THE SMOOTH SOUNDING GRAPH. A Manual for Field Work ... - BGR

52
If j' is negative, i.e. the disturbing leakage acts against the j (I) than in the
sounding graph we will get in the ρa-values a trend steeply downwards
ending in negative resistivities.
But from experience we know, that already before reaching these final
stages, so-called "clouds" in ρa-values will be a signal, that there is some-
thing wrong.
We have to ask for the origin of these "clouds" .Looking at the last equa-
j'
tion where is assumed to be a constant and the surface resistivity ρ1=
aI
10 000Ωm as well and take into account influences from outside (e.g.
wind and rain) than remembering the very low voltages concerned we
should not be surprised, that a few raindrops may change these parame-
ters. Repeating the measurement will bring then of course different ρa-
values. This is the result of simple physics. If we try to get out of the diffi-
j'
culties caused by the factor the only chance would be increasing the
aI
distance a of the potential electrodes. This can only be done up to a=L/3
(Wenner-arrangement). So-called "Over-Wenner" would bring the poten-
tial electrodes into the neighbourhood of the current electrodes A and B
causing additional difficulties not to be discussed here. Increasing the
spacing a of the potential electrodes will bring at the utmost a factor 10,
increasing the underground influence from 1‰ to 1%.
Comparing this effect with the ratio 1:1000 in ρ1, this will not be a real
help. Increasing I will cause an increasing of the disturbing current den-
sity j' and therefore be no help at all.
The only chance is to look for a surface layer with higher conductivity be-
fore starting a measurement. If there is no chance for finding a centre
point at a surface