Geophysics (also called terrestrial physics) is a predominantly experimental science, which deals in general with the application of physical measurements and methods to the study of the physical properties and phenomena of planet Earth. Although it is an autonomous science, has an eminently interdisciplinary structure, as it uses the observations and results of many disciplines that study the Earth, such as astronomy, geodesy, geology, geochemistry, physical geography, in addition of course to physics that provides the general laws, methods and tools for the analysis and interpretation of various phenomena.
Geophysics is traditionally divided into three major branches: the geophysics of the solid Earth or lithophysics (the geophysics in the strict sense), the geophysics of the atmosphere or aerophysics, the geophysics of the hydrosphere or hydrophysics. Each branch is then subdivided into several other sectors, referring to specific problems, for example gravimetry, seismology, volcanology, meteorology, aeronomy, oceanography, hydrology, glaciology. Concerning the methods of investigation and the purpose of research, geophysics is usually divided into pure and applied.
Studies the properties and physical conditions of the Earth in order to achieve a thorough description of physical phenomena in their general aspects and to interpret the causes that produce them. In geophysics, unlike other physical disciplines, observational data prevail over experimental data: it is in fact difficult, if not virtually impossible, to reproduce the natural phenomena that affect geophysics in the same scale, spatial and temporal, in which they occur, in addition to natural phenomena involve many changing environmental elements, not always known or repeatable in the laboratory.
Another characteristic of geophysics is to have to interpret some properties or phenomena of the Earth through theoretical models that are not always built with the data of direct observation: typical example is the definition of the value of the density of the Earth’s interior, for which we must use indirect data because it is impossible to get materially beyond a certain depth. A model will be considered as much closer to reality, the more numerous the natural elements will be explainable with it: in the case mentioned, models are assumed in which the Earth is attributed to increasing density towards the nucleus so that it is explainable the average density of the Earth (known element), there is no contradiction with the average density of surface rocks (known element) and it is explainable the trend of the speed of propagation of seismic waves (known element).
The method of investigation of pure geophysics is therefore essentially to collect as many elements as possible, to process them mathematically and to build models that, by successive approximations, give reason for the phenomena observed. One can therefore understand how geophysics takes advantage when the research and the observed data are very numerous and extended to the entire surface of the earth. For this purpose each nation creates a network of geophysical stations that work in close collaboration with each other and with geophysical observatories of other countries, often implementing common programs. A particularly interesting and extensive international program, including among other things the crossing of Antarctica, was that of the International Geophysical Year 1957-58.
It proposes to use the properties and physical phenomena studied by pure geophysics for practical purposes; its fields of application are many; just think of the possibility of exploiting the earth’s thermal energy or the conversion of dynamic tidal energy into electrical energy, investigations into the nature of the subsoil, studies of soil stability, archaeological research, the production of artificial rain in arid regions.
The field of research in which applied geophysics offers the best results is currently that of subsurface prospecting. In these researches are used methods that allow to eliminate, as far as possible, subjective evaluations, remote actions, selectivity of the results, that is, sensitive variation of the observed values according to the medium in which the properties occur; such methods are gravimetric, magnetic, seismic, electrical, radioactive and geothermal.