GB1587386A

GB1587386A - Magnetic field gradient measuring device

Info

Publication number: GB1587386A
Application number: GB413678A
Authority: GB
Original assignee: INST ZEMNOGO MAGNET IONOSFERY
Current assignee: INST ZEMNOGO MAGNET IONOSFERY
Priority date: 1978-02-01
Filing date: 1978-02-01
Publication date: 1981-04-01

Description

(54) MAGNETIC FIELD GRADIENT MEASURING DEVICE (71) We, INSTITUT ZEMNOGO MAGNETIZMA IONOSFERY I RASPROS TRANENIA RADZIOVOLN AKADEMII NAUK SSSR., (IZMIRAN), d Akademichesky gorodok, Moskovskaya blast, USSR., a USSR corporate body, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: - The present invention relates to a magnetic field gradient measuring device. The invention is applicable to geophysics, where it can be used for detecting magnetic anomalies from an aircraft. The invention can also be used for measuring the variations in magnetization of rock and other substances, as well as for mapping permanent and variable magnetic fields inherent in or generated by objects.

According to the present invention there is provided a magnetic field gradient measuring device, comprising two sensors which in use are arranged at spaced points in a magnetic field which are to be investigated, the outputs of the two sensors being connected to inputs of a phase detector, each of the two sensors comprising an absorption cell filled with atoms of a working substance exhibiting magnetic transition resonance, each said absorption cell having arranged at an input thereof a light source suitable for optical pump- ing of atoms of the working substance, and at an output thereof a photoconverter responsive to the beam of light passing through the cell, each said absorption cell being arranged inside a coil energized with radio frequency current of a variable frequency close to the magnetic transition resonance frequency of atoms of the working substance, determined by the intensity of the magnetic field at the location of the respective sensor, the radio frequency coils of both sensors being energized by a common means.

It is convenient for the function of the means for supplying variable-frequency current to the radio frequency coils to be performed by a variablevfrequency oscillator.

Alternatively, the means for supplying variabile-ifrequency current to the radio frequency coils may be a feedback circuit provided in one of the sensors, the parameters of the feedback circuit being selected so as to ensure a phase balance and a resonance frequency amplitude balance in the feedback circuit, which circuit is electrically connected to the input of the radio frequency coil of the other sensor.

Alternatively again, the function of the means for supplying variablezfrequency current to the radio frequency coils may be performed by an additional sensor identical with the first two sensors and provided with a feedback circuit whose parameters are selected so as to ensure a - phase balance and a resonance frequency amplitude balance, which circuit is electrically connected to the inputs of the radio frequency coils ob the first two sensors.

In this latter case it is advantageous for the device to include a frequency (or phase) detector connected to the output of the additional sensor, and a filter tuned to the frequency of noise to be compensated and connected with its input to the output of the frequency (or phase) detector, whereas the output of the filter is connected to the input of the radio frequency coil of the additional sensor and, via a signal level regulator, to the inputs of the radio frequency coils of the first two sensors.

The gradient meter of the present invention makes it possible to measure small magnetic field gradients of the order of 10-8 Gauss.

Interaction between the sensors at small abase distances is ruled out due to the faot that the proposed device operates at a single frequency coherent for all the sensors, the effectiveness of filtering the output signal of the gradient meter is not affected by the value of the magnetic field gradient, which value may be close to half the width d the magnetic resonance line.

The proposed gradient meter features a high immunity to noise generated by electric networks, and a high vibration resistance.

The invention will be better understood from the following detailed description of pre ferred embodiments thereof, taken in conjunc tion with the accompanying drawings, wherein: Figure 1 is a block diagram of a magnetic field gradient measuring device, wherein the means for supplying variable-frequency current to the radio frequency coils is a variable frequency oscillator; Figure 2 is a block diagram of a magnetic field gradient measuring device, wherein the means for supplying vatiable4requency current to the radio frequency coils is a feedback circuit provided in one of the sensors; Figure 3 is a block diagram of a magnetic field gradient measuring device, wherein the means for supplying variable-frequency current to the radio frequency coils is an additional sensor; Figure 4 is a block diagram of a magnetic field gradient measuring device provided with a circuit intended to eliminate electric network noise.

