Thank you for your excellent question regarding Soil Resistivity testing using induced magnetic fields.
The short answer to your question is that, yes, it is possible to measure soil resistivity using Electromagnetic Fields. However, there are some short comings to these methods that you should be aware of. First of all, there are two (2) primary techniques in use today, Electromagnetic (EM) and Nuclear Magnetic Resonance (NMR).
Electromagnetic (EM) Soil Resistivity Measurements are conducted using a variety of instruments, from hand-held devices resembling long poles, to plates that are dragged behind a vehicle, to airborne arrays attached to helicopters and airplanes designed to survey large areas. The system works by generating a dipole electromagnetic field to gather data, and then using some inverse mathematical techniques is able to calculate out both soil conductivity and soil resistivity. The primary drawback with these systems is penetration depth. Generally speaking, these systems are only good for 30-ft depths, and often more like 10-20-ft.
Nuclear Magnetic Resonance (NMR) is another technique capable of reaching depths as far as 2,000-ft. It involves setting up a large magnetic coil, which similar to an MRI machine, slowly aligns the magnetic poles of the molecules beneath the surface, and upon release of the magnetism is able to monitor numerous geological aspects of the earth below that point, including resistivity.
Both techniques have serious problems penetrating clay.
India has some very interesting soil conditions in that vast stretches of the continent tends to have a lot of volcanic material near the surface, with clay beneath. In the central part of India, in particular the area stretching from New Delhi to Calcutta, there is a well known field of clay. With India’s proclivity for very deep soil resistivity soundings and the abundance of measurement stopping clay, Electromagnetic Soil Resistivity and/or Nuclear Magnetic Resonance imaging technologies may not be the best choice. You could try some airborne measurements in the rocky area’s and see if any clay is found, but in order to get really deep readings or to get measurements past layers of clay, you may find yourself back using the Wenner method.
The Engineering Team at E&S Grounding Solutions
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