The Earth’s magnetic field can be thought of as having three components:
- down in the direction of gravity
- to true North (not magnetic North)
- and an East-West component at right angles to true North
The East – West component causes what is called ‘declination’, or the compass pointing to “magnetic north” instead of “true North”. The true North component is in the horizontal plane, but the down component is much stronger than the North component!. If a compass is not level then it will have a large error because it is starting to read the down component of the Earth’s field. That’s the reason why conventional compasses should always be kept perfectly level.
The total value of the vector resulting from these three components depends on the geographical location with its maximum values around the two Poles and minimum values along the Equator – look at figure 4 on page 9 of the ampm-opt.pdf document.
The direction of this total vector is North-South with an ‘inclination’ from the horizontal also depending only on the location on Earth (50° to 70° North in Europe and Northern America but 0° on the Equator) – look at figure 3 on page 8 of the the ampm-opt.pdf document.
The earth magnetic field modeling is the following: mag field calculator
Some underground or underwater objects or structures with permanent or thermo-remanent magnetism are able to locally change these average value and direction. If you bury a permanent magnet and you measure the earth magnetic field over and around it, you will find that the field greatly concentrates in its immediate vicinity. Actually, any ferromagnetic object or matter will produce the same effects as well but with much less magnitude than a magnet.
Magnetometers are all measuring the natural or human-made disturbances of the earth magnetic field. These disturbances are called ‘magnetic anomalies’.
Important Note:
There is some confusion in the literature over physical units used in magnetism. SI units are now the preferred units over the older CGS. In the older CGS system (still very much in use in USA), the magnetic flux density was expressed in GAUSS (G) while in SI system, it is expressed in Tesla (T) or, more practically, in nanoTesla (nT = 10-9T) with the following relation between the two:
1 Gauss = 0.0001 Telsa or 1 Gauss = 100,000 nT.
As an approximate reference, the nominal earth magnetic field is on average around 0.5 Gauss or 50,000 nT (while a small modern permanent magnet can easily give 3000 Gauss!!) with its maximum values around the two Poles (70,000 nT) and minimum values along the Equator (25,000 nT).
Actually, the important measurement units in geophysics applications are Field Gradients expressed in nT/meter. This is the variation of the total field value ( or vertical component of it) measured between two points distant of one meter. Magnetic anomalies are detected by the value of the field gradient they produce when surveying around and above them; higher the gradient, bigger the anomaly. It also mean that small anomalies require very sensitive instruments and also a large number of measurements close to each other. There is absolutely no chance to detect an anomaly of 1 nT with an instrument whose sensitivity or resolution would be 10 nT and an anomaly or object with a dimension of 10 cm x 10 cm will probably be missed with a 5 m x 5 m search grid.