Syscal, Promis, Numis
In sedimentary context, the groundwater is generally located in the pore space of permeable rock layers. Depending on the rock porosity, rock permeability and sometime fracture and karstic network, the number of ground water table, their thickness and their spatial heterogeneity may vary greatly. In order to locate the most favorable site to setup a drilling, several geophysical investigations can be performed.
The first and simplest geophysical method is the electrical sounding. It is performed using four electrodes (a quadrupole), centered on the investigation point. Several measurements are performed, each time increasing the electrode spacing. In that way, the investigation depth is increased at each measurement so that one can obtain after data inversion a log of resistivity. Although this method is based on 1D assumption (no lateral variation of resistivity), it has been showed to be very powerful for decades. It can be easily performed using the Syscal instrument range (Syscal Junior / Syscal R1 / Syscal R2 / Syscal Pro) or the VIP or TIPIX transmitters range associated to the Elrec range of receivers.
In cases where the geology is more complex (faults, inclusion, clay lenses, etc.), the groundwater level may vary laterally. Electrical Resistivity Tomography may therefore be a good tool to assess the best location of a drilling in this context. This method has been applied in a very large number of hydrological contexts and is often interpreted jointly with hydrological data (Koch et al., 2009). The Syscal Switch range of equipment are adapted for this type of application for shallow to deep aquifers (Syscal Junior Switch / Syscal R1 Switch / Syscal Pro Switch).
Finally, the most adapted method to locate and characterize groundwater table is the Nuclear Magnetic Resonance (NMR, Legchenko, 2013). This method consists in exiting the molecule of water in the soil and to measure their electromagnetic response. Contrary to the electrical resistivity, this measure is directly linked to the aquifere characteristics (water content, permeability). NMR is able to provide data on the water content and permeability of the soil, avoiding you to locate a drilling in a high water content but low permeable layer that would result in a low productivity. This instrument has been proven reliable in a large number of contexts (karsts, Vouillamoz et al., 2003; hard-rock aquifers, Legchenko et al., 2006, fractured aquifers, Wyns et al., 2004; coastal aquifers, Vouillamoz et al, 2007, water-filled cavities, Girard et al., 2005; glacier, Vincent et al., 2015, etc.). The Numis range equipment allows to realize NMR sounding up to 150m of depth.
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