User:Gralex

Vertical Electrical Sounding

Vertical Electrical Sounding (VES) ist the geophysical method for an investigation of the geological medium. The method is based on the estimation of the electrical conductivity or resistivity of the medium. The estimation performed based on the measurement of voltage of electrical field induced by the distant grounded electrodes (current electrodes). The Figures 1.-4. show the possible configuration of the measurement setup. The electrodes A and B are current electrodes which are connected to a current source, N and M are potential electrodes which are used for the voltage measurements. As source the direct current or low frequency alternating current is used.

Figure 1. Electrical profiling using four-electrode probes in Wenner configuration.

Figure 2. Electrical profiling using four-electrode probes in Schlumberger configuration.

Figure 3. Three-electrode array profiling.

Figure 4. Dipole-dipole electrode array profiling.

The interpretation of the measurements can be performed based on the apparent resistivity values. The depth of investigation depends on the distance between the current electrodes. In order to obtain the apparent resistivity as the function of depth, for each position the measurements are performed with several different distances between current electrodes. The apparent resistivity is calculated as

$\rho _{k}=k{\frac {U_{MN}}{I_{AB}}}$

here, k geometric factor, ${U_{MN}}$ — voltage between electrodes М and N, ${I_{AB}}$ — current in the line AB. The geometric factor is defined by

$k={\frac {2\pi }{{\frac {1}{r_{AM}}}-{\frac {1}{r_{BM}}}-{\frac {1}{r_{AN}}}+{\frac {1}{r_{BN}}}}}$

here r — distance between electrodes.

Interpretation of gathered data is performed based on the dependency ρ_k(AB/2).

The application of large electrode arrays allows to reconstruct complex 3D structure of geological media (see Electrical resistivity tomography). However the interpretation for such measurement is rather difficult. In this case advanced interpretation the techniques based on numerical methods can be applied.

Freeware for numerical calculations

• Solution of inverse problem
• Solution of forward problem

See also

• Magnetotellurics
• Telluric current
• Seismo-electromagnetics
• Electrical resistivity tomography

References

Category:Geophysical imaging Category:Inverse problems