Gal (unit)

The Gal (named after Galileo Galilei) is the unit in which most of the gravitational acceleration is given on earth. There is no SI unit (although in the geosciences in use ), but based on the CGS system of units. It is no longer used since 1 January 1978 for the statutory statement of acceleration. In official documents in Germany and Austria only information is provided in the SI unit m / s ² is allowed.

Numerical examples

At various points of the earth's surface due to the gravitational acceleration varies flattening planet up to 0.5 percent. It amounts to

• At the equator in the middle 978.03 Gal
• At the North Pole 983.22 Gal
• At the South Pole about 982.5 Gal (due to 3 km altitude)
• At 45 ° latitude approximately 980.6 Gal, 1)

What each nor the vertical gradient of about -0.3086 mGal / m coming.

In geophysics and geodesy to measure the acceleration of gravity to determine the structure of the earth's crust, their deposits and the geoid. Here we calculate the deviations of the true from the theoretically calculated the gravitational field of the central Erdellipsoides. These gravity anomalies reach about ± 0.3 Gal, but are given in milligal. They can be modern gravimeters to ± 0.001 mGal = 0.01 microns / s ² accurately measure, ie 1: 1 billion.

For example, the largest gravity anomaly is in the Eastern Alps? G = -200 mGal, because the earth's crust further reaches down under large mountain ranges in the mantle than elsewhere. But even cause the loose sediments of a broad Alpine valley and the Vienna Basin and the Upper Rhine plain changes to 20 mGal or close to 100 mGal.

Use in gravimetry

The milligal (abbreviated mgal ) is one thousandth of Gal and a handy unit for gravimetry small changes in Earth's gravity. It is used for gravity anomalies and the so-called area reduction ( topographic reduction) of gravity measurements.

Practical unit

The milligal is in geodesy and geophysics still most commonly used, and only reluctantly by the SI derived unit microns / s ² replaced. Because it has the most suitable for gravimetric practice dimension:

• The area reduction is in the hills a few mGal
• And in the high mountains up to 30 mGal. You can, however, due to the uncertain inside the mountain rock density hardly more precise than 1 mGal calculate (see Digital Terrain Model ).
• The accuracy in the measured values ​​of conventional gravimeters ( meters according to the principle of the spring balance ) is in practical use in rough terrain usually also in mGal range or just below.
• Therefore, ultimately, the accuracy of the sought gravity anomalies also at 0.5 to 2 mGal. Only in very stable conditions (eg in gradiometry on a measuring pillars in air-conditioned basement) can ( 0.1 microns / s ²) actually exploit the measurement accuracy of about 0.01 mGal.

Milligal as a unit in severity maps and databases

The representation of the gravimetric results in milligal preferred - For the aforementioned reasons, therefore, to this day - five decades after the introduction of the SI.

For example, the Bouguer anomaly is in the center of the Eastern Alps from -150 to -200 mGal and can interpolate locally on about 1-5 mGal. A similar resolution have the gravity maps and databases, which provides the Applied Geophysics for the Exploration ( pre-investigation ) of veins, oil and other deposits.