Electrical resistivity is one of the oldest geophysical methods, dating back to 1830 when Robert Fox observed what he realized were self-potentials in copper mines in Cornwall. Work on resistivity itself started in the 1890’s, when Alfred Williams and Leo Daft did research on earth resistivity and patented methods that were used in Sweden from 1906. Conrad Schlumberger published on self-potential in 1918 following his field applications of the method from 1912.
Frank Wenner introduced apparent resistivity in 1915. Vertical electrical sounding (VES) was established in the 1920’s and did not change much since the Schlumberger Group and several other researchers later developed the field techniques and curve-matching data interpretation tools in the 1920’s and 1930s.
Computer-based interpretation techniques were introduced in the early 1970s and later automatic inversion in the late ‘70s. In Kenya these techniques were introduced by the TNO Institute for Applied Geosciences at the turn into the 80’s and I owe my knowledge to the institute.
While 2D electrical resistivity imaging has gained currency since the mid-1990s, the cost of instrumentation remains prohibitive and the 1D method is still widely used. When applied and interpreted rightly, it is still a very useful method, but this is sometimes not the case.
I encounter situations where practitioners use 1D VES with limited depth of field technique and interpretation. Ironically, this could be attributed to its simplicity. The problem is compounded when practitioners not grounded in 1D techniques progress to 2D/3D imaging. It’s more like driving a car without principles of internal combustion/electric alternators and power transmission.
For best results with 1D VES, and indeed any method, it is important to have a good idea of what you are looking for. This helps in understanding and interpreting the feed back you get from the field data and modifying your approach to get what you want. Always remember that whether the method gives you good or noisy/poor data, it is telling a story about the ground you are investigating that you need to pay attention to.
In one such occurrence while working in sediment-on-sandstone terrain in a remote part of Somalia I was not able to progress my current half electrode separation beyond 30 or 40 meters without negative readings, for several days. Having done all I could to ensure the instrument was working well and the contact resistance was good (clay pot, nested electrodes etc.) to no avail, I turned to the geological structure.
From studying grainy satellite imagery downloaded in the field through a phone modem, I noticed a gridded fracture pattern in the sandstone, with many fractures traceable for up to 20 kilometers. My suspicion was that while the ground contact was good, the current loop could not be completed due to scatter, whenever electrodes were on opposite sides of a deep regional fracture. I therefore located on the image a point for investigation, georeferenced it and noted the orientation of the fracture on which it lay.
Back in the field I traced the point and conducted a short electric profile to locate the exact fracture position. I then had a line cut exactly on the fracture and orientated in its bearing (220 degrees) so that at no moment would any electrode be misplaced. On this occasion my team managed 250 metres half-AB separation and completed the sounding, resulting to successful drilling and a fresh water borehole.
My point here is that by knowing the principles of VES and seeking to understand the ground conditions I was able to use the method to my advantage and that of the community that desperately needed the water.
I once encountered a similar problem, but with 2D environmental surveying. Using the gradient protocol with a 4-channel instrument on small electrode separation of 2 metres, the instrument would not settle on a reading, all the while attempting to send various magnitudes of current to no avail.
I turned to the soil and found that it was chromium-rich. Now, chromium has weak ferrimagnetism that causes multiple magnetization reversal when an external current is applied. Could this be the problem, that when all four channels were working, I had 16 active electrodes at any one time compounding the chaotic ionic alignment that would not result to normal DC resistance protocol? Not having the luxury to explore further since I had a 2-day window in which to finish my work, I went for a single-channel instrument on the assumption that the 4 electrodes will result to less magnetic noise and completed my assignment without further hitches. It is apparent that even though the electrical resistivity method has been here for over a century, there’s room for research and knowledge development. For example, numerical analysis and machine learning offer opportunities for improved geophysical data interpretation even for simple techniques like VES.