One of the problems I have with these documents is that most of them talk about the 3D resistance measurements to determine the depth of the objects to, in these cases they use a variable distance between the current probes. This complicates the theory behind it significantly.
For the 2D resistance map we see the 4-probe movable system unit used by the geo-physiscs teams a fixed distance between the probes is used.
I think it's important to distinguish between 2D and 3D so tings don't get to complicated and resulting in something that is over-engineered (and expensive).
Another thing is that these documents seems like they are written by some very skilled theoretical physics academics, that's making it even more difficult for me. There's loads of stuff I don't understand in these documents.
I understand that the galvanic currents generated by the soil might have an impact, but I'm not sure how significant this is. But AC should be better than DC in this case.
Also, I don't think pondering over Kelvin measurement is necessary. Kelvin probes is used to measure very low resistance (practically what we consider a dead short) or very high currents. I expect that earth resistance will be in the mega-ohm range, and a few milli-ohms will not affect the readings.
Same goes for the voltage readings as a measurement is taken with very little current the probe and wire resistance is insignificant.
The current is dependent on the earth resistance, and the voltage is changed to limit the current through the outer probes.
A little about constant current
here.
It would be interesting to know how much current we are talking about here, and if the voltage really needs to go all the way up to 100V. Perhaps 100V is used in the large array 3D measurements?
The 2D measurement systems we see used is normally battery operated so I don't think we are talking about much energy here.
At some point I will need to get off my butt and try some measurements.
As I mentioned, calibration might be a issue if you are going to use data and/or software for the commercial products, but it shouldn't be to difficult to make some software to make a image from the sampled data.
It would be quite nice to know a bit more about the data collected by the commercial systems.