Gravity and Magnetic Methods have been applied for many years and still show promising results in various geoscience investigations. Even in Petroleum exploration industry these methods form the two among the three (3) principle geophysical tools in searching for these world's and useful fuel resources at which the seismic methods complete this list. If we tried to examine these two techniques we will discover that they have much in common.
Reason of why gravity and and magnetic methods are called Potential field methods.
And the fact that in many geophysical surveys their measured field magnitudes have a tendency of decreasing at a distance from their sources. Then they are regarded as Potential field techniques (methods). To make it clear that Potential fields are those related to vector quantities (having both magnitude and direction), at which magnitudes decrease from distance away from the source.
Despite this similarity, in geophysics gravity method and Magnetic method are technically work differently. In order to remove this confusion it is better to make it clear and in simplified form, their pretty differences.
What is the difference between gravity and magnetic method in geophysics?
Let's see through this post how these techniques agree with each other that they are distinctly different! The following is the list of some factors that I described to make clear to you, the difference between gravity method and Magnetic method in geophysics
5 key differences between gravity and magnetic methods
1: Basic Operating Principle
When it comes to how these methods apply physical laws and principles, The concept of gravity method involves mostly two (2) Laws of force used in physics,
i/ Universal Gravitational Law
F = GMm/r2
ii/ Newton's second law of motion
F = ma
Then to determine value of g, F = W = mg
Then compare two equations of force
GMm/r2 = mg
Then, g = GM/r2
When we assume the body is at the Surface of the Earth, we can compute the value of acceleration due to gravity (g)
g = GMe/re2
where,
G is the universal gravitational Constant: 6.6732 x 10-11 Nm2/kg2
re is the radius of the Earth: (6.367 x 106) m
Me is the mass of the Earth: (5.976 x 1024) kg
Then, g = 9.84ms-2
Note that, in most gravity surveys units conversion used are as follows,
1 Gal = 1 cms-2 = 0.01 m/s2
1 mGal = 0.001 Gal
1 gravity unit (g.u) = 0.1 mGal.
While Magnetic method relied on the principle the same like that around a bar, magnetic flux exists indicated by the lines of forces converges at ends.With the force between two magnetic poles given by
F = Mm/4Ï€Ur2
Where, U is the magnetic permeability of the medium separating the poles.
The magnetic field intensity is measured in units such as
1nT = 10-9 T
1nT = 10-5 gauss
1nT = 1 c.g.s
2: Measured Parameter
Gravity method is used to measure the spatial variation of strength of geo-gravitational field while the magnetic method relies on measurement of spatial variation of the strength of geomagnetic field.
For example we can simply represent the gravity at point x as 15 mGal, while for the case of Magnetic intensity of a certain point due to rocks will be 352 nT.
N:B, I used the values above for just as example there is no any significant relationship or intended physical meaning behind, what I wanted you to know is only mGal for gravity as the strength of geo-gravitational field while nT for Magnetic as the strength of geomagnetic field.
3: Nature of fields sources
As you have already known that these are potential fields, they are measured as total fields due to interactions with other fields above the earth. The source nature of the field for gravity is single polarity (Monopolar) while that of magnetic is two polarities that may exist as like or unlike magnetic poles, hence termed as Dipoles.
You can see the figures below for Monopole and Dipole respectively to grasp a concept
4: The Operative Physical Property
The physical property is that property at which a particular geophysical technique is sensitive.The gravity technique is based on the sensing of the Rock density.
However in practice it is the density contrast between the body that we are interested in, and its surrounding materials is what we are looking for. While in magnetic surveys the physical property objectived is that the ability of a certain material to become magnetized is known as Magnetic susceptibility.
5: Measuring Sensor instrument
Gravity strength is measured by the sensing instrument known as Gravity meter (gravimeter) while the magnetic strength is measured by Magnetometer. If the spatial variation of either gravity or magnetic strength is measured at different heights known as Field gradient, we use the term Gradiometer to refer to the appropriate measuring instrument. Such that for gravity is Gravity Gradiometer while that of Magnetic is Magnetic Gradiometer.
All in all, Magnetic and gravity field methods have much in common as both are Potential fields. However they are technically dissimilar from acquisition to data reduction. You have to consider the above five (5) key and basic factors as the guide for the difference between gravity method and Magnetic method in geophysics
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