According to the needs, the laboratory includes: aerial magnetometer, ground magnetometer, borehole magnetometer, ocean magnetometer and high sensitivity magnetometer.
Magnetometers can be divided into relative measuring instruments (such as suspended wire vertical magnetometer, which measures the relative difference of vertical component z of geomagnetic field) and absolute measuring instruments (such as proton precession magnetometer, which measures the absolute value of the total intensity of geomagnetic field, and can also measure the relative value or gradient value) according to the parameters and sizes of geomagnetic field measured.
4.4. 1. 1 fluxgate magnetometer
Permalloy is a soft magnetic alloy with high permeability and low coercivity. Under the action of external magnetic field (the hysteresis line is narrow and steep), it quickly reaches saturation magnetization, so fluxgate is also called saturation magnetometer. That is, the external magnetic field changes little, the intensity of the induced magnetic field changes greatly, and the instrument is very sensitive. Making permalloy into a closed magnetic circuit; The output pulse voltage of the external excitation magnetic coil and the signal receiving winding is proportional to the external magnetic field. There are many kinds of magnetometers, such as aviation, ground magnetometers, magnetic susceptibility measuring instruments and so on.
4.4. 1.2 proton precession magnetometer
A container in a solenoid that can generate a magnetic field is filled with a liquid rich in hydrogen (such as water). When the magnetic field is generated by electrification, the excited hydrogen nuclei (protons) spin to generate magnetic moments, which are arranged in parallel along the spiral direction, resulting in paramagnetic macroscopic magnetic moments. When the solenoid magnetic field perpendicular to the geomagnetic field stops, the macroscopic magnetic moment of the hydrogen nucleus makes larmor precession around the direction of the total intensity (f) of the geomagnetic field, and the relationship between precession frequency and the geomagnetic field (f) is as follows.
Introduction to environmental geophysics
It shows that the precession frequency f is proportional to f, and the coil that generates the instrument excitation magnetic field is also the receiving coil, which is tuned to the precession frequency f, so in a certain intensity geomagnetic field, the magnetic moment of proton precession will generate an induced voltage in the coil, which is the intensity signal of geomagnetic field.
4.4. 1.3 optically pumped magnetometer
According to the principle that atoms are excited after gaining energy and jump from low energy level to high energy level. The optically pumped magnetometer uses the helium (4He) atomic lamp to emit light with the wavelength of 1.08μm, and makes a parallel beam in the same direction as the geomagnetic field (the measured magnetic field). After passing through an absorption chamber filled with 4He, 4He absorbs 1.08 micron light and forms positive ions, which jump from low energy level to high energy level (called optical pumping). The magnetic moments of these 4He atoms are arranged in parallel. The relationship between transition magnetic moment frequency f0 and geomagnetic field T is as follows
Introduction to environmental geophysics
Because f0 in the formula is much higher than F in (4.4. 1), it is beneficial to improve the sensitivity of the instrument. A modulated magnetic field perpendicular to the incident direction of light is added to the absorption chamber of the instrument, so that the frequency of the incident magnetic field can automatically track the change of geomagnetic field and realize automatic measurement.
4.4. 1.4 superconducting magnetometer
Josephson proposed in 1962 and it was proved by experiments that there is an insulating layer between two superconductors 10 ~ 30 A, and superconducting electrons can pass through unimpeded, and there is no voltage drop at both ends of the insulating layer, which is called superconducting tunnel junction (Josephson junction). This phenomenon is called Josephson effect of superconducting tunnel junction.
Superconducting magnetometer is a measuring instrument based on Josephson effect. Its measuring device is a closed loop made of superconducting material with one or two superconducting tunnel junctions. The interception area of the joint is very small. As long as a small current (10-1~10-6a) passes, the junction will reach the critical current Ic. The superconductivity of Ic is destroyed, that is, the maximum superconducting current that superconducting tunnel junction can bear. Ic is very sensitive to magnetic field and fluctuates periodically with the magnitude of external magnetic field. Its amplitude gradually decays. The critical current Ic is also a periodic function of the magnetic energy φ penetrating the superconducting junction. It uses the periodic response of the device to count the change of magnetic energy (proportional to the change of external magnetic field), and the magnetic field value can be calculated when the area of the ring is known.
Superconducting magnetometer is a highly sensitive magnetometer developed by superconducting technology in the mid-1960s. Its sensitivity is several orders of magnitude higher than other magnetometers (up to 10-6nT), and it can measure the magnetic field of 10-3nT. Wide measuring range, high frequency response of magnetic field and stable and reliable observation data. In geomagnetism, it is used to study the disturbance of geomagnetic field. In magnetotelluric and electromagnetic methods, it is used to measure weak magnetic field changes. In rock physics, it is used for the study of rock magnetism.
Because the probe of this instrument needs low temperature, it is often cooled by helium in Dewar bottle, which makes the equipment complicated and expensive. At present, it is mainly used in laboratories. However, with the continuous progress of superconducting technology research, it is believed that in the near future, its application in environmental geophysics will be more and more.