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 benefits and limitations, discuss AGI’s technology option, and take a look at relevant case studies.
Benefits Of Vertical Electrical Sounding
The primary advantage of vertical electrical sounding is that it is relatively easy to perform and provides good result when certain criteria for the survey are fulfilled.
The ASTM G57 standard is largely a good thing in the geophysical community, as it ensures the data is collected in the same way and allows for comparisons between sites. However, there are some difficulties associated with standards—for example when technology moves forward, standards are no longer adequate.
Limitations Of Vertical Electrical Sounding
There are two critical limitations associated with VES. For the interpretation of the VES data to be accurate:
- The geological layers have to be horizontal, and each layer must have a consistent thickness across the surveyed area. In other words, the layers need to look like a layer cake. Of course, nature is not that exact in many instances. If the survey is on or beside a mountain range (or even a hill), the layers could be upturned or even flipped. They may even pinch out or expand at a certain point.
- Each layer must be homogeneous in terms of resistivity. Again, nature doesn’t always follow these rules. There could easily be caverns, boulders, or voids in the ground that interrupt homogeneity and create an issue with the VES resistivity solution.
If these two elements are not in place, the data measurement will not fit with the ground-truthing. In fact, in the 1980s and early 1990s, electrical resistivity was being phased out of the geologic community because too many erroneous results had given it a bad name. The mathematical formula is exact, but the false assumption that all geologic layers are homogenous and horizontal led to many inaccurate groundtruth samples and unhappy clients. While many people have moved to more modern, automated geophysical testing methods like 2D or 3D electrical resistivity imaging surveys, VES is still commonly used in parts of Asia, South America, and Africa. In the United States, VES is being phased out—but is still used for cathodic protection studies using the Wenner electrode configuration as described in the ASTM G57 standard.
One final limitation of VES is that it is quite labor-intensive. There are four electrodes used in VES. If the survey is performed with the Wenner electrode array, all four electrodes must be moved for every measurement. If it’s performed with the Schlumberger electrode array, at least two electrodes must be moved for every measurement. This requires two to four people to consistently move electrodes simultaneously, or one person moving each electrode one-by-one throughout the survey. This is both time-consuming and prone to human error.
AGI offers both electrical resistivity equipment and corresponding inversion modeling programs for vertical electrical sounding. EarthImager 1D can process VES data collected using Schlumberger, Wenner, dipole-dipole, pole-pole, and various other arrays, and presents it in a graphed chart.