Gal (unit)

Physical unit
Unit name Gal
Unit symbol ${\ displaystyle \ mathrm {Gal}}$
Physical quantity (s) acceleration
Formula symbol ${\ displaystyle a}$
dimension ${\ displaystyle {\ mathsf {L \; T ^ {- 2}}}}$
system CGS system of units
In SI units ${\ displaystyle \ mathrm {1 \, Gal = 0 {,} 01 \; {\ frac {m} {s ^ {2}}}}}$
In CGS units ${\ displaystyle \ mathrm {1 \, Gal = 1 \; {\ frac {cm} {s ^ {2}}}}}$
Named after Galileo Galilei
Derived from Centimeter , second

The Gal (named after Galileo Galilei ) is the unit in which the gravitational acceleration on earth is often given in the geosciences . It is not an SI unit , but is based on the CGS system of units . Therefore, since January 1, 1978, the Gal is no longer used for the legal specification of the acceleration, in official documents in Germany and Austria only the specification in the SI unit m / s² is permitted.

• 1 Gal = 1 cm / s² = 0.01 m / s², i.e. about one per thousand of the average acceleration due to gravity of approx. 9.81 m / s² ≈ 10 m / s² = 1000 Gal.
• 1 milligal = 1 mGal = 0.01 mm / s² = 10 µm / s², i.e. approx. One millionth of the acceleration due to gravity.

Numerical examples

At different points on the earth's surface, the acceleration due to the flattening of our planet varies by up to 0.5  percent ( flattening of gravity ). The acceleration due to gravity is

In addition, there is a vertical gradient of around −0.3086 mGal / m. The acceleration due to gravity is z. B. at the South Pole about 982.5 Gal and thus slightly less than at the North Pole, because the South Pole is about 3 km above sea level.

In geophysics and geodesy one measures the acceleration due to gravity and calculates the deviations of the measured, true gravity field from the theoretically calculated gravity field of the central ellipsoid of the earth . From these gravity anomalies , the structure of the earth's crust , its deposits and the geoid can be determined.

Gravity anomalies reach around ± 0.3 Gal, with modern gravimeters they can be measured with an accuracy of ± 0.001 mGal = 0.01 µm / s², i.e. to 1: 1  billion . For example, the largest gravity anomaly in the Eastern Alps is Δg = −0.2 Gal, because the earth's crust extends deeper into the mantle under large mountains than elsewhere. But even the loose sediments of a wide alpine valley cause changes of up to 0.02 gal, the Vienna Basin or the Upper Rhine Plain of almost 0.1 gal.

1)This value at 45 ° latitude was used to define the previous unit of force, the kilopond , by setting it to be 9.80665  Newtons .

Use in gravimetry

The milligal is still used most frequently in geodesy and geophysics - five decades after the introduction of the SI - and is only reluctantly replaced by the derived SI unit µm / s² (1 mGal = 10 µm / s²). Because the milligal is a handy unit of small changes in the earth's gravitational field and therefore has the most suitable size for gravimetric practice :

• The terrain reduction (topographical reduction of gravity measurements) amounts to a few mGal in the hill country and up to 30 mGal in the high mountains . However, because of the uncertain rock density inside the mountains, it can hardly be calculated more precisely than 1 mGal (see digital terrain model ).
• The measurement deviation in the measured values of conventional gravimeters (measuring devices based on the spring balance principle ) is usually also in the mGal range or just below that when used in rough terrain.
• Therefore, the accuracy of the gravity anomalies sought is ultimately also between 0.5 and 2 mGal. The measuring accuracy of about 0.01 mGal = 0.1 µm / s² can only actually be used with very stable conditions (for example with gradiometry on a measuring pillar in an air-conditioned basement) .

For example, the Bouguer anomaly in the center of the Eastern Alps is −150 to −200 mGal and can be locally interpolated to about 1–5 mGal . The gravity maps and databases that applied geophysics provide for the exploration (preliminary exploration) of veins , oil and other deposits have a similar resolution .