Page 64 - Geochemical Remote Sensing of The Sub-Surface

Geoelectrochemistry and stream dispersion 41

C~m
2 3

Co "'~ . . . . .

0 >I;

Fig. 2-19. Dependence of concentration C and mass m of accumulated metal in an element
collector on time ~: 1- pure diffusion accumulation (j=0; Ed~=0); 2- migration under the action of
an external electric field (j>>0); 3- diffusion accumulation with action of a double electrical layer
of semi-permeable membrane (Em~:0) (reproduced with permission from Putikov, 1993).

is a linear function of time (Fig. 2-19, curve 2):

upy
C- h C~ (2.22)

In the case of MDE the extemal current is absent (j=0) and the equation (2.19) takes
the form,

dC k (c _c )+ U Eo,('c )C (2.23)
da: --h o ~ o

If we neglect the influence of the double electrical layer (Edl=_0) we can obtain the
following solution to equation (2.23),

k
C - C o (1 - e ) (2.24)

This is the case of the pure diffusion accumulation of metal, its concentration in the
element-collector increasing smoothly with time up to the equilibrium value Co (Fig. 2-
19, curve 1 ). In the general case, solution of equation (2.23) considering the influence of
the double electrical layer, metal concentration in the element-collector is time-
dependent (Fig.2-19, curve 3).
In its usual configurations CHIM is applied in ground and borehole modes. It is
possible to use different kinds of installations in the ground mode, including those shown
Fig. 2-20. In the case of homogeneous rocks with a constant concentration of metal, C1,
the mass of this metal accumulated in the element-collector, m, is a function of time x,