Page 96 - Geochemical Remote Sensing of The Sub-Surface

Geoelectrochemistry and stream dispersion 73
consider that the end faces of this cylinder carry a double electrical layer with a potential
difference equal to the difference of the potential of two sequential electrochemical
reactions Aq~. Then from the coincidence of the experimental curve [Aq~a ] =f( [ Y-Yext 1)
with the theoretical curve (Fig. 2-48C) it is possible to estimate values of r0, h and Aq).
For the second cathodic reaction at the southern part of the ore body we obtain h = r0 =
320 m, Aq~ = -170 mV. For the same electrochemical process at the northern part of the
ore body, h = r0 = 220 m, Aq) = -160 mV. The estimated values of h and r0 are somewhat
greater than the real values, probably because of the complicated shape of the ore bodies.
For an ore body that contains pyrite, the potential of the second cathodic process can
be calculated. For this it is necessary to add the potential of the first cathodic reaction of
pyrite, q~ =-0.50 V, and the potential difference between the second and first cathodic
reactions, Aq~ = -0.16 V. Thus q)2 ---- q)l+mq ) -- -0.66 V. This value of potential corresponds
to the first cathodic reaction of chalcopyrite (-0.60 + 0.10 V), which is actually present in
the ore body (Ryss, 1983).
Application of the CLPC method is effective at the detailed prospecting stage for
checking geophysical and geochemical anomalies and for locating mineral-enriched
zones. The accuracy of determination of mineral composition, concentration and
reserves of ore bodies by means of the CLPC method is much lower than in the case of
the CPC method.

Polarographic logging (PL)

Polarographic logging belongs to a group of non-linear polarisation
geoelectrochemical methods that are based on the acquisition and interpretation of
voltammograms (in the case of PL, polarograms). These describe the non-linear
dependence of the current on voltage between two special electrodes immersed in the
medium under investigation (Heyrovsky and Kuta, 1965; Ryss, 1973).
For obtaining borehole water polarograms, a dipping sonde is used (Putikov, 1977).
The sonde consists of a mercury-dropping working electrode (WE), an auxiliary lead
electrode and a mercury container which prevents mercury from escaping and causing
pollution. In contrast to laboratory polarography (Heyrovsky and Kuta, 1965), the PL
method does not require an additional supporting neutral electrolyte of high
concentration. The PL sonde accomplishes in situ qualitative and quantitative analysis of
water in boreholes, lakes and seas up to depths of 2 km.
Two modes of polarographic logging have been developed: direct current
polarographic logging (DCPL) (Uvarov, 1981); and pulse polarographic logging (PPL)
(Uvarov, 1984). The potential of the mercury-dropping electrode is a linear function of
time in the DCPL mode. In the PPL mode an additional pulse is introduced by means of
a synchronisation system which enhances sensitivity approximately ten-fold in
comparison with the DCPL mode.