* Optically pumped magnetometer and superconducting magnetometer

(1) Optically pumped magnetometer

After proton precession magnetometer, the optically pumped magnetometer was applied to geophysical work in the mid-195s. It is a high sensitivity and high precision magnetometer.

1. Physical principle of optically pumped magnetometer

(1) Zeeman splitting and energy level transition. Atoms in an external magnetic field can be split into (2f+1) magnetic sub-levels with the same f value (total angular quantum number) due to the action of the magnetic field, which is called Zeeman splitting. The energy difference between adjacent magnetic sub-levels is proportional to the external magnetic field, which makes it possible to measure the geomagnetic field T.

when electrons gain energy from the outside world or release appropriate energy to the outside world, that is, they jump from one energy level to another, and the change of atomic energy level is called atomic jump.

(2) optical pumping. In the optically pumped magnetometer, helium is used as the working substance, and the process of pumping the energy state of atoms to the same energy level by using light energy is called optical pumping.

2. Tracking optical pump magnetometer to measure geomagnetic field T

In the probe device of optical pump magnetometer, helium lamp is filled with 4He with higher air pressure. Excited by high-frequency electric field, it emits monochromatic light of 183.75nm, which passes through convex mirror, polarizer and quarter-wave plate to form circularly polarized light of 1.8μm and irradiates the absorption chamber. The optical axis of optical system should be consistent with the direction of geomagnetic field (measured magnetic field). The absorption chamber is filled with 4He with low air pressure, and excited by high frequency electric field, its 4He atom becomes metastable positive helium, which is magnetic. The circularly polarized light from the helium lamp interacts with metastable positive helium, resulting in atomic transition. The transition frequency f has the following relationship with geomagnetic field t:

InTroduction to geophysical exploration

In the formula, t takes nt as the unit. That is to say, circularly polarized light orients the magnetic moments of atoms in the absorption chamber, and then the light emitted by the helium lamp can pass through the absorption chamber, be focused by the convex mirror, and irradiate the photosensitive element to form a photocurrent.

RF electromagnetic field (modulation field) is applied in the direction perpendicular to the optical axis, and its frequency is equal to the atomic transition frequency f. Due to the interaction between radio frequency magnetic field and the magnetic moments of atoms in directional arrangement, the arrangement of atomic magnetic moments in the absorption chamber is disturbed (called magnetic * * * vibration). At this time, the circularly polarized light from the helium lamp will interact with the disordered atomic magnetic moments, and cannot penetrate the absorption chamber, so the photocurrent is the weakest. By measuring the radio frequency f at this time, the value of geomagnetic field T can be obtained. When the geomagnetic field changes, the frequency of the RF field is changed accordingly, so that the light passing through the absorption chamber is the weakest, that is, the frequency of the RF field automatically tracks the change of the geomagnetic field to realize continuous and automatic measurement of the T value.

(2) Superconducting magnetometer

It is a highly sensitive magnetometer developed in the mid-196s. Its sensitivity is several orders of magnitude higher than that of other magnetometers, which can reach 1-6 nt. It has wide measuring range, high magnetic field frequency response and stable and reliable observation data.

the basic principle of superconducting magnetometer is as follows: when the temperature of some metals such as tin, lead, zinc, niobium, tantalum and some alloys drops below a certain temperature near absolute zero, their resistance suddenly drops to zero. This property of sudden disappearance of resistance at low temperature is called superconductivity, and substances with this property are called superconductors. The temperature when the resistance is zero is called critical temperature Tc, such as tin (3.7K), lead (7.2K) and niobium (9.2K).

In p>1962, Josephson proposed and it was proved by experiments that there is an insulating layer between two superconductors with a diameter of 1 nm to 3 nm, so that superconducting electrons can pass through unimpeded, and there is no voltage drop at both ends of the insulating layer. This insulating layer is called superconducting tunnel junction (Josephson junction), and this phenomenon is called Josephson effect of superconducting tunnel junction.

The superconducting magnetometer measures the magnetic field by using Josephson effect, and its measuring device is a closed loop made of superconducting material with one or two superconducting tunnel junctions. The cross-sectional area of the junction is very small. As long as a small current (1-4a ~ 1-6a) is passed, the critical current Ic will be reached at the junction (exceeding the superconductivity of Ic, that is, the maximum superconducting current that the junction can bear). Ic is very sensitive to magnetic field. It fluctuates periodically with the magnitude of external magnetic field, and its amplitude gradually decays. The critical current Ic is also a periodic function of the magnetic flux φ penetrating into the superconducting junction. It uses the periodic response of the device to the external magnetic field to count the change of magnetic flux (which is proportional to the change of external magnetic field); Given the area of the ring, the magnetic field value can be calculated.

In the field of applied geophysics, superconducting aeromagnetic gradiometer can be made. It can be used to study the perturbation of geomagnetic field in geomagnetism; It can be used to measure weak magnetic field changes in magnetotelluric current method; It can also be used in the study of rock magnetism. Because the probe of this instrument needs low temperature, it is often cooled by liquid helium in Dewar bottle, which is complicated in equipment and expensive.