Indirect effect

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In earth measurements and geophysics, the indirect effect is the fact that the level surfaces of the gravitational field change when masses are removed or shifted.

The effect is called indirect because it is unwanted by the processor and can only be treated afterwards ( i.e. only iteratively ).

If the terrain is leveled mathematically during the evaluation of gravity measurements - see topographical reduction - the level surface in the vicinity of the measuring point can rise or fall by a few centimeters to decimeters . As a result, the deviation from the perpendicular also changes - because the perpendicular is now less attracted by the mountain - up to a “reduced deviation from the perpendicular”. On steep slopes in high mountains , this change can be up to 30 "in the Alps and about 50" (0.015 °) in the most rugged parts of the Andes or Himalayas .

Example of the "leveling" of Eurasia

If the entire mainland mass of Eurasia were to be removed, as far as it is above sea ​​level , the geoid in Central Asia would sink by around 1,600 meters. This result, initially unimaginable for experts, goes back to an estimate made in 1880 ( Friedrich Robert Helmert : The mathematical and physical theories of higher geodesy ). It showed the old master of geodesy 125 years ago that very large-scale processes in the earth's crust have to be compensated for by an effect still unknown to geophysics .

Helmert's “feel” for these geophysical relationships may be one reason why his Helmert ellipsoid, calculated in 1906, comes much closer to the parameters of the earth ellipsoid used since 1985 than the parameters of his colleagues up to 1960.

Pressure equilibrium in the earth's crust

A few decades later, Helmert's assumption about a compensated earth's crust was confirmed with evidence of its more than 90 percent isostasis : the continents react to loading and unloading in the same way as a very viscous semolina and rise or fall, depending on the positive or negative load . Therefore Helmert's fictitious leveling of Eurasia would be almost equalized after a few 10,000 years by uplifting the middle of the continent.

In fact, a similar effect can be observed in Scandinavia , where a kilometer-thick ice sheet lay in the Ice Age . Since it melted , the south of Finland has risen 1–2 cm from the Baltic Sea every year because the earth's crust has been relieved again, but will only be in equilibrium again after a long relaxation period .

One of the causes of such phenomena is the indirect effect. However, it would only last if the earth were a completely rigid body .

The significant postglacial land elevation - which causes the coastline to recede by several meters every year - helped Finnish geodesy to play a leading role in geosciences in the 20th century . A figurehead of this interdisciplinary research , in which geodesy, geophysics, geology and oceanography work together, was the geodesist Weikko A. Heiskanen - the main author of the well-known textbook Physical Geodesy published in 1967 .

See also

literature

Individual evidence

  1. ^ Edition Minerva, Frankfurt / M. 1962/63 (unchanged reprint of the Teubner edition, Leipzig 1880/84, 2 vols.)
  2. ↑ Weikko A. Heiskanen, Helmut Moritz : Physical geodesy . Freeman Books, San Francisco 1967.