Sub-ranges of geophysics

Since the earth is the subject of investigation for geophysicists and it is not possible to look inside of the earth we depend on various techniques of measurement at the earth's surface. With the measurements of the physical properties of the different rocks and materials (such as the density, the velocity of propagation of waves, the electric conductivity and the magnetic properties of the earth) it is possible to make conclusions about the structure of the earth's interior.

Therefore the following sub-ranges of geophysics are distinguished:

 

 

These sub-ranges deal with different methods of measurement and mainly with a variety of methods to analyse, interpret and graph the data. Thereby each sub-range offers the possibility for a complete and independant analysis, but to avoid ambiguities and to control the freedom of interpretation, very often neighbouring disciplines are involved for comparison. Hence geophysics presents a wide variety of field of activity, but also demands a high degree of interdisciplinary cooperation.

The main field of application of geophysics is the exploration of resources, particularly the exploration of hydrocarbon. Here the seismics contributes the most, because she is able to image the underground soonest clearly and accurate. However the seismics is also by far the most expensive and data bulked method.

 

In seismics the travel-times of sound-waves (elastic waves, seismic waves), that are created synthetically on the surface or within boreholes, are analysed. In the earth's interior these waves experience diffraction and reflexion at discontinuities and their travel-times are recorded with seismographs (seismometer, geophone). The two sub-ranges refraction seismics and reflexion seismics result from the different wave paths a seismic wave is able to travel through the underground. Seismics is based on the different velocities of propagation in different media and on the reflectivity of the layers.

 

On the basis of earthquake-records seismology analyses the structure and the properties of the earth and tries to make a prediction for the occurence and the effects of earthquakes. Therefore not only the place, the depth and the time of the earthquakes are important, but also the magnitude, the directions of the radiation of energy, the frequency and the ... and orientation of the fraction.

 

The method of geo-sonar is based on the same principle as reflexion seismics. The difference is that with a ground-penetrating radar electromagnetic waves are sent into the earth via an antenna and are registrated by a receiver-antenna at a distant location. The velocity of propagation, that in the air is almost equal to light speed in vacuum, varies in rocks. Because of their high relative permittivity water and cavities show particularly high and distinctive velocities compared to rocks. As a result of the low penetration of the waves the geo-sonar explores only a couple of meters of the ground, but there it works very detailed.

 

Another electromagnetic method is the magnetotellurics. Via timed-varying magnetic fields electric (telluric = concerning the earth) currents are induced in conducting structures of the earth's interior. The electric field generated by the currents in turn induces a secondary magnetic field. The stimulating primary magnetic fields may be of natural origin (electric-current systems in the ionosphere and magnetosphere, radiation of storm-flashs) as well as of artificial origin. By a very wide range of the period-spectra of the alternating fields (from 10^-8 s to 10^6 s) this method is - via the frequency-depending depth of penetration - able to make a statement about the distribution of the electric conductivity within the different crust-layers up to the upper Earth mantle.

 

In gravimetric analysis the gravitational field of the earth is studied and exiguous changes in gravitation caused by differences in density in the underground are measured with very sensitive measuring instruments. So not only local interferences caused by baffles such as salt domes can be detected, but also the shape of the earth as well as its bowl-shaped structure can be investigated.

 

The basis of geomagnetics is the existence of a terrestrial magentic field. Via an induction process each material receives its own magnetic properties or - with other words - an induced magnetisation. The real characterising material-specific property is the magnetic susceptibility. The induced magentic field superposes with the general earth field and creates anomalies in the general field. The survey of those anomalies gives us the possibility to locate magnetized bodies and objects.

 

With the methods of rockmagnetism we are able to study the different types and processes of magnetisation. The methods of paleomagnetism (with is by the way the only sub-range that deals with the past) give us the possibility to suggest the type and the direction of the former magnetic field of the Earth via cores. Herefrom we can make statements about the continental drift.

 

The geoelectricity deals with the determination of the resistivity of rocks and other materials. For these purposes continuous currents or low-frequency alternating currents are injected into the ground by electrodes and the demanded dimensions are calculated from the measured voltage or the induced magnetic field. Because electric and magnetic fields do affect each other and interdepend respectively, it is very helpful to combine the two sub-ranges geoelectricity and magetics. For instance the magnetotellurics works with the currents, that are induced in the earth by the magnetic field that is brought along from the solar wind to suggest therefrom the electric conductivity.

 

The geothermy looks at the temperature field and the thermal processes in the underground. Therefore temporal constant (static) as well as temporal changing processes are interesting. As a static process the heat flow from the Earth's interior has to be mentioned here; unsteady processes are for example the periodic heating with circadian- or annual rhythm caused by the penetration of temperature waves in the underground. Depending on the process the rock-specific parameters thermal conductivity and thermal diffusivity may be very important.

 

Geodynamics treat the natural processes of motion in the Earth's interior and on the surface respectively. At the same time this sub-range investigates the driving mechanisms and forces that are associated with the displacements.

 

The physics of the high atmosphere investigates the atmospheric composition (composition of high air films) and deals with the interaction of particles far away from the earth (e.g. cosmological radiation, solar wind)

 

The small sub-range of extraterrestrical geophysics tries to assign the knowledge about the structure of the earth to other planets of our solar system.