(I) Optical Pump Magnetometer
Following the proton magnetometer, the optical pump magnetometer began to be used in geophysical work in the mid 1950s. It is a kind of high sensitivity, high precision magnetometer.
1.Physical principle of optical pump magnetometer
(1) Seeman splitting, energy level jump. Atoms in the external magnetic field, due to the action of the magnetic field, the same F value (total angular quantum number) of the energy level, can be split into (2F + 1) magnetic sub-energy level, called the Seeman split. The energy difference between neighboring magnetic sub-energy levels is proportional to the external magnetic field, which provides the possibility of determining the geomagnetic field T.
When an electron receives energy from the outside world or gives out appropriate energy to the outside world, i.e., leaps from one energy level to another, the change in the atomic energy level, is called the leap of an atom.
(2) optical pump action. In the light pump magnetometer there are helium as a working substance, the use of light energy, the energy state of the atom pump excitation to the same energy level of the process, it is called the light pump action.
2. Tracking optical pump magnetometer to determine the geomagnetic field T
In the optical pump magnetometer probe device, helium lamp is filled with higher pressure 4He. excitation by the high-frequency electric field, issued by the 1083.075nm monochromatic light, which is through the convex mirror, polarizer and 1/4 wavelength piece, the formation of 1.08μm circular polarization of the light irradiation to the absorption chamber. The optical axis of the optical system should be consistent with the direction of the geomagnetic field (the magnetic field being measured). The absorption chamber is filled with 4He at a lower pressure, which is excited by a high-frequency electric field, and its 4He atoms are changed into substable positive helium with magnetic properties. Circularly polarized light from a helium lamp interacts with the stabilized positive helium to produce an atomic jump. The frequency of the jump, f, is related to the geomagnetic field, T, as follows:
Introduction to Geophysical Exploration
The formula: T is measured in nT. This means that the circularly polarized light causes the magnetic moments of the atoms in the absorption chamber to align directionally, after which 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.
In the perpendicular direction of the optical axis plus radio frequency electromagnetic field (modulation field), its frequency is equal to the atomic jump frequency f. Due to the interaction of the radio frequency magnetic field and directional arrangement of the atomic magnetic moments, thereby disrupting the arrangement of the atomic magnetic moments in the absorption chamber (called magnetic **** vibration). At this time, the helium lamp from the circularly polarized light will be with the chaotic arrangement of atomic magnetic moments, can not penetrate the absorption chamber, the photocurrent is the weakest, the determination of this time the radio frequency f, you can get the value of the geomagnetic field T. When the local magnetic field changes, the corresponding change in the frequency of the radio frequency field to keep the weakest light through the absorption chamber, that is, the frequency of the radio frequency field to automatically track the changes in the geomagnetic field to realize the continuous automatic measurement of the T value.
(B) superconducting magnetometer
It is a highly sensitive magnetometer developed in the mid-1960s. Its sensitivity is several orders of magnitude higher than other magnetometers, up to 10-6nT. It has a wide range of measurement, high frequency response of the magnetic field, stable and reliable observation data.
The basic principle of superconducting magnetometer is as follows: some metals such as tin, lead, zinc, niobium, tantalum and some alloys, when their temperatures drop to a temperature below absolute zero near a certain temperature, its resistance suddenly drops to zero. This low-temperature conditions, the sudden disappearance of resistance properties, known as superconductivity, with this property of the material called superconductor. The temperature at which the resistance is zero is called the critical temperature Tc, such as tin (3.7K), lead (7.2K), niobium (9.2K).
In 1962, Josephson proposed and experimentally confirmed that in the middle of the two superconductors sandwiched between 1nm ~ 3nm insulation, superconducting electrons can pass through unimpeded, no voltage drop at the two ends of the insulating layer, the insulating layer is called a superconducting tunnel junction (Josephson junction), a phenomenon known as superconducting tunnel junction of the Josephson effect.
Superconducting magnetometer is the use of the Josephson effect to measure the magnetic field, the measurement device is made of superconducting material closed loop, there are one or two superconducting tunnel junction. Junction of the cross-sectional area is very small, as long as through a small current (10-4A ~ 10 -6A), the junction at the critical current Ic (more than Ic superconductivity is destroyed, that is, the junction can withstand the maximum superconducting current). Ic is very sensitive to the magnetic field, which with the size of the external magnetic field is a periodic ups and downs, and the amplitude of its gradual attenuation. The critical current, Ic, is also a periodic function of the magnetic flux Φ penetrating the superconducting junction. It uses the periodic response of the device to the external magnetic field to count the flux change (which is proportional to the change in the external magnetic field); with the area of the ring known, the value of the magnetic field can be calculated.
In the field of applied geophysics, it can be made into a superconducting aeromagnetic gradiometer; in geomagnetism, it can be used to study the perturbations of the geomagnetic field; in the magnetic earth current method, it can be used to measure weak magnetic field changes; it can also be used in rock magnetism research. Because the probe of this instrument requires low temperature conditions, commonly used in the Dewar's bottle of liquid helium for cooling, complex equipment, high cost.