Geoscience Reference
In-Depth Information
site to exchange site produces a mechanism by which electric current is essentially transmitted elec-
trolytically through a soil along clay mineral and organic matter surfaces. As previously indicated,
assuming conditions are wet enough, the presence of clay minerals and organic matter in significant
amounts will increase the capability of the soil to deliver electric current, corresponding to a ρ value
lower than what would have been obtained if the clay minerals and organic matter were absent.
The mechanism of electrolytic current transfer facilitated by clay minerals and organic mat-
ter depends on the number of negatively charged surface sites. Accordingly, if this mechanism of
electric current delivery dominates, which is certainly not always the case, then a strong inverse
correlation between the cation exchange capacity (CEC) and ρ is to be expected. However, different
clay minerals exhibit different CEC values. Furthermore, pH governs part of the clay mineral CEC
and all of the soil organic matter CEC. The resulting implication regarding CEC dependence on
clay mineral type and pH is that an inverse relationship between ρ and the total amount of clay min-
erals and organic matter present, although anticipated, probably will not be as strong as the inverse
relationship between CEC and ρ, again assuming this clay mineral and organic matter facilitated,
surface-based, electrolytic current transport mechanism dominates.
Water molecules orient themselves within the electric field adjacent to clay mineral and organic
matter surfaces. This phenomenon prevents freezing of the soil solution portion near the clay min-
eral and organic matter surfaces. Furthermore, as temperatures drop below 0°C, dissolved ions tend
to migrate out of the soil solution portion that begins to freeze, causing the unfrozen soil solution
next to clay mineral and organic matter surfaces to become much more concentrated with dissolved
ions. The main consequence for soils containing significant amounts of clay minerals and organic
matter is that these soils maintain their ability to deliver electric current, even when temperatures
drop below 0°C for prolonged periods. Therefore, when temperatures stay at 0°C or below for an
extended time, soils containing significant amounts of clay minerals and organic matter will typi-
cally have lower ρ values than soils having little clay mineral and organic matter material.
The following equation for soil resistivity, valid for temperatures above 0°C, was developed by
Rhoades et al. (1976), and takes into account clay mineral and organic matter facilitated, surface-
based, electric current:
( )
+
θθ
ρ
z
z
1
1
5
6
=
(5.5)
ρ
ρ
W
S
The ρ
S
value in Equation (5.5) is the clay mineral and organic matter resistivity component, and
z
5
and
z
6
are empirically derived constants. The validity of Equation (5.5) is supported, in part,
because of similarities to Equation (5.6) (Schlumberger Wireline and Testing, 1991), which is com-
monly used in the petroleum industry to determine the resistivity value of a shaly (clayey) sand.
−
( )
+
()
z
z
φ
S
1
V
WV
R
1
23
SH
SH
=
(5.6)
ρ
z
ρ
1
W
SH
For Equation (5.6), the variables ρ, ρ
W
, φ, and
S
along with constants,
z
1
,
z
2
, and
z
3
were previously
defined, and
V
SH
is a shale (clay) volumetric characteristic, the value of
W
is a function of
S
, and
R
SH
is a quantity related to the shale resistivity. The first terms to the right of the equal signs in Equa-
tion (5.5) and Equation (5.6) certainly exhibit a degree of equivalence to one another, because each
contains θ in the numerator and ρ
W
in the denominator. (With respect to Equation (5.6), remember,
θ = φ
S
.) The quantities that make up the second term on the right side of Equation (5.6) indicate that
this term is a resistivity component related to the presence of clay minerals, much the same as the
second term on the right side of Equation (5.5).