Referring to the attached drawings, the pro posed magnetic field gradient measuring device (gradient meter) comprises two identical sensors 1 and 2 (Figure 1). Each of the sensors 1 and 2 oomprises an absorption cell 3 filled with a working substance exhibiting magnetic resonance, for example the saturated vapour of an alkali metal, for example, cesium, and provided with means to prevent deposition of atoms of the working substance. The cell 3 is arranged within a radio frequency coil 4. The beam of light emitted by a spectro scopic lamp 5 performing the function of a light source travels through a light guide 6, a lens 7 and an interference polarization plate 8 to the input of the cell 3. Arranged at the output of the cell 3, downstream of the beam of light, are a lens 9 and a photocell 10 con nected with its output to the input of an amplifier 11. The spectroscopic lamp 5 is powered by a generator 12.

The outputs of the amplifiers 11 form the outputs of the sensors 1 and 2 and are con nected to inputs of respective frequency multi pliers. The output of that frequency multi plier 13 which is connected to the output of the sensor 2 is coupled via a phase shifter 14 to one input of a phase detector 15. The output of that frequency multiplier 13 which is connected to the output of the sensor 1 is directly connected to the other input of the phase detector 15.

The frequency multipliers 13 are conven tional devices built around, for example, an automatic phase frequency control circuit. The known frequency multipliers may be com plemented by units arranged to stabilize the output signal phase with respect to the input signal phase The radio frequency coils 4 of both sensors 1 and 2 are supplied with variable-frequency current by a common means of which the output is connected to the inputs of the radio frequency coils 4. In the embodiment of Figure 1, this means is a variable-frequency oscillator 16. The frequency of the oscillator 16 is selected so as to be close to the magnetic transition resonance frequency of atoms of the working substance determined by the intensity of the magnetic field at the points the transmitters 1 and 2 are located.

The sensors 1 and 2 are mounted on a common rigid base (not shown in Figure 1) and spaced at a base distance of from a few centimeters to several meters, depending on the purpose of magnetic field gradient measurements.

Figure 2 is a block diagram of another embodiment of the proposed device. Unlike the device of Figure 1, the means for supplying variable-frequency current to the radio frequency coils 4 is a feedback circuit 17 provided in the sensor 2. The parameters of the feedback circuit 17 are selected so as to ensure a phase balance and an amplitude balance at the magnetic transition frequency of atoms of the working substance. The feedback circuit 17 is electrically connected to the input of the radio frequency coil 4 of the sensor 1.

Unlike the device of Figure 1, the means for supplying variable-frequency current to the radio frequency coils 4 of the embodiment of Figure 3 is a third sensor 18 which is identical to the sensors 1 and 2. The sensor 18 is provided with a feedback circuit 17 whose parameters are selected so as to ensure a phase balance and an amplitude balance at the magnetic transition frequency of atoms of the working substance. The feedback circuilt 17 is electrically coupled to the inputs of the radio frequency coils 4 of the sensors 1 and 2.

Unlike the device of Figure 3, the embodi- ment of Figure 4 includes a frequency (or phase) detector 19 for noise signal separation, connected to the output of the sensor 18.

The output of the frequency (or phase) detector 19 is connected via a filter 20, tuned to the noise frequency range to the radio frequency coil 4 of the sensor 18 and via a signal level regulator 21 to the radio frequency coils 4 of the sensors 1 and 2. The output of tne sensor 18 is also connected to all of the radio frequency coils 4 via isolation circuits 22, for example, active resistors.

The proposed magnetic field gradient measuring device operates as follows.

As the device is switched on, the highfrequency generator 12 produces a discharge in the spectroscopic lamp 5 (Figure 1).

The working substance of the spectroscopic lamp 5 is the same as in the absorption cell 3, so in the course of the discharge the spectroscopic lamp 5 emits a beam of light which is in resonance with the atoms contained in the absorption cell 3.

In order to carry out optical pumping of atoms of the working substance in the cell 3, the beam d light of the spectral lamp 5 is directed through the light guides 6, lenses 7 and interference polarization plates 8 which separate a desired resonance radiation line for optical pumping and effect its circular polarization with respeot to the absorption cells 3.

The cells 3 containing saturated vapour of an alkali metal, for example, cesium, are filled with a buffer gas to prevent deposition of atoms of the working substance on the walls of the cells 3 in the course of optical pump ing. Another way d preventing deposition is to coat the walls of the cells 3 with a thin layer of paraffin.

As the radio frequency coils 4 are energized by the variable-frequency oscillator 16, a varying magnetic field is produced in the cells 3.

If the frequency ob the oscillator 16 is close to the frequency of transition between the magnetic sublevels of atoms of the working substance, between which the optical pumping produces an inverse population, and if the angle between the direction of the magnetic field Ho and the axis of the beam of light in the cells 3 is close to 45 , the beam. of light that passes through the cells 3 is amplitude-modulated. Maximum modulation occurs when the frequency of the variable frequency oscillator 16 is equal to that od magnetic transition.

