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Throughout most of the 20th century, Vertical Electrical Sounding (VES) was the dominant geophysical resistivity method. It has been used all over the world for three primary purposes: geotechnical investigation, groundwater exploration, and mineral exploration. VES is performed using either the Wenner electrode configuration described in the ASTM G57 standard or using the Schlumberger electrode configuration. (The Schlumberger method is most commonly used for groundwater and mineral exploration, because it is less labor intensive than the Wenner method.)
Below, we’ll describe VES’s...
Soil resistivity testing is performed for a variety of important applications, including:
- Testing grounding grids: Grounding grids are primarily used in industrial settings—like electrical substations—where expensive machinery needs to be protected. The U.S. National Electric Code sets forth that every substation must have at least one grounding rod to protect equipment from lightning damage and to ensure that a stray current from the neutral doesn’t electrocute a person nearby.
- Cathodic protection: Cathodic protection is a way to protect underground or...
Cathodic protection is a method used to reduce steel oxidation through an electrochemical process. Cathodic protection is used to protect buried or submerged pipelines, bridges, and large steel structures from corrosion, breakdown, and rust when an electrolyte (like water with salt and minerals) is present. The electrolyte serves as a conduit for the electron flow from the anode to the cathode. In steel structures where no electrolyte is present (like a steel building), they are best protected by galvanizing (zinc coating) or simply painting.
For small steel structures, anode...
Aggregate materials—like sand, gravel, crushed stone, slag, and recycled concrete—provide bulk and strength to concrete or asphalt. Large aggregate quarries and sand and gravel pits are located around most populated areas because of the high cost of transporting aggregate. (In fact, the cost of transportation from the mine to the consumer may even be higher than the actual cost of the aggregate.)
There are many different types of aggregate materials found in various geographic locations:
- Alluvial deposits are clay, sand and gravel beds deposited by flowing water. These...
The ASTM G57 standard (also known as the four-pin Wenner method) is used to determine the properties of soil.
Soil tests can be performed in many ways and for a number of reasons.
- In agricultural applications, a soil test may be useful for analyzing nutrients and moisture content.
- In environmental analysis, soil testing is used to detect hazardous material present in the ground.
- In geotechnical investigations, soil tests may involve cone penetrometer tests to examine certain soil properties.
- In laboratory settings, soil tests are performed to...
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).
VES uses four electrodes...
Having an accurate image of the subsurface beneath a body of water is beneficial in myriad use cases, including:
- Determining sub-bottom geology for dredging purposes or communication cable lay-out.
- Monitoring leakage or mapping sediments in a dam.
- Mapping freshwater and saltwater interfaces near shorelines or offshore.
- Characterizing estuary boundaries.
- Measuring water column salinity variations.
- Mapping underwater mineral exploration.
- Locating freshwater springs at sea.
- Determining sediment load (dredging survey...
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.
Why is the IEEE Standard 81 fall-of-potential grounding test important?
Grounding tests are mainly used at lightning protection systems, electrical substations, and industrial sites where expensive machinery needs to be...
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.
To understand the need for land subsidence testing and monitoring, think of the need for both MRIs and X-rays. Just because a patient says his shoulder hurts doesn’t mean a...
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.”