I went to the field with two different resistivity meters (OHMEGA and ABEM Terrameter SAS 4000) and recorded negative resistance values from a particular subsurface interval.
There are many reasons why you would measure negative resistance at particular intervals along the profile. Here are a few ones, among others. The first one is a flip between electrodes within the sequence file (e.g. a flip between M and N). The second one is a short along one of the cables. The third one is the presence of buried lines/pipes/structures which would result in (approximately) very low differences in potentials (voltage) between M and N and, possibly, negative measured voltages. Dipole-dipole array is especially sensitive to this particular point since the measure is made with low voltages (relative to other protocols).
Hello dear Alex. I agree with Gregory, in particular point 3. It is possible that is buried metal (rod, steel wire) or the presence of an electrical power line, this buried or in land surface (I had, an experience so, a house had protection of wire with current around all construction) this puts at cut off circuit the system and hence low voltages and therefore negative resistances are generated. You should check if the current sent by your transmissor to the ground is very low, if so, open the electrodes MN.
Thanks Raymundo and Gregory. I have checked all that.But the situation is that after such intervals and after increasing the current electrode separation for some time it changed back to positive.
Dear Alex, If the resistance values go from positive to negative and back to positive, maybe there is some kind of electrical generator that is working intermittently. Check for water pumps near the survey area. This has happened to me several times. Good luck. Antonio Correia
Alex, in same way of Antonio, may be the mine site at about 20km around your area or there is a water in the survey area that can also affect the potential snake (wire). The potential wire can have a problem of junction where water can come into the potential electrodes system. That could be also the case if it change from positive to negative.
Dear Alex, negative resistance means negative total energy dissipation or, equivalently, that the "subsurface" generates energy. This cannot happen in the strictly passive geophysical sounding domain and does not afford a physical interpretation. Ergo, if your equipment works OK, look for a source. The downside of this thing is that if none is found, then the source may be some weird conductivity configuration that generates a reactive component (an eddy, for instance). Burried metals may be a possibility. In this case, you will never be able to interpret this particular resistivity section - period. Try to see if the problem persists along different directions. If it does, give up. If it doesn't, and you cannot identify any other agent like burried metal, you may have stumbled on an anisotropic structure, in which negative tensor elements are allowed along certain directions, provided that the trace and determinant of the conductivity tensor remain strictly positive (energy must be dissipated). If there is anisotropy the structure is interpretable, but you will need additional fieldwork and specialized software... Good luck...
Thanks Andreas. I truly appreciate your insightful comments and remarks.A few days ago, I ran into a drilling operation a few metres from the site where this very sounding was carried out. Over 300ft have been drilled . Apart from the sandy overburden, from 20ft to 300ft is just a continuous sequence of black / dark plastic shale..
Dear Mr.Alax opara,
The [-] resistance may be due to the presence of a highly conductive formations or high resistive formations like massive charnockite. It may be possible due to skinning effect. If the contact resistance [RG] is more than 5 mega ohms it is possible to get minus values. The contact resistance may be reduced by driving the electrodes deep or saline water may be poured. If it possible to measure the contact resistance, it can be checked out. In Cuddalore & Vanur sandstones of Tamilnadu, India, there may be drastic variations & fluctuations in resistivity values while conducting VES. The VES curve pattern may be haphazard. It may be common in sandstone areas with massive laterite cappings.
Badrinarayanan T.S
I am frequently getting negative values in Deccan trap area with inter-layered Red and Green bole in between the flows. I want to know could these red and green boles be the the reasons for negative values? These intertrappean layers at time contain water but most of the times they are dry.
I encountered the same problem in a typical forest areas (ie Rg is negative). In such areas there is no possibility of metals beneath the surface. I tried two different geological environment and I found that Rg is negative for high conductive materials such as clays. In sands, the 2-D machine works ok and Rg is positive. I am still in the field and I have no option on how to conduct 2-D survey in such conductive formation. 1-D works and the resistivity is as low as 3 ohm-m.
May be the moisture content is zero, sometimes you have to wet the area of investigation if such need arises....
Resistivity values are always positive
There are no negative resistivity values
Interesting case. Have you checked the wiring? maybe if it's really negative just use an inverting circuit.
Also, I got some negative resistivity values during the 2D ERT field survey in a garden area (spacing between electrodes was 1 meter), I did not find the result of that negative values?
Electrode's polarization could result in negative readings, changing the current electrodes' polarity (in your SAS instrument replace the negative electrode instead of positive one and vice versa) would solve this technical problem.
Thank you Mr. Hayder for your answer. This is possible in the VES survey but in 2D ERT you can not have current and potential electrodes separately. Changing the current electrodes' polarity or replace the negative electrode instead of the positive one is impossible. Thanks
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