Electrical resistivity tomography, also known as electrical resistivity imaging, has been a technology in development for many years. However, its importance is only now beginning to be appreciated.
How it works
The first thing you need to understand is the word resistivity. This simply refers to how much resistance a material has to electricity. It is the opposite of conductivity; a word you may be more familiar with. The resistivity of various naturally occurring materials is known, so the idea is to inject a current through the ground and measure the resistance found in the underground area. The data for resistivity can then be translated into images.
The images can be displayed on a screen, and with modern imaging equipment, various colors can be employed so that the materials being sought after can be quickly identified visually.
The basic technique
The fundamental way to measure the resistivity of material in the ground is through the use of electrodes. The first step is to drill a number of holes for the electrodes you’re using. With the use of a direct current power source, a voltage is applied. Through the application of Ohm’s law, once you know the voltage and current, you can easily calculate the resistance, and this will tell you what is beneath the ground.
In its two dimensional form ERT was first understood more than 50 years ago, but it wasn’t until the development of computers that a method could be employed that produced important and usable results.
The applications of this type of imaging system are wide ranging, and are only limited by an engineer’s imagination, but some of the more common applications are found throughout the world. The location of water has become and important part of EMT, because fresh ground water is becoming scarce all over the world. Oil companies are also using this technology to verify the amount of oil underground before drilling begins. A third application is in the area of earthquake research. EMT can be employed to discover and map the location and size of faults found throughout the earth’s subsurface.
Generating images and mapping the subsurface of our planet will only continue to grow in both use and importance as the world’s population grows, and the demand for resources grows. If you have not been familiar with this technology in the past, keep your eyes and ears open as you will likely be hearing more about it in the future.