Upon passing through the cells 3, the amplitude-ntodulated beam of light is detected by the photocells 10 and amplified.

In the absence of a magnetic field gradient, the phase shift between the signals at the out puts of the identical sensors 1 and 2 is zero.

The presence of a gradient AH accounts for a phase shift Acp related to the magnetic field gradient as follows: 900 AH.A Af [degrees], where A is an atom constant, whereby the magnetic transition frequency is related to the magnetic field intensity; and Af is the width of the magnetic transition line, which is dependent upon the transverse relaxation time of atoms in the cells 3 and the intensity of the pumping light.

The phase shift Ao is measured by the phase detector 15. In order to improve the resolution of the device, signals from the sensors 1 and 2 are applied to said phase detector 15 via the identical frequency multipliers 13.

Normal operation of the phase detector 15 requires a certain phase shift between signals applied to its inputs. This shift is determined by the phase shifter 14.

The gradient meter of Fig. 1 makes it possible to measure small gradients with small distances between the sensors. However, high effectiveness requires the presence of a magnetic field which would be stable with time, and a low level of variable magnetic noise which causes amplitude noise modulation of sensor signals.

In order to measure the gradient of a magnetic field that varies with time, it is more convenient to use the gradient meter of Fig.

2, wherein the means for supplying variable- frequency current to the radio frequency coils 4 is the feedback circuit 17 provided in the sensor 2 and connected to the input of the radio frequency coil 4 of the sensor 1. The parameters of the feedback circuit- 17 are selected so as to ensure a phase balance and a resonance frequency amplitude balance; as a result, the sensor 2 operates in the selfoscillation mode, its oscillation frequency following variations in the external magnetic field, whereby normal functioning of the sensor 1 is ensured.

The gradient meter of Fig. 2 makes it possible to measure gradients of magnetic fields which vary with time; however, this gradient meter is sensitive to variable magnetic noise that causes amplitude noise rnodulation of signals of the transmitters 1 and 2.

In order to measure magnetic field gradients in the presence of magnetic noise, it is preferable that use should be made of the gradient meter shown in Fig. 3.

In the embodiment of Fig. 3, the function of the means for supplying variarbleafrequency current to the radio frequency coils 4 is performed by the sensor 18 provided with the feedback circuit 17 which connects the output of said sensor 18 to the radio frequency coils 4 of all the three sensors 1, 2 and 18. The parameters of the feedback circuit 17 of the sensor 18 are selected so as to ensure a phase balance and a resonance frequency amplitude balance; as a result, the sensor 18 operates in the self-oscillation mode, its oscillation frequency following variations in the intensity of the external magnetic field, which automatically ensures normal functioning of the sensors 1 and 2.

The permissible gradient of the magnetic field between the three sensors 1, 2 and 18 must be of a value at which the self-oscillation frequency of the sensor 18 should be found within the interval between the resonance frequencies ob the sensors 1 and 2.

The gradient meter of Fig. 3 makes it possible to measure gradients of magnetic fields which vary with time at variable magnetic noise levels that are commensurable, in terms of magnetic units, with the width of the magnetic transition line.

If the magnetic noise level is higher, for example, because of the presence of electric networks, the best accuracy of measurements is provided by the gradient meter of Fig. 4.

The device of Fig. 4 includes a frequency (phase) detector 19 intended for noise separation and connected to the output of the sensor 18. The frequency detector 19 may have different circuitries; one of the possible versions is an automatic phase frequency control circuit operating as a narrow-band follow filter. An error signal is applied from the output of a phase detector, incorporated in the abovementioned automatic phase frequency control circuit, to the filter 20 tuned to the noise frequency.

The output signal of the filter 20 is applied to the radio frequency coil 4, whereby in the cell 3 of the sensor 18 there is produced a magnetic field which is in antiphase with the magnetic field of the noise; thus the noise is compensated.

In the sensor 18, the degree of noise suppression is determined by the transfer factor K of the feedback circuit from the frequency detector 19 to the radio frequency coil 4.

The same noise, acting upon the sensors 1 and 2, is compensated by the magnetic field at the noise frequency by connecting the radio frequency coils 4 to the output of the filter 20 via the signal level regulator 21. If the intensity of the magnetic compensation field is equal to half the sum total of the intensities of the noise fields acting upon the sensors 1 and 2, any change in the events can be detected at the output of the device.

The noise suppression circuit of the sensor 18 reduces the noise signal (K + 1)-fold. If similar circuits are introduced in the sensors 1 and 2, the noise gradient is reduced accordingly, which makes it unnecessary to adjust the noise compensation level.

