**2. Gravitational acceleration**

The acceleration (a) of a mass (m2) due to the attraction of a mass (m1) a distance (r) away can be obtained simply by dividing the attracting force F by the mass (m2).

$$\mathbf{F} = \frac{Gm\_1m\_2}{R^2} \tag{2}$$

*Gravitational Field - Concepts and Applications*

$$\mathbf{g} = \mathbf{a} = \frac{F}{m\_2} = \frac{Gm\_1}{R^2} \tag{3}$$

The acceleration is the conventional quantity used to measure the gravitational field acting any at point [1].

Is the Earth's gravitational acceleration is constant? No, it is not, that is due to the variation of mass distribution and variation of the diameters of the Earth.

In the cgs system, the dimension of acceleration is (cm/sec2 ). Among geophysicists this unit is referred as the (Gal) (in honor of Galileo, who Conducted pioneering research on the earth's gravity).

The gravitational acceleration at the earth surface is about (980 cm/sec<sup>2</sup> ) or (980 Gal), but in exploration work we are likely to be measuring differences in acceleration. 10�<sup>7</sup> of the earth's field. The unit which is more convenience in working with gravity data for geological and geodetic studies is milli-gal (milligal) or

$$(\text{mGal}) = \frac{1}{1000} \text{ gal} = 10^{-3} \text{ gal}$$

This unit has come to be the common unit for expressing gravitational accelerations.

There is another unit called gravity Unit (g.u.) which is equal to 0.1 mGal. g.u. = 0.1 milligal = 10�<sup>4</sup> gal.

and micro- gal = 10�<sup>6</sup> gal.
