What is a magnetometer

What is a magnetometer?

Magnetometer is a general term for measuring the intensity and direction of magnetic field. Magnetometers for measuring geomagnetic field strength can be divided into absolute magnetometer and relative magnetometer. The accuracy of the absolute magnetometer is determined by some physical constants of the instrument itself; the accuracy of the relative magnetometer can only be determined by comparing with the absolute magnetometer. The main purpose is to collect magnetic anomaly data and determine rock magnetic parameters. Since the 20th century, magnetometer has experienced the development process from simple to complex, mechanical principle to modern electronic technology.

Development history of magnetometer

Cal Friedrich Gauss invented the first magnetometer in 1833. Later, magnetometer was widely used in geomagnetic field measurement and geophysical exploration to measure various types of magnetic anomalies. Magnetometers are also used in modern smart phones (Compass APP), space satellites, and military applications to detect submarines. Some countries, such as the United States, Canada and Australia, classify super sensitive magnetometers as military technology, and strictly control the sale of products. Magnetometer can detect magnetic (iron) metal, and the detection depth of this kind of metal is deeper than that of conventional metal detector; compared with the general metal detector, the detection range of magnetometer is large, and it can detect large targets, such as cars tens of meters long, while the detection range of general metal detector is rarely more than two meters. Magnetometer has gradually developed from the application of traditional mining and oil industry to high-precision aeromagnetic survey and space spacecraft to detect planets.
According to the development history of magnetometer and the physical principle of its application, it can be divided into three parts:

• The first generation magnetometer is based on the principle of interaction torque between permanent magnet and geomagnetic field, or using induction coil and auxiliary mechanical device. Such as mechanical magnetometer, induction aviation magnetometer, etc.
• The second generation magnetometer uses NMR characteristics, high permeability soft magnetic alloy and special electronic circuits. Such as proton magnetometer, optical pump magnetometer, and fluxgate magnetometer.
• The third generation magnetometer uses the low temperature quantum effect, namely superconducting magnetometer.

Measuring principle of magnetic field

The magnetic field is a vector field with magnitude and direction. There are mainly two kinds of magnetometers. The vector magnetometer is used to measure the magnetic field component, and the total field magnetometer (scalar magnetometer) measures the vector field. The magnetometer measures the earth’s magnetic field according to the magnetic declination and inclination, and reflects each component. Absolute magnetometer measures the absolute amplitude or vector of the magnetic field, using internal calibration or known physical parameters of the sensor. Relative magnetometer measures scalar or vector of magnetic field with a fixed and uncalibrated reference line, which is also called magnetometer.

Working principle of magnetometer

Proton precession magnetometer uses hydrogen atom to generate precession signal. It uses certain liquids, such as kerosene, because they provide a very high hydrogen density and are not dangerous to operate.
Let the polarized DC current pass through a coil wound around the liquid sample, which will produce an auxiliary flux density of 100 Gauss. The proton is polarized to a strong net magnetization, which is in thermal equilibrium with a strong magnetic flux density.
When the auxiliary flux is terminated, the polarized protons spin and rearrange to the normal flux density. According to the following formula, the precession frequency f0 of proton is directly related to the flux density B (Tesla Tesla, t):

f0=(p/2)B p/2=42.5763751MHz/T

The measurement of proton precession must be carried out in sequence. That is, there is an initial polarization, then the frequency is measured, and then the cycle is repeated. This is different from the continuous measurement method of precession while the hydrogen nucleus is polarized.

Classification of magnetometer

According to its internal structure and working principle, magnetometer can be divided into:

• ① mechanical magnetometer. For example, suspension type magnetic scale, blade type magnetic scale, etc.;
• ② electronic magnetometer. Such as proton magnetometer, optical pump magnetometer, flux gate magnetometer, etc. If they are used in the field of magnetometers, they can be divided into: ground magnetometers, aviation magnetometers, marine magnetometers and in-hole magnetometers.

According to the parameters and the measurement values of the geomagnetic field, the magnetometer can be divided into:

• ① relative measuring instruments, such as suspension type vertical magnetometer, etc., it is the relative difference of the vertical components of the geomagnetic field;
• ② the measuring instrument, such as proton magnetometer, is the value of measuring the total strength of the geomagnetic field; however, the gradient value can be measured.

