• Resistivity governs the amount of current that passes through a rock when a specific potential difference (voltage) is applied.
• Electro-chemical methods deal with the chemical activity with respect to the electrolytes in the ground.
• Dielectric constant methods give information on the capacity of rock materials to store electric charge. These methods use high frequency alternation current (AC) introduced into the earth surface by conductive or inductive means.

Basics of Resistivity

Ohm's Law relates Current (I), Resistance (R), and Voltage (V)
V = IR

Resistance is proportional to Length/Area of a wire. The constant that relates Resistance to its length and area is the Resistivity (p).
R = p L/A
Resistivity is defined by the resistance of a cylinder with a cross sectional area and length. The accepted unit of Resistivity in the metric system (kms) is ohm-meters.

The conductivity of a material is defined as. Conductivity = 1/p.

Electrical Properties of Rocks

Rocks have resistivity ranges from 1E-5 to 1 E15.
Rocks and Minerals with a resistivity from 1 E -5 to 0.1 Ohm-meters are considered good conductors, from 1 to 1E7 Semiconductors (or intermediate conductors) and from 1 E 8 to 1E15 Ohm-meters insulators (poor conductors).

Types of Electrical Resistivity Surveys

Vertical Electrical Sounding (VES): The objective of VES is to find variation in electrical conductivity with depth.
Common Types of VES Surveys:

• Wenner Sounding
• Schlumberger Sounding

Electrical Mapping: used to determine lateral variations in conductivity of the ground.
Common types of mapping surveys:

• Wenner
• Schlumberger