Weight reduction

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Under gravity reduction is understood in the Geophysics and Geodesy a calculated reduction of measurements of the force of gravity to unwanted disturbances of the terrain , the geological eliminate substrate or to the mounting height.

In principle, you proceed in three steps:

  1. Topographical reduction by dividing the area into a fine grid. The more precise the gravity measurements , the more precisely the shape of the terrain has to be recorded; either as a template with circular ring sectors or with a digital terrain model . The result is the gravity value that would prevail on "leveled" terrain.
  2. Consideration of the height of the measuring point: gravity decreases upwards with an average of 0.308 mGal per meter; this vertical gradient is multiplied by the point height and added to the measured value. This is how the open-air anomaly is obtained . The name stands for the idea that a measuring point on a mountain peak is lowered to sea level together with it.
  3. Consideration of the topographic masses between the measuring point and the geoid (or NN). There are several variants for this, depending on the purpose of the gravity measurements.
In the simplest case, the standard value 2.67 g / cm³ is used for the rock density and reduced by the influence of a flat or spherical "Bouguer plate". The result is the Bouguer anomaly , e.g. For example, values ​​of −150 to −200 mGal are achieved in the high mountains. It is an indication of "missing masses" under the earth's crust - see isostasy - and an important key figure for estimating the interference potential .
With a more detailed approach for the vertical gradient one obtains somewhat different reduced gravity values and therefore also other gravity anomalies . The best known variant is the "Prey Gradient", which roughly corresponds to every one in a vertical shaft . With the "Prey anomaly" one can e.g. B. locate underground cavities , but it is also needed for some problems of geoid determination .

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