The first magnetic measurements were aquired with essentially a basic compass. These measurements were of great importance in the art of off-shore navigation and surrendered many secrets of the Earth, but, in general, consisted of only the direction of the geomagnetic field. In later times (the early 1900's), magnetic surveys used magnetic variometers. These instruments were capable of measuring the the geomagnetic elements Z, H and B. There were several different types, including the torsion head magnetometer and the Schmidt vertical balance, but all were essentially just bar magnets suspended in the Earth's field. These devices required accurate levelling and a stable platform for measurement, and thus the readings were extremely time-consuming and limited to sites on land. Most modern survey instruments are designed only to measure B. The precision generally required is ± 1 nT (which is approximately one part in 50,000 of the background field). This may initially seem to be a high level of precision, but it is noticably less than that necessary for gravity measurements. This new generation of instruments provides nearly instantaneous readings and require only coarse orientation. Now magnetic measurements can be easily and efficiently taken on land, at sea, and in the air.
The first of these new generation of instruments to be developed was the fluxgate magnetometer. The fluxgate burst onto the scene during the second world war where it was used extensively in the detection of submarines from the air. The instrument consists of two identical ferromagnetic cores with properties such that the geomagnetic field can induce a magnetization that is substantial in the cores. Identical primary and secondary coils are wound in opposite directions around the cores and an alternating current (50-1000 Hz) is passed through the primary coils (thereby generating an alternating magnetic field). The alternating magnetic field induces an alternating voltage in the secondary coils. Since the coils are wound in opposite directions, the voltage in the coils is equal and opposite in sign and their combined output is zero IF there is no external field. In the presence of an external field, i.e. the Earth's magnetic field, the magnitude of the combined voltage is proportional to the amplitude of the external field component. A fluxgate magnetometer can be used to measure Z or H by aligning the cores in these directions, but the required accuracy of orientation is some eleven seconds of arc to achieve a reading accuracy of ± 1 nT . The total geomagnetic field can be measured to ± 1 nT with far less precise orientation. The fluxgate magnetometer is a continuously recording instrument and is relatively insensitive to magnetic field gradients over the length of the cores. The instrument may be temperature sensitive, however, and thus require corrections.
The most commonly used magnetometer for both survey work and observatory monitoring is currently the nuclear or proton precession magnetometer. The sensing device for this instrument is a container filled with a liquid rich in hydrogen atoms, such as kerosene or water, surrounded by a coil. Hydrogen nuclei (protons) act as small dipoles and align parallel to the ambient geomagnetic field (think about the iron fillings around a strong magnet). A current is passed through the coil to generate a magnetic field that is much greater in amplitude than the Earth's magnetic field, and in a different direction. The protons in the fluid then re-align in this new field direction. The current to the coil is then switched off such that the polarizing field is removed rapidly. The protons then return to their original alignment (parallel to the geomagnetic field) by spiralling, or precessing, in phase around the geomagnetic field direction. The frequency of this precession is related to the amplitude of the Earth's magnetic field. Field instruments usually have a precision of about ± 1 nT for the total magnetic field, although much greater precision can be attained if necessary.