The accuracy of the measured value of magnetometer is determined by the instrument itself, and the accuracy of the relative magnetometer measurement value can be determined only after comparing with the magnetometer. The common magnetometers are as follows:

• 1. Geomagnetic induction instrument. An instrument for measuring the inclination of geomagnetic. It was made by W.E. Weber in 1837 on the basis of the principle of electromagnetic response. The measurement accuracy can reach several seconds.
• 2. Magnetic deflector. An instrument for measuring the geomagnetic bias angle. It is mainly composed of magnetic system, suspension wire, collimation telescope and horizontal disk. The measurement accuracy can reach several seconds.
• 3. Quartz wire horizontal strength magnetometer. A relative magnetometer for measuring the horizontal strength of the geomagnetic field. It was designed by Danish scholar D. lakur in 1936 based on the principle of balance between torsional moment and magnetic moment. The main part of the instrument is a fine quartz wire and magnetic needle. This instrument can be used for field magnetic measurement and can also be used for the geomagnetic record correction of geomagnetic station. Before using, it must be calibrated with magnetometer.
• 4. Zero magnetic scale. A relative magnetometer for measuring the vertical strength of the geomagnetic field. It was made by lakur in 1942 by using the interaction between two magnetic needles according to the principle of balance between heavy torque and magnetic moment.
• 5. Flux gate magnetometer. A relative magnetometer for measuring the intensity and direction of the geomagnetic field. The instrument is composed of independent flux gate probes. Each flux gate probe can independently detect the strength of the geomagnetic field in a certain direction, and combine the three probes vertically with each other, and then three components of the geomagnetic field strength can be measured simultaneously. Magnetometer is developed in the Second World War to detect enemy submarines from aircraft. It has been widely used in geomagnetic station, land magnetic survey, aeromagnetic survey, satellite magnetic survey and so on.
• 6. Proton precession magnetometer. Magnetometer for measuring the total strength of the geomagnetic field. The strong magnetic field polarizes the protons in water or carbon hydride. When the strong magnetic field is suddenly removed, the proton spins around the earth magnetic field at an angular velocity W. The total strength of the geomagnetic field can be calculated by measuring the precession frequency of the proton. This instrument is not afraid of vibration, and is suitable for loading on ships, balloons, aircraft, artificial satellites and other vehicles.
• 7. Optical pump magnetometer. Magnetometer made according to the principle of light pump. The magnetic moment arranged by the action of optical pump will produce resonance absorption and disrupt the arrangement of atoms under the action of alternating electromagnetic field of a specific frequency. The frequency of electromagnetic field which occurs resonance absorption is proportional to the intensity of the external magnetic field at the point where the sample is located. Therefore, the value of the external magnetic field can be measured by measuring the frequency. The common working elements are: potassium (K39); rubidium (Rb87, rb85); cesium (cs133); helium (HE4, He3), etc.

Main technical specifications of magnetometer

Technical indicators are the technical parameters reflecting the overall performance of the instrument, usually including: sensitivity, precision, accuracy, stability, range, etc.

Sensitivity

Sometimes referred to as the ability of a magnetometer to reflect changes in magnetic field intensity (z). For the instrument reading magnetic field value with digital display (such as proton magnetometer), the small discernible change of Z estimated on its reading device is called display sensitivity (or reading ability), such as 1nt / word, 0.1nt/word, etc. Because the instrument has a problem of noise level, there is a conceptual difference between sensitivity and display sensitivity.

Precision

It is an index to measure the repeatability of the instrument, which refers to the Z small reliable value that the instrument can achieve by measuring the magnetic field itself. Expressed by the square deviation of a set of measured values from the mean. It is called self repetition precision in the instrument manual.

Accuracy

It refers to the ability of the instrument to determine the true value, that is, the total error compared with the true value. In the work of magnetic exploration, precision and accuracy are usually not distinguished and collectively referred to as precision.

Composition and brief working process of proton magnetometer

Taking CZM-2 proton magnetometer as an example, the instrument adopts integrated circuit and program control. It has the characteristics of small volume, light weight, digital display, wide measuring range, strong anti-interference ability, low power consumption, stable performance and high precision. It is a product of China in the mid-1980s. It is mainly used for ground magnetic survey, daily variation station and geomagnetic station in earthquake prediction.

CZM-2 proton magnetometer
The composition of the instrument proton magnetometer is generally composed of instrument host, probe and battery box. The main engine adopts semi sealed structure, the probe adopts fully sealed structure, and adopts antimagnetic material.
The main part of the magnetometer is a digital frequency meter which can measure the proton precession signal with 2 × 10-5 accuracy. Its frequency range is 1360hz-3040hz (corresponding magnetic field 31942-71401nt). When the micro switch is pressed once, the magnetization system starts to work under the control of the program controller, that is, the magnetizing current flows through the probe, and the probe coil generates n × 103a / M magnetizing magnetic field. After a few seconds, the magnetization system is shut down and the magnetization field disappears. The proton magnetic moment is rotated under the action of geomagnetic field t, and the cutting coil generates the induction signal in the probe. By properly selecting the probe matching capacitance C and the probe coil inductance L to resonate at the precession frequency f, the input end of the frequency selection amplifier can obtain the z-large signal. After frequency selection and amplification, it is sent to the frequency multiplier, which multiplies f by 128 times, and then it is sent to the counter through the electronic gate, and the value of geomagnetic field t is directly displayed on the display.

Comparison of several optical pumping magnetometers

Source: China Permanent Magnet Manufacturer www.rizinia.com

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