Severity gradient
The change in the acceleration of gravity in space is called gravity gradient in earth measurements . A distinction is made between vertical and horizontal gravity gradients.
Vertical gradient of severity
The change in the acceleration of gravity with altitude is called the vertical gravity gradient . The latter refers - depending on the calculation model - to the geoid or to the reference area of a land survey or the mean earth ellipsoid .
below the surface of the earth the gravity increases up to the core-mantle boundary
- In the lowlands the vertical gradient averages −0.3086 mGal / meter = -3.086 µm / (s² · m) and is called the open-air gradient , the acceleration of gravity decreases with increasing altitude (see altitude dependence of normal gravity ).
- In the hill country it varies by a few percent,
- in high mountains, however, 10 times more because it is strongly influenced by the shape of the terrain . In a very steep, narrow valley it can take on almost the same value that prevails in a borehole (about −0.1 mGal / m).
- In the earth's interior in the earth's crust , the Prey - or "inner gradient" is about −0.085 mGal / m, with increasing depth (i.e. decreasing height) the acceleration due to gravity increases slightly due to the high density of the earth's core .
On the one hand, the vertical gradients are important for geological and geophysical investigations. On the other hand, they are required to reduce measured gravity values, because measuring the earth's gravity field only makes sense if the data are then converted to a uniform height. The differences obtained in this way from a regional mean are called gravity anomalies .
The Bouguer gradient (according to Pierre Bouguer ) is a specialty . It is approximately −0.19 mGal / m and results from the open-air gradient if the terrain plate below the measuring point is excluded. This gives the bouguer anomalies , which indicate a possibly deviating rock density in the subsurface. They are used in geophysics and to search for deposits .
Vertical gradients also occur at a greater distance from the earth and can be used to orient a satellite in an orbit ( gravitational stabilization ). This also occurs naturally with any elongated body in orbit.
Horizontal gradient of severity
The horizontal severity gradient is of lesser importance and is also much smaller than the vertical severity gradient. Special projects in gravimetry and earth measurement use the horizontal change in gravity to examine details about the upper crust of the earth or the course of the geoid. The most common unit used is the Eötvös , named after the Hungarian Roland Eötvös , who designed the rotary balance in the 1920s . It was especially in the oil - exploration used before starting about 1960 the modern gravimeters arose.
The most recent application of horizontal gravity gradients comes from satellite geodesy . Special geodetic satellites such as GRACE and the former GOCE measure the changes in gravity within the probes in three to six directions with gradiometers ; also precise microwave - distance measurements between two consecutively-orbiting satellites can detect these gradients. This enables a regional geoid determination , which on average reaches an accuracy of one centimeter over approximately 100 km × 100 km and is planned to record long-term changes in the earth.
The gradiometry (measurement of gravity gradient) will continue the traditional geoid determination with gravimetric or in flat countries Geodetic Astronomy replace. In the mountains , however, the effects of the terrain on gravity are difficult to determine, which is why terrestrial gravimetry should continue to be used there .