Resistivity Methods - Agricultural Geophysics - page 91

Equation (5.5) and Equation (5.6) imply a soil model having two separate but continuous elec-
tric current pathways, effectively corresponding to two parallel resistors within an electric circuit,
where the first of the pathways involves electrolytic current delivery strictly through the bulk soil
solution, and the second pathway involves electrolytic current delivery along clay mineral and
organic matter surfaces (Knight and Endres, 2005). Here, the term “bulk soil solution” is used
to indicate all of the soil solution excluding the thin layers of soil solution adjacent to clay min-
eral and organic matter surfaces, which contain the “cloud” of cations attracted to the negatively
charged surface sites. Rhoades et al. (1989) developed a soil resistivity equation based on another
somewhat different model having two separate electric current pathways, where the first pathway
is constrained to the larger soil pores and involves electrolytic current transfer strictly through the
“mobile” portion of the bulk soil solution. The second pathway is constrained to the smaller pores
and electrolytic current transfer through the “immobile” portion of the bulk soil solution alternates
with electrolytic current transfer along clay mineral and organic matter surfaces. As an electric
circuit, this second model again corresponds overall to two resistors in parallel, but the difference
with respect to the first model is that one of these parallel resistors is in effect representative of two
resistors in series. The second model considers current flow to be negligible via a “continuous”
pathway involving just the electrolytic current transport along clay mineral and organic matter sur-
faces. More information on the soil resistivity equation derived from this second model is provided
in Chapter 2 and Chapter 4. Regardless of the model, there is general agreement that the flow of
electric current in soil occurs by two mechanisms; electrolytic current transmission through the
bulk soil solution and electrolytic current transmission along particle surfaces composed of clay
minerals and organic matter.
Discussions involving Equation (5.3), Equation (5.4), and Equation (5.5) imply that the soil
resistivity, ρ, is influenced by complex interactions among a number of different factors. These
complex interactions make it entirely possible for the correlation between one specific factor and ρ
to be much weaker or even the inverse of what might be expected (Allred et al., 2005; Banton et al.,
1997; Johnson et al., 2001). The occurrence of this type of result simply indicates that there are
other factors, either individually or as a group, that have a greater impact on ρ than the specific fac-
tor being considered. Some factors affecting ρ, such as soil temperature and soil volumetric water
content, are very transient, often exhibiting substantial changes over periods of a few hours or days.
Other factors affecting ρ, if they vary temporally at all, do so at a slower rate, and in this category
are soil properties including pH, organic matter content, clay content, CEC, and specific surface.
Factors like nutrient level and salinity sometimes exhibit little variation over long periods, but will
then change rapidly with an irrigation or fertilizer application event. Changes in soil temperature
and shallow hydrologic conditions can cause the average ρ value within an agricultural field to
increase or decrease (Allred et al., 2005). Soil resistivity spatial patterns within an agricultural field,
however, have been found to remain relatively consistent over time, regardless of temperature and
shallow hydrologic conditions, indicating that ρ spatial patterns are governed predominantly by the
spatial variations in soil properties (Allred et al., 2005, 2006; Lund et al., 1999).
5.4 theoRetICAl CURRent floW In A hoMoGeneoUS eARth
And AppARent ReSIStIvIty
Resistivity methods produce three-dimensional patterns of electric current flow and electric poten-
tial within the subsurface. Figure 5.3 two-dimensionally illustrates the distributions of current flow
(solid black arrowhead lines) and potential (circular dashed black lines) in a vertical plane that inter-
sects the positions of four electrodes, C 1 , C 2 , P 1 , and P 2 located at the ground surface. For simplic-
ity, the lines of equal potential shown in Figure 5.3a represent only the electric field due to current
applied at C 1 , and the lines of equal potential shown in Figure 5.3b represent only the electric field
due to current collected at C 2 . Figure 5.3a shows that electric current, +I , applied at the surface