Resistivity survey measures the electrical conductivity of the various geological materials in subsurface by simply applying 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. However the survey design depends on the geology of the site and the expected end goal of the survey.
Electrical Resistivity gained attention especially in searching for groundwater resources, due to promising direct relationship between electrical conductivity of groundwater and electrical Resistivity.
Groundwater exploration is the systematic process of searching for groundwater resources (aquifers) by applying geologic, geochemical and geophysical techniques.
Groundwater refers to the water that occupies the subsurface spaces and interacted with subsurface geologic environment.
There are three (3) common methods of conducting electrical resistivity surveys for groundwater exploration, these are electrical sounding , electrical profiling and sounding-profiling (combined).
Although when it comes to choose any of these methods to apply you have to consider, what do you want to achieve example , when your goal is to map the depths and thickness of underlying layers then the sounding method is your best option. But if you want to map lithologic contacts, then you can consider the profiling method.
This post will inform you on how these three (3) methods give to the rise of three (3) basic operation modes of resistivity surveying for groundwater exploration. These modes includes Vertical Electrical Sounding (VES), Constant Separation Traversing (CST) and Electrical Resistivity Tomography (ERT).
1. Electrical sounding
This technique measures vertical variations of resistivity, such that resistivity varies with depth. It is also commonly known as Vertical Electrical Sounding (VES). It means that resistivity is plotted against electrode separation. It means any horizontal discontinuity can be mapped.
The basic principle behind this technique is that the separation distance between current (outer) electrodes affects the depth of current penetration into the subsurface. Such that as the separation distance between current electrodes increases, depth of penetration of injected currents into subsurface also increases, which in turn increases the depth of exploration.
We can simply say this as, the wider the electrode separation, the deeper the penetration of injected currents, lead to deeper depth of exploration, and vice versa is true.
You may look at the figure 1, below as it illustrates that as the separation distance between current electrodes (C1 and C2) increases also the depth of penetration for both injected currents and exploration increases. P1 and P2 are potential electrodes and the figure is an example of the model of Schlumberger array configuration.
Figure 1: simple representation of Schlumberger array.
This technique applies two most common arrays during surveying, the Schlumberger and Wenner arrays. Each of these arrays has its own geometry and configuration.
2. electrical profiling
This technique measures horizontal variations of resistivity. It means that the resistivity is plotted against horizontal distance.
The basic principle behind this technique is that resistivities are recorded horizontally along a profile at regular intervals as shown in figure 2. However the electrode separation is kept constant during such movement.
This made this technique to be known as Constant Separation Traversing (CST). It means any vertical discontinuity such as dykes,soil profiles can be mapped.
Figure 2: Simple representation of profiling survey.
Figure 3: The simple resistivity profile
This technique applies two most common arrays during surveying, Wenner and Dipole - dipole arrays. Each of these arrays has its own geometry and configuration.
3. Electrical Resistivity Tomography (ERT)
ERT is an advanced geophysical method that utilizes electrical resistivity techniques to image variation of subsurface resistivity either on ground or through the boreholes. It is termed as Geotomography technique at which 3D mapping (imaging) of the subsurface distribution of electrical resistivity can be determined.
The technique applies a multi-channel electrode array system at which both lateral and vertical subsurface variation of electrical resistivity can be revealed. Hence it is regarded to deploy "Combined sounding - Profiling" resistivity survey Modes.
This means that you can apply this technique to address issues the same as how basic electrical resistivity methods do. However the good news is that with ERT you can solve even those problems with both horizontal and vertical variation of resistivity and when the geology of your site is complex.
However its basic working principle remain the same as that of Basic electrical resistivity, where by four (4) Co-Linear electrodes are utilized along the survey array, where electric current is sent into the ground through two (2) current electrodes and the ground potential difference (P.d) is recorded through two (2) potential electrodes. You can see the figure 4 below (simple Schlumberger array) to get a basic insight on how this arrangement works.
All in all you can choose any of these technique to apply in your survey, However when you faced a challenge where geology is complex, example in contaminant plumes such as resistivity varies in both vertical and horizontal direction, here it is a good idea to combine both techniques, so as to solve a such problem.
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You can check here below the simple lists of equipment and tools that can suit your groundwater and metal searching Project!
BASIC RESISTIVITY EQUIPMENTS and TOOLS
- Electrical resistivity meter
- Geological Hammer (The friend of Geologists)
NEED METAL DETECTORS
- Lightweight, High accuracy Metal Detector.
- Gold Detector for Mineral Exploration.
- Professional underground Gold digger - Detector.
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