In the early 1940s, the first fluxgate marine magnetometers were used to detect submarines and other ‘large' submerged or buried objects. For these applications, the maximum obtainable accuracy of ±1nT was sufficient. Since their development, magnetometers have evolved from the proton types, proton-free precession technology to Overhauser-Abragam and optically pumped caesium magnetometers, and, currently, more accurate systems are available with an absolute scalar reading of the total magnetic field to an accuracy of ±0.1nT.
Today, the most commonly used magnetometer is the nuclear precession or proton magnetometer. Since the early 1950s, magnetometers were based on the ‘Overhauser effect', named after the American physicist Albert Overhauser who discovered the transfer of energy from electrons to protons in hydrogen atoms. At about the same time, A. Abragam discovered the same effect, although he used a different method. This invention relates to nuclear magnetometers for measuring weak magnetic fields, based on dynamic polarisation of nuclei and a free radical substance for use therein.
Overhauser-Abragam magnetometers are vastly more energy efficient than their predecessors, proton precession magnetometers, which relied on excitement of protons by a direct current source. Overhauser magnetometers also have faster sampling frequencies (>10 magnetic measurements per second) and higher sensitivities than the older proton precession magnetometers.
Caesium magnetometers use the alkali metal caesium and are optically pumped. A cell containing the gaseous form of the metal caesium is polarised (or pumped) by exposure to light of a very specific wavelength. The light depopulates one electron energy level in the gas by pumping the electrons to a
higher energy level. These electrons spontaneously decay to both energy levels, and, eventually, a lower energy level is fully populated. Next, the cell is ‘depolarised' by shifting the electrons in the lower energy level back to their original position using lower wavelength radiofrequency (RF) power.
The energy required to repopulate this energy level varies with the ambient magnetic field, according a principle called the ‘Zeeman effect'. Therefore, the frequency of the depolarising RF power corresponds to the magnetic field value.
Caesium magnetometers offer high sensitivity (0.001nT) at very high sample frequencies (>30Hz). For high-precision surveys, two or three magnetometers are towed in a fixed geometric shape. The sensor outputs are compared and a value for the magnetic gradient can be calculated. This enables the user to locate the target in 2D. By using a third sensor, vertical distance can also be calculated.
When properly used, especially in combination with sidescan sonars, sub-bottom profilers and bathymetric systems, the magnetometer can be a useful tool for all kinds of detection surveys.