Resistivity Methods - Agricultural Geophysics - page 103

resistivity depth model from which syn-
thetically generated apparent resistivity
data are in good agreement with actual
field measurements. An important issue
needing special emphasis with respect to
both forward and inverse computer mod-
eling is that within the limits of measure-
ment error for ρ a , there are often several
resistivity depth models, differing from
one another to a certain extent, that are
able to provide a good fit between the mea-
sured apparent resistivity data and synthet-
ically generated apparent resistivity data.
Consequently, the resistivity depth model
obtained through these computer analysis
procedures may not always be “unique,”
and it is worth keeping in mind the pos-
sibility that other models exist that can
adequately account for the observed apparent resistivity data.
The apparent resistivity measurements from an azimuthal rotation survey carried out at a par-
ticular location are plotted on a polar coordinate graph (Figure 5.13). Each plotted ρ a measurement
shown in Figure 5.13 is associated with an angle and a radial length. Given a specific ρ a measure-
ment, the orientation of the electrode array, clockwise from true north, is specified by the angle.
As an example, a ρ a value plotted with an angle of 45° represents a measurement obtained with a
northeast-southwest-oriented electrode array. Figure 5.13 is representative of an azimuthal rotation
survey where ρ a measurements were made as the electrode array was pivoted about a stationary
point in increments of 15° through a complete sweep of 360°. The radial length from the center of
the graph to a data point represents the magnitude of the ρ a measurement.
For any location where an azimuthal rotation survey is conducted, a circular pattern for the ρ a
measurements plotted on a polar graph implies that resistivity is the same in all directions (resistiv-
ity is isotropic). If the plotted ρ a measurements instead have the pattern of an ellipse, then resistivity
varies with direction (resistivity is anisotropic). The principle (longest) axis of the ellipse indicates
the electrode array orientation for which the maximum ρ a value or values, ρ a - Max , were acquired dur-
ing the azimuthal rotation survey. The orientation of this principle axis often corresponds with the
trend of aligned features in the subsurface. The fact that ρ a - Max is found along an orientation coincid-
ing with linear subsurface trends is somewhat counterintuitive but is explained by the “anisotropy
paradox” (Keller and Frischknecht, 1966; Parasnis, 1986). Taylor and Fleming (1988) determined
azimuthal rotation resistivity surveys to be useful for characterizing fracture systems in glacial till.
For vertical fractures with an average length greater than the length of the electrode array, the over-
all fracture system trend was found to coincide with the principle axis of the polar graph ρ a ellipse
(the orientation corresponding to ρ a - Max ). The magnitude of ρ a - Max proved to be a good indicator of
fracture density. The presence of two major fracture systems results in the polar graph of ρ a having
two superimposed ellipses and, therefore, two principle axes (Taylor and Fleming, 1988).
Two of the more detailed and comprehensive data analysis products from resistivity surveying
include horizontal (areal) apparent resistivity (or apparent electrical conductivity) maps and resistiv-
ity (or electrical conductivity) depth sections. Measurements from a constant separation traversing
survey are commonly collected along a set of equally spaced transects covering a study area. With a
complete data set (all ρ a or σ a , EC a values for all transects), interpolation and contouring procedures
can be applied to produce a map showing horizontal changes in ρ a or σ a , EC a .
Figure 5.14 shows two horizontal EC a maps of the same agricultural test plot located in Colum-
bus, Ohio. Measurement transects were spaced 3.1 m apart. The data used to produce Figure 5.14a

0° (N)

270° (W) 90° (E)

180° (S)

fIGURe 5.13 Azimuthal rotation apparent resistivity data
plotted on a polar coordinate graph.