of Fig. 3 (b) frequency deviators, frequency deviator Δ fsetCalculation procedure is as follows：
A) setting search Δ fsetScope be [- 20,20] Hz and step-size in search 1Hz；
B) calculated according to nuclear magnetic resonance principle, and according to the real and imaginary parts for emulating aqueous layer model calculating MRS signals；
C) MRS signal real part absolute values and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) frequency deviator Δ f corresponding to the maximum of the L1 norms of the real and imaginary parts of MRS signals is foundsetOptimal value,
Now the real and imaginary parts of the MRS signals under all pulse squares have reached optimum value.
Step 3：Positive bias tranmitting frequency f is setT1=fL+Δfset；
With reference to Fig. 2 (b) off resonance excitation sequence schematic diagrames, the present invention forms excitation sequences using two artificial magnetic fields are launched
Off resonance detection is carried out, positive bias tranmitting frequency f is set firstT1With fLDiffer Δ fset, i.e. fT1=fL+Δfset, frequency be present
Deviator Δ fT1=fT1-fLarmor=Δ fset+Δfuk。
Step 4：With positive bias tranmitting frequency fT1Transmitting, collection signal S1=x1+iy1；
By transmitting coil to underground tranmitting frequency be fT1Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil1=x1+iy1；
Step 5：Negative bias tranmitting frequency f is setT2=fL-Δfset；
With reference to Fig. 2 (b) off resonance excitation sequence schematic diagrames, negative bias tranmitting frequency f is setT2With fLDifference-Δ fset, i.e.,
fT2=fL-Δfset, frequency deviator Δ f be presentT2=fT2-fLarmor=Δ fset-Δfuk。
Step 6：With negative bias tranmitting frequency fT2Transmitting, collection signal S2=x2+iy2；
By transmitting coil to underground tranmitting frequency be fT2Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil2=x2+iy2。
Step 7：Signal averaging
The MRS signal real parts x of twice emitting collection is added divided by 2, imaginary part y subtract each other divided by 2, obtains signal
Step 8：Obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, with reference to the MRS signal simulation figures under Fig. 4 resonant probes and off resonance detection, MRS is produced
The magnetic moment of signal is perpendicular to earth's magnetic field B0Component m on direction⊥, it is m that magnetic moment is excited in the case of off resonance⊥,off=my+
imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequency fT=fL±Δfset, caused off resonance excites magnetic moment m⊥,off's
myComponent is identical, mxComponent amplitude is equal, but symbol is opposite.
With reference to Fig. 2 (b) off resonance excitation sequence schematic diagrames, tranmitting frequency fT1Artificial alternating magnetic field, produce MRS signals
Magnetic moment be m⊥,off,1=my+imx(shown in such as Fig. 4 (a), 4 (b) and 4 (c)), tranmitting frequency is f againT2Artificial alternation
Magnetic field, the magnetic moment for producing MRS signals is m⊥,off,2=my-imx(shown in such as Fig. 4 (a), 4 (b) and 4 (c)), therefore will send out twice
The MRS signals real part for penetrating collection is added divided by 2, and imaginary part is subtracted each other divided by 2, equivalent to off resonance Δ f twiceT1With Δ fT2Spy
Survey result is positive and negative to counteract original unknown Δ fukInfluence to MRS signals, acquisition only exist setpoint frequency deviator Δ fsetUnder
Off resonance detectable signal.
