BGR Bundesanstalt für Geowissenschaften und Rohstoffe

Potential methods

Gravimetry and magnetometry belong to the potential methods. Potential fields can be described by a certain mathematic formalism (Laplace-Equation). The potential is the ability of the gravity or magnetic field, to carry out work. If, for example, in a gravity field a mass is transported along a certain way, this can only be done by a defined amount of work.

In gravimetry and magnetometry, the spatial and temporal variations and the absolute values of the earth’s gravity and magnetic field are measured, determining either the single components in the three spatial directions (vector measurement) or the total value (scalar measurement).

Gravimetry

The spatial variation of the gravity field yields information on the density distribution of masses in the earth’s interior. Thus, for example, continental crust has a lower density than oceanic crust, sedimentary rock a lower density than bedrock. The measuring of the spatial density variation allows, together with geological knowledge on an investigation area, a modelling of the underground. Unfortunately, such modellings are not unambiguous and have to be supported by further, independent methods. Gravity measurements depend on the place and their height above ground, thus, the observations are usually calculated back to a mean level. This mean level is the average undisturbed sea level. At the same time, the deduction of a latitude-dependent earth model highlights the spatial deviations from the normal field (anomalies). A strong influence of topography can still be found in these data. In order to reduce this influence in support of deeper inhomogeneities, further corrections of the topographic effect are carried out (Bouguer Plate correction and terrain correction). Finally, only structures generated from inside the earth’s crust and the earth’s mantle remain visible. Taking for granted, furthermore, that the earth’s crust is swimming on the earth’s mantle, isostatic equalization calculations are possible, making visible further details of the gravity field’s anomalies. These methods are applied globally in order to get a detailed picture of the earth’s inner build-up, to explore raw materials locally and regionally and to support environmentally relevant studies.

The temporal variations of the gravity field are mainly generated by tidal effects and by natural oscillations, for example at the core-mantle boundary. New, ultra-exact measuring techniques, however, allow to determine temporally changing structures, as for example groundwater level fluctuations.

Application areas of gravimetry are: the firm earth’s surface (terrestrial gravimetry), on ships (marine gravimetry) and on aeroplanes (airborne gravimetry). In the last few years there have also been satellite missions for the survey of the earth’s gravity field.

An article on gravimetry can also be found in a subchapter of ground geophysics.

Magnetometry

Magnetometric surveys mainly concentrate on measuring the magnetic field. The earth’s magnetic field is generated by dynamo effects between the earth’s core and the earth’s mantle and, to the outside, forms as a first proximation a dipole field, with the main axis penetrating the earth at the northern and the southern magnetic pole, respectively. Contrary to the gravity field, the earth’s magnetic field is subject to great temporal variations that can lead to a breakdown of the field or to a field reversal. These variations occur in periods of several millions of years. Much shorter variations of hours and minutes are caused by the interaction of the earth and the sun. Electrically charged sun particles hit the earth’s magnetic field depending on the activity. Here, the magnetic field acts as a shield, only at the magnetic poles the charged particles can penetrate the shield, causing polar lights. After their intrusion, the charged particles continue their journey in the earth’s magnetic field, causing the polar electrojets and the equatorial electrojet, that are generated in the ionosphere of the earth. Measuring and interpretation of the temporal variations do not belong to the potential methods. In practice, however, the temporal variation is part of the measurement and has to be corrected accordingly. The temporal variations of the magnetic field are used in methods using the electromagnetic induction (cf. magnetotellurics and geomagnetic deep sounding within the electromagnetic methods).

If superheated, liquid rock solidifies in the earth’s magnetic field, the magnetic field of this time is “frozen” while the rock is cooling, depending on the rock’s properties (paleomagnetics).This is especially important for the examination of the ocean floor. At the ridge systems, hot mantle material is pressed up to the surface and cools down there. Multiple field reversals cause positive-negative striped patters, that can be mapped accordingly and yield information on the age of the crust. Over land, the underground can be explored, similar as with gravimetry. Also in magnetometry, however, near-surface effects have great influence, either caused by magnetite veins in the earth’s crust or by man-made factors in the form of contamination.

An article on magnetometry can be found in a subchapter of ground geophysics.

Rock magnetometry

The magnetization of rocks depends strongly on their composition. If, for instance, the rock contains much magnetite or iron, it can build up a relatively strong remanent (“frozen”) and (by relocation in the recent magnetic field) induced own magnetic field. This field is stable only up to a certain maximum temperature (Curie temperature). General surveys of the variation of the earth’s magnetic field are called geomagnetics, if special samples are examined, this is called rock magnetics.

Magnetometry is applied on the earth’s firm surface (terrestric magnetometry und bore hole magnetometry), from ships (marine magnetometry) and from aeroplanes (airborne magnetometry). Special satellite missions determine the earth’s outer magnetic field.

Direct current electrics also numbers among the potential methods, it is discussed, however, among the electromagnetic methods.

Contact

    
Dr. Uwe Meyer
Phone: +49-(0)511-643-3212
Fax: +49-(0)511-643-3662

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