Page 89 - Geochemical Remote Sensing of The Sub-Surface

66 O.F. Putikov and B. Wen

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- [mZ/A] (2.45)
h" 100 J lira

In this case the transient coefficient kl00 has a clear physical meaning, the inverse
value of the limiting current density. In the general case the relationship between kl00 and
ji~n is more complicated and has a statistical character.
For determination of transient coefficient k~00 values, Ryss (1973, 1983) used tens of
known deposits with different mineral compositions, at different depths and in different
geological situations, but with known geological surface So, mineral concentration Ci
and Ii~ i values, which were obtained by CPC polarisation curves for each ore body. It
was found that for cathodic electrochemical reactions, on average, k~00 = 500 m2/A and
for the anodic electrochemical reactions kl00 = 200 m2/A for pH>5 (in dry regions) and
kl00 = 100 m2/A for pH<5 (in wet regions).
The Ryss formula (2.44) is the basic equation of the CPC method for determination
of the quantitative parameters of an ore body. The polarisation curve of the ore body
reflects a number of electrochemical reactions (i = 1, 2,..., n). Thus we can obtain the
limiting current value Ilim i for each reaction and concentration of the corresponding
mineral (as a result of the core investigation). With respect to the dependence on current
that is analogous to (2.44), we can calculate a partial meaning of the total ore body
surface that corresponds to the reaction number i,

Ilimi
Soi - klo o 100 (2.46)
Ci

where, Ilim i = limiting current for the reaction of mineral i in A, reduced to the standard
excitation velocity a'st = 15 mA/s, kl00 = the Ryss transient coefficient, m2/A, Ci =
concentration of the mineral i in %.
In accordance with equation (2.39) the reduced limiting current Ilim i is defined as,

ast
Ilim i --/lim mes.i 9 (2.47)
a

where Ilim mes i is the limiting current of the reaction i, measured from the polarisation
curve for the excitation current rate a ~
It is sufficient, in principle, to perform the CPC investigations only in one borehole,
in which case the exact concentration value for the mineral i, as a rule, is unknown.
Deflections from their exact values of the known concentrations Ci for different minerals
are accidental and the calculated surface S0~ is also accidental. The most probable value