# Gravity Methods in Geophysics

Gravity Survey - Measurements of the gravitational field at a series of different locations over an area of interest. The objective in exploration work is to associate variations with differences in the distribution of densities and hence rock types. Occasionally the whole gravitational field is measured or derivatives of the gravitational field, but usually the difference between the gravity field at two points is measured Geophysical interpretations from gravity surveys are based on the mutual attraction experienced between two masses* as first expressed by Isaac Newton in his classic work Philosophiae naturalis principa mathematica (The mathematical principles of natural philosophy). Newton's law of gravitation states that the mutual attractive force between two point masses**, m1 and m2, is proportional to one over the square of the distance between them. The constant of proportionality is usually specified as G, the gravitational constant. Thus, we usually see the law of gravitation written as shown to the right where F is the force of attraction, G is the gravitational constant, and r is the distance between

the two masses, m1 and m2.

*As described on the next page, mass

is formally defined as the

proportionality constant relating the force applied to a body and the accleration the body undergoes as given by Newton's second law, usually written as F=ma. Therefore, mass is given as m=F/a and has the units of force over acceleration.

**A point mass specifies a body that has very small physical dimensions. That is, the mass can be considered to be concentrated at a single point.

Gravitational Acceleration

When making measurements of the earth's gravity, we usually don't measure the gravitational force, F. Rather, we measure the gravitational acceleration, g. The gravitational acceleration is the time rate of change of a body's speed under the influence of the gravitational force. That is, if you drop a rock off a cliff, it not only falls,...

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