Weight reduction
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:
- 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.
- 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.
- 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 .
See also
- Gravimetry , gradiometry
- Normal field , reduction (measurement)
- Interference potential , indirect effect
literature
- Karl Ledersteger : Astronomical and physical geodesy . Handbook of Surveying Volume 5, 10th edition. Metzler, Stuttgart 1969
- Wolfgang Torge : Gravimetry . Textbook 477 p., De Gruyter-Verlag, Berlin 1990