Geophysics simply means the application of principles of physics in the study of the earth. The application of geophysics is vast. It can be used to study the entire Earth (Global geophysics), or for localized survey of the crust e.g for Engineering purposes.
Geophysical survey is broadly classified into two based on the source of energy.
Natural field: This uses the electric, magnetic, electromagnetic and gravitational fields of the Earth.
Artificial source: Involves the generation of artificial electrical or electromagnetic fields into the earth by a means of a D.C.
Magnetic method is applied when searching for buried magnetite ores because of their high magnetic susceptibility.
Electrical method is used when exploring for ground water because rocks resistances differ according to their water saturation levels.
Seismic method is used for hydrocarbon exploration.
It is worth noting that these methods can be combined in a survey such as in metallic mineral exploration in which electromagnetic and airborne magnetic methods are jointly used. The essence of combining different methods in one survey is to remove ambiguity.
If it were possible to know precisely and certainly the physical properties of the Earth, the magnitude of any particular geophysical measurement taken at the Earth’s surface could be predicted easily.