Embodiment 2
A kind of unknown Larmor frequency of the present invention carries out the block diagram of the embodiment 2 of ground nuclear-magnetism off resonance detection method such as
Shown in Fig. 1 and 5.Because earth's magnetic field changes over time, Larmor frequency also changes therewith, therefore unstable in Larmor frequency
When, the embodiment comprises the following steps that：
Step 1：Estimate Larmor frequency fL；
Because earth's magnetic field changes over time, Larmor frequency also changes therewith, and Larmor frequency is unstable, the present invention
Though not needing true Larmor frequency, its estimate can not differ too big with actual value, therefore, the present invention proposes the following two kinds
Estimate Larmor frequency fLScheme it is as follows：
Scheme 11：Geomagnetic field measuring, Ran Houqiu are carried out to the multiple positions for measuring place in different time using magnetometer
The discreet value f to average as Larmor frequencyL；
Scheme 12：When magnetometer can not measure, carried out according to data with existing storehouse (such as China and US Geological Survey)
Discreet value f of the mathematic interpolation as Larmor frequencyL；
With reference to Fig. 2 (a) resonant excitation sequence diagrams, Larmor's discreet value fLWith Larmor's actual value fLarmor(t) between
Unknown frequency deviator Δ f be presentuk(t)=fL-fLarmor(t)。
Step 2：Setpoint frequency deviator Δ fset；
By using Larmor frequency discreet value fLAs frequency deviator Δ fsetSelection gist, ensure two transmitting frequency
Rate is in real Larmor frequency both sides, simultaneously so that the real and imaginary parts of MRS signals all reach optimum value.
With reference to figure 3 (a) and the optimization process figure of Fig. 3 (b) frequency deviators, frequency deviator Δ fsetCalculation procedure is as follows：
A) setting search Δ fsetScope be [- 20,20] Hz and step-size in search 1Hz；
B) calculated according to nuclear magnetic resonance principle, and according to the real and imaginary parts for emulating aqueous layer model calculating MRS signals；
C) MRS signal real part absolute values and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) frequency deviator Δ f corresponding to the maximum of the L1 norms of the real and imaginary parts of MRS signals is foundsetOptimal value,
Now the real and imaginary parts of the MRS signals under all pulse squares have reached optimum value.
Step 3：Positive bias tranmitting frequency f is setT1=fL+Δfset；
With reference to Fig. 5, when Larmor frequency is unstable, off resonance excitation sequence schematic diagram, the present invention are proposed using transmitting two
Individual artificial magnetic field composition excitation sequence carries out off resonance detection, sets positive bias tranmitting frequency fT1With fLDiffer Δ fset, i.e. fT1=
fL+Δfset, frequency deviator Δ f be presentT1(t)=fT1-fLarmor(t)(ΔfT1(t)=Δ fset+Δfuk(t))。
Step 4：With positive bias tranmitting frequency fT1Transmitting, collection signal S1=x1+iy1；
By transmitting coil to underground tranmitting frequency be fT1Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil1=x1+iy1；
Step 5：Negative bias tranmitting frequency f is setT2=fL-Δfset；
With reference to Fig. 5 when Larmor frequency is unstable off resonance excitation sequence schematic diagram, negative bias tranmitting frequency f is setT2
With fLDifference-Δ fset, i.e. fT2=fL-Δfset, frequency deviator Δ f be presentT2(t)=fT2-fLarmor(t)(ΔfT2(t)=Δ
fset-Δfuk(t))。
Step 6：With negative bias tranmitting frequency fT2Transmitting, collection signal S2=x2+iy2；
By transmitting coil to underground tranmitting frequency be fT2Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil2=x2+iy2。
Step 7：Signal averaging
The MRS signals real part of twice emitting collection is added divided by 2, imaginary part is subtracted each other divided by 2, obtains signal
Step 8：Obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, with reference to the MRS signal simulation figures under Fig. 4 resonant probes and off resonance detection, MRS is produced
The magnetic moment of signal is perpendicular to earth's magnetic field B0Component m on direction⊥, it is m that magnetic moment is excited in the case of off resonance⊥,off=my+
imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequency fT=fL±Δfset, caused off resonance excites magnetic moment m⊥,off's
myComponent is identical, mxComponent amplitude is equal, but symbol is opposite.
With reference to Fig. 5 when Larmor frequency is unstable off resonance excitation sequence schematic diagram, tranmitting frequency fT1People's industry and traffic
Varying magnetic field, the magnetic moment for producing MRS signals is m⊥,off,1=my+imx(shown in such as Fig. 4 (a), 4 (b) and 4 (c)), launches frequency again
Rate is fT2Artificial alternating magnetic field, produce MRS signals magnetic moment be m⊥,off,2=my-imx(such as Fig. 4 (a), 4 (b) and 4 (c)
It is shown), therefore the MRS signals real part of twice emitting collection is added divided by 2, imaginary part is subtracted each other divided by 2, equivalent to partially common twice
Shake Δ fT1And Δ f (t)T2(t) result of detection is positive and negative to counteract original unknown Δ fuk(t) to the influence of MRS signals, obtain
Only exist setpoint frequency deviator Δ fsetUnder off resonance detectable signal.
Embodiment 3
A kind of unknown Larmor frequency of the present invention carries out the block diagram of the embodiment 3 of ground nuclear-magnetism off resonance detection method such as
Shown in Fig. 1 and 6.When in measured zone earth's magnetic field, spatial and temporal distributions are uneven, i.e. fLarmorMultiple values, and f be present simultaneouslyLarmorPresent
Certain distribution, the embodiment comprise the following steps that：
Step 1：Estimate Larmor frequency fL；
When in measured zone earth's magnetic field, spatial and temporal distributions are uneven, i.e. fLarmorMultiple values, and f be present simultaneouslyLarmorIt is presented one
Fixed distribution, though the present invention does not need true Larmor frequency, its estimate can not differ too big with actual value, therefore, this
Invention proposes that the following two kinds estimates Larmor frequency fLScheme it is as follows：
Scheme 11：Geomagnetic field measuring, Ran Houqiu are carried out to the multiple positions for measuring place in different time using magnetometer
The discreet value f to average as Larmor frequencyL；
Scheme 12：When magnetometer can not measure, carried out according to data with existing storehouse (such as China and US Geological Survey)
Discreet value f of the mathematic interpolation as Larmor frequencyL；
With reference to Fig. 2 (a) resonant excitation sequence diagrams, Larmor's discreet value fLWith multiple actual value fLarmorBetween exist
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, ensure two transmitting frequency
Rate is in real Larmor frequency both sides, simultaneously so that the real part x and imaginary part y of MRS signals reach optimum value.
With reference to figure 3 (a) and the optimization process figure of Fig. 3 (b) frequency deviators, frequency deviator Δ fsetCalculation procedure is as follows：
A) setting search Δ fsetScope be [- 20,20] Hz and step-size in search 1Hz；
B) calculated according to nuclear magnetic resonance principle, and according to the real and imaginary parts for emulating aqueous layer model calculating MRS signals；
C) MRS signal real part absolute values and imaginary part absolute value sum are drawn with Δ fsetThe curve of change；
D) frequency deviator Δ f corresponding to the maximum of the L1 norms of the real and imaginary parts of MRS signals is foundsetOptimal value,
Now the real and imaginary parts of the MRS signals under all pulse squares have reached optimum value.
Step 3：Positive bias tranmitting frequency f is setT1=fL+Δfset；
With reference to Fig. 6, when measured zone earth's magnetic field spatial and temporal distributions are uneven, off resonance excitation sequence schematic diagram, the present invention adopt
Off resonance detection is carried out with two artificial magnetic field composition excitation sequences of transmitting, positive bias tranmitting frequency f is set firstT1With fLDifference
Δfset, i.e. fT1=fL+Δfset, frequency deviator Δ f be presentT1(n)=fT1-fLarmor(n)=Δ fset+Δfuk(n) (n=1,
2…)。
Step 4：With positive bias tranmitting frequency fT1Transmitting, collection signal S1=x1+iy1；
By transmitting coil to underground tranmitting frequency be fT1Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil1=x1+iy1；
Step 5：Negative bias tranmitting frequency f is setT2=fL-Δfset；
With reference to Fig. 6 when measured zone earth's magnetic field spatial and temporal distributions are uneven off resonance excitation sequence schematic diagram, set negative bias
Put tranmitting frequency fT2With fLDifference-Δ fset, i.e. fT2=fL-Δfset, frequency deviator Δ f be presentT2(n)=fT2-fLarmor(n)=
Δfset-Δfuk(n) (n=1,2 ...).
Step 6：With negative bias tranmitting frequency fT2Transmitting, collection signal S2=x2+iy2；
By transmitting coil to underground tranmitting frequency be fT2Artificial alternating magnetic field, excite the Hydrogen Proton in underground water, enter
Row off resonance is detected, and MRS off resonance signals S is gathered using receiving coil2=x2+iy2。
Step 7：Signal averaging
The MRS signal real parts x of twice emitting collection is added divided by 2, imaginary part y subtract each other divided by 2, obtains signal
Step 8：Obtain and differ Δ f with true Larmor frequencysetSignal；
According to nuclear magnetic resonance principle, with reference to the MRS signal simulation figures under Fig. 4 resonant probes and off resonance detection, MRS is produced
The magnetic moment of signal is perpendicular to earth's magnetic field B0Component m on direction⊥, it is m that magnetic moment is excited in the case of off resonance⊥,off=my+
imx, setpoint frequency deviator is Δ fsetTwo tranmitting frequency fT=fL±Δfset, caused off resonance excites magnetic moment m⊥,off's
myComponent is identical, mxComponent amplitude is equal, but symbol is opposite.
With reference to Fig. 6 when measured zone earth's magnetic field spatial and temporal distributions are uneven off resonance excitation sequence schematic diagram, tranmitting frequency
For fT1Artificial alternating magnetic field, produce MRS signals magnetic moment be m⊥,off,1=my+imx(such as Fig. 4 (a), 4 (b) and 4 (c) institute
Show), tranmitting frequency is f againT2Artificial alternating magnetic field, produce MRS signals magnetic moment be m⊥,off,2=my-imx(such as Fig. 4
(a), shown in 4 (b) and 4 (c)), therefore the MRS signals real part of twice emitting collection is added divided by 2, imaginary part is subtracted each other divided by 2,
Equivalent to off resonance Δ f twiceT1And Δ f (n)T2(n) result of detection is positive and negative to counteract original unknown Δ fuk(n) to MRS
The influence of signal, acquisition only exist setpoint frequency deviator Δ fsetUnder off resonance detectable signal.
The foregoing is merely illustrative of the preferred embodiments of the present invention, is not intended to limit the invention, all essences in the present invention
All any modification, equivalent and improvement made within refreshing and principle etc., should be included in the scope of the protection.
Claims (3)
1. a kind of unknown Larmor frequency carries out ground nuclear-magnetism off resonance detection method, it is characterised in that
Step 1：Estimate discreet value f of the Larmor frequency as Larmor frequencyL；
Step 2：Setpoint frequency deviator △ fset；
Step 3：Positive bias tranmitting frequency f is setT1=fL+△fset；
Step 4：With positive bias tranmitting frequency fT1Transmitting, collection signal S1=x1+iy1；
Step 5：Negative bias tranmitting frequency f is setT2=fL-△fset；
Step 6：With negative bias tranmitting frequency fT2Transmitting, collection signal S2=x2+iy2；
Step 7：Signal averaging
Step 8：Obtain and differ △ f with true Larmor frequencysetSignal；
Setpoint frequency deviator △ fsetThe step of it is as follows：
A) search rate deviator △ f are setsetScope and step-size in search；
B) calculated according to nuclear magnetic resonance principle, and according to the real and imaginary parts for emulating aqueous layer model calculating MRS signals；
C) MRS signal real part absolute values and imaginary part absolute value sum are drawn with frequency deviator △ fsetThe curve of change；
D) frequency deviator △ f corresponding to the maximum of the L1 norms of the real and imaginary parts of MRS signals are foundsetOptimal value.
2. according to the method for claim 1, it is characterised in that carry out setpoint frequency deviator respectively in Larmor frequency both sides
Off resonance excite.
3. according to the method for claim 1, it is characterised in that
Estimate Larmor frequency fLMethod be:Earth magnetism is carried out to the multiple positions for measuring place in different time using magnetometer
Field measurement, then discreet value f of the averaged as Larmor frequencyL；
When magnetometer can not measure, discreet value f of the mathematic interpolation as Larmor frequency is carried out according to data with existing storehouseL。
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