In resistivity surveying, vertical electrical sounding (VES) and electrical resistivity tomography (ERT) are two of the most common variants being deployed routinely in geophysical investigations. VES is essentially a 1D survey whereas ERT is a 2D survey. Many are familiar with both VES and ERT. If you are one of them, there is no need to scroll further, stop and skip. However, if you are a student or someone who has just been introduced to geophysics (the resistivity method in particular), you need to read on.
I have been approached by students not long ago wondering what went wrong with their resistivity inversions. I thought this was just an isolated case. However, when I was again approached by a student with the same issue, I thought that this has to stop. What these students were doing was inverting VES data using 2D resistivity software. Of course, the output was weird, wrong and unacceptable. This led me to think that these students were not properly taught and guided on the differences between VES and ERT. This issue should be easily has been resolved if they had fully understood the differences. There is really no necessity to bring this up with someone they hardly know who are far away from where they are !
I will explain the differences between VES and ERT but I will not delve into details. VES is making a resistivity measurement at a particular point/location using a particular array, the most popular being Schlumberger. There other arrays such as Wenner, dipole-dipole, gradient etc. can also be used for VES. However, the Schlumberger array is synonymous with VES. VES requires the electrodes to be expanded to reach deeper depths. The most critical thing to remember is that this measurement is only representative at the point of measurement. Each VES point will typically be like a borehole location. There will be generally be quite a number of VES points which can then be connected to form a 1D subsurface resistivity profile. 1D inversion (of course with 1D inversion software, not 2D) is carried out to obtain true resistivity, layer thickness and depths at the POINT OF MEASUREMENT.
ERT is a multi-electrode system, typically deploying >20 electrodes laid out over a line of specific length. The electrodes are often spaced out at regular interval. The arrays used are generally similar to VES. The output are 2D images showing subsurface changes along the survey line and also with depth. It's for this reason that they are referred to as 2D survey. ERT is also capable of picking up 2D subsurface features (eg. bolder & cavity) which VES will not be able to do. ERT inversion is carried out with 2D resistivity inversion software. It will not work with 1D software !
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