Which method to find deep underground water?
To locate groundwater accurately and to determine the depth, quantity, and quality of the water, several techniques must be used, and a target area must be thoroughly tested and studied to identify hydrologic and geologic features important to the planning and management of the resource.
From measurements, the type, size and quality of the aquifer is deduced and specified, or perhaps only presumed, but with a high probability.
There are two main types of methods, which are sometimes used successively :
a) The measurement of electrical resistivity using direct current.
This is the most widely used method as it is suited to the greatest number of situations.
It consists in sending direct current into a geological structure on a given site (50 to 400 volts depending on its resistivity - conductivity) using two electrodes (A and B).
There are several possible electrode arrays (Wenner, Schlumberger, 4 terminals, etc.).
The area investigated must not be too large and must be relatively flat and free of buildings which may cause interference and would make it impossible to have AB lines of the required length (over 300m).
b) Methods for measuring this reactivity by magnetic means
Easier to implement, such as the VLF methods, these methods measure electromagnetic signals due to magnetic induction phenomena. They don't need any contact with the ground and thus no electrodes. They make it possible to measure the soil's reactivity to electromagnetic excitation. However, they cannot be used on all types of grounds or for aquifers over 20 metres deep, or even less. Their use seems to have dwindled.