Electromagnetic (EM) and Electrical Resistivity methods are two different geophysical techniques and frequently applied to infer the subsurface electrical properties. However there are some situations where these techniques cause confusion due to their similarity as you can't explain one of these methods without falling on the term electrical. This is where the confusion arose and this is why I have decided to make this clearance difference to you through this post in a simple way as possible even a layman with limited geophysical knowledge can grasp the concept.
However I have to make clear and specific to you that the EM technique is the general term but what I intend in this article is Active EM techniques such as Frequency Domain and Time Domain and not those of passive origins such as Magnetoterullic (MT). I hope this is clear to everyone!
Okay! Let us grasp the basics and see the following that makes the difference between these two geophysical techniques,
Basic Operation Principle
Electromagnetic (EM) technique applies the primary EM field that 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.
This eddy current which is Alternating in nature produces a secondary EM field which is detected by the receiver. We can simply note that this technique applies the Principle of electromagnetic induction.
Electrical Resistivity Method simply applies the direct current into the ground through current electrodes and recording the potential difference between potential electrodes to give the electrical resistivity of the subsurface. The electric current is injected into the ground through current electrodes which are connected to power sources such as batteries and the potential difference resulting in the earth by this current is measured at the potential electrodes which are connected to a sensitive voltmeter device. It is then possible to determine an effective or apparent resistivity of the subsurface. This technique applies the basic Ohm's law.
Mode of deployment
Electromagnetic (EM) methods can be deployed through air by Aircraft or helicopter such as Airborne EM surveys (AEM). If It is done through Aircraft the flying height normally ranges from 60 - 120m, but this may vary depending on survey design.
The primary EM field is generated by the transmitter, while Secondary EM field is picked up by the receiver coil.
However In typical frequency domain helicopter EM systems (HEM), both transmitting and receiving coils are housed in a rigid boom or "Bird" that is towed beneath the helicopter.
While the unique fact of resistivity surveying is that metal electrodes should be directly plugged into the ground hence Ground based Surveys and can't be deployed through Air. However you have to remember that even Electromagnetic (EM) can be done on ground but there is no need for direct contact of the receiver - transmitter system with the ground surface.
Measured physical parameter
Electromagnetic (EM) measures the electrical conductivity of the subsurface materials by simply relating the measured electric and magnetic fields as these will give us the impedance which in turn can extract the electrical conductivity. However this relation will depends on the nature of the layers (substratum) as given by (Zhdanov and Keller, 1994) as described here below
For perfectly insulating substratum
Z = E/B, and Z = 1/S
For E - electric Components , B - Magnetic components, Z - Impedance, σ - conductivity, d - layer thickness, Where S = Σ(σd) from n = 1 to n = N
For perfectly conducting substratum
Z = - iwμ0D
For w - angular velocity, Where D = Σ(d) from n = 1 to n = N
While the Resistivity technique measures the electric resistance as the result of passing Direct current (DC), when you multiply this resistance by a given geometric factor it gives us the apparent Resistivity which is the electrical resistivity of the subsurface.
Diffusive Vs Electrical method
Electromagnetic (EM) are considered as Diffusive techniques because the primary EM fields diffuses (passing) into the medium from the surface as it obeys the diffusion equation and this makes this technique associated with diffusive Depth. The secondary EM field is measured by the receiving sensor enclosed within a device known as magnetometer
DC resistivity is considered the Electrical method as the modified electric signals are measured by the voltmeter receiver housed inside the instrument known as resistivity meter (receiver). Hence it is associated with depth of current penetration as this depend with mode of deployment you can see more here previous post concerned this.
All in All the primary objective of these two Geophysical methods will remain to explore the nature of subsurface conductivities. While their depth of penetration is based on geometric sounding at which the separation distance between the transmitter - receiver is used to control such penetration.
At the end of the road there is nothing to say than to give Electromagnetic methods (EM) more credits over DC resistivity when one is dealing with the same objective provided that other factors have been considered.
Thank you for having interest in this blog!
Follow this blog on Telegram group
You can SUBSCRIBE this blog to have Updates in your mailbox. Don't forget to verify your subscription in your mailbox after subscribing.
SHARE! with your colleagues and friends if you found it interesting!
RELATED LINKS >>> See More
- Digital Bench LCR Meter Bridge with 0.1% accuracy
- Three component geophone string sensor
- Digital Electrical Tester with EM display
- AC Voltage Testing with Auto Range
- Handheld Electromagnetic detector - Multifunction
SCIENCE ACADEMIC ASSISTANCE >>> See More
- Physics
ENTERTAINMENT
 and Electrical Resistivity methods){kind=link}
0 Comments