Early Methods Of Groundwater Exploration

To best understand how electrical resistivity surveys for groundwater exploration work today, it’s important to understand where resistivity testing began. 

Brothers Conrad and Marcel Schlumberger performed the very first investigation of how electrical current moves through the ground and what you can detect using an electrical resistivity survey in the early 1900s. From their research came what is now one of the most common methods of groundwater exploration, vertical electrical sounding (VES).

Having an accurate image of the subsurface beneath a body of water is beneficial in myriad use cases, including: 

The Institute of Electrical and Electronics Engineers (IEEE) Standard 81-2012 “Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System”, suggests the fall-of-potential grounding test to be used to evaluate the capacity of an electrical grounding system—it is often used by subcontractors to power engineers. 

Land subsidence is the act of land moving downward, or subsiding. In many cases, land subsidence can signify the formation of a sinkhole, which you can read about in this article. Land subsidence could also signify the presence of an expansive clay. 

One way that sinkholes form is when water in the atmosphere reacts with carbon dioxide and forms a weak carbonic acid. As the slightly acidic rainwater moves through fissures in the limestone, it begins to dissolve and widens the fissures—which eventually creates air or water filled pockets. When those pockets become expansive, they’re called “caves” or “voids.” This is a common natural phenomenon in limestone or dolomite known as karstification—but it can be dangerous, expensive, and life-threatening when the ceiling of a void weakens and caves in. This is known as a “sinkhole.” 

At the most basic level, electrical resistivity (ER) (as used in geophysics) is the measurement of ground variations gathered by applying a small and highly controlled electric current across an array of electrodes.

Electrical resistivity imaging (ERI)—also known as electrical resistivity tomography (ERT)—is a geophysical technique used to create an image of a specific portion of the Earth’s subsurface. It is created through the use of automated geophysical instruments that gather thousands of resistivity measurements via an electrode cable and multiple electrodes.