VERTICAL ELECTRICAL SOUNDING
Vertical Electrical Soundings (VES) are probably the original electrical resistivity method from which all other techniques have evolved.
VES is a direct-current resistivity method typically carried out with a four-electrode array. By progressively increasing the spacing between the current electrodes and/or the potential electrodes, greater investigation depths can be achieved. Shorter electrode spacings measure the resistivity distribution of shallow subsurface layers, while larger spacings provide information from deeper levels. By taking measurements at progressively larger distances between electrodes around a fixed central point, VES generates one-dimensional vertical profiles of the subsurface’s electrical resistivity.
The technique has two main limitations:
It should only be applied when geological formations present a subhorizontal distribution. Therefore, it is not suitable for highly tectonized zones.
The method assumes that each subsurface layer has a uniform electrical resistivity. Hence, it should not be used where lateral variations within the same layer are expected.
Due to these limitations, its use today is largely restricted to hydrogeological studies where only an approximate characterization of deep aquifer levels is required.
On the other hand, it is a very cost-effective technique that requires neither extensive personnel nor sophisticated equipment, which has made it one of the most widely used and in-demand methods worldwide.
Main Applications
Investigation of subsurface geoelectrical stratification (hydrogeology).
Aquifer studies: depth, thickness, and properties (e.g., salinity).
Aquifer monitoring (e.g., contamination studies).
Detection of sedimentary formations where a transition from clay to gravel increases both permeability and resistivity.
Determination of thicknesses in alluvial materials, provided a contrast exists with underlying finer-grained layers.
Bedrock depth determination.
Ground resistivity assessment for grounding system design.
Current flow lines and equipotential distributions in an apparent resistivity reading of a Vertical Electrical Sounding.
Fit between field-measured apparent resistivities (black curve) and calculated values (red curve) for a 1D subsurface resistivity model (blue curve).
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