Page 84 - Geochemical Remote Sensing of The Sub-Surface

Geoelectrochemistry and stream dispersion 61

irlil

N

Fig. 2-39. The basic mode of the contact polarisation curve (CPC) field installation: 1- ore body;
2- borehole; 3- compensation resistor; 4- electric current source; 5- resistance of shunt; 6-
potentiometer; 7- recorder; A- current electrode; B- auxiliary current electrode; M- measuring
electrode; N- non-polarisable measuring (reference) electrode (reproduced with permission from
Ryss, 1983).

other pole, electrode B, is placed in the host rocks beyond the ore body. Recording
comprises the simultaneous measurement of current, which flows across the ore body
surface, and the electrode potential of the ore body. The current is supplied as a linear
function of time. In order to measure electrode potential of the ore body, one measuring
electrode, M is positioned inside the ore body and another measuring electrode, N, is
placed in the host rocks directly in contact with the ore body. In practice we cannot put
electrode N at the desired point, so it is impossible to measure directly the electrode
potential of the ore body in the field. However Ryss (1969, 1973) found that by putting
another measuring electrode on the ground surface in the host rocks beyond the ore
body, it is possible to eliminate the difference between the measured voltage and the
electrode potential of the ore body by means of the compensation method (Fig. 2-39). In
this case the difference of potentials AUMN between the two measuring electrodes is,

AUMN =(a+~AU;

where, q~ is the electrode potential of the ore body (the useful signal), and )-".AUi is the
sum of the potential differences in different parts of the measuring circuit (in the ore
body, in the host rocks and so on) which comprises the disturbance. Usually
l yAu, I>> and this does not allow us to observe the polarisation effects without
using a special technique of potential separation, which is based on the different natures
of the potentials q~ and ~AUi. Since q~ is produced by electrochemical reactions, whereas
~AUi satisfies Ohm's law, q~ is a non-linear function of current, but Y.AUi is a linear
function. This difference between q~ and ~AUi is used in CPC for elimination of the