Whereas composition is the dominant control of density variation in metamorphic and igneous rocks. For example, the density of an igneous rock increases from acid to basic due to a decrease in silica content. When evaluating gravity data, a knowledge of the theory of gravity, and an appreciation of the aquisition and processing of gravity data should prove fruitful when interpretating subsequent geological anomalies. The power with which 2 masses will attract one another is proportional to the given mass and conversely proportional to the square of the distance. This is known as 'Newton's Law of Gravitation' as Newton framed it in 1686.
Force is related to mass by acceleration.
The Gal is derived from using the basic law of mechanics. Acceleration a , in which the velocity 'v' increases in the time unit by 1 is called 1 Gal. However g does vary. This is because the Earth is not a perfect homogenous sphere. If it was, then the gravity field at any point on the surface would be identical.