Electromagnetic methods exploit how the ground reacts when electromagnetic fields pass through it. These fields consist of alternating electric intensity and magnetizing force. To create the primary electromagnetic fields, an alternating current is run through a tightly wound coil of wire or a loop made of wire.
When these primary fields interact with the ground, they generate secondary electromagnetic fields. We can detect these resulting fields by measuring the alternating currents they induce in a receiver coil, utilizing the principle of electromagnetic induction.
Electromagnetic methods:
Exploring different frequencies is a fascinating way to investigate the Earth and other planets, often proving more effective than just looking at the distance between sources and receivers. Consider the concept of “skin depth.” In some instances, these techniques are employed in the frequency domain, utilizing either the natural frequencies present in the environment or, with the help of a controlled source, a selection of fixed frequencies—this approach is known as the FDEM method or frequency domain electromagnetic surveying.
Fourier theory’s fascinating principles often play a crucial role, especially when analyzing transient signals like a step function, which inherently contains multiple frequencies. Researchers and engineers frequently utilize this approach in the TDEM method, or “time domain electromagnetic,” which has gained significant popularity in recent times.
In a low-frequency range, the currents from conduction are more significant than those from displacement. This is the opposite of Ground Penetrating Radar (GPR), which relies on wave propagation and can be best understood through the lens of geometrical optics. Additionally, low-frequency electromagnetic waves are effective in addressing the diffusion equation.
The system depends on a mix of controlled sources, like the transmitter integrated into its instrumentation, and uncontrolled sources. While nature predominantly provides the latter, it can also come from the Department of Defense.
- EM does not require direct contact with the ground. So, the speed with which EM can be made is much greater than that of electrical methods.
- EM can be used from aircraft and ships as well as down boreholes.
Types of Electromagnetic Methods:
Two types of Electromagnetic Methods are
Passive Electromagnetic Methods:
Utilizing natural ground signals (magnetotellurics)
Active Electromagnetic Methods:
This involves the use of artificial transmitters in the near field (such as ground conductivity meters) or in the far field (using remote high-power military transmitters such as VLF mapping at 15-24 kHz).
Electromagnetic Method can be classified as either :
- Time – Domain (TEM): TEM makes measurements as a function of time
- Frequency- Domain (FEM): FEM use either one or more frequencies.
Factors Affecting EM Signal:
The signal at the Receiver depends on :
- The material
- Shape
- Depth of The Targ
- Structure and position of the transmitting coil and receiving coil
The magnitude of the current induced by the transmitter depends on:
- Number of magnetic field lines passing through the loop (magnetic flux)
- Rate of change of this number
- The material of the loop.
Magnetic flux Depends on :
- The Strength of the magnetic field at the Loop
- Area of the Loop
- Angle of the loop relative to the field
Flux Ø = Magnetic field X cos Ө X area X number of turns
Applications of the Electromagnetic Method:
- Mineral Exploration
- Mineral Resource Evaluation
- Groundwater Surveys
- Mapping Contaminant Plumes
- Geothermal Resource Investigation
- Contaminated Land Mapping
- Landfill surveys
- Detection of Natural and Artificial Cavities
- Location of geological faults
- Geological Mapping
Advantages of the Electromagnetic Method:
- Lightweight & easily portable.
- Measurement can be collected rapidly with a minimum number of field personnel.
- Accurate
- Good for groundwater pollution investigations.
Limitations of the Electromagnetic Method:
- Cultural noise
- Complex Geological Settings
- Depth Penetration Limits
Principle of EM surveying:
EM field can be generated by passing an alternating current through a small coil comprising many turns of wire or a large loop of wire.
The electromagnetic (EM) radiation spectrum covers a vast range of frequencies. It begins below 15 Hz, which includes atmospheric micropulsations, extends through radar bands ranging from 10^8 to 10^11 Hz, and reaches into the X-ray and gamma regions, exceeding 10^16 Hz.
In geophysical applications involving frequencies below a few thousand hertz, the wavelengths are typically between 15 and 100 kilometers. However, the distance between the source and receiver tends to be much smaller, usually around 4 to 10 meters.
The primary EM field travels from the transmitter coil to the receiver coil via paths above and below the surface.
In the presence of a conducting body, the magnetic component of the EM field penetrating the ground induces alternating or eddy currents to flow in the conductor.
The eddy currents create a secondary electromagnetic field that moves toward the receiver. The variations found between the transmitter and receiver fields indicate the presence of the conductor, offering insights into its shape and electrical characteristics.
Conclusion:
Electromagnetic methods are a valuable and adaptable tool in geophysical exploration and many other fields. This is crucial for obtaining information about the subsurface while reducing environmental impact, highlighting its importance to modern science and industry. As technology advances, the range of these techniques is expanding, further highlighting their vital role in understanding the structure of the Earth.
