Magnetometry

Magnetic Measurement Source: BGR

Magnetic powers have fascinated mankind from ancient times, certainly because magnetic fields can not be felt directly by our senses. As a scientific discipline, magnetics is one of the basic methods of geophysics. It was probably the first geophysical method that was applied already in the first half of the 17th century for the exploration of deposits, here especially iron ores, in Scandinavia.

The classic surveying of the earth’s magnetic field using magnetized sample bodies (especially needles) used to be painstaking and time-consuming. The probably strongest magnetic anomaly at all, caused by the greatest iron ore deposit on earth at Kursk in the Ukraine, was discovered reportedly in year 1778 with the help of a magnetic ore sample tied to a thread. Nearly fifty years ago, people learned how to use the magnetic properties of elementary particles for the measurements, since then, magnetometry has become one of the methods in applied geophysics, that can be applied most easily and fast in fieldwork.

The magnetic disturbing field is defined, like in gravimetry, as a deviation of the measured values from a normal field, the latter being basically a dipole field called International Geomagnetic Reference Field (IGRF). In small-scale surveys, often simply an empirically determined regional field is deducted, calculated from, for example, existing measured values.

The cause of magnetic field anomalies are magnetized disturbing bodies in the underground, often built up from basic or ultra basic rock like gabbro, diorite, basalt, serpentinite and the like. The strongest disturbing fields are found over deposits of magnetite and pyrrhotine. The magnetic susceptibility, which is the relevant physical parameter for magnetization, can vary by many decimal powers (unlike the corresponding parameter ‘density’ in gravimetry), thus very different magnetic field anomalies can be found. It is, however, impossible, to identify a certain type of rock directly by the strength and form of the anomaly it generates.

Magnetic total field over a dipping thin plate, inclination = 60° Source: Redford, 1964

Field example from Uruguay: magnetic and gravimetric profile Laguna Merín. Source: BGR

Generally, magnetic anomalies are much more complicated than gravimetric anomalies, normally they consist of a minimum and a maximum. Their form depends strongly on the geographical and magnetic latitude, more precisely, the inclination of the magnetic field vector at the site of the measurement. Measurements are also affected by the dip of the disturbing bodies and the strike direction of the measuring profile.

The interpretation of magnetic anomalies is usually again done with the help of model calculations. In former times, also graphic methods were very important, i. e. adequate geometric values are read off the gradients; with the help of evaluation diagrams it is possible to determine the parameters of the generating disturbing bodies, like depth, width and dip.