Principle of Electromagnetic (EM) Method

Electromagnetic (EM) Method

Electromagnetic (EM) Method is the one among the common techniques, which is widely used in different applications such as Mineral exploration, groundwater contamination studies, just to mention a few. Let you grasp the basic Principle behind this technique.

Electromagnetic (EM) waves

The term Electromagnetic (EM) implies the combination of Electric and Magnetic fields components.

Electromagnetic waves require no medium during their propagation, They consist of both electric and magnetic fields components that are both perpendicular to each other and to the direction of propagation.

Examples of these waves are radio waves, X - rays, gamma rays, Ultraviolet light just to mention a few.

However when it comes to geophysics, these waves are more utilized in geophysical techniques such as Electromagnetic (EM), GPR and others.

Electromagnetic wave equation

Let's see Basic mathematical derivation of how this works, but we have to assume that our wave is Planar.

In order to move smoothly and grasp the concept, I'm assuming that you have the basic idea on Partial derivatives.

Since Electromagnetic waves composed of Electric and magnetic components, they obey Maxwell equations (simplified form) such as these two below,

∂E/∂x = - ∂B/∂t....................(1)

∂B/∂x = - μoεo∂E/∂t.................(2)

If we take the Partial derivative of equation 1, such as

∂ [∂E/∂x]/ ∂x = ∂ [- ∂B/∂t]/∂x

∂2E/∂x2 = - ∂ [∂B/∂x]/∂t

Then substitute ∂B/∂x from equation 2 into equation above, we get

∂2E/∂x2 = - ∂ [- μoεo∂E/∂t]/∂t

∂2E/∂x2 = (μoεo)∂2E/∂t2..................(3)

Also the same if we take Partial derivative of equation 2, then combine with equation 3

∂2B/∂x2 = (μoεo)∂2B/∂t2...................(4)

Equation 3 and 4 is Electromagnetic wave equation for Electric (E) and Magnetic (B) components respectively in free space.

If you compare any of the above equation with general Linear wave equation, you will find,

V = 1/√(μoεo).

Then insert  μo= 4π × 10-7 Wb/A and εo= 8.85418 × 10-12 C2/N.m2

Then V = 2.99792 × 108 m/s, is the speed of Electromagnetic waves.

Then the value of V is approximately the same as the speed of light, this implies that Electromagnetic waves travel with speed equal to that of light in free space.  

Alternative 2: we can derive the Electromagnetic wave equation by simply using plane sinusoidal wave equation such as

B = Bm cos(kx - wt ) and E = Em cos(kx - wt ), the same as described in the derivation of linear wave equation.

It is geophysics! but the basic laws and principles are those of physics!

Classes of Electromagnetic (EM) methods

There are two (2) broadly classes of EM methods based on nature of EM source fields.

1. Passive source EM methods

These utilize natural EM fields originating from outer space, Example Magnetotelluric (MT) method

2. Active source EM methods

These utilize active EM fields produced from artificial sources, Example controlled source EM (CSEM) method, Very Low Frequency (VLF)

Basic Principle of Electromagnetic (EM) Method

This technique applies the principle of Electromagnetic Induction.

This phenomena means an electromotive force (e.m.f), is produced whenever there is change in magnetic flux linked to that circuit as described by Faraday Law of induction.

How does it works?

The primary EM field is generated by passing an alternating current (Ac) through a wire coil known as a transmitter.

This primary EM field propagates above and below the ground. If there is a conductive material buried in the ground the magnetic component of the EM wave induces  Eddy currents in that conductor according to Faraday Law of induction.

This eddy current which is alternating in nature produces a secondary EM field which is detected by the receiver, as shown on Figure 1.

Figure 1: Basic principle of Electromagnetic method

The EM system must consist of Transmitter and Receiver either separated or placed in a single unit.

RECEIVER, This detect Resultant Field such as a combination of Primary and Secondary EM fields and Primary EM field itself

N.B: The Primary and Secondary EM fields in most cases differ in phase and Amplitudes.

Figure 2: phase difference in primary and secondary EM field.

The Secondary EM field depends on Coil spacing, Operating Frequency and ground conductivity.

The difference in the Resultant field from the primary EM field provides information about the geometry, Size and the Electrical properties of the subsurface Conductor.

TRANSMITTER, This records the out of phase component (Quadrature phase) and in phase component. See figure 2 above

Rule of thumb

The depth of interest depends on the separation distance between receiver and transmitter such as if distance is tens of meters then the depth of interest below ground will range at tens of meters.

If receiver and transmitter are collocated such that they are mounted in one unit then the depth of interest will depend on inter coil spacing.

Advantages of Electromagnetic (EM) Method.

i/ It is successful that  can be applied even when the ground is resistive such as in Arid zones.

ii/ High Penetrating depth

Transient has an additional advantage that the secondary EM field can be measured when the primary EM field is switched off. Such that Transient  has high penetration depth compared to frequency domain.

Disadvantages (Limitations) of Electromagnetic Method

i/ It does not provide a clearly defined signals

ii/ Signal resolution decreases with increase of depth of penetration.

Depth Penetration

The EM energy is attenuated as it propagates through the conductor.

Factors at which penetration depends are   Frequency and Conductivity of the materials

Penetration is described by Skin depth.

SKIN DEPTH:

Skin Depth is the depth at which the Amplitude of a plane wave decreases to 1/e or 37% relative to its initial amplitude (Ao).

i.e Az = Ao/e

Then skin depth, z (m),

z =  √(2/wK)

where, w - Angular Velocity, K - Conductivity (s/m)

But , w = 2πf

Then,  z =  √(2/2πfK)

From the formula above It means that skin depth (z) increases as frequency and conductivity decreases as shown in the figure 3 below

Applications (uses) of Electromagnetic (EM) methods

Electromagnetic (EM) methods are applied in the following areas,

- Mineral exploration

- Groundwater contamination, to detect contaminants (Greenhouse and Harris,1983)

- Hydrogeological investigations by Everett and Meju (2005). - Salt water intrusion - Mapping geology and soil - Locating buried objects such as pipes, Barrels, tanks, walls - Archeological studies. - Locating gravel - Locating cavities such as caves, abondoned underground mines.

Now I think you have got this basic idea about Electromagnetic method.

References

Best, M. E. (1992). Geological Association of Canada Short Course Notes Volume 10. Wolfville, Nova Scotia, May 28–29

Everett, M.E. and M.A. Meju, (2005), Near-surface controlled source electromagnetic induction: background and recent advances, Chapter 6 in Hydrogeophysics, Eds. Y. Rubin and S. Hubbard, Springer, Netherlands

Greenhouse, J. P. and R. D. Harris, (1983) Migration of contaminants in groundwater at a landfill: a case study. J. Hydrology, 63,177–197.

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