1)Geophysical exploration techniques that employ both gravity and magnetic are passive. By this, we simply mean that when using these two methods we measure a naturally occurring field of the earth. Collectively, the gravity and magnetic methods are often referred to as potential methods and the gravitational and magnetic fields that we measure are referred to as potential fields.

Similarities Between Gravity and Magnetics

2) Identical physical and mathematical representations can be used to understand magnetic and gravitational forces.

For example, the fundamental element used to define the gravitational force is the point mass. An equivalent representation is used to define the force derived from the fundamental magnetic element. Instead of being called a point mass, however, the fundamental magnetic element is called a magnetic monopole.

3) The acquisition ,reduction, and interpretation of gravity and magnetic observations are very similar.4) Vary from place to place.5) Both are Vector fields6) Both fields are force fields.7) Drift correction is applied in both methods.8) Both fields exert force with a speed equal to the speed of light.

1)The fundamental parameter that controls gravity variations of interest to us as exploration geophysicists is rock density.

The highest densities we typically observe are about 3.0 gm/cm3and the lowest densities are about 1.0 gm/cm3.

The fundamental parameter controlling the

magnetic field variations of interest to us, is

magnetic susceptibility.This variation is not only present amongst different rock types, but wide variations in susceptibility also occur within a given rock type.

Differences Between Gravity and Magnetics

2) Gravitational field is always perpendicular to the surface of earth. While Direction of Magnetic field changes from place to place (it might be same at different places but unlike g it is not always perpendicular).

3) Value of g is less near the equator than at the poles (about 5 gals) for two reasons (1-The distance to the earth's center is greater 2-The rotation of the earth introduces centripetal acceleration V2/r ).

Magnetic field varies from the equator to the poles in a range that goes from 20000 nT to 70000 nT.

4) Gravity (mass) is monopolar it always attracts.

Magnetism is dipolar; like poles repel while unlike poles attract.

5) All matter has mass therefore contributes to g.

While main magnetic field is due to convection currents in the core; it effects on some minerals not all.

6) Gravitational field is measured by Mechanical Instrument. While magnetic field is measured by purely electronic Instruments.

7) Gravimeter is relative instrument; magnetometer is absolute.

8) tides are only external effect on value of g and can be corrected; effect of magnetic storms cannot be removed9) A properly reduced gravitational field is always generated by subsurface variations in rock density. ; A properly reduced magnetic field, however, can have as its origin at least two possible sources. It can be produced via an induced magnetization, or it can be produced via a remanent magnetization.

10) Unlike the gravitational field, which does not change significantly withtime; the magnetic field is highly time dependent.