Magnetics
Magnetic forces have fascinated people since ancient times, certainly because magnetic fields cannot be directly perceived by our senses.
Magnetic forces have fascinated humans since ancient times, certainly because magnetic fields cannot be directly detected by our senses. As a scientific discipline, magnetism is one of the basic methods of geophysics. It was arguably the first geophysical method used in the exploration of mineral deposits, specifically iron ore in Scandinavia, as early as the first half of the 17th century. The traditional measurement of the Earth's magnetic field using magnetized specimens (primarily needles) used to be laborious and time-consuming. The strongest magnetic anomaly ever, caused by the largest iron ore deposit on Earth, the Kursk Mine in Ukraine, was allegedly discovered in 1778 with the help of a magnetic ore sample suspended from a thread. Since people learned to use the magnetic properties of elementary particles for measurements almost fifty years ago, magnetometry has developed into one of the simplest and fastest methods of applied geophysics that can be used in the field. Similar to gravimetry, the magnetic interference field is defined as the deviation of the measured values from a standard field; the latter is essentially a dipole field and is referred to as the International Geomagnetic Reference Field (IGRF). For smaller-scale surveys, an empirically determined regional field is usually simply subtracted, e.g., calculated from the available measured values.
Magnetic field anomalies are caused by magnetized disruptive bodies in the subsurface, which are often composed of basic or ultrabasic rocks such as gabbros, diorites, basalts, serpentinites, and the like. The strongest disruptive fields are observed over magnetite and pyrite deposits. Magnetic susceptibility, the physical parameter responsible for magnetization, can vary over many orders of magnitude (unlike the corresponding parameter of density in gravimetry), and magnetic field anomalies of varying strength can be observed accordingly. However, it is by no means possible to directly determine the causative rock from the strength or shape of the anomalies. Magnetic anomalies are generally much more complex than gravimetric ones; they usually consist of a minimum and a maximum. Their shape depends strongly on the geographical or magnetic latitude, or more precisely, on the inclination or inclination of the magnetic field vector at the measurement location. The dip of the disruptive bodies and the strike direction of the measurement profiles also have an influence (see graphic on the right). The interpretation of magnetic anomalies is usually carried out using model calculations. Graphical methods also played a significant role in the past. Suitable geometric quantities are read from the measurement curves, and the parameters of the disruptive bodies causing the disturbance, such as depth, width, and dip, can then be determined using evaluation diagrams.