In a Schlumberger Vertical Electrical Sounding (VES) survey for groundwater exploration, how is depth of investigation progressively increased into deeper geological strata?
Options:
(A) By increasing the spacing between the outer current electrodes ($AB$) while maintaining relatively small potential electrode spacing ($MN$)
(B) By decreasing the electrical current to zero
(C) By submerging the electrodes into a water tank
(D) By rotating the sounder $90^\circ$ horizontally without expanding electrode spacing
Answer Key:
A
Step-by-Step Solution & Explanation
In the Schlumberger array, expanding the outer current electrodes ($A$ and $B$) forces electric current lines to penetrate deeper into the subsurface, enabling measurement of apparent resistivity at progressively greater depths. Option A is correct.