Differences Between Gravity and Magnetics

Unfortunately, despite these similarities, there are several significant differences between gravity and magnetic exploration. By-in-large, these differences make the qualitative and quantitative assessment of magnetic anomalies more difficult and less intuitive than gravity anomalies.

• The fundamental parameter that controls gravity variations of interest to us as exploration geophysicists is rock density. The densities of rocks and soils vary little from place to place near the surface of the earth. The highest densities we typically observe are about 3.0 gm/cm^3 , and the lowest densities are about 1.0 gm/cm^3. The fundamental parameter controlling the magnetic field variations of interest to us, magnetic susceptibility, on the other hand, can vary as much as four to five orders of magnitude*. This variation is not only present amongst different rock types, but wide variations in susceptibility also occur within a given rock type. Thus, it will be extremely difficult with magnetic prospecting to determine rock types on the basis of estimated susceptibilities.
• Unlike the gravitational force, which is always attractive, the magnetic force can be either attractive or repulsive. Thus, mathematically, monopoles can assume either positive or negative values.
• Unlike the gravitational case, single magnetic point sources (monopoles) can never be found alone in the magnetic case. Rather, monopoles always occur in pairs. A pair of magnetic monopoles, referred to as a dipole, always consists of one positive monopole and one negative monopole.
• 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. For any given set of field observations, both mechanisms probably contribute to the observed field. It is difficult, however, to distinguish between these possible production mechanisms from the field observations alone.
• Unlike the gravitational field, which does not change significantly with time**, the magnetic field is highly time dependent.

*One order of magnitude is a factor of ten. Thus, four orders of magnitude represent a variation of 10,000.

**By this we are only referring to that portion of the gravity field produced by the internal density distribution and not that produced by the tidal or drift components of the observed field. That portion of the magnetic field relating to internal earth structure can vary significantly with time.

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