Groundwater Hydrology

In surface electrical resistivity surveys for groundwater exploration (Schlumberger and Wenner electrode arrays):

Options:

• (A) Current is injected into the ground through outer electrodes $A$ and $B$
• (B) Potential drop is measured across inner electrodes $M$ and $N$
• (C) Apparent resistivity $\rho_a = K \frac{\Delta V}{I}$ where $K$ is the geometric configuration factor
• (D) Direct current cannot be used under any circumstances