The Similarities and the Differences Between Geomagnetic and Gravity Survey

Tri Wulaningsih Why gravity geomagnetic methods are called potential field method??

The  potential field method  refers to using the principles of  gravity f ield and Earths magnetic f ield. Gravity surveys measure the acceleration due to gravity ( g), while gravitational attraction depends on density of underlying rocks, so (g) varies across surf ac ace to Earth. Magnetic survey may use a single sensor to measure the total magnetic f ield strength, or may use two (sometimes more) spatially separated sensors to measure the gradient of the magnetic f ield (the difference between the sensors). They are used to give geoscientists an indirect way to ace by sensing different physical properties of  rocks see beneath the Earths surf ac (magnetization and density, respectively). Hence, the expression potential f ield. The similarities and the differences between geomagnetic and gravity survey The similarities

The similarities between geomagnetic and gravity surveys are: y

y

y

Geophysical exploration techniques that employ both gravity and m agnetic are passive. By this, we simply mean that when using these two methods we measure a naturally occurring f ield of the earth: either the Earth's gravitational or magnetic f ields. Collectively, the gravity and magnetic methods are often referred to as potential methods, and the gravitational and magnetic f ields that we measure are referred to as potential f ields. Identical physical and mathematical representations can be used to understand magnetic and gravitational forces. For example, the fundamental element used to def in ine the gravitational fo rce is the point m ass. A n equivalent representation is used to def in ine the force derived f rom the fundamental magnetic element. Instead of being called a point mass, however, the fundamental magnetic element is called a magnetic monopole. Mathematical representations for the point mass and the magnetic monopole are identical. The acquisition, reduction, and interpretation of  gravity and m agnetic observations are very similar.

The differences Despite

these similarities, there are several signif icant differences between gravity and magnetic exploration. By-and-large, these differences make the qualitative and quantitative assessment of magnetic anomalies more diff icult and less intuitive than gravity anomalies. Differences Fundamental parameter

Gravity survey 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 f rom place to place near the surf ace of the earth. The highest densities we typically observe are 3 about 3.0 gm/cm , and the lowest 3 densities are about 1.0 gm/cm .

Force

The gravitational force attractive or active.

Point source

Single magnetic (monopole)

Field

always generated by subsurf ace variations in rock density Does not change signif icantly with time

Time

point

is

always

sources

Magnetic survey The fundamental parameter controlling the magnetic f ield variations of  interest to us, magnetic susceptibility, on the othe r hand, can vary as much as four to f ive o rders 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 diff icult with magnetic prospecting to determine rock types on the basis of  estimated susceptibilities. The magnetic force can be attractive or repulsive, passive. M athematically, monopoles can assume either positive or negative values. Monopoles always occur in pairs (dipole), one positive monopole, one negative monopole can have as its origin at least two possible sources The magnetic f ield is highly time dependent.