The fluxgate magnetometer was originally designed and developed during World War II. It was built for use from low-flying aircraft as a submarine detection device. Today it is used for making borehole measurements, and the transducers are found in electronic compasses and in laboratory devices for measuring remanent magnetisation. Much airborne magnetic surveying was carried out using fluxgate detectors between 1945 and 1985, and hand-held portable devices were used for making vertical-component ground measurements between 1965 and 1985. A schematic of the fluxgate magnetometer is shown below.
The fluxgate magnetometer is based on what is referred to as the magnetic saturation circuit. Two parallel bars of a ferromagnetic material are placed closely together. The susceptibility of the two bars is large enough so that even the Earth's relatively weak magnetic field can produce near magnetic saturation* in the bars.
Each bar is wound with a primary coil, but the direction in which the coil is wrapped around the bars is reversed. An alternating current (AC) is passed through the primary coils causing a large, artificial, and varying magnetic field in each coil. This produces induced magnetic fields in the two cores that have the same strengths but opposite orientations, at any given time during the current cycle.
If the cores are in an external magnetic field, one component of the external field will be parallel to the core axes. As the current in the primary coil increases, the magnetic field in one core will be parallel to the external field and so reinforced by it. The other will be in opposition to the external field and so smaller. The field will reach saturation in one core at a time different from the other core (and fall below saturation, as the current decreases, at a different time. This difference is sufficient to induce a measurable voltage in a secondary coil that is proportional to the strength of the magnetic field in the direction of the cores.
The secondary coil surrounds the two ferromagnetic cores and the primary coil. The magnetic fields induced in the cores by the primary coil produce a voltage potential in the secondary coil. In the absence of an external field (i.e., if the earth had no magnetic field), the voltage detected in the secondary coil would be zero because the magnetic fields generated in the two cores have the same strength but are in opposite directions (their effects on the secondary coil exactly cancel). In the presence of an external field component, the behaviour in the two cores differs, by an amount which depends on the external field.
Thus, the fluxgate magnetometer is capable of measuring the strength of any component of the Earth's magnetic field by simply reorienting the instrument so that the cores are parallel to the desired component. Fluxgate magnetometers are capable of measuring the strength of the magnetic field to about 0.5 to 1.0 nT. These are relatively simple instruments to construct, hence they are relatively inexpensive ($5,000 - $10,000). Unlike the commonly used gravimeters, fluxgate magnetometers show no appreciable instrument drift with time.
| FAQ | Intro GP | Outline | Next Page | Previous Page |
|---|