The Basic Principle of Electrical Resistivity Survey

Resistivity method is one of the oldest geophysical survey techniques used until today. It is one of the versatile method among the others.

Electrical resistivity survey aim to determine the distribution of subsurface's resistivity by making measurement on ground surface.

True resistivity of the subsurface can be estimates from this measurement.

There are a few parameters that influence the ground resistivity such as mineral and fluid content, porosity, and the degree of water saturation in the rock.

These parameters make resistivity survey suitable in geotechnical, mining, hydrogeological, environmental and even in hydrocarbon exploration.

The fundamental physical law used in resistivity surveys is Ohm's Law.

The Measurement (Basic Setup)

Basically, the basic setup for resistivity measurement involved four active electrode (stainless steel bar). These electrodes planted into the ground.

The electric current will injected into the ground (earth) using two current electrodes (C1 and C2). The resulting voltage difference at two points on the ground surface measured by two potential electrodes (P1 and P2).

Changes in the ground resistivity will cause deviations in the current flow and the resulting measured voltage difference.

As it's origin, resistivity method is known as resistivity sounding. This is one dimensional (1D) survey where only take count vertical changes.

2D Electrical Resistivity Survey

The greatest limitation of resistivity sounding method can be overcome by using two dimensional (2D) model where resistivity changes in vertical direction and horizontal direction along the survey line are take in count.

However, 3D model or resistivity survey is more accurate. At present time, 2D resistivity survey is more practical in term of cost and accurate result.

Two dimensional electrical resistivity surveys are usually carry out using large number of electrodes or also known as multi-electrode system. This multi-electrode system can cover wider area in shorter time.

A resistivity meter system with internal microprocessor controlled circuitry together with an electronic switching unit is commonly used to automatically select the relevant four electrodes for each measurement.