The gradient meter of Fig. 4 makes it possible to measure gradients of magnetic fields varying with time at levels of variable magnetic noise which are in excess, in terms of magnetic units, of the width of the magnetic transition line.

WHAT WE CLAIM LS:- 1. A magnetic field gradient measuring device, comprising two sensors which in use are arranged at spaced points in a magnetic field which are to be investigated, the outputs of the two sensors being connected to inputs of a phase detector, each of the two sensors comprising an absorption cell filled with atoms of a working substance exhibiting magnetic transition resonance, each said absorption cell having arranged at an input thereof a light source suitable for optical pumping of atoms of the working substance, and at an output thereof a photoconverter responsive to the beam of light passing through the cell, each said absorption cell being arranged inside a coil energized with radio frequency current of a variable frequency close to the magnetic transition resonance frequency of atoms of the working substance, determined by the intensity of the magnetic field at the location of the respective sensor, the radio frequency coils of both sensors being energized by a common means.

2. A magnetic field gradient measuring device as claimed in claim 1, wherein the means for supplying variableifrequency current to the radio-frequency coils is a variablefrequency oscillator.

3. A magnetic field gradient measuring device as claimed in claim 1, wherein the means for supplying variable 4requency current to the radio frequency coils is a feedback circuit provided in one of the sensors, the parameters of the feedback circuit being selected so as to ensure a phase balance and a resonance frequency balance in this feedback circuit which is electrically connected to the input of the radio frequency coil of the second sensor.

4. A magnetic field gradient measuring device as claimed in claim 1, wherein the means for supplying variable-frequency current to the radio frequency coils is a third sensor identical with the two sensors and provided with a feedback circuit whose parameters are selected so as to ensure a phase balance and a resonance frequency balance, the feedback circuit being electrically connected to the inputs of the radio frequency coils of the first and second sensors.

5. A magnetic field gradient measuring device as claimed in claim 4, provided with a frequency (phase) detector connected to the output of the third sensor, and with a filter tuned to the frequency of the noise to be compensated, the filter being connected with its input to the output of the frequency (phase) detector, and with its output to the input of the radio frequency coil of the third transmitter and via a signal level regulator to the inputs of the radio frequency coils of the first and second sensors.

6. A magnetic field gradient measuring device, substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. (phase) detector 19 intended for noise separation and connected to the output of the sensor 18. The frequency detector 19 may have different circuitries; one of the possible versions is an automatic phase frequency control circuit operating as a narrow-band follow filter. An error signal is applied from the output of a phase detector, incorporated in the abovementioned automatic phase frequency control circuit, to the filter 20 tuned to the noise frequency. The output signal of the filter 20 is applied to the radio frequency coil 4, whereby in the cell 3 of the sensor 18 there is produced a magnetic field which is in antiphase with the magnetic field of the noise; thus the noise is compensated. In the sensor 18, the degree of noise suppression is determined by the transfer factor K of the feedback circuit from the frequency detector 19 to the radio frequency coil 4. The same noise, acting upon the sensors 1 and 2, is compensated by the magnetic field at the noise frequency by connecting the radio frequency coils 4 to the output of the filter 20 via the signal level regulator 21. If the intensity of the magnetic compensation field is equal to half the sum total of the intensities of the noise fields acting upon the sensors 1 and 2, any change in the events can be detected at the output of the device. The noise suppression circuit of the sensor 18 reduces the noise signal (K + 1)-fold. If similar circuits are introduced in the sensors 1 and 2, the noise gradient is reduced accordingly, which makes it unnecessary to adjust the noise compensation level. The gradient meter of Fig. 4 makes it possible to measure gradients of magnetic fields varying with time at levels of variable magnetic noise which are in excess, in terms of magnetic units, of the width of the magnetic transition line. WHAT WE CLAIM LS:-

1. A magnetic field gradient measuring device, comprising two sensors which in use are arranged at spaced points in a magnetic field which are to be investigated, the outputs of the two sensors being connected to inputs of a phase detector, each of the two sensors comprising an absorption cell filled with atoms of a working substance exhibiting magnetic transition resonance, each said absorption cell having arranged at an input thereof a light source suitable for optical pumping of atoms of the working substance, and at an output thereof a photoconverter responsive to the beam of light passing through the cell, each said absorption cell being arranged inside a coil energized with radio frequency current of a variable frequency close to the magnetic transition resonance frequency of atoms of the working substance, determined by the intensity of the magnetic field at the location of the respective sensor, the radio frequency coils of both sensors being energized by a